Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.arcetri.astro.it/~lt/preprints/nirsur2/nirsur2.ps.gz Äàòà èçìåíåíèÿ: Tue Sep 11 16:56:49 2007 Äàòà èíäåêñèðîâàíèÿ: Sat Dec 22 07:42:36 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: photosphere

A&A manuscript no.
(will be inserted by hand later)
Your thesaurus codes are:
08 (08.06.2; 13.09.3; 13.09.6)
ASTRONOMY
AND
ASTROPHYSICS
Near­infrared images of star forming regions containing
masers
Las Campanas observations of 31 southern sources ?
Leonardo Testi 1;2 , Marcello Felli 3 , Paolo Persi 4 and Miguel Roth 5
1 Division of Physics, Mathematics and Astronomy, California Institute of Technology, MS105­24, Pasadena, CA 91125, USA
2 Dipartimento di Astronomia e Scienza dello Spazio, Universit`a degli Studi di Firenze, Largo E. Fermi 5, I­50125 Firenze, Italy
3 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I­50125 Firenze, Italy
4 Istituto di Astrofisica Spaziale, C.N.R., C.P. 67, I­00044 Frascati, Italy
5 Las Campanas Observatory, Casilla 601, La Serena, Chile
the date of receipt and acceptance should be inserted later
Abstract. We present sensitive high resolution near in­
frared (NIR) broad band (J, H, and K) observations of a
sample of 31 Star Forming Regions (SFRs) which contain
H 2 O and OH maser sources.
The observations are aimed at the detection and char­
acterization of Young Stellar Objects (YSOs) which may
be the source of excitation of the maser emission. In spite
of the large number of sources detected in the regions, us­
ing positional coincidence and NIR colours we are able to
reliably identify K­band sources related to the masing gas
in a large fraction of the observed regions.
The NIR infrared sources selected from close positional
coincidence with the maser show strong NIR excesses and
most probably represent the YSOs still embedded in their
parental cocoon where the maser emission occurs.
Key words: masers ­ star formation ­ circumstellar mat­
ter ­ infrared: ISM, stars
1. Introduction
H 2
O masers were found in SFRs since the earliest spectral
line surveys (Genzel & Downes 1977) and were recognized
to be closely related with young massive stars. More re­
cently Palla et al. (1993) and Codella et al.( 1994) sug­
gested that water maser activity is indeed present in the
earliest evolutionary phases of a high mass (proto­)star,
even before the onset of an UCHII region. Nevertheless, it
was clear that the comparison of the low resolution data
(e.g. single dish maser observations, radio continuum ob­
servations of the free­free emission from HII regions and
Send offprint requests to: Testi: Caltech, lt@astro.caltech.edu
? Based on observations collected at Las Campanas Obser­
vatory, Carnegie Institution of Washington
IRAS observations of cool dust clouds around luminous
stars) could give but a general indication. Only higher
resolution ( !
¸ 1 00 ) observations can enable one to disen­
tangle the complexity of high mass SFRs and allow one
to search for the stellar source directly connected to the
maser. In fact, according to the shock excitation model of
Elitzur et al. (1989), the exciting stellar source should be
located very close to the maser (Ÿ 10 4 AU or Ÿ 10 00 at 1
kpc). Consequently, arcmin coincidences have very little
significance.
On the radio side of the spectrum the NRAO Very
Large Array (VLA) offers the possibility of obtaining sub­
arcsec resolutions for the water masers and for the ra­
dio continuum emission from UCHII regions (Forster &
Caswell 1989, hereafter FC89; Tofani et al. 1995; Jen­
ness et al. 1995; Hofner & Churchwell 1996). One of the
most important results of these studies has been that H 2 O
masers and UCHII regions, although generally found in
the same SFR, are not necessarely closely related to each
other, and that the powering sources of the free­free con­
tinuum and of the maser radiation have to be searched
in distinct objects, most probably YSOs belonging to the
same SFR but in different evolutionary phases.
Consequently, to identify the stellar source powering
the maser, arcsecond resolution observations are required,
in particular in the far infrared and submillimeter regions
of the spectrum. In fact, it is in these wavebands that the
embedded YSOs are expected to emit most of their en­
ergy. Although some attempts in this direction have been
carried out (Jenness et al. 1995), the current instrumen­
tation does not allow one to attain the required resolution
and sensitivity in these spectral ranges.
Another powerful probe that became available in the
past few years is molecular line interferometry at centime­
ter and millimeter wavelengths (Cesaroni et al. 1994; Ce­
saroni et al. 1997; Codella et al. 1997; Hofner et al. 1996;
Olmi et al. 1996) which enables arcsecond resolution of

2 L. Testi et al.: NIR images of galactic masers
hot molecular clumps and cold dust clouds surrounding
the water masers. These studies, focused on a few selected
sources, confirm that the water masers are indeed associ­
ated with hot molecular clumps and millimeter continuum
sources, and that the expected luminosities of the objects
embedded inside these cores is that of high mass stars.
However, this method is not feasible for large surveys.
In order to search for the stellar sources physically as­
sociated with the masers we decided to undertake a sys­
tematic NIR imaging survey of a sample of SFRs con­
taining masers. This is motivated by the fact that the
modern NIR array detectors offer the required sensitiv­
ity and resolution even with small telescopes and with
short integration times. We were also guided by the ex­
pectation that in its early evolutionary stages high mass
stars could be detectable at NIR wavelengths even if still
deeply embedded in a dust cloud, due to the emission
of hot dust around the star (Testi et al. 1997). Previ­
ous attempts to look for near infrared sources close to
H 2 O masers had been carried out with single element pho­
tometers and large diaphragms (Evans et al. 1979; Moor­
wood & Salinari 1981a,b; Epchtein & L'epine 1981; Braz &
Epchtein 1982) and could not reach the sensitivities and
resolutions needed to separate the various YSOs in each
region.
In this paper we present NIR observations with arcsec
resolution of a sample of H 2 O/OH masers in SFRs taken
from the list of FC89. The complete radio data were kindly
provided by R. Forster (1992 private communication).
The SFR type of the masers in our sample (to distin­
guish them from those around late type stars) relies on the
selection criteria used by FC89, basically the presence of a
thermal continum radio source (an HII region) close to the
maser. As an additional check, we have derived for those
fields that contain an IRAS point source (27 out of 31)
the (25¯m/12¯m) and the (60¯m/12¯m) colours. All the
points fall in the box of the (25¯m/12¯m)­(60¯m/12¯m)
diagram that defines the locus of UCHII regions (Wood
and Churchwell 1989). This is a further confirmation of
the star forming nature of the selected fields, even though,
strictly speaking, it is not directly related to the nature of
the star associated with the maser itself.
In Testi et al. 1994 (hereafter Paper I) we have pre­
sented the results obtained toward a subsample of 17 SFRs,
which is in fact the first attempt to compare maser and
NIR observations at arcsec resolution. Assuming that a
NIR source is physically associated with the maser if it
lies within 10 00 from it, we were able to identify a NIR
source in ¸ 85% of the cases. In ¸ 80% the NIR source
was closer than 5 00 . In this paper we present new higher
resolution and higher sensitivity observations of 31 SFRs
from the list of FC89 (5 regions overlap with that pre­
sented in Paper I and are indicated with an \Lambda in Tab. 1).
Three of the regions in the present sample (G351.41+0.64;
G009.62+0.19;G035.20\Gamma1.73) have been studied also with
high resolution observations at other frequencies (Persi et
al. 1996, Testi et al. 1997 and Persi et al. 1997). In all
these cases the physical association of the selected NIR
source with the maser is amply confirmed by independent
criteria. In Sect. 2 the observational parameters and the
data reduction procedures are summarized. In Sect. 3 the
results are shown. In Sect. 4 we show that close positional
coincidence alone is not sufficient to establish a physical
association, and that the NIR colour is the complemen­
tary information. In fact, the NIR sources associated with
the masers all show a strong NIR excess, while the back­
ground NIR sources are distributed along the reddening
line. In Sect. 5 the main conclusions are summarized.
2. Observations
The observed H 2 O and OH masers are listed in Table 1.
For each source we report the FC89 name (the 5 presented
in Paper I and reobserved in June 1994 are marked with
an \Lambda), the 1950 coordinates of the H 2 O maser and the
distance from the Sun (from FC89), the IRAS name of
the IRAS--PSC source (if any within 1 arcmin from the
maser position), and the limiting magnitudes achieved in
our observations (see below). All the sources were observed
in J, H and K (1:25 ¯m, 1:65 ¯m and 2:2 ¯m respectively)
NIR broad bands using the Las Campanas NIR camera
(Persson et al. 1992) equipped with a NICMOS3 256\Theta256
HgCdTe array detector. The sources were observed in two
runs at the 1 m telescope (July 1992) and at the 2.5 m
telescope (June 1994). The observational and data reduc­
tion techniques were the same as for the preliminary sam­
ple presented in Paper I. The plate scale on the detec­
tor was 0.92 00 /pix at the 1 m telescope and 0.35 00 /pix at
the 2.5 m. During both observing runs the seeing was
in the range 0:8 00 --1:5 00 . The data reduction and analysis
were performed using the IRAF 1 and ARNICA 2 (Hunt et
al. 1994) software packages.
Photometric calibration was achieved by observing a
set of the UKIRT faint standard stars (Casali & Hawardeen 1992)
each night. The calibration accuracy is in the range 5%--
10% depending on the weather conditions. The limiting
magnitudes obtained in each field (3oe in a 4 pix aperture)
are listed in Table 1. Photometry was performed using the
IRAF DAOPHOT package.
The key aspect of this project is the comparison of the
NIR images with sub­arcsec resolution positions of the
masers. Consequently, it is important to perform an accu­
rate astrometric calibration of the NIR images. This was
performed following the procedure outlined in Testi 1993
using the Digitized Sky Survey provided by the Space
1 IRAF is made available to the astronomical community
by the National Optical Astronomy Observatories, which are
operated by AURA, Inc., under contract with the U.S. National
Science Foundation
2 ARNICA can be obtained from the Osservatorio Astrofisico
di Arcetri at ftp://150.217.20.1/pub/arnica/

L. Testi et al.: NIR images of galactic masers 3
Table 1. Parameters of the observed regions.
Name ff (1950) ffi (1950) D (kpc) IRAS mlJ mlH mlK
Las Campanas 1 m telescope, July 1992
G351.41+0.64 17:17:32.34 \Gamma35:44:02.5 1.9 17175\Gamma3544 16.5 15.8 14.6
G351.78\Gamma0.54 17:23:20.32 \Gamma36:06:44.0 2.2 17233\Gamma3606 16.6 16.1 15.3
G353.41\Gamma0.36 17:27:05.56 \Gamma34:39:21.7 4.5 17271\Gamma3439 16.5 15.9 15.0
G355.34+0.15 17:30:12.54 \Gamma32:45:56.6 2.0 17302\Gamma3245 16.3 15.2 14.3
G359.62\Gamma0.25 17:42:27.70 \Gamma29:22:21.2 10.0 -- 16.5 15.1 14.2
G005.88\Gamma0.39 17:57:26.83 \Gamma24:03:56.5 2.6 17574\Gamma2403 16.5 15.5 14.5
G009.62+0.19 18:03:15.99 \Gamma20:31:54.9 2.0 18032\Gamma2032 16.7 15.4 14.5
G011.03+0.06 18:06:42.56 \Gamma19:21:56.0 3.2 18067\Gamma1921 16.0 15.0 14.5
G010.62\Gamma0.38 18:07:30.56 \Gamma19:56:28.8 6.0 18075\Gamma1956 16.5 15.7 14.8
G012.22\Gamma0.12 18:09:43.81 \Gamma18:25:05.7 16.1 18097\Gamma1825 16.1 15.0 14.4
G015.04\Gamma0.68 18:17:29.91 \Gamma16:13:11.0 2.5 18174\Gamma1612 16.0 15.1 15.0
G019.61\Gamma0.23 18:24:50.25 \Gamma11:58:31.7 3.8 18248\Gamma1158 15.5 14.5 14.5
G023.43\Gamma0.19 18:31:55.53 \Gamma08:34:03.9 7.8 -- 16.0 15.1 15.0
G023.01\Gamma0.41 18:31:56.10 \Gamma09:03:03.7 12.8 -- 16.3 15.3 14.7
G024.78+0.08 18:33:30.43 \Gamma07:14:43.2 7.7 18335\Gamma0713A 16.0 15.1 15.0
Las Campanas 2.5 m telescope, June 1994
\Lambda G345.01+1.79 16:53:17.28 \Gamma40:09:23.2 3.0 16533\Gamma4009 19.3 18.3 17.0
G343.12\Gamma0.06 16:54:44.03 \Gamma42:47:36.6 3.2 16547\Gamma4247 19.2 18.2 17.2
\Lambda G345.51+0.35 17:00:53.55 \Gamma40:40:18.5 2.1 17008\Gamma4040 19.0 17.4 17.1
\Lambda G345.00\Gamma0.22 17:01:40.14 \Gamma41:24:58.5 3.2 17016\Gamma4124 19.5 18.5 17.2
\Lambda G345.40\Gamma0.90 17:06:02.18 \Gamma41:32:06.3 2.3 17059\Gamma4132 17.5 16.0 15.5
G349.07\Gamma0.01 17:13:25.55 \Gamma38:01:58.8 1.0 17134\Gamma3802 18.1 16.5 16.4
G350.02+0.43 17:14:22.13 \Gamma37:00:03.2 5.0 17143\Gamma3700 18.9 17.1 16.9
G348.73\Gamma1.06 17:16:39.83 \Gamma38:54:13.6 1.8 17167\Gamma3854 19.0 17.3 16.2
G351.58\Gamma0.35 17:22:03.27 \Gamma36:10:05.5 9.9 17220\Gamma3609 19.0 17.2 17.0
G353.45+0.54 17:23:33.02 \Gamma34:05:54.6 8.4 -- 19.5 17.0 16.8
G352.51\Gamma0.15 17:23:50.87 \Gamma35:17:00.9 8.0 17238\Gamma3516 19.3 17.4 16.7
G354.61+0.48 17:26:59.99 \Gamma33:11:38.2 4.2 17269\Gamma3312 19.0 17.2 16.8
G359.14+0.03 17:40:13.96 \Gamma29:37:57.9 5.0 17402\Gamma2938 19.2 16.9 15.4
G359.97\Gamma0.46 17:44:09.22 \Gamma29:10:57.7 10.0 17441\Gamma2910 18.3 16.8 15.6
G002.14+0.01 17:47:28.19 \Gamma27:04:59.2 8.9 17474\Gamma2704 18.6 16.5 16.2
\Lambda G035.20\Gamma1.73 18:59:13.24 +01:09:13.5 2.9 18592+0108 19.6 17.6 17.0
Telescope Science Institute. We reached an accuracy bet­
ter than 1 00 rms.
3. Results
In Fig. 1a­p the J, H and K images for all the observed
fields are presented. The fields are presented in the same
order as in Tab. 1. The greyscale display is logarithmic
in all cases. The crosses and the open squares represent
the positions of the H 2 O and OH maser spots detected
by FC89. The circles represent the peak positions of the
UCHII regions, when detected by FC89 (however, note
that their continuum observations are not sensitive to sources
weaker than ¸ 150 mJy at 22 GHz). The large ellipse rep­
resents the IRAS--PSC error box (note that due to an
error in the plotting routine the ellipse for G035.20\Gamma1.73
in Fig 2 of Paper I was plotted west of north instead of
east of north). For each NIR image the 1950 coordinate
grid was derived from the astrometry.
3.1. Maser morphology
Following the reasoning of Paper I, in order to compare the
radio data with the NIR images we have grouped the H 2 O
and OH maser spots detected by FC89 in each field into
components of ¸ 1 00 size. In this way, in the 31 fields we
find a total of 68 H 2 O and 40 OH maser components. The
components have been identified following the convention
of Paper I.
3.2. Close coincidences between NIR sources and masers
The necessary (but not sufficient) requirement to establish
a physical association between a maser and a NIR source
is close positional coincidence. For each maser we have
searched for the closest source in the K­band. The NIR
source was considered to be physically associated with the
maser component if closer than 5 00 (see below). We decided
to use the nearest neighbour approach in the association in
order to avoid any a priori bias on the associated sources
(see the discussion in Paper I).

4 L. Testi et al.: NIR images of galactic masers
Table 2. a) NIR sources associated with the maser emission, 1992 observations.
Name ff (1950) ffi (1950) mJ mH mK Maser d ( 00 ) d (mpc)
g35141­1 17 : 17 : 32:26 \Gamma35 : 44 : 04:6 15:80 \Sigma 0:18 13:30 \Sigma 0:04 10:02 \Sigma 0:01 H2O­A 2.3 21.2
H2O­B 7.4 68.2 D
OH­A 2.7 24.9
g35178­1 17 : 23 : 20:49 \Gamma36 : 06 : 40:2 13:60 \Sigma 0:02 12:98 \Sigma 0:02 11:73 \Sigma 0:02 H2O­A 4.3 45.9
H2O­B 2.4 25.6
H2O­C 3.2 34.1
H2O­D 9.9 105.6 D
OH­A 6.7 71.5 D
OH­B 5.0 53.3
g35341­N ? 16:5 ? 15:9 ? 15:0 H2O­A -- --
OH­A -- --
g35341­1 17 : 27 : 08:94 \Gamma34 : 39 : 33:9 15:00 \Sigma 0:08 14:11 \Sigma 0:07 11:90 \Sigma 0:06 H2O­B 1.7 37.1
g35534­1 17 : 30 : 12:57 \Gamma32 : 45 : 56:2 16:10 \Sigma 0:23 14:48 \Sigma 0:16 13:52 \Sigma 0:20 H2O­A 0.6 5.8
g35534­2 17 : 30 : 12:39 \Gamma32 : 45 : 41:8 15:30 \Sigma 0:10 12:23 \Sigma 0:02 10:67 \Sigma 0:01 H2O­B 5.3 51.4 D
OH­A 0.6 5.8
g35962­1 17 : 42 : 28:07 \Gamma29 : 22 : 21:5 14:40 \Sigma 0:05 11:56 \Sigma 0:01 9:20 \Sigma 0:01 H2O­A 4.8 232.7
