Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.arcetri.astro.it/~lt/preprints/proplyds/proplyds.ps.gz Äàòà èçìåíåíèÿ: Tue Sep 11 16:56:49 2007 Äàòà èíäåêñèðîâàíèÿ: Sat Dec 22 08:00:43 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: ionization front

Disk Mass Limits and Lifetimes of
Externally Illuminated Young Stellar Objects
Embedded in the Orion Nebula
John Bally 1;4 , Leonardo Testi 2;5 , Anneila Sargent 2;6 , and John Carlstrom 3;7
ABSTRACT
We present 1.3 millimeter wavelength interferometric observations of two
fields containing 6 externally illuminated young stellar objects embedded within
the Orion Nebula that have been observed with Hubble Space Telescope. We
derive upper bounds on the dust mass from the absence of continuum emission
and upper bounds on the gas mass from the lack of CO emission. These limits
imply circumstellar disk masses less than 0.015M fi for the observed sources and
upper bounds on the column density of 13 CO of N( 13 CO) ! 1:5 \Theta 10 15 cm \Gamma2 .
Comparison with lower bounds on the dust content derived from the visibility
of the circumstellar material in silhouette against the background nebular light
and the extinction towards the embedded central star implies that 13 CO is at
least an order of magnitude less abundant in these circumstellar environments
than in normal molecular clouds. The non­detection statistics are combined
with estimates of UV radiation­induced mass loss rates to derive an upper
bound on the UV irradiation time for these young stellar objects. The young
stellar objects in the Orion Nebula that are still surrounded by circumstellar
material have been exposed to external UV radiation for less than 10 5 years and
possibly for as little as 10 4 years.
1 Department of Astrophysical and Planetary Sciences and Center for Astrophysics and Space Astronomy,
University of Colorado, Campus Box 389, Boulder, CO 80309­0389
2 Division of Physics, Mathematics and Astronomy, California Institute of Technology, MS 105­24,
Pasadena, CA 91125
3 Department of Astronomy and Astrophysics, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637
4 bally@casa.colorado.edu --- http://casa.colorado.edu/¸ bally
5 lt@astro.caltech.edu
6 afs@astro.caltech.edu
7 jc@hyde.uchicago.edu

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Subject headings: stars: pre­main sequence ­ formation ­ individual (Orion
Nebula);
1. INTRODUCTION
Evidence for accretion disks surrounding young stellar objects (YSOs) comes from the
infrared excesses, IR spectra, from the large column density of dust detected at millimeter
and sub­mm wavelengths combined with the low extinction towards the central stars,
and from direct imaging of the gas and dust with millimeter wavelength interferometers
(Sargent 1995 and references therein). However, the most compelling observations of disks
surrounding young stars come from the Hubble Space Telescope (HST) which provides
angular resolution better than 0.05 00 . HST images of the Orion nebula reveal over 150 young
stars with extended circumstellar structure (O'Dell et al. 1993; O'Dell & Wen 1994). These
authors coined the term ``proplyd'' (an acronym for PROto PLanetarY Disk) to describe
the apparent nature of these objects.
The special nature of these systems was first indicated by Laques & Vidal (1979) who
observed seven compact emission line sources very close to the O7pe star ` 1 Orionis C.
Subsequently, Churchwell et al. (1987) and Garay et al. (1987) detected radio continuum
emission from about 30 sub­arcsecond diameter sources in the region, including all of
the Laques and Vidal objects. They argued that these are young stars surrounded by
circumstellar material that is partially ionized by ` 1 Orionis C. Indeed a more complete
survey of M42 by Felli et al. (1993) demonstrated that many of the radio sources consist of
cusps facing ` 1 Orionis C. Many of the radio sources show unresolved near­infrared emission
which McCaughrean & Stauffer (1994) argue are young stars of relatively low mass.
A narrow band HST imaging survey of the inner part of M42 at 0.1 00 angular resolution
resulted in the discovery of about 150 proplyds (O'Dell & Wong 1996). McCaughrean &
O'Dell (1996) discuss the properties of 6 objects seen in silhouette against the nebula. A
more detailed study of the inner parts of the nebula south of the Trapezium utilized the
higher resolution Planetary Camera (PC) detector of WFPC2 (Bally et al. 1998) that has a
pixel scale of 0.045 00 . These images provide the best available data for probing the structure
of the YSO environments embedded in M42.
The mass of ionized circumstellar material associated with the proplyds is small. O'Dell
& Wen (1994) estimated values of about 10 \Gamma5 M fi while Churchwell et al. (1987) found 10 \Gamma6
to 10 \Gamma7 M fi . Based on lower limits to the dust optical depth, O'Dell & Wen (1994) and
McCaughrean & O'Dell (1996) determined lower bounds of 3:0 \Theta 10 \Gamma7 to 2:2 \Theta 10 \Gamma3 M fi for

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six of the dark proplyds. Most recently, using the NICMOS camera on HST, McCaughrean
et al. (1998) have estimated a lower bound of at least 5\Theta10 \Gamma4 M fi for the large, edge­on
silhouette disk, 114\Gamma426.
The long wavelength thermal emission from grains can also be used to determine the
dust mass. Using the BIMA array at 2.6 mm, Mundy, Looney, & Lada (1995) searched
for continuum emission from the externally illuminated YSOs embedded in the Orion
Nebula. Their observations covered a 45 00 radius region centered about 20 00 northwest of
the Trapezium that contains 33 YSOs associated with externally illuminated circumstellar
material and 108 Trapezium cluster stars. Four sources that are bright at centimeter
wavelengths were also detected at 2.6 mm, but the nature of the short wavelength emission
is uncertain. In fact, there may be a strong free­free component with little thermal emission
from dust. The detections and upper limits led these authors to argue that the mass
of circumstellar material surrounding the externally illuminated objects in the observed
field of view is less than 0.15M fi . Statistical analyses of the entire sample of possible
sources suggested that the average disk mass is less than 0.03M fi . Evidently, high mass
circumstellar disks are not present in the inner portion of the Trapezium cluster.
With the IRAM Plateau de Bure interferometer Lada et al. (1996) re­observed at
1.3 mm two 30 00 diameter fields of view that contain 15 cometary externally illuminated
YSOs near the Trapezium. The first field contained 170­337 (HST 2) 170­334, 170­337,
169­338, 171­340, 173­341, and 177­341, and the second 161­322 (LV 4), 158­323 (LV 5),
157­323, 161­328, 158­326, and 158­327 (LV 6 = HST 4). Their sensitivity was about an
order of magnitude greater (0.005M fi at the 9oe level) than that achieved by Mundