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Astronomy & Astrophysics manuscript no. 9917 May 27, 2008

c ESO 2008

The outskir ts of Cygnus OB2
´ F. Comeron
1

1

, A. Pasquali , F. Figueras , and J. Torra

2

3

3

European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany e-mail:

fcomeron@eso.org pasquali@mpia-hd.mpg.de jordi@am.ub.es, cesca@am.ub.es

2

¨ ¨ Max-Planck-Institut fur Astronomie, Konigstuhl 17, D-69117 Heidelberg, Germany e-mail: Departament d'Astronomia i Meteorologia, Universitat de Barcelona, E-08028 Barcelona, Spain e-mail:

3

Received; accepted
ABSTRACT Context. Cygnus OB2 is one of the richest OB associations in the local Galaxy, and is located in a vast complex containing several

other associations, clusters, molecular clouds, and HII regions. However, the stellar content of Cygnus OB2 and its surroundings remains rather poorly known largely due to the considerable reddening in its direction at visible wavelength.
Aims. We investigate the possible existence of an extended halo of early-type stars around Cygnus OB2, which is hinted at by near-

infrared color-color diagrams, and its relationship to Cygnus OB2 itself, as well as to the nearby association Cygnus OB9 and to the star forming regions in the Cygnus X North complex.
Methods. Candidate selection is made with photometry in the 2MASS all-sky point source catalog. The early-type nature of the

selected candidates is con rmed or discarded through our infrared spectroscopy at low resolution. In addition, spectral classi cations in the visible are presented for many lightly-reddened stars.
Results. A total of 96 early-type stars are identi ed in the targeted region, which amounts to nearly half of the observed sample. Most

of them have featureless near-infrared spectra as expected from OB stars at the available resolution. Another 18 stars that display Brackett emission lines can be divided between evolved massive stars (most likely Be stars) and Herbig Ae /Be stars based on their infrared excesses. A component associated with Cygnus OB9/NGC 6910 is clearly identi ed, as well as an enhancement in the surface density of early-type stars at Cygnus X North. We also nd a which is probably the same as identi ed in recent studies of the inner 1 eld population, consisting largely of early B giants and supergiants, circle around Cygnus OB2. The age and large extension of

this population discards a direct relationship with Cygnus OB2 or any other particular association.
Conclusions. Earlier claims of the possible large extent of Cygnus OB2 beyond its central, very massive aggregate seem to be

dismissed by our region.

ndings. The existence of a nearly ubiquitous population of evolved stars with massive precursors suggests a

massive star formation history in Cygnus having started long before the formation of the currently observed OB associations in the

Key words. stars: early type - open clusters and associations: individual: Cygnus OB2, Cygnus OB9.

1. Introduction
Cygnus OB2 is one of the richest OB associations known in our Galaxy, and it is the most nearby one of its kind at less than ¨ 2 kpc from the Sun (Knodlseder 2003). It is also a unique resource for the observational study of the upper end of the stellar mass function (e.g., Herrero et al. 1999): O stars, Wolf-Rayet (WR) stars, Luminous Blue Variables (LBVs), B[e] stars... are all classes with representatives in Cygnus OB2. This association thus provides, in a single complex, the most complete and accessible showcase of the variety found among the hottest and most massive stars. U BV photometry reveals the presence of hundreds of early-type, heavily-obscured stars in Cygnus OB2 (Massey & Thompson 1991; Kiminki et al. 2007). However, a more real measure of its richness is given by star counts in the near infrared, where the large, patchy foreground extinction in the diBased on observations collected at the Centro ´ Astronomico

rection of the association is greatly reduced. Using this tech¨ nique, Knodlseder (2000) has inferred a content of



2600 OB

stars, including over 100 O-type stars and evolved stars having massive progenitors. Support for this estimate has been pro´ vided by Comeron et al. (2002) using near-infrared low resolution spectroscopy. High signal-to-noise ratio spectra in the visible of the least obscured objects of this sample carried out by Hanson (2003) has con rmed the classi cation of the vast ´ majority of the early-type star candidates listed by Comeron et al. (2002). However, it has also shown that a large fraction of them are giant and supergiant B stars rather than O stars. This suggests a picture more complex than that of a single, recent starburst less than 3 Myr ago giving rise to the association (Massey & Thompson 1991). It also casts doubts on the actual membership of these stars in Cygnus OB2, given their older ages and their extended spatial distribution when compared to the strongly-clustered, earliest-type stars at the center of the association. Since the precursors of the extended population dominated by early B-type giants and supergiants must have been O-type stars, the region must have been producing massive stars long before the formation of the major young clusters and associations currently observed there.

´ Hispano-Aleman (CAHA) at Calar Alto, operated jointly by the Max¨ Planck Institut fur Astronomie and the Instituto de Astrof´ sica de Andaluc´ (CSIC) a Visiting astronomer at the Vatican Observatory


2

´ F. Comeron et al.: The outskirts of Cygnus OB2

We investigate the nature of this population and its possible relationship with Cygnus OB2 by studying the early-type stellar content in a region lying between 1



an 2



from the center

of the association. Earlier results of this work have already revealed the existence of interesting objects possibly related to Cygnus OB2 well beyond the boundaries of the central cluster. Examples are WR 142a, a WC8 Wolf-Rayet star about 1.3



from the center of Cygnus OB2 (Pasquali et al. 2002), and the very massive O4If runaway star BD+43 3654 more than two ´ degrees away (Comeron & Pasquali 2007). The study of the outskirts of Cygnus OB2 is also motivated by its overlap with other structures tracing recent star formation, most notably the Cygnus OB9 association and several compact HII regions and embedded clusters belonging to the Cygnus X molecular cloud complex (Schneider et al. 2006). We use the same technique based on 2MASS J H K colors ´ used in Comeron et al. (2002) to identify OB stars in the targeted region. Infrared spectroscopy allows us to identify new young intermediate-mass stars and evolved massive stars, both characterized by their emission in Br, as well as other likely early-type stars without noticeable emission lines. We also provide spectral types of many of these objects whose relatively light obscuration makes them accessible to spectral classi cation in the blue. This paper can thus be regarded as a continuation of the previous ´ work of Comeron et al. (2002) and Hanson (2003), now probing the largely uncharted region beyond the limits of Cygnus OB2 explored thus far. The reader is referred to the recent review by Reipurth & Schneider (2008) for a detailed description of the current state of knowledge of the vast extent of Cygnus star forming complexes, and, in particular, of the Cygnus OB2 association and the Cygnus X region. the number of candidate OB stars in the region between 1 2
Fig. 1. ( J



-


between 1



(2000)

=

H ), ( H - K ) diagram of the eld surrounding Cygnus OB2 and 2 from the approximate center of the association, at h m s 20 32 26 , (2000) = +40 52 . The plotted magnitudes
S

are from the 2MASS catalog. Only objects brighter than K its origin near ( J

=

10 are

included. Candidate early-type stars are located along a band that has

-

H)

=

0, ( H

-

K)

=

0 and traces the reddening vector.





and

from the center of Cygnus OB2 is comparable to that found

within 1



from the center. In addition, the spread in extinctions

2. Target selection
The use of the ( J

is larger than in the central region, indicating that many OB stars may be very obscured and thus inaccessible to survey in the vis-

-

H ), ( H

-

KS ) diagram to distinguish dis-

ible. To con rm the nature of individual stars lying in the earlytype locus described above and their likely membership in the association, we obtained low-resolution near-infrared spectroscopy of 232 stars in the region between 1 1.70( H

tant early-type stars from late-type evolved stars is founded on the difference in intrinsic colors between both populations. The interstellar reddening law keeps the loci occupied by bright, reddened early- and late-type stars separated by a gap of in ( J

0.

4 mag



and 2



from the

-

H ) at any given ( H

-

KS ) color, in the sense of the ( J

-

H)

center of the association. The selected stars have ( J

-

H)

-
8,

´ color being bluer; see, e.g., Comeron & Pasquali (2005) for an application of this technique to the identi cation of the star responsible for the ionization of the North America and Pelican nebulae. The location of a star along the band de ned by the reddened colors of early-type stars obscured by different amounts is not a proof of its early spectral type, since a certain degree of contamination of this locus is caused by evolved stars with particular photometric properties. Nevertheless, the signature of a rich association such as Cygnus OB2 in the ( J

-

KS )

< 0.

20, implying a closer position to the locus

of early-type stars than to that of late-type stars; and KS

< 8.

allowing us to obtain, with a reasonable investment of observing time, a large number of K -band spectra. We also obtained classi cation-quality spectra in the blue of 30 of the stars with near-infrared spectra consistent with early types and membership in the association, and whose foreground obscuration was relatively low as judged from their B magnitude listed in the USNO-B catalog (Monet et al. 2003). We adopt here the rather loose denomination 'Cygnus OB2 halo members' to refer to early-type stars and young stellar objects located in the area under discussion, and suspected to lie at the same distance as the association. This is not meant to indicate con rmed membership in the Cygnus OB2 association. Moreover, some of the stars we discuss are actually located within the boundaries of Cygnus OB9 (including some members of the open cluster NGC 6910 belonging to that association) or are associated with star forming regions in Cygnus X like DR 17 or DR 21 that are normally not considered to be a part of Cygnus OB2. Hanson (2003) has produced a detailed discussion on the distance to Cygnus OB2 based on current available calibrations of the intrinsic properties of upper main sequence stars. Following these conclusions, we adopt a distance of 1.45 kpc (distance

-

H ), ( H

-

KS )

diagram is clearly indicated by the presence of an enhancement ´ of the population of this locus (see, e.g., Fig. 2 of Comeron et al. 2002). This makes the early-type population easily identi able even in regions where its areal density is far below that of cool giants, thus extending the sensitivity threshold to the earlytype population of the association well beyond the where it stands out in infrared starcounts. The presence of early-type stars beyond 1 2MASS ( J 1



radius



from the center

of Cygnus OB2 is clearly indicated in Fig. 1, where we plot the tween 1 and 2 from the approximate center of Cygnus OB2 at



-

H ), ( H



-
s

KS ) diagram of point sources located be-



(2000)

=

20 32 26 ,

h

m

(2000) = +

40 52 . A band of objects



with varying reddening runs along the locus of early-type stars, well separated from the dominant band de ned by cool background stars, which runs above it. It is interesting to note that


´ F. Comeron et al.: The outskirts of Cygnus OB2

3

modulus 10.80) to the region. We also assume that all the structures discussed here belong to a physically-coherent region lying at approximately that distance, rather than belonging to disconnected entities along the line of sight, as further discussed in Sect. 4.4.

posures for wavelength calibration, to minimize the effects of instrument exure. The frames containing the raw spectra were at eld, subtracted from bias and divided by a spectroscopic

and the spectra were subsequently extracted from each one of them. The individual wavelength-calibrated spectra were coadded after identi cation and removal of cosmic ray hits. The coadded spectra were then normalized to the interpolated continuum,

3. Observations
3.1. Near-infrared spectroscopy

to facilitate feature recognition and comparison to spectral atlases of these generally heavily-reddened objects.

We carried out near-infrared spectroscopic observations during two observing runs at the Calar Alto observatory. The rst one took place from 16 June to 1 July 2002 at the 1.23m telescope, and the second one from 31 July to 2 August 2004 at the 2.2m telescope. The instrument used was in both cases MAGIC, a NICMOS3-based near-infrared camera and spectrograph. We obtained the spectra using the resin-replica grism providing a resolution

4. Results
4.1. Infrared spectral classi cation and membership

Despite the limited resolution and signal-to-noise ratio of our infrared spectra, it is possible to broadly classify our entire sample of stars into eight classes: Reddened stars with featureless spectra: the absence of spectral features at the resolution and signal-to-noise ratio of our spectra is expected from hot stars earlier than mid-B type (Hanson et al. 1996), at which the Brackett lines begin to be clearly visible. Their signi cant foreground reddening and the relatively bright limiting magnitude of our sample ensures that these are luminous early-type stars. Whereas Gtype stars would also appear featureless in our spectra, their infrared colors would in general exclude them from our sample (Sect. 2), and main sequence G stars would be too faint at the distances where reddening begins being noticeable. The sample of reddened stars with featureless spectra (see examples in Fig. 2) is thus expected to have little or no contamination by objects other than O and B stars, as con rmed by our visible spectroscopy of some of the least heavily-reddened stars in this category. A similar conclusion was reached by Hanson (2003), who obtained visible spectroscopy of 14 out ´ of the 31 stars selected in the same manner by Comeron et al. (2002), and con rmed that all of them are indeed earlytype stars. Given the rarity of such objects in the eld, we consider that this class provides a largely uncontaminated sample of members of the Cygnus region. This is the class with most members in our sample, and the 66 stars belonging to it are listed in Table 1 . Visible spectral classi cations exist for 28 of these stars, of which 24 are presented in this work for the rst time.
2

/ =

240 over the 1.50-2.40

µ

m range with the 1

slit used. Each star was observed at six positions along the slit, with exposure times per position ranging from 20 s (stacking 10 integrations of 2 s) for the brightest stars to 60 s (stacking 20 integrations of 3 s) for the faintest, with exposure times determined by the 2MASS KS magnitude and the aperture of the telescope used. We carried out the extraction and calibration of the spectra with dedicated IRAF
1

scripts. The frames obtained at consec-

utive slit positions were subtracted from each other to cancel out the sky contribution to the spectrum, and were divided by a at eld frame. A one-dimensional object spectrum was then at- elded frame. We perextracted from each sky-subtracted,

formed the wavelength calibration of each individual spectrum with the OH airglow lines in each frame as a reference (Oliva & Origlia 1992). The wavelength-calibrated spectra extracted at each sky position were then coadded, with deviant pixels due to detector defects or cosmic ray hits automatically clipped off. Cancellation of telluric features was achieved by ratioing the object spectra by those of the nearby G5IV star HD190771, which is expected to be featureless at the resolution employed, reduced in the same manner. Finally, we performed relative ux calibration by multiplying the reduced spectra by that of a 5,700 K blackbody, which should be a good approximation to the spectral energy distribution of an unreddened G5IV star in the wavelength range covered by our spectra.

3.2. Visible spectroscopy

Reddened stars without measurable absorption lines, but with emission lines (particularly Br): stars in this category are expected to belong to two distinct, unrelated classes. The rst one is composed of evolved massive stars such as B[e], Of, LBV stars or, more likely, early Be stars (Morris et al. 1996), all of which have massive precursors. The second class is composed of intermediate-mass Herbig Ae/Be stars. We consider both classes of objects as possible members of the Cygnus complex as well, and we discuss in Sect. 4.2 how we can distinguish between both. We list in Table 2 the 19 stars in this category, whose spectra are plotted in Fig. 3. Visible spectra are available for 5 of them, and a sixth one, the Wolf-Rayet star WR 142a, has been classi ed as WC8 by Pasquali et al. (2002) from its infrared spectrum. Six of those stars, including WR 142a, had been previously recognized in objective prism surveys as emission-line stars. Lightly or no reddened stars with featureless spectra: A total of 31 stars in our sample show featureless infrared spectra,
2

Spectroscopy in the visible of 26 targets suspected to be reddened early-type stars from their near-infrared photometric and spectroscopic properties was obtained at Calar Alto using CAFOS, the facility visible-light imager and low-resolution spectrograph, in an observing run between 1 and 12 July 2005. We obtained spectra of four additional stars with an identical instrumental setup at the same telescope in an observing run between 17 and 25 August 2006. The grism used covered the range shortwards of

=

6350 å at a resolution of

/ =

1000 and

with a 1 5 slit. We based the exposure times on the B magnitude listed in the USNO-B catalog, and ranged from 10 min to 180 min. For integration times longer than 30 min, the exposures were split in blocks of 30 min, and the individual extracted and reduced spectra were then stacked together. Spectra of three lamps of HgCd, He, and Rb were taken between subsequent ex1

IRAF is distributed by NOAO, which is operated by the Association

of Universities for Research in Astronomy, Inc., under contract to the National Science Foundation. Tables 1 to 9 are only available in the online version of this paper.


4

´ F. Comeron et al.: The outskirts of Cygnus OB2

but their colors indicate little reddening in their direction, thus casting doubts as to their membership in the Cygnus complex. Fortunately, spectral classi cations in the visible, as well as photometric classi cations based on Vilnius photometry, are available for most of these stars, showing that the sample can be split into two groups: O and early B stars with light reddening, whose apparent magnitudes are consistent with the adopted distance modulus of Cygnus OB2. We consider these stars as likely members of the Cygnus complex, placed in the least reddened parts of the region, and we list them in Table 3. We have included in it star #30, classi ed as B8 on the basis of its Vilnius photometry. Such spectral type is unlikely, however, as a B8 star should show noticeable Brackett-series lines in our near-infrared spectra. An earlier B-type seems more likely, and we thus consider it as a possible member of the complex, although with an uncertain spectral type. Virtually unreddened F-G stars, some of which have high proper motions. We consider those as non-members and
Fig. 2. Typical examples of suspected reddened, early-type stars. These stars appear featureless at the resolution and signal-to-noise ratio of our near-infrared spectra.

we list them in Table 4. Only 11 stars in this class, mostly faint, have not been observed spectroscopically and it is not possible to assign them to either of those two subclasses. To preserve our sample of Cygnus complex members as uncontaminated as possible, we list these stars separately within Table 4 and we do not consider them among the possible members. Stars with obvious Br



absorption, and possibly other

Brackett lines as well: these are unambiguously classi ed as late-B, A or early-F stars. Their infrared colors generally indicate little or no foreground extinction, suggesting distances closer than those of the members of Cygnus OB2. With very few exceptions, spectral classi cations in the visible exist in the literature for most of these stars, con rming the above range of spectral types. We thus consider the 48 lightly-reddened objects that we identify in this category, listed in Table 5, as foreground stars. A few exceptions are found: 4 stars have clearly reddened colors and still display obvious Brackett absorption lines indicating spectral types in the late-B to early-F range. Since their apparent magnitudes and reddening indicate an intrinsically high luminosity, these objects are probably giants or supergiants. Their possible membership in the Cygnus region is unclear, and we will not include them in our census of possible members. We list them separately in Table 5 and display some typical spectra in Fig. 4. Red giant branch stars: Although the infrared colors of these stars normally place them on the locus of reddened late-type stars, infrared excesses at 2 of interest of the ( J

µ

m can move them to the region

-

H ), ( H

-

KS ) diagram. This seems to

be the case of most of the 16 stars in our sample that clearly display CO bands starting at 2.29

µm

despite having been

color-selected. They are listed in Table 6, and display a wide
Fig. 3. Near-infrared spectra of the stars in our sample that display Brackett-series lines in emission. For convenience, the spectra are sorted by increasing reddening from top to bottom. The spectrum of an additional emission-line star, the Wolf-Rayet WR 142a, can be found in Pasquali et al. (2002). The sample shown here is expected to contain representatives of two different classes, classical Be stars and intermediate-mass Herbig Ae/Be stars, with the reddest stars belonging to the latter category.

range of foreground reddening, as is obvious in the representative examples shown in Fig. 4. We note, however, that one member of this class, star #221, is projected on the DR 21 star forming region, appears extended at 2.2

µ

m and displays

a spectral energy distribution at longer wavelengths (Davis et al. 2007) characteristic of the Class I/Class II transition (Adams et al. 1987). It is thus likely to be a young stellar object belonging to DR 21, and still surrounded by substantial amounts of gas and dust with its CO absorption being produced in its circumstellar envelope. At the available resolution and signal-to-noise ratio its spectrum (Fig. 5) is typi-


´ F. Comeron et al.: The outskirts of Cygnus OB2

5

B-F stars

Red giants

Carbon stars

Long-period variables

Fig. 4. Typical near-infrared spectra of the different classes of stars contaminating our sample of color-selected, early-type candidate members of the Cygnus OB2 region, which are described in Sect. 4.1.

gram occupied by early-type stars. They are easily recognized by the numerous features in the K window, in addition to the CO bands, and, most particularly, by the prominent C band at 1.77

µ

m just below the telluric H2 O cuto

ff

2

(Lancon ¸

& Wood 2000; Fig. 4). We

nd 29 such stars in our sample,
S

which are listed in Table 7. Only the two brightest ones in K

had been previously recognized as carbon stars, and most of the others are IRAS point sources. We also note that one of the stars that we classify as a red giant in Table 6, star #9, has been identi ed as a carbon star by Chen & Chen (2003), although we do not identify its characteristic features in the near-infrared. Long-period variables: the infrared colors of Mira and semiregular long-period variables makes them another potential contaminant of the early-type locus. The 14 members of this class that we identify in our sample (Table 8; Fig. 4) are easily distinguished by their prominent CO bands as well as by the marked broad absorption due to H2 O in
Fig. 5. Spectrum of the likely young stellar object #221 in the DR 21 star forming region.

their extended envelopes that dominates the spectral energy distribution in the region above and below the telluric feature separating the H and K bands. Protostellar sources: two objects in our sample display ex-

cal of a cold photosphere, except perhaps for its broader CO bands. Carbon stars: Their frequent infrared excesses also place carbon stars in the region of the ( J

tremely red colors ( H (Table 9). The

-

K

S

= 2.

76 and 4.28, respectively)

and featureless spectra, except for hints of Br



emission

rst one is identi ed as the well-known source

-

H ), ( H

-

KS ) dia-

AFGL 2591, a high-mass protostellar object that has been


6

´ F. Comeron et al.: The outskirts of Cygnus OB2

Fig. 6. Spectra of the extremely red sources #158 and #235. Star #158 is the well-studied massive protostellar object AFGL 2591 (see text). Note the similar appearance of the weak Br

Fig. 7. Color-color diagram of all the objects in our sample, with the exception of foreground stars.Circles: stars with featureless spectra; squares: emission-line stars; triangles: massive young stellar objects; 3-pointed asterisks: red giant branch stars; 4-pointed asterisks: carbon stars; and 8-pointed asterisks: long-period variables. The length of the reddening vector (dotted line) corresponds to 20 mag of visual extinction. The dashed line separates the loci of classical Be from Herbig Ae/Be stars.



emission line at 2.166

µm.

the subject of numerous studies; see, e.g., Benz et al. (2007), Poelman & van der Tak (2007), as well as the review by Reipurth & Schneider (2008) for recent discussions on this source. The second one is identi ed as IRAS 20249+3953, for which no detailed observations have been published thus far. The obvious similarity of its near-infrared spectrum to that of AFGL 2591, as seen in Fig. 6, leads us to classify it as a massive protostar also, perhaps in an earlier evolutionary stage than AFGL 2591, given its much redder colors. The source IRAS 20249+3953 was undetected in the search for SiO maser emission by Nakashima & Deguchi (2003).

servational lines of evidence indicate that it most usually results from spin-up of the star via mass transfer from a companion, near the end of the main sequence lifetime (McSwain & Gies 2007). On the other hand, Herbig Ae/Be stars are premain sequence stars that display strong infrared excesses produced by dust in their circumstellar accretion disks, particularly in the puffed-up walls delimiting their inner rims (Dullemond ´ et al. 2001). As shown by Hernandez et al. (2005), the J H K color-color diagram is a useful tool for distinguishing between both classes of star and our own sample con rms the split of both classes according to their position in the ( J rameter Q

4.2. Infrared color-color diagram

The distribution of objects of our sample in the near-infrared color-color diagram is presented in Fig. 7. The most abundant class of stars, those displaying featureless near-infrared spectra, are well aligned along the reddening vector, displaying the colors of reddened normal early-type photospheres without indications of near-infrared excess. Based on the position of the reddest stars with the colors and near-infrared spectra of normal photospheres, we estimate a maximum reddening toward the halo of Cygnus OB2 of AV
max

-

H ), ( H

-

K)

diagram, or more precisely according to the reddening-free pa-

=

(J

-

H)

- 1.

70( H

-

K ) already used in Sect. 2.

Classical Be star candidates cluster around at Q as Q

-0.

18

<

Q

< 0.

07,

whereas Herbig Ae/Be candidates cover a broad range starting

= - 0.

22 and extending to Q

= -1.

38. The adopted divid-

ing line between classical Be and Herbig Ae/Be stars, de ned

= -0.

20, is indicated in Fig. 7. Since Herbig Ae/Be ful-

15 mag, or A

K max

1.7 mag.

ll our selection criteria due to their infrared excess rather than to their photospheric colors, by including them in our sample of candidate Cygnus OB2 halo members we extend it to stars less massive than the OB stars on which we focus. However, including the Herbig Ae/Be in our discussion is still useful as they are additional tracers of recent star formation. Two of the massive young stellar objects that we include in our sample stand out in Fig. 7 due to their extreme infrared excess. Star #158 (=AFGL 2591) is represented by the triangle at (J

The bluest emission-line stars tend to appear slightly to the right of the limiting reddening vector having its origin at the position of the earliest unreddened stars, thus indicating the existence of moderate amounts of circumstellar excess. However, a break is easily identi ed at ( H

-

K)

> 0.

8 as emission-line stars

with redder colors are all far more removed from the limiting reddening vector. We interpret this as a consequence of the existence of two classes of emission-line stars in our sample, as discussed in Sect. 3.1. Classical Be stars have infrared excesses produced by free-free and free-bound transitions in the ionized circumstellar gas, which is thought to be distributed in a decretion disk formed as a consequence of mass loss from the atmosphere of a star rotating at nearly the break-up speed (Sigut & ¨ Jones 2007; Ekstrom et al. 2008). Although the Be phenomenon may appear at all stellar ages, and also among young stars with a rapid initial rotation speed (Zorec & Briot 1997), several ob-

-

H)

= 1.

24, ( H

-

K)

= 2.

76, whereas the even more ex-

treme star #235 (=IRAS 20249+3953) appears near the upper right corner of the diagram. The candidate massive young stellar object #221 shows a modest amount of excess by comparison, occupying a position similar to that of some Herbig Ae/Be stars, which probably indicates a more evolved status. We note the apparent existence of a third object with extreme infrared colors, star #141, which we classi ed among the red giant candidates on


´ F. Comeron et al.: The outskirts of Cygnus OB2

7

Fig. 8. Visible spectra of the stars for which a new MK spectral classi cation is given in this work. Note the strong interstellar feature centered at 4428 å, due to the strong extinction. The main criteria used for classi cation are outlined in Sect. 4.3.

the basis of the well-de ned CO absorption bands longwards of 2.29

4.3. Spectral classi cation in the visible

µm

characteristic of a late-type star. However, an inspection

of the 2MASS images at the position of this star clearly shows the presence of a nearby star of similar brightness at KS , which dominates at shorter wavelengths; in fact, star #141 is not noticeable in the 2MASS J image. The strong apparent infrared excess is thus almost certainly an artifact due to contamination of the photometric measurement by the dominating bright companion.

The new spectroscopic classi cations of 29 stars presented for the rst time in this paper are listed in Col. 7 of Tables 1 and 2.
3

Classi cations were based on a comparison between our spectra and the atlas of Walborn & Kirkpatrick (1990), which contains an abundant selection of early stars providing a very nelygrained coverage of the two-dimensional classi cation of the earliest spectral types, as well as an extensive discussion of the classi cation criteria. In the range of spectral types covered by our stars, spectral subtype classi cation is mainly based on the relative strengths of the HeII features with respect to HeI, most notably HeII



4541 vs. HeI



4471, for O-type stars in which

HeII lines are visible; and on the SiIV/SiIII and SiII/SiIII ratios, as well as the HeI/H ratios, for the early B stars. In turn, the The cool stars in our sample that we classify as non-members appear almost entirely removed from the loci occupied by the candidate members of the Cygnus OB2 halo, having in general much redder colors. This is partly caused by their intrinsically redder spectral energy distributions, but the dominant factor is their larger average distances due to their brighter infrared absolute magnitudes. Whereas we expect OB stars in Cygnus OB2 to have absolute magnitudes in the M luminosity class is largely derived from the appearance of the lines produced by ionized metals (CIII, SiIV) in the proximities of H and, most notably, the intensity of the CIII+OII feature at 4650 å. Because of the large extinction in their direction, all our stars show prominent interstellar absorption features, most notably the broad band centered at 4428 å and narrower lines at 4501 å, 4726 å, and 4762 å. To estimate the uncertainty in the spectral classi cation, each spectrum was separately classi ed by each of the authors using the same reference atlas. A comparison of the individual results obtained then led us to estimate
3

-

3

<

M

K

< -5.

5 range

(Martins & Plez 2006), cool red giants are expected to reach
K

= -7

(e.g., Ferraro et al. 2000); long period variables are

in the range

-6.4 >

M

K

bon stars usually have M

K

> -8.2 < -8

(Knapp et al. 2003); and car(Weinberg & Nikolaev 2001;

We inadvertently also observed star #044, already classi ed by

Demers & Battinelli 2007). They are thus easily detectable at the distance of the Perseus arm and beyond in a magnitude-limited sample like ours.

Hanson (2003), as noted in Table 1. For consistency, we use here our own classi cation. Note in any case that the difference between both (O9.7II in Hanson (2003), B0II according to our own classi cation) is minimal.


8

´ F. Comeron et al.: The outskirts of Cygnus OB2

an uncertainty in each dimension of the spectral classi cation amounting to approximately one luminosity class and less than one spectral subtype, respectively. The spectral types listed in Tables 1 and 2 are the averages of the classi cations attributed by each author. The spectral types obtained for the new members of the region are encompassed within a very narrow range going from O7 to B2, with the vast majority of the members having subtypes between O9 and B1. In contrast, stars can be found in all the luminosity classes from I to V. Our sample includes both objects with featureless spectra in the near-infrared (Table 1) and objects observed to display lines of the Brackett series in emission (Table 2). Since we restricted our visible spectroscopy to targets with relatively bright magnitudes in the blue, we only expect classical Be stars in the latter category to be present in the sample observed in the visible. Indeed, none of the stars that we observed in the visible displays the strong infrared excess char´ acteristic of Herbig Ae/Be stars (Hernandez et al. 2005). On the other hand, of all the classical Be star candidates only one, star #128, also displays noticeable emission in the visible in the form of a faint emission core at the center of the Balmer lines.
Fig. 10. Location of all the 98 candidate early-type stars overlaid on a

NGC 6910

DR 4 DR 7 DR 17 DR 5

Cyg OB9 Cyg OB2
DR 6

W75N DR 21

DR 23 DR 22

DR 9 DR 13

LDK 10

4.4. Spatial distribution

radiocontinuum image of the region at 1420 MHz, from the Canadian Galactic Plane Survey (Taylor et al. 2003). Stars are represented by the following symbols: red squares, stars with featureless near-infrared spectra; black circles, stars with Brackett emission lines; blue triangles, massive young stellar objects; and diamond, Wolf-Rayet star WR 142a. The large inner and outer blue dashed circles are respectively 1 and 2 in radius, and they delimit the region in the outskirts of Cygnus OB2 covered by the present study. The location of the main HII regions, as well as the W75N star forming region and the NGC 6910 open cluster, are indicated. The HII regions noted here with pre x 'DR' were identied by Downes & Rinehart (1966). The pre x 'LDK' refers to the list ¨ of embedded clusters of Le Duigou & Knodlseder (2002). The position of the compact HII region DR 7 is marked here, although it is probably ´ more distant and associated to the background Perseus arm (Comeron & Torra 2001) .

Schematic plots with the spatial distributions of stars that we consider as likely members and non-members of the Cygnus OB2 halo are presented in Fig. 9. Candidate members are those listed in Tables 1, 2, 3, 9, as well as the young stellar object #221. Both populations have clearly different distributions: the non-members are roughly uniformly spread out, as we may expect from a population composed by old stars. Such uniform distribution implies that there are no signi cant largescale differences in extinction along the line of sight at least out to the typical distances of those stars, which as noted above are greater on the average than those of early-type stars of the same apparent magnitude. The stars that we have considered as likely members, on the other hand, do display structure in their spatial distribution. Although OB stars are found all over the eld, two areas of surface density greater than the average appear toward the (galactic) northwest (upper right in the left panel of Fig. 9) and toward the east (left in that same gure). Figure 10 identi es the main structures known in the region, superimposed on a 1420 MHz radio continuum map from the Canadian Galactic Plane Survey (Taylor et al. 2003) tracing the emission by ionized gas. A comparison to previously-known structures in that region shows that the northeastern concentration corresponds to the eastern boundary of the Cygnus OB9 association (e.g., Garmany & Stencel 1992), and in particular to the cluster NGC 6910 (the tight concentration of stars near l

and DR 23 are all structures belonging to Cygnus X and lying at the same distance as Cygnus OB2, although we note that a distance of 2-3 kpc to W75/DR21, signi cantly larger than the one adopted here, has been often adopted in the literature (Kumar et al. 2007). The concentration of early-type stars on the eastern side is particularly obvious at the position of DR 17, composed by the earliest-type members of the embedded clusters hosted by ¨ this HII region (Le Duigou & Knodlseder 2002). However, it is more widespread and extends beyond the boundaries of the compact HII regions. A similar result has been reported recently by Kumar et al. (2007), who nd that many young stars revealed by their infrared excesses in the Spitzer bands are located outside the boundaries of the main molecular complexes in the region. We note, in particular, the concentration of emission-line stars toward this general region, with a preference toward the surroundings of DR 21, although DR 21 itself is too obscured for its embedded stars to be included in our magnitude-limited sample (Nadeau et al. 1991). Almost all these stars have clear infrared excess placing them among the Herbig Ae/Be stars as discussed in Sect. 4.2, with the only exception of star #118, appearing in the direction of W75N and whose near-infrared colors place it in the classical Be star locus. Only another candidate classical Be star is possibly associated with a compact HII region, namely

=

78.7, b

= +2.

0, which are recovered by our candidate mem-

ber selection procedure). On the eastern side, the broad enhancement of stellar surface density contains the chain of compact HII regions DR 17, DR 21 in the W75 complex, DR 23, and DR 22, all of which are embedded in the Cygnus X North complex. The reality of Cygnus X as a single physical entity, rather than as a chance alignment of unrelated structures lying at different distances along a spiral arm seen nearly end-on, has been widely debated in the literature (see, e.g., Odenwald & Schwartz 1993). Recent detailed observations of the molecular gas in the region, mainly by Schneider et al. (2006, 2007), strongly argue for a real interconnection among these structures and present evidence for their interaction with the massive stars of Cygnus OB2. Based on those results, we consider here that DR 17, W75/DR 21, DR 22,


´ F. Comeron et al.: The outskirts of Cygnus OB2

9

Fig. 9. Left panel: Location of all the candidate members identi ed through their infrared spectra (Tables 1, 2, 3), including the protostellar candidates listed in Table 9 and star #221. Symbols are as follows: open circles, stars with featureless near-infrared spectra; with Brackett emission lines; three classes of non-members; open circles, red giant stars; lled circles, carbon stars; and open triangles, long-period variables. lled circles, stars lled triangles, massive young stellar objects; and open triangle, Wolf-Rayet star WR 142a. Right panel: same for

star #122 near DR 6, although the spatial coincidence between them is not precise.

4.5. Evolutionar y status

It is thus possible to identify three distinct components in the The southwestern (lower right) part of the area covered by this study South of the galactic equator appears particularly devoid of early-type stars. This is interesting since that region coincides with Cygnus X South (Schneider et al. 2006), and recent stellar surface density estimates based on extinction-corrected starcounts (Bontemps et al., in prep; preliminary results shown in Reipurth & Schneider 2008) indicate an enhanced stellar density in that direction, which appears to be an extension of Cygnus OB2. The starcount enhancement detected by Bontemps et al. is contained within 1 from the center of Cygnus OB2 and is in rough agreement with the distribution of early-type stars in ´ the same region found by Comeron et al. (2002; see Fig. 14 in that paper), which preferentially extends toward the west. The same distribution is well matched by that of A0V-A5V stars recently identi ed by Drew et al. (2008). As discussed by those authors, the magnitudes of these stars suggest an average age somewhat older than that of the Cygnus OB2 cluster if they lie at the distance of 1.45 kpc assumed here, in agreement with the results of Hanson (2003), and it is most likely the lower mass ´ counterpart of the sample discussed by Comeron et al. (2002). The fact that we detect no traces of it in our color-selected sample beyond the 1 area targeted by this study. The rst one is the population associated with the eastern edge of Cygnus OB9, particularly the cluster NGC 6910. The second component extends over the region occupied by the DR 17, W75/DR 21, DR 22, and DR 23 complexes in Cygnus X North. The third component is a distributed population whose members are stars that do not belong to either of those two groups, which may be related to the extended population in the central 1 -radius circle around Cygnus OB2 ´ reported by Comeron et al. (2002) and Hanson (2003), and probably also by Drew et al. (2008). The new availability of detailed spectral classi cations for many stars in these three groups allows us to investigate in more detail their nature and evolutionary status. To this purpose we have adopted for each O-type star an effective temperature T Martins et al. (2005), using their observational effective temperature scale. To our knowledge no similar work is available to date covering the early B spectral interval, in particular the whole range of luminosity classes covered by our spectra. We have thus used the T
ef f ef f





based on the T

ef f

-spectral type calibration of

-spectral type compilation by



Tokunaga (2000) for luminosity classes I and V, applying a scaling factor to the temperatures so as to force agreement with Martins et al.'s (2005) observational temperature scale at spectral types O9I and O9.5V respectively. The calibrations of Tokunaga (2000) and Martins et al. (2005) closely match each other in the overlapping late-O range, implying that our arbitrary scaling factors deviate from unity by less than 4%. Because the compilation of Tokunaga (2000) does not include luminosity class III early-type stars, we have estimated their temperatures by means of a weighted interpolation between the temperatures adopted for classes I and V. The weight factor is derived once again by forcing agreement between the temperatures derived in this way for spectral type O9III and the corresponding value given by Martins et al. (2005). Approximate temperatures of B

circle suggests that it either does not reach

beyond that distance, or that its possible extension does not contain massive stars picked up by our selection criterion. On the other hand,
13

CO maps of that region (Simon et al., in prep; pre-

liminary results shown in Reipurth & Schneider 2008) indicate the presence of large amounts of molecular gas in the zone where we do not detect a corresponding overdensity of early-type stars. It thus appears that any star formation currently going on in that region of Cygnus X is not producing massive stars. It should be noted that this does not apply to the entire Cygnus X South region, which extends well beyond the boundaries of the area considered here and does contain massive embedded clusters and HII regions.


10

´ F. Comeron et al.: The outskirts of Cygnus OB2

stars of luminosity classes II and IV have then been computed by averaging the temperatures adopted for stars of luminosity class I and III, and III and V, respectively. We estimate that the systematic errors introduced in this way are below 2000 K for all the spectral types represented in our sample. A summary of T
ef f

effects can brighten O stars by as much as



MV

= 0.
-1
.

6 (Meynet

& Maeder 2000) for an initial velocity of 200 km s

The situation is less clear in Cygnus X North, as the higher extinction in the area prevented us from obtaining visible spectra of many stars. The presence of several Brackett emission-line stars with the infrared characteristics of Herbig Ae/Be stars suggests recent star formation over a widespread area. The ve stars with spectral classi cation in the zone are all lightly reddened and may be on the near side of the complex. The luminosity classes (Ib-III) of four of them indicates that they are not related to the ongoing star formation in the area, with the possible exception of the O9V star #100 that is very close to DR 21. It can be seen in Fig. 11 that a physical association of this star with DR 21 would support the distance adopted here, rather than the 2-3 kpc often used in the literature (Davis et al. 2007), as a greater distance would move it even further away from the main sequence. In contrast, most of the population outside those two areas, which we consider as the eld component, is lightly reddened, and visible spectroscopy is available for 20 of its members, of which 14 are found to have luminosity classes placing them above the main sequence. Of the remaining six stars, four are classi ed as B1V or B2V and thus main sequence lifetimes longer than 10 Myr; and only three stars, #150 (O8V; Morgan et al. 1955), #152 (O9.5V; this work) and WR 142a (WC8, for which Pasquali et al. (2002) estimate an age of at least 3 Myr) indicate more recent massive star formation. The rst two are also the ones closest to the main sequence plotted on Fig. 11, in agreement with the common distance adopted for all our stars. Besides WR 142a, 7 out of 9 emission line stars in the region appear to be classical Be stars, with the only exceptions of stars #178 and #180. A comparison of the position of the stars belonging to the eld component with the evolutionary tracks in the temperature-luminosity diagram indicates ages older than 6 Myr for this population. It may be noted that several stars classi ed as B1V or B2V appear up to 2 mag above the main sequence in the temperaturemagnitude diagrams for both Cygnus OB9 and the eld population. This is considerably more than what one may expect from the effects of rotation, binarity, or the spread in intrinsic magnitudes within their spectral types. In fact such stars are not expected to be present in our sample, as normal main sequence stars later than B1V at the distance of Cygnus OB2 should have KS magnitudes fainter than our target selection limit (see Sect. 2). In principle, these may be unrelated foreground stars at much closer distances. However, an inspection of their spectra in Fig. 8 shows interstellar features of a depth similar to those of the other stars in the region, indicating a similar level of extinction that is con rmed by their infrared photometry. A misclassi cation of the luminosity class thus appears as a more likely cause, which may be con rmed with better quality spectra. However, we have decided not to modify the spectral types given in this paper so as not to bias our classi cation with a posteriori knowledge.

-spectral type calibrations for B-type stars presented by

Fitzpatrick & Massa (2005) shows that existing systematic uncertainties in this domain exceed this value. We thus consider that the ad-hoc procedure that we have described to assign effective temperatures to our stars does not introduce large additional uncertainties. The large scatter in absolute magnitudes at a given spectral ´ type among O and early-B stars (e.g., Jaschek & Gomez 1998) cautions against the use of the spectral classi cation alone to derive the positions of our stars in the temperature-absolute magnitude diagram, and compare them to the predictions of evolutionary tracks to assess their evolutionary status. Instead, we have preferred to adhere to the underlying hypothesis already noted in Sect. 4.4 that all our stars have essentially the same distance modulus of 10.80 mag, and derive individual absolute magnitudes as

M

V

=

K

-

10.80

- 0.659[( J - -

K)

-

(J

-

K )0 ]

+

(V

-

K )0

where ( J

-

K )0 and (V

K )0 are the intrinsic, unreddened color

indices and the factor 0.659 is derived from the extinction law of Rieke & Lebofsky (1985). We have used the intrinsic colors derived by Martins & Plez (2006) for O stars and those compiled by Tokunaga (2000) for B stars. We estimate that interpolations needed to cover all the spectral types and luminosity classes in our sample introduce a systematic error in M
V

not exceeding

0.1 mag. The scatter in intrinsic colors among actual stars is likely to be a more important source of errors, as illustrated by the results of Robert et al. (2003). The T
ef f

- MV diagrams are shown in Fig. 11 for the easteld population. In these gures, we also plot

ern edge of Cygnus OB9, the region in Cygnus X North around DR 21, and the the isochrones calculated by Lejeune & Schaerer (2001) in their Case e with solar metallicity, high mass loss and no rotation, for initial stellar masses (60), 40, 25, 20, 15, 12 and 9 M ted lines). Individual T and M (solid lines) and stellar ages 2, (3.1), 4, 6, 8, 10, 15 and 25 Myr (dotdifferent regions are listed in Table 10. There are signi cant differences between the distributions of stars in the temperature-luminosity diagrams among the three components considered. The Cygnus OB9 region contains abundant O-type main sequence stars, particularly in the cluster NGC 6910 whose members display other signatures of youth (Delgado & Alfaro 2000), and only four stars outside the main sequence (the B giants #181, 193, and 206 and the B1.5IV star #142) are found within this region in our sample. The class V Otype stars appear mostly above the main sequence, which may indicate that they lie at a shorter distance than that adopted by us. Garmany et al. (1992) ulus D M nd a very uncertain distance modef f V

derived for each star in the

=

10.0 to NGC 6910, in contrast with the better

determined value for the rest of the association, D M

=

11.0.

However, this latter value is in closer agreement with the distance found by Delgado & Alfaro (2000) for NGC 6910 itself, D M

5. Discussion
Our observations sample an interesting region where the outskirts of the Cygnus OB2 association merge with neighbor structures, allowing us to explore the possible existence of links among them and the history of star formation in the whole region. Such questions are rendered all the more relevant given the increasing evidence for their common distance (Schneider et al. 2007) and the possibility that the distinct structures rec-

=

11.2

± 0.

2, which solves the discrepancy noted by

Garmany & Stencel (1992) and is only marginally larger than that adopted by us. A value signi cantly shorter than D M

=

10.8

thus does not seem supported by other observations. The position of the class V O-type stars above the lowest isochrone may be at least partly due to binarity, as well as to rotation, whose


´ F. Comeron et al.: The outskirts of Cygnus OB2

11

-7 -6 2 -5 -4 -3 -2 4.6 4.5 40 25 20

4 CygnOB9 Cyg OB9 6 8 10 15 25 15 12 9 4.4 4.3

-7 -6 2 -5 -4 -3 -2 4.6 4.5 40 25 20

4 DR21 Cyg X North 6 -6 8 10 15 25 15 12 9 4.4 4.3 -2 4.6 4.5 -4 -3 -5 40 25 20 2 -7 60 6 8 4

3.1

Field FIELD

10 15 25 15 12 9 4.4 4.3

Fig. 11. Temperature-absolute magnitude diagram with the location of the stars for which spectral classi cation is available in each of the three regions discussed in Sect. 4.5. Circles indicate luminosity class V, crosses class IV, triangles class III, squares class II, and asterisks class I. The evolutionary tracks and isochrones from Lejeune and Schaerer (2001) are plotted for comparison, highlighting the older age of the We believe that most of the stars in the Cygnus X North area actually belong to the part of the eld population. eld population lying along that line of sight.

ognized thus far are parts of a single association, particularly Cygnus 1, 8, and 9 (Mel'nik & Efremov 1995).

+5

distribution of OB stars around Cygnus OB2 and probably beyond, selected through their near-infrared colors and con rmed on the basis of their near-infrared spectrum. Visible spectroscopy con rms the very high reliability of infrared spectroscopy at modest resolution and signal-to-noise ratio in identifying earlytype stars, as already found by Hanson (2003) when investigat´ ing the sample of candidates identi ed by Comeron et al. (2002). Our results also show a remarkable coincidence with those of Hanson (2003) in that the newly-identi ed early-type stars do not have, as a rule, spectral types as early as those at the center of Cygnus OB2. Instead, they are dominated by massive stars that have already evolved o

galactic latitude

One of our main results is the identi cation of an extended

0

-5 90

85

80

75

70

galactic longitude
Fig. 12. Surface density of OB stars selected according to the criteria K

ff

the main sequence (i.e., giants,

< 8.

8,

-0.

15

<

(J

-

H)

- 1.

70( H

-

KS )

< 0.

20. The shade of grey

supergiants, and a Wolf-Rayet) or are thought to be at the end of their main sequence lifetime (the classical Be stars). Such evolved stars are dominant among the eld component described in Sect. 4.5. Their presence also among the populations that we identi ed as belonging to the complex that includes DR 17, DR 21, DR 22, and DR 23, on one side, and Cygnus OB9, on the other, is not surprising since our allocation of stars to those groups is purely based on position. A certain level of contamination of those two components by stars actually belonging to the eld population is thus to be expected. What is the relationship between the eld component and

scales with the number of stars per square degree, ranging from zero (white) to 100 or more (black). The Cygnus OB2 central cluster is the black square near the center, with a density of 176 stars per square degree. A pedestal surface density of 12.5 stars per square degree has been subtracted from each cell to account for the estimated contamination by cool and foreground stars that are also selected by the color-based criteria.

the results of Sect. 4.1 as a way to remove statistically the contamination by cool background stars and by foreground stars. 70 We have carried out this exercise over the area limited by



Cygnus OB2, if there is one? Hanson (2003) already pointed out that the spatial distribution of the evolved component differed markedly from that of the most massive stars of Cygnus OB2 in being much more evenly distributed over the studied area. In fact, based on both the wider spatial distribution and the gap in age Hanson suggested that the evolved component is unrelated to Cygnus OB2. Our new observations allow us to reevaluate this conclusion by extending the analysis of the spatial distribution to a larger area. Unfortunately, the somewhat different selection of candidate ´ early-B stars carried out in Comeron et al. (2002) and the present paper prevents a direct comparison between the stellar densities inside and outside the 1

<

l

<

90 ,



-5 <

b

< +5



, which has Cygnus OB2 near

its center and includes the entire Cygnus OB9 region as well as parts of Cygnus OB1 to the West and Cygnus OB7 to the East. Stars were selected according to the criteria KS

<

8.8,

-0.

15

<

(J

-

H)

- 1.

70( H

-

KS )

<

0.20. The results of

the spectroscopic follow-up discussed in Sect. 4.1 indicate that these criteria select approximately 12.5 cool (red giants, longperiod variables, and carbon stars) and foreground (mostly late B to F-type) stars per square degree, in addition to OB stars. We assume that these contaminants are uniformly distributed across the eld, and subtract this number density from the total number density of selected objects. The result is displayed in Fig. 12, which is similar to the surface density maps presented by ¨ Knodlseder (2000) and Bontemps et al. (in prep.; presented by Reipurth & Schneider 2008). The main difference is that Fig. 12 is restricted to early-type stars; the surface densities are thus much lower and the resolution much coarser than in their contour



radius. It is however possible to per-

form the color selection of candidates over a much larger area that includes the central 1



radius around Cygnus OB2, the re-

gion covered by this study, and the surrounding regions, using


12

´ F. Comeron et al.: The outskirts of Cygnus OB2

maps. As one may expect, Cygnus OB2 is the most prominent feature appearing as a very tight concentration near the center of the gure, with a peak surface density reaching 176 OB stars per square degree in the central 0.25 square degrees. The stellar density remains high toward the western half occupied by Cygnus OB9 next to Cygnus OB2, and then Cygnus OB1. The easternmost quarter of the gure is occupied by the more nearby association Cygnus OB7 (Garmany & Stencel 1992). Despite the overall similarity, there are signi cant differences between our map and the general starcount-based map of Bontemps et al.: Cygnus OB9 appears much more prominently in our map, whereas the DR 17/DR 21/DR 22/DR23 region, while easily recognizable as a stellar density enhancement in our Fig. 10, stands out much more clearly in Bontemps et al.'s map. Interestingly, Fig. 12 shows that a non-zero surface density of early-type star candidates pervades the region even outside the boundaries of the already-known OB associations. By extrapolating the results obtained in the area between 1 and 2 from the center of Cygnus OB2, we tentatively identify this as the eld population mainly composed of evolved stars, noting that this population does not appear to be associated with Cygnus OB2 nor with any other speci c feature in the region. The massive precursors required by this widespread eld population suggests that massive star formation has been taking place in the Cygnus region for a time exceeding 10 Myr, and long before the formation of the currently observed associations like Cygnus OB2, whose age is less than 4 Myr (Massey et al. 1995). The fact that such a population appears as an extended component with no apparent concentration is not entirely surprising, given the high density of OB associations in the region, if we assume that this density was similarly high in the past. The typical center-tocenter distance among the four associations currently de ned in

ena such as the apparent large-scale expansion pattern noted by ´ Comeron et al. (1998) in the proper motions of stars in the area.

6. Conclusions
´ We have extended previous work carried out mainly by Comeron et al. (2002) and Hanson (2003) in characterizing the population of early-type stars in the vicinity of Cygnus OB2. We have done this by identifying a magnitude-limited sample of candidate early-type stars through near-infrared imaging in the area between 1



and 2



from the center of Cygnus OB2, con rming

them through near-infrared spectroscopy, and providing an accurate spectral classi cation of many of them through spectroscopy in the visible. Our main results can be summarized as follows: The studied area is rich in early-type stars, including evolved massive emission-line stars, likely Herbig Ae/Be stars, and some deeply-embedded massive young stellar objects. As in previous studies, a color-based selection criterion is found to be e





fficient

in selecting candidate early-type stars, although

with a considerable degree of contamination by other types of objects, mainly cool stars and foreground late-B, A, and F stars. Near-infrared spectroscopy at modest resolution and signal-to-noise ratio suffices to identify the actual emissionline stars. Our early-type selection criteria recovers many stars belonging to the Cygnus OB9 association and its cluster NGC 6910, which are partly included in the studied area. We also nd a signi cant number of reddened early-type stars and Herbig Ae/Be star candidates in the area occupied by the Cygnus X North complex, which includes the compact HII regions DR 17, DR 21, DR 22, and DR 23. The massive star distribution is, however, not con ned to the HII regions. Many early-type stars are found not to be associated with any known structure. Visible spectroscopy is available for most of this eld population, and shows that it is strongly dominated by giant and supergiant early-B stars. The characteristics of this population appear to be the same as discussed by Hanson (2003) for the additional early-type stars identi´ ed by Comeron et al. (2002) in the vicinity of Cygnus OB2. It most probably represents a population signi cantly older than the massive stellar cluster at the center of Cygnus OB2. We have extended the color-based selection criterion to estimate the surface density of early-type candidates in an area of 200 square degrees centered on Cygnus OB2, statistically correcting for the unrelated background and foreground populations. We nd that the eld population is not particularly related to Cygnus OB2 nor to any other structure of the region, being all-pervasive. The need for massive precursors to produce this extended eld population suggests that massive star formation in the whole Cygnus region has proceeded for a long time, well before the currently identi ed OB associations were formed. Such interpretation rests on the assumption underlying this work that all the early-type stars are located at similar distances, and it would be challenged if the eld population were either foreground or background to the OB associations, or if it spread over a wide range of distances. Although this possibility needs to be clari ed by future studies, we do not nd evidence in our temperatureabsolute magnitude diagrams for a systematic difference between the distance to the members of this population and the adopted distance of Cygnus OB2. We believe that the present study conclusively dismisses the case for a large extent of Cygnus OB2 much beyond the

the galactic longitude interval 74 < l < 82 (Cygnus OB1, OB2, OB8, and OB9) is 2 5. At the distance of 1.45 kpc
adopted throughout this paper, an internal velocity dispersion of 3.5 km s

-1

would su

ffi

ce to make them overlap after 10 Myr if

they are gravitationally unbound, even if they occupied a small initial volume. This velocity dispersion is comparable to the internal velocity dispersion derived for members of Cygnus OB2 by Kiminki et al. (2007), 2.44

± 0.

07 km s

-1

. Moreover it can be

regarded as an upper limit, since even very young OB associations like Cygnus OB9 occupy a sizeable volume comparable to the center-to-center angular distance quoted above, and the eld population contains members older than 10 Myr leaving more time for dispersal of the original associations. This population may be the equivalent to an extended component also observed in other galaxies (Pasquali & Castangia 2008), and it can thus be accounted for by assuming a previous generation of OB associations, now dispersed, with characteristic similar to those of the current associations. It appears unlikely that the eld population has its origin in bound clusters having become unbound in their early evolutionary stages (Lada & Lada 2003): recent observational evidence discussed by Gieles & Bastian (2008) indicates dispersal timescales at least one order of magnitude longer than the age of the eld population in a wide range of environments, including our galactic neighborhood. Our results rather suggest that those massive OB stars originate in either isolation or unbound aggregates. The sustained massive star forming activity in Cygnus must have been essential in arranging the large-scale distribution of the interstellar gas and in driving the current generation of star formation. It may also be at the origin of the large Cygnus Supperbubble observed in X rays (Cash et al. 1980, Bochkarev & Sitnik 1985, Uyaniker et al. 2001) and possibly related phenom-


´ F. Comeron et al.: The outskirts of Cygnus OB2

13

boundaries of its central concentration, which was already sug´ gested by Garmany & Stencel (1992) and supported by Comeron et al. (2002), but then questioned by Hanson (2003). Previous studies such as those by Massey & Thompson (1991) and ¨ Knodlseder (2000) clearly show that most of the massive stellar content of the association still awaits identi cation and classi cation (see also Kiminki et al. 2007, for recent work on the con rmation of new OB members in the inner regions of Cygnus OB2). Our results indicate that future studies of Cygnus OB2 aiming to characterize its upper main sequence should concentrate on heavily-reddened members near its core, rather than on identifying new members at large distances from it. We note that over half of the early-type stars for which we have obtained con rming near-infrared spectroscopy do not yet have a detailed spectral classi cation available. This is particularly important for the new candidates identi ed in Cygnus X North, whose proximity to compact HII regions, some of which contain embedded clusters, leads us to suspect a young age for most of them. If con rmed, the extent and composition of this group may warrant its status as an OB association in its own right, although we feel that a better characterization is still needed before taking that step. On the other hand, the as yet unclassi ed stars located elsewhere offer a potential for the discovery of new interesting members deserving detailed study, such as WR 142a or BD+43 3654. The fact that the vast majority of stars that form the basis for the present study are recognized here as early-type members of the region for the rst time is a reminder of the large amount of observational work still needed to characterize the stellar population in the Cygnus region. The importance of this task is stressed by the rapid progress being made at different wavelengths in the characterization of its interstellar content.
Acknowledgements. Once again it is a pleasure to thank the staff of the Calar Alto observatory, and especially Santos Pedraz and Ana Guijarro, for their unfailingly competent and friendly support while observing on Calar Alto. We are very thankful to Bo Reipurth and Nicola Schneider for making available to us their chapter on the Cygnus region for the Handbook of Star Forming Regions prior to publication. We also thank the anonymous referee for constructive comments that helped improve both the style and content of this paper. FC warmly acknowledges the hospitality of the Vatican Observatory during the preparation of this paper. AP acknowledges support from the OPTICON Network. JT and FF acknowledge support by the Spanish Ministry of Science and Technology under contract AYA2006-15623-C02-02. This research has made use of the SIMBAD database operated at CDS, Strasbourg, France. It also makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation; and of data from the Canadian Galactic Plane Survey, a Canadian project with international partners, supported by the Natural Sciences and Engineering Research Council.

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´ F. Comeron et al.: The outskirts of Cygnus OB2 , Online Material p 1 Table 6. Red giant candidates Star 1 2 3 4 9 84 87 127 141 162 168 182 184 221 225 247 248 Table 8. Likely long-period variables K Notes 1 Star 8 40 51 2 3 54 61 80 89 91 111 121 124 160 163 4 177



(2000)



(2000)

S



(2000)

(2000)
39 59 56.7 39 36 35.2 40 30 33.0 42 42 35.4 39 50 59.3 42 23 14.0 40 46 47.6 39 44 59.8 39 42 01.6 41 40 12.6 39 33 49.0 41 49 12.5 41 32 02.4 42 01 17.5

K

S

Notes

20 25 41.39 20 27 41.05 20 24 17.99 20 35 18.04 20 39 47.55 20 30 40.10 20 36 40.68 20 31 00.20 20 39 22.70 20 33 53.54 20 22 39.72 20 35 28.30 20 24 22.85 20 38 46.36 20 40 35.63 20 40 46.98 20 32 20.89

40 00 33.5 39 51 44.0 41 13 09.9 41 53 24.5 40 47 04.1 42 22 46.6 40 07 29.4 42 20 16.1 41 13 11.4 42 58 14.8 40 31 40.4 42 53 00.6 39 58 23.6 42 24 39.7 42 10 17.0 39 50 05.4 42 32 49.1

4.186 4.231 4.309 4.436 4.885 7.511 7.553 7.976 8.145 8.337 8.388 8.471 8.473 8.713 8.736 8.839 8.845

20 35 49.22 20 34 55.26 20 37 23.92 20 32 35.80 20 29 10.72 20 28 01.84 20 26 47.81 20 34 15.36 20 27 23.51 20 25 49.61 20 39 10.34 20 29 07.75 20 27 46.56 20 40 09.21

4.801 6.977 7.129 7.178 7.284 7.483 7.563 7.603 7.832 7.917 7.931 8.328 8.345 8.441

Notes: 1: HD 229291, classi ed in the visible as M0 (Neckel 1958). 2: HD 196360, classi ed in the visible as K0III (Bidelman 1957). 3: Classi ed as a carbon star by Chen & Chen (2003); the C feature is barely visible in our near-infrared spectrum. 4: Possible Class I/II member of DR 21 (Davis et al. 2007).
2

1.77

µ

m

Table 7. Carbon stars Star 5 6 11 24 25 43 47 52 55 76 79 90 98 102 103 105 113 114 115 117 119 123 151 165 167 187 200 213 224



(2000)



(2000)

K

S

20 23 10.08 20 40 58.42 20 22 56.90 20 41 25.51 20 35 47.30 20 31 58.75 20 36 10.71 20 28 38.36 20 24 36.14 20 38 24.09 20 27 26.95 20 41 08.34 20 26 26.64 20 33 11.10 20 41 18.63 20 29 14.98 20 25 28.25 20 30 50.87 20 27 04.60 20 23 16.58 20 27 27.82 20 28 24.74 20 41 26.61 20 34 45.66 20 38 57.69 20 38 18.30 20 33 05.99 20 29 36.10 20 39 37.54

40 57 41.7 40 33 47.8 41 17 01.7 40 15 21.8 42 40 53.2 42 48 08.0 39 13 07.6 42 35 47.7 40 51 30.9 39 51 52.3 40 24 19.3 40 24 14.2 40 21 20.5 39 02 24.3 40 21 53.7 39 19 23.5 41 41 09.4 42 38 14.4 42 38 31.4 41 07 44.0 42 43 42.7 42 20 10.2 41 44 32.2 42 49 13.3 41 09 34.5 41 42 39.5 39 01 16.1 41 49 24.9 42 11 32.4

4.653 4.676 5.174 6.413 6.432 7.014 7.084 7.150 7.190 7.453 7.460 7.567 7.695 7.731 7.734 7.750 7.851 7.852 7.865 7.876 7.909 7.924 8.262 8.359 8.373 8.529 8.601 8.674 8.729


´ F. Comeron et al.: The outskirts of Cygnus OB2 , Online Material p 2 Table 2. Emission line stars Star 46 48 66 67 112 116 118 120 122 125 128 136 144 148 178 180 198 199 218



(2000)



(2000)

J 8.574 9.220 8.703 9.077 8.286 12.058 8.239 8.686 8.499 9.865 8.883 10.106 10.666 13.160 13.083 12.325 9.735 10.198 9.222

H 7.762 8.077 7.880 7.986 7.999 9.687 8.018 8.191 8.120 8.635 8.392 8.937 9.603 10.307 10.379 10.207 9.067 9.291 8.894

K

S

Vis. classi cation

Other IDs AS 415 WR142a

Notes 1 2 3

20 24 38.42 20 24 06.20 20 26 52.54 20 36 44.24 20 28 15.21 20 38 08.27 20 35 16.63 20 38 59.18 20 27 25.52 20 38 13.55 20 28 39.20 20 23 16.63 20 38 45.88 20 38 57.19 20 30 41.44 20 25 28.57 20 25 59.59 20 38 30.39 20 38 12.88

41 14 09.9 41 25 33.9 41 58 38.2 41 53 17.4 42 25 39.1 42 38 36.5 42 50 47.1 42 02 39.4 39 29 24.5 41 44 16.3 42 46 28.1 41 09 25.0 42 07 04.8 42 22 40.9 39 45 15.5 41 08 26.1 40 10 18.4 42 28 26.4 40 57 16.9

7.055 7.089 7.340 7.348 7.838 7.874 7.906 7.909 7.923 7.952 7.997 8.120 8.165 8.214 8.464 8.464 8.586 8.599 8.699 B2V B0IIIe B0Ib B0.2IV B2V

BD+41 3762 4, 5 BD+42 3793 AS 434 6 7 8 AS 419 9 5 5 5, 10 5, 11 5 12 5, 13

Notes: 1: Identi ed as a star with very strong H & Shevchenko (1990). 3: Already recognized as an emission line star by Dolidze (1975) (star 20-091). 4: In the W75N star forming region, strong excess at 8



emission by Merrill & Burwell (1950).

2: Wolf-Rayet star, infrared spectral type WC8; see Pasquali et al. (2002). It had been previously recognized as an emission-line star by Melikian

µ

m (Davis et al. 2007).

5: Suspected Herbig Ae/Be star basedon its infrared excess. 6: Double star, separation 11: type estimated as G3IV-V by Straizys et al. (1989) based on Vilnius photometry. 7: Identi ed as a star with weak H 8: In DR 6. 9: Identi ed as a star with strong H 10: In the HII region DR 21. 11: Possibly associated to IRAS 20288+3934. 12: Already recognized as an emission line star by Dolidze (1975) (star 20-089). In the HII region DR 5. 13: In DR 21.



emission by Merrill & Burwell (1950). emission by Merrill & Burwell (1950).



Table 3. Lightly reddened OB stars Star 27 30 31 50 64 68 71 142 150 238



(2000)



(2000)

J 6.836 7.047 7.105 7.167 7.334 7.727 7.271 8.056 8.415 8.888

H 6.623 6.670 6.802 7.147 7.351 7.473 7.327 8.129 8.316 8.848

K

S

Other IDs HD 194649 BD+41 3833 V455 Cyg HD 194779 HD 195229 BD+40 4146 HD 195089 HD 194194 BD+40 4179 BD+39 4263

Notes 1 2 3 4 5 6 7 8 9 10

20 25 22.12 20 38 10.94 20 26 20.91 20 25 55.09 20 28 30.24 20 23 10.46 20 27 36.77 20 22 44.75 20 27 43.62 20 38 28.89

40 13 01.2 42 10 26.0 39 40 10.3 41 20 11.8 42 00 35.3 40 45 52.4 42 02 07.6 40 42 52.8 40 35 43.6 40 09 56.8

6.495 6.556 6.580 7.116 7.318 7.353 7.384 8.152 8.250 8.810

Notes: 1: Possibly associated to DR 5. Classi ed as O6.5 by Morgan et al. (1955). 2: Classi ed as B8 by Straizys et al. (1989) based on Vilnius photometry. 3: Eclipsing binary (Giuricin et al. 1983), classi ed as B2p?e? by Morgan et al. (1955). 4: Classi ed as B3II by Morgan et al. (1955). 5: Classi ed as B0.2III by Walborn (1971). 6: In cluster NGC 6910; classi ed as B1 by Walker & Hodge (1968). 7: Classi ed as B2IV by Guetter (1968). 8: In cluster NGC 6910; classi ed as spectral type B1.5IV by Walborn (1971). 9: Classi ed as O8V: by Morgan et al. (1955). 10: Ultraviolet-bright star, type B estimated by Carruthers & Page (1984).


´ F. Comeron et al.: The outskirts of Cygnus OB2 , Online Material p 3 Table 4. Foreground intermediate-type stars and lightly reddened, unclassi ed stars Star



(2000)



(2000)

J

H

K

S

Other IDs

Notes

Foreground stars 28 35 57 58 69 73 85 92 174 203 20 29 28.37 20 38 13.10 20 24 36.12 20 37 29.72 20 25 39.03 20 24 03.81 20 29 46.12 20 39 34.11 20 29 52.91 20 40 07.90 42 19 00.3 39 32 49.5 41 30 02.8 42 33 14.9 41 53 45.6 40 20 22.7 39 39 01.1 42 01 30.3 42 21 27.4 41 15 10.8 6.814 6.941 7.603 7.651 7.673 7.653 7.842 7.784 8.714 9.067 6.579 6.705 7.281 7.305 7.428 7.454 7.572 7.670 8.481 8.708 6.521 6.660 7.192 7.219 7.370 7.391 7.516 7.616 8.423 8.615 HD 195405 HD 196790 LTT 15972 HIP 101763 HD 229293 HD 229244 BD+39 4212 BD+41 3844 BD+41 3776 BD+40 4272 1 2 3 4 5 6 7 8 9 10

Lightly reddened stars with no classi cation in the visible 139 191 197 201 210 214 216 231 234 239 249 20 35 35.00 20 38 46.80 20 26 36.09 20 24 22.65 20 24 16.06 20 39 28.12 20 24 25.50 20 30 02.17 20 23 59.70 20 39 10.53 20 27 52.05 40 01 29.5 42 01 06.4 40 28 29.1 41 26 08.5 40 29 26.6 40 36 15.1 39 49 28.4 42 14 34.1 40 07 46.8 40 20 06.4 41 31 20.0 8.380 8.908 8.810 8.901 8.964 9.020 8.831 9.053 8.952 9.149 9.143 8.211 8.615 8.606 8.652 8.686 8.747 8.748 8.845 8.820 8.861 8.895 8.128 8.552 8.583 8.612 8.642 8.677 8.695 8.767 8.783 8.813 8.847 HD 229237 HD 229266 HD 229252 BD+40 4266 HD 229258

Notes: 1: Classi ed as G0Iab: (Nassau & Morgan 1952) and as G2IV (Bidelman 1957). 2: Classi ed as F8IV (NordstrÜm et al. 2004). 3: High proper motion, horizontal branch star; see e.g. Behr (2003). 4: Close binary, high proper motion star; Fabricius & Makarov (2000). 5: Classi ed as K0 in the HD catalog. 6: Classi ed as G0 in the HD catalog. 7: Classi ed as F8 in the SAO catalog. 8: Classi ed as F0V by Straizys et al. (1989) based on Vilnius photometry. 9: Classi ed as F5:V by Straizys et al. (1989) based on Vilnius photometry. 10: Nearby, high proper motion star at 75 pc according to the Hipparcos catalog. Suspected metal-poor subdwarf by Reid et al. (2001).


´ F. Comeron et al.: The outskirts of Cygnus OB2 , Online Material p 4 Table 5. Late B-F stars with Brackett absorption lines Star



(2000)



(2000)

J

H

K

S

Unreddened or lightly reddened stars 23 26 38 41 49 53 59 77 78 83 95 99 104 106 107 126 145 147 149 153 154 155 157 161 166 171 172 173 176 188 192 195 207 208 211 212 215 217 223 226 228 233 236 237 241 243 245 251 20 39 33.32 20 25 26.34 20 26 3.85 20 36 50.43 20 40 53.3 20 25 51.05 20 26 43.77 20 35 12.48 20 31 26.34 20 34 33.06 20 26 51.15 20 25 28.57 20 28 42.56 20 26 46.23 20 25 15.68 20 26 43.76 20 23 21.83 20 36 24.25 20 29 23.57 20 27 30.84 20 31 59.74 20 38 34.33 20 32 44.44 20 42 17.28 20 27 13.68 20 40 40.74 20 25 44.55 20 27 3.49 20 32 43.5 20 27 34.89 20 33 46.89 20 37 55.33 20 25 51.15 20 42 1.19 20 25 15.1 20 31 33.58 20 37 22.33 20 28 10.76 20 37 34.28 20 41 59.45 20 36 34.73 20 24 58.78 20 39 38.95 20 33 36.33 20 29 59.66 20 28 27.27 20 32 14.14 20 30 34.23 40 34 46.4 41 54 37.4 40 24 6.3 42 36 30.1 41 12 9. 39 37 53.0 39 29 45.4 42 51 15.3 39 56 19.7 41 59 47.9 41 51 33.5 39 47 36.7 39 28 25.1 39 27 44.1 40 59 55.1 40 20 50.1 41 30 37.7 39 11 40.7 40 5 56.5 41 26 1.2 42 3 38.3 41 9 37.4 39 13 15.2 41 29 4. 42 37 46.5 40 37 43.8 40 3 59.2 42 17 57.1 39 16 15.8 40 32 21.9 39 56 34.1 42 7 51.1 40 35 54.2 40 14 42.8 40 1 9.8 42 12 14.2 39 29 39. 39 12 47.8 39 48 27.8 41 26 58.9 41 48 8.9 39 50 21.7 40 35 11. 39 48 49.5 39 34 26.6 39 22 28.4 39 44 0.1 39 16 51.9 6.223 6.603 6.755 7.203 7.012 7.540 7.154 7.765 7.481 7.532 7.889 7.625 7.915 7.719 7.912 7.848 8.246 8.207 8.164 8.329 8.276 8.469 8.508 8.947 8.482 8.602 8.46 8.383 8.365 8.838 8.697 8.794 8.591 8.701 8.547 8.558 8.688 8.746 9.088 8.865 8.923 8.713 8.733 9.083 8.901 8.824 8.927 9.023 6.351 6.573 6.847 7.04 7.082 7.231 7.214 7.595 7.481 7.542 7.682 7.703 7.744 7.749 7.82 7.932 8.212 8.191 8.181 8.276 8.286 8.28 8.338 8.512 8.418 8.428 8.446 8.402 8.457 8.605 8.55 8.633 8.62 8.665 8.65 8.635 8.679 8.758 8.847 8.787 8.779 8.776 8.812 8.862 8.858 8.843 8.85 8.921 6.375 6.487 6.873 6.986 7.097 7.153 7.219 7.457 7.46 7.499 7.651 7.698 7.743 7.778 7.783 7.967 8.192 8.213 8.225 8.265 8.276 8.279 8.301 8.336 8.369 8.409 8.41 8.414 8.433 8.54 8.553 8.579 8.629 8.633 8.655 8.66 8.691 8.696 8.723 8.737 8.74 8.776 8.803 8.804 8.822 8.824 8.825 8.855

Reddened stars 056 109 179 240 20 28 58.75 20 36 57.14 20 31 19.00 20 38 21.43 40 13 30.3 42 42 10.0 42 2 56.0 40 30 39.7 8.505 9.688 9.595 10.548 7.666 8.442 8.870 9.416 7.192 7.788 8.464 8.815


´ F. Comeron et al.: The outskirts of Cygnus OB2 , Online Material p 5 Table 9. Extremely red objects Star 158 235



(2000)



(2000)

J 12.325 17.200

H 11.082 13.069

K

S

IRAS source IRAS 20275+4001 IRAS 20249+3953

Notes 1 2

20 29 24.36 20 26 44.43

40 11 15.6 40 02 57.8

8.320 8.793

Notes: 1: Source AFGL 2591. 2: Possibly associated to DR 5.