H2O­B 9.3 450.9 D
g00588­1 17 : 57 : 26:91 \Gamma24 : 03 : 58:1 16:00 \Sigma 0:29 13:03 \Sigma 0:05 10:19 \Sigma 0:01 H2O­A 3.4 42.9
H2O­B 2.2 27.7
H2O­C 2.4 30.3
H2O­D 3.3 41.6
H2O­E 6.9 87.0 D
OH­A 5.7 71.8 D
OH­B 1.1 13.9
OH­C 4.8 60.5
g00588­N ? 16:5 ? 15:5 ? 14:5 H2O­F -- --
g00962­2 18 : 03 : 15:94 \Gamma20 : 31 : 53:6 15:50 \Sigma 0:07 15:00 \Sigma 0:20 14:00 \Sigma 0:20 H2O­A 1.4 13.6
H2O­B 1.9 18.4
OH­A 1.5 14.5
OH­B 4.2 40.7
g00962­1 18 : 03 : 16:15 \Gamma20 : 32 : 00:0 ? 16:4 ? 15:4 12:90 \Sigma 0:08 H2O­C 0.7 6.8
H2O­D 2.7 26.2
OH­C 3.7 35.9
g01103­1 18 : 06 : 42:69 \Gamma19 : 21 : 55:5 14:90 \Sigma 0:10 13:34 \Sigma 0:07 12:30 \Sigma 0:07 H2O­A 2.1 32.6
OH­A 1.8 27.9
g01062­2 18 : 07 : 30:90 \Gamma19 : 56 : 28:1 16:50 \Sigma 0:24 15:08 \Sigma 0:10 13:50 \Sigma 0:09 H2O­A 4.9 142.5
H2O­B 6.4 186.2 D
H2O­C 8.5 247.3 D
OH­A 3.3 96.0
OH­B 1.4 40.7
g01062­1 18 : 07 : 31:24 \Gamma19 : 56 : 28:3 15:30 \Sigma 0:08 14:38 \Sigma 0:06 14:25 \Sigma 0:18 H2O­D 2.5 72.7
g01222­2 18 : 09 : 43:94 \Gamma18 : 25 : 07:8 13:70 \Sigma 0:03 11:61 \Sigma 0:02 10:75 \Sigma 0:02 H2O­A 5.5 429.3 D
H2O­B 3.8 296.6
g01222­1 18 : 09 : 48:74 \Gamma18 : 25 : 15:0 14:20 \Sigma 0:05 12:66 \Sigma 0:03 12:34 \Sigma 0:06 H2O­C 4.0 312.2
OH­A 4.2 327.8
g01504­1 18 : 17 : 29:88 \Gamma16 : 13 : 15:8 13:80 \Sigma 0:09 12:59 \Sigma 0:06 12:03 \Sigma 0:10 H2O­A 4.8 58.2
g01504­N ? 16:0 ? 15:1 ? 15:0 H2O­B -- --
g01504­2 18 : 17 : 31:54 \Gamma16 : 13 : 02:7 13:20 \Sigma 0:12 10:50 \Sigma 0:02 9:18 \Sigma 0:01 OH­A 5.0 60.6
g01961­1 18 : 24 : 50:50 \Gamma11 : 58 : 34:5 ? 15:5 12:76 \Sigma 0:07 11:04 \Sigma 0:03 H2O­A 4.1 75.5
H2O­B 5.6 103.2 D
H2O­C 6.8 125.3 D
H2O­D 9.4 173.2 D
OH­A 3.5 64.5
g02301­1 18 : 31 : 55:82 \Gamma09 : 03 : 03:0 13:70 \Sigma 0:03 13:73 \Sigma 0:05 12:99 \Sigma 0:05 H2O­A 4.2 260.6
OH­A 2.2 136.5
g02343­N ? 16:0 ? 15:1 ? 15:0 H2O­A -- --
H2O­B -- --
OH­A -- --
g02343­1 18 : 31 : 55:65 \Gamma08 : 33 : 52:5 12:40 \Sigma 0:01 11:80 \Sigma 0:01 11:39 \Sigma 0:02 OH­B 3.2 121.0
g02478­1 18 : 33 : 30:17 \Gamma07 : 14 : 36:0 16:20 \Sigma 0:30 14:80 \Sigma 0:10 13:23 \Sigma 0:07 H2O­A 8.2 306.1 D
OH­A 6.8 253.8 D
g02478­N ? 16:0 ? 15:1 ? 15:0 H2O­B -- --

L. Testi et al.: NIR images of galactic masers 5
Fig. 1. a)­p) J, H and K images of all the observed regions. The greyscale is logarithmic in all cases, crosses and open squares
represent the H2O and OH maser spots detected by FC89, respectively. Open circles mark the position of the radio continuum
peak (where detected by FC89). The large ellipses represent the IRAS--PSC error boxes. The coordinate grid, derived from the
astrometry, are at 1950.
Table 2. b) same as Tab. 2.a but for the 1994 observations.
Name ff (1950) ffi (1950) mJ mH mK Maser d ( 00 ) d (mpc)
g34501­1 16 : 53 : 17:26 \Gamma40 : 09 : 19:7 ? 19:3 ? 18:3 16:13 \Sigma 0:11 H2O­A 3.5 50.9
g34501­2 16 : 53 : 18:91 \Gamma40 : 09 : 28:3 ? 19:3 ? 18:3 15:31 \Sigma 0:03 H2O­B 1.8 26.2
g34501­3 16 : 53 : 19:72 \Gamma40 : 09 : 38:6 15:64 \Sigma 0:01 14:72 \Sigma 0:01 14:29 \Sigma 0:03 H2O­C 2.7 39.3
g34501­4 16 : 53 : 19:63 \Gamma40 : 09 : 45:7 ? 19:3 ? 18:3 15:46 \Sigma 0:08 OH­A 2.4 34.9
OH­B 1.1 16.0
OH­C 0.2 2.9
g34313­1 16 : 54 : 43:79 \Gamma42 : 47 : 35:0 17:19 \Sigma 0:03 15:85 \Sigma 0:03 15:18 \Sigma 0:05 H2O­A 1.6 24.8
H2O­B 1.3 20.2
H2O­C 3.1 48.1
OH­A 0.5 7.8
g34551­1 17 : 00 : 53:52 \Gamma40 : 40 : 17:2 19:12 \Sigma 0:13 ? 17:4 16:11 \Sigma 0:07 H2O­A 1.3 13.2
g34551­2 17 : 00 : 53:42 \Gamma40 : 40 : 14:8 ? 19:0 ? 17:4 12:98 \Sigma 0:03 OH­A 0.6 6.1
g34500­1 17 : 01 : 39:74 \Gamma41 : 24 : 58:4 ? 19:5 17:68 \Sigma 0:09 14:32 \Sigma 0:03 H2O­A 3.9 60.5
g34500­2 17 : 01 : 40:19 \Gamma41 : 24 : 58:5 ? 19:5 ? 18:5 13:86 \Sigma 0:01 H2O­B 0.6 9.3
OH­A 2.2 34.1
g34541­1 17 : 06 : 02:28 \Gamma41 : 32 : 06:5 14:96 \Sigma 0:11 13:96 \Sigma 0:09 12:71 \Sigma 0:08 H2O­A 1.2 13.4
g34541­2 17 : 06 : 03:92 \Gamma41 : 32 : 10:1 14:77 \Sigma 0:04 13:24 \Sigma 0:03 11:02 \Sigma 0:01 OH­A 0.9 10.0
g34907­1 17 : 13 : 25:59 \Gamma38 : 01 : 58:3 13:69 \Sigma 0:01 12:63 \Sigma 0:01 12:02 \Sigma 0:01 H2O­A 0.7 3.4 C
OH­A 0.8 3.9 C
g35002­1 17 : 14 : 22:17 \Gamma37 : 00 : 02:6 17:76 \Sigma 0:10 16:60 \Sigma 0:16 13:21 \Sigma 0:01 H2O­A 0.8 19.4
OH­A 0.3 7.3
g34870­1 17 : 16 : 38:96 \Gamma38 : 54 : 16:8 17:62 \Sigma 0:07 17:11 \Sigma 0:12 15:83 \Sigma 0:23 H2O­A 3.7 32.3
g34870­2 17 : 16 : 39:76 \Gamma38 : 54 : 12:8 18:53 \Sigma 0:19 17:34 \Sigma 0:15 15:60 \Sigma 0:10 H2O­B 4.2 36.7
H2O­C 2.0 17.5
H2O­D 2.0 17.5
H2O­E 1.2 10.5
g35158­1 17 : 22 : 02:81 \Gamma36 : 10 : 06:3 ? 19:0 ? 17:2 17:28 \Sigma 0:25 H2O­A 2.5 120.0
OH­A 2.5 120.0
g35158­2 17 : 22 : 03:37 \Gamma36 : 10 : 08:3 13:80 \Sigma 0:02 12:89 \Sigma 0:04 12:36 \Sigma 0:03 H2O­B 3.0 144.0
OH­B 3.2 153.6
OH­C 2.8 134.4
g35252­1 17 : 23 : 50:80 \Gamma35 : 17 : 02:0 18:29 \Sigma 0:10 16:18 \Sigma 0:07 14:80 \Sigma 0:06 H2O­A 1.3 50.4 C
OH­A 0.7 27.1 C
g35346­1 17 : 23 : 33:00 \Gamma34 : 05 : 53:8 17:31 \Sigma 0:03 15:25 \Sigma 0:03 12:79 \Sigma 0:01 H2O­A 0.8 32.6
OH­A 0.6 24.4
g35461­1 17 : 27 : 00:29 \Gamma33 : 11 : 37:3 ? 19:0 ? 17:2 15:21 \Sigma 0:04 H2O­A 3.9 79.4
OH­A 3.8 77.4
g35914­1 17 : 40 : 13:99 \Gamma29 : 37 : 57:5 16:44 \Sigma 0:03 12:32 \Sigma 0:02 10:41 \Sigma 0:02 H2O­A 0.6 14.5 C
H2O­B 1.8 43.6 C
OH­A 0.4 9.7 C
g35997­1 17 : 44 : 09:18 \Gamma29 : 10 : 57:3 17:26 \Sigma 0:12 13:65 \Sigma 0:02 9:98 \Sigma 0:01 H2O­A 0.7 33.9
OH­A 0.9 43.6
g00214­1 17 : 47 : 28:29 \Gamma27 : 05 : 01:1 18:49 \Sigma 0:12 ? 16:5 13:45 \Sigma 0:02 H2O­A 2.3 99.2
OH­A 2.0 86.3
g03520­1 18 : 59 : 13:06 +01 : 09 : 11:2 ? 19:6 ? 17:6 12:82 \Sigma 0:01 H2O­A 1.0 14.1
OH­A 1.7 23.9

6 L. Testi et al.: NIR images of galactic masers
In Table 2a and 2b the parameters are listed for the
NIR sources associated with the maser components. For
each NIR source we list an identifyer derived from the
galactic coordinates (e.g. g35141­1 for the NIR source as­
sociated with the first maser component in G351.41+0.64),
the 1950 coordinates, the magnitudes with errors in the
J, H and K bands, the maser component with which it is
associated, the separation between the maser component
and the NIR source in arcseconds and in milliparsecs (i.e.