´ F. Comeron et al.: The outskirts of Cygnus OB2 , Online Material p 6 Table 1. Stars with reddened, featureless spectra

Star 10 33 36 37 39 42 44 45 60 62 65 70 72 74 75 82 93 94 96 97 100 101 108 110 129 130 131 132 133 134 135 137 138 140 143 146 152 156 159 164 169 175 181 183 185 186 189 190 193 196 202 204 205 206 209 219 220 222 227 229



(2000)

(2000)
42 25 53.2 40 24 00.1 41 56 56.4 42 17 10.9 42 25 57.0 42 29 41.9 42 02 42.3 42 12 26.1 41 44 06.8 40 09 22.5 42 20 22.6 40 29 39.3 39 51 51.9 42 47 25.5 39 55 19.5 40 35 49.6 42 03 13.7 40 48 44.5 40 05 57.0 41 57 06.8 42 06 07.8 39 44 32.6 40 45 19.3 41 33 46.7 39 21 22.4 40 22 45.1 40 17 04.0 41 56 31.4 40 17 00.4 42 18 42.6 39 47 26.5 40 46 09.0 40 46 03.6 41 16 56.5 42 22 02.4 40 46 55.2 41 15 46.8 41 52 06.9 40 01 41.4 39 51 42.5 41 17 34.0 42 31 16.0 40 02 22.4 39 36 15.8 41 13 50.7 42 55 08.1 39 59 38.7 42 01 23.7 40 23 31.7 40 40 38.6 40 13 42.7 42 16 24.3 42 23 48.4 40 12 54.2 39 45 02.3 40 27 41.2 39 36 47.2 40 27 52.0 42 11 14.2 41 21 26.2

J 6.212 7.200 7.839 8.870 9.359 8.222 7.815 7.534 8.152 7.581 7.829 8.734 8.745 8.963 8.526 7.931 9.243 8.337 8.955 8.755 8.470 8.100 8.157 8.758 9.538 9.775 9.239 9.092 8.705 9.534 9.271 8.480 8.621 10.080 9.417 8.504 9.172 10.218 9.044 9.344 10.329 9.056 9.282 9.793 9.785 9.615 9.675 10.726 9.140 8.938 10.601 10.669 10.052 9.058 10.015 9.085 10.054 10.580 9.964 9.460

H 5.467 6.852 7.086 7.554 7.801 7.426 7.298 7.264 7.580 7.421 7.510 7.862 7.847 7.977 7.806 7.650 8.156 7.900 8.073 8.037 7.984 7.822 7.932 8.140 8.523 8.606 8.456 8.379 8.310 8.580 8.482 8.244 8.310 8.800 8.613 8.254 8.581 8.988 8.569 8.723 9.098 8.626 8.746 8.910 8.973 8.901 8.925 9.332 8.753 8.673 9.299 9.308 9.075 8.750 9.116 8.806 9.189 9.351 9.162 8.981

K

S

Vis. classi cation B1Ib

Other IDs BD+39 4179

Notes

20 31 42.15 20 25 27.27 20 38 20.40 20 39 15.58 20 35 08.66 20 35 42.96 20 31 08.39 20 32 02.20 20 27 52.95 20 23 22.84 20 35 10.62 20 39 24.00 20 26 10.44 20 32 33.51 20 41 05.74 20 25 06.52 20 40 29.78 20 25 33.19 20 26 46.79 20 38 21.72 20 39 45.06 20 27 17.57 20 23 14.54 20 23 02.91 20 29 29.76 20 40 33.68 20 28 45.54 20 37 17.74 20 27 49.25 20 39 44.16 20 33 52.19 20 23 07.58 20 24 05.15 20 39 34.10 20 35 22.96 20 23 07.30 20 27 37.87 20 41 15.71 20 26 24.88 20 26 19.75 20 39 40.24 20 29 02.47 20 26 11.89 20 29 18.68 20 22 50.95 20 34 52.07 20 33 50.76 20 41 12.17 20 22 54.49 20 27 39.82 20 28 31.87 20 35 29.68 20 35 18.14 20 25 28.89 20 39 34.16 20 22 54.30 20 36 32.51 20 37 30.82 20 38 05.37 20 27 20.99

5.039 6.652 6.686 6.830 6.966 6.993 7.029 7.049 7.271 7.286 7.326 7.373 7.388 7.443 7.448 7.490 7.619 7.639 7.655 7.680 7.712 7.727 7.783 7.825 8.003 8.028 8.043 8.053 8.079 8.094 8.105 8.126 8.127 8.140 8.155 8.206 8.264 8.289 8.322 8.349 8.389 8.428 8.464 8.471 8.519 8.524 8.541 8.548 8.553 8.579 8.613 8.618 8.620 8.627 8.634 8.700 8.713 8.723 8.740 8.746

B0II 1 2 3 B0II B0.2III B0.2III HD 229202 B1III LS III 5 6 A41 BD+41 3794 4

+42

17

O9V B1III

BD+40 4159 7 8

B0III O9V LS II B1V B1III

+39

53

9 10

BD+40 4149

B1III 11 10, 12 B0.5V LS III

+40

15 13 14 10

O9.5V 15 O9.5V O7V 17 B1III B0III 18 19 16

20 B0.2III B1V 21 22 B0III LS III

+40

32

23

HD 229167 24

B1V


´ F. Comeron et al.: The outskirts of Cygnus OB2 , Online Material p 7 Table 1. continued.

Star 230 232 242 244 246 250



(2000)

(2000)
41 31 45.2 40 45 39.4 42 27 44.8 41 59 32.4 40 59 46.9 41 23 53.5

J 10.090 9.131 9.876 9.615 11.198 10.901

H 9.224 8.905 9.145 9.128 9.652 9.565

K

S

Vis. classi cation B1V B1II

Other IDs

Notes 25 26

20 41 43.65 20 22 58.94 20 38 26.96 20 31 06.24 20 26 40.87 20 40 10.22

8.767 8.774 8.823 8.824 8.829 8.852 27

Notes: 1: In DR 21, possibly associated to the radiosource WSRTGP 2037+4206 (Taylor et al. 1996). ¨ 2: In DR 17. 3: In DR 17. Brightest star in cluster 14 of Le Duigou & Knodlseder (2002) ´ 4: In Comeron et al. (2002); classi ed as O9.7II by Hanson (2003). 5: Classi ed as O8V: by Morgan et al. (1955); X-ray emission (1989) 6: Possibly related to the HII region G081.3+01.1 in DR17. ¨ 7: In DR 23. Brightest star in cluster 15 of Le Duigou & Knodlseder (2002). 8: Possibly associated to DR 5. 9: Classi ed as O7V: by Vijapurkar & Drilling (1993). 10: Member of cluster NGC 6910 11: In DR 21. 12: Classi ed as B0.5V by Hoag & Applequist (1965) ¨ 13: In DR 22, member of cluster 11 of Le Duigou & Knodlseder (2002). ¨ 14: In DR 17, member of cluster 12 of Le Duigou & Knodlseder (2002). 15: In cluster associated to DR 23 (Bica et al. 2003). 16: Possibly associated to DR 5. 17: In DR 22. 18: Possibly associated to DR 5. 19: Possibly associated to the radiosource MITGJ2029+3936 (Gri 20: Possibly associated to DR 23. 21: In DR 17. 22: In DR 17. 23: Possibly associated to DR 5. 24: Probably associated to IRAS 20346+3926A. 25: In cluster NGC 6910 (Delgado & Alfaro 2000). 26: In DR 21. 27: Possibly associated to DR 22.

ffi

th et al. 1991).


´ F. Comeron et al.: The outskirts of Cygnus OB2 , Online Material p 8 Table 10. Adopted temperatures and absolute magnitudes Star Sp. type MV log T (K)

ef f

Cygnus OB9 area 27 62 68 82 101 108 137 138 142 159 164 181 193 206 232 O6.5V O8V B1V O9V O7V B1V B0.5V B0.5V B1.5IV O9.5V O7V B0III B0.2III B0III B1V -5.53 -4.69 -4.52 -4.58 -4.32 -4.09 -3.80 -3.89 -3.26 -3.92 -4.10 -3.69 -3.42 -3.28 -3.09 4.578 4.542 4.414 4.517 4.567 4.414 4.448 4.448 4.344 4.504 4.567 4.419 4.403 4.419 4.414

Cygnus X North (DR 21) area 36 65 97 100 120 B0II B1III B0III O9V B0Ib -5.67 -4.47 -4.65 -4.57 -4.19 4.409 4.329 4.419 4.517 4.399

Field area 10 44 45 50 60 64 71 94 110 112 122 128 133 150 152 175 196 218 229 244 B1Ib B0II B0.2III B3II B0.2III B0.2III B2IV B1III B1III B2V B0.2IV B0IIIe B1III O8V O9.5V B1III B1V B2V B1V B1II -7.14 -5.08 -4.85 -4.03 -4.89 -4.27 -3.96 -4.28 -4.25 -4.00 -4.11 -4.21 -3.80 -3.64 -4.10 -3.45 -3.28 -3.18 -3.35 -3.13 4.301 4.409 4.403 4.191 4.403 4.403 4.312 4.329 4.329 4.354 4.436 4.419 4.329 4.543 4.504 4.329 4.414 4.354 4.414 4.315