projected on the plane of the sky using the distance re­
ported by FC89). When a source is not detected in one
or more wavebands the limiting magnitude is indicated
and the lower limit is identified with a ``?'' symbol. When
a maser group was not coincident with any NIR sources,
the NIR identifyer is marked with an ``N'' and the limiting
magnitudes in the three bands are given.
It is worth noting that all the maser components in
the 1994 sample have a NIR source within 5 00 . This is pri­
marly due to the much higher sensitivity of the observa­
tions with respect to those of the 1992 run and especially
those presented in Paper I. For this reason the possibility
of spurious coincidence should be reconsidered in view of
the higher quality of the data available (see Sect. 4). An­
ticipating the results of the following section, we decided
to retain as good physical associations the coincidences
within 5 00 and mark as dubious all the coincidences at
larger distances (marked with ``D'' in Tab. 2). Extremely
crowded regions in which the probability of spurious coin­
cidences is very high have been indicated in Tab. 2b with
a ``C'' in the last column.
For the following discussion we will merge the data pre­
sented in this paper with that of Paper I. Due to the better
resolution and sensitivity, for the 5 sources of Paper I that
have been reobserved in June 1994 we will consider only
the observations presented here. Consequently, the total
number of SFRs is 43 and the numbers of H 2 O and OH
masers are 82 and 53, respectively. In Figure 2 we show
the distribution of the separations between the first K­
band neighbour and the OH and H 2 O maser components,
in arcseconds and in physical separation projected on the
plane of the sky (assuming the distance quoted in Tab. 1).
Note that in the figure we have considered also the asso­
ciation with 5 00 ! d ! 10 00 , to be consistent with Paper I.
In the following section we will discuss the reliability of
such associations.
In Figure 3 we show the difference in separation of the
H 2 O and OH maser components associated with the same
NIR source. The histogram is strongly peaked around zero
and does not show any significant asymmetry, indicating
that there does not seem to be any trend for one of the two
maser types to form closer to the associated NIR source.
3.3. Comparison between the 1991 and 1994 observations
Five of the sources have been observed both in 1991 at the
1 m telescope with the NICMOS2­based camera (Paper I)
and in 1994 at the 2.5 m telescope with the new detector
(this paper).
In several cases (sources: g34500­1; g34501­1,2,3,4;g34541­
1,2; g34551­1) the new higher resolution and higher sensi­
tivity observations enabled detection of new sources (closer
to the maser components than those reported in Paper I)
fainter than the old detection limit or confused because
they are embedded in diffuse nebulositities, which pre­
vented a reliable detection in the lower resolution obser­
vations. In one case (g34500­2) the point­like source is
surrounded by a diffuse halo (unresolved in the 1991 ob­
servations) and hence the integrated magnitude (that of
Paper I) is 1 magnitude brighter than that reported in
Tab. 2. The other two sources (g03520­1 and g34551­2)
are in excellent agreement with that of Paper I, taking
into account the photometric calibration uncertainties.
We thus conclude that in a few cases the new observa­
tions found a NIR source associated with the maser com­
ponent where no sources were found in Paper I and in
other cases the agreement between the two sets of obser­
vations is rather good. However, in some cases the source
associated with the maser components in Paper I is not
the closest to the maser when higher sensitivity data are
considered. This fact implies that (especially for the asso­
ciations with larger separation) the confusion due to unre­
lated background sources should be carefully considered.
4. Nature of the NIR sources
Due to the higher data quality the sources surface density
at K­band near the maser positions in the 1994 observa­
tions is a factor 3--20 (depending on the field) higher than
in the 1991 observations (see Paper I). As a consequence,
for all the maser components we can find a NIR source
within 5 00 . Hence we do not have strong statistical support
for the uniqueness of the association between the maser
components and the NIR sources based only on positional
coincidence. In fact, due to the high densities of sources we
expect some contamination from background/foreground
sources. The fields in which the mean separation between
the K­band sources is much smaller than 5 00 have been
marked in Tab. 2 with a ``C'' in the last column (note
that in all those cases the distance between the maser
components and the NIR sources is !
¸ 2 00 ).
In order to find additional criteria to establish a phys­
ical association we have examined the NIR colours of the
close coincident sources and those of background sources.
Since many of the sources coincident with the maser
components are not detected at J and H, we will use the
(H--K,J--K) colour­colour diagram to investigate the na­
ture of the infrared sources, as in Paper I.
Since there is no systematic difference in the colour
characteristics of the NIR sources associated to the H 2 O
and OH masers (in fact in many cases both masers are as­
sociated with the same NIR source), we will discuss the na­
ture of the sources associated with both masers together.

L. Testi et al.: NIR images of galactic masers 7
Fig. 2. Distribution of separations between NIR sources and maser components. Continuous line: H2O maser components;
dotted line: OH masers components. On the left: observed projected separation on the sky in arcsecond; on the right: projected
separation on the plane of the sky in milliparsecs. The distances quoted in Tab. 1 have been used.
Fig. 3. Distribution of the difference of the separations between the NIR source and the H2O and OH maser components. On
the left: separation difference in arcsecond; on the right: in milliparsecs.
In Fig. 4 the (H--K,J--K) colour­colour diagram and
the (H--K,K) colour magnitude diagram for the sources
coincident with the maser components are presented. The
colours of main sequence (MS) stars are from Koornneef (1983),
the magnitudes from Schmidt­Kaler (1981) and the red­
dening vectors have been plotted assuming a Rieke &
Lebofski (1985) extinction law. The K magnitudes of all
the sources have been normalized to a distance of 5 kpc
from the Sun, assuming the distances quoted in Tab. 1.
In Fig. 5 similar plots are given for the sources detected
within 1 arcmin from the maser reference of FC89, pre­
sumably field stars.
From a comparison of Fig. 4 and 5 it can be clearly
seen that the field stars are distributed close with the lo­
cus of main sequence and reddened main sequence stars,

8 L. Testi et al.: NIR images of galactic masers
while almost all the sources coincident with the maser
components show a conspicuous infrared excess.
Consequently, independent support to a physical as­
sociation between maser components and NIR sources is
given by the fact that the NIR sources close coincident
with the maser components show NIR excesses, while the
field stars are located along the reddening line.
This provides proof of a distinct property of the maser
associated sources. As discussed in Paper I and in Testi et
al. (1997) the NIR excess emission is probably the result
of the superposition of (at least) three main contributions:
i) the heavily extincted stellar photosphere of a massive
young stellar object (the ultimate energy source of the
entire system), ii) a contribution from free­free and free­
bound continum emission from ionized gas and iii) the
emission of the hot dust surrounding the YSO. In par­
ticular, in the earliest evolutionary stages, when dust is
still present very close to the exciting star, a hot dust
shell dominates the emission at wavelengths longward of
¸ 1:5 ¯m and the object is effectively detectable at K band
even if still embedded in the parental molecular clump.
Strong support for the association procedure is also
provided by the three sources of this sample that have
been studied in detail by Persi et al. (1996; 1997) and Testi
et al. (1997). The arcsecond resolution comparison of the
NIR images with mid infrared and/or thermal molecular
line emission and mm­wave continuum observations show
that the NIR sources associated with the maser emission
using the ``nearest neighbour'' criterion are also associ­
ated with high density molecular clumps and dense cool
dust clouds, which are other indicators of the presence of
a YSO. Due to the absence of a UCHII region (in spite of
the high luminosities inferred from the IRAS far infrared
observations), the three NIR sources discussed above are
believed to be in a very early phase, when the size of the
ionized region around the young star is so small that the
radio continuum emission is strongly self­absorbed. Conse­
quently, the UC HII region is undetectable at radio wave­
lengths (see also the discussion in Testi et al. 1997), while
hot dust emission can be detected even through strong
absorption.
In a forthcoming paper (Testi et al. 1997 in prepara­
tion) we will discuss the NIR properties of the sources
associated with the masers in the framework of a model
which takes into account the various emission components.
5. Conclusions
We have presented near infrared observations at J, H and
K broad bands of the regions surrounding 31 SFR galactic
H 2 O and OH masers. The main conclusions, taking into
account also the results of Paper I, are as follows:
1. we identified a K­band source within 5 00 from the H 2 O
maser components in 73% of the cases (91% within
10 00 ) and in 88% in the case of OH masers (95% within
10 00 );
2. the NIR sources close to the masers and which are
believed to be physically associated with them, have
colours which differs from those of the field stars, show­
ing in most cases a remarkable near infrared excess at
2.2¯m;
3. the separation between the maser components and the
associated NIR sources is in most cases less than 10 4
AU;
4. the NIR sources associated with the maser components
appear to be young massive stars in their earliest evo­
lutionary stages.
One of the major results of this work is that even in
fields overcrowded by foreground/background stars it is
possible to find the NIR source physically associated with
the masers by examining the positional coincidence and
the NIR colours. We argue that this is the general situa­
tion, namely that the masers are always associated with
a NIR source with strong infrared excess and that this is
the ultimate energy source for the maser action.
Acknowledgements. We thank R. Forster for providing unpub­
lished VLA data. Support from C.N.R.--N.A.T.O. Advanced
Fellowship program and from NASA's Origins of Solar Systems
program (through grant NAGW--4030) is gratefully aknowl­
edged.
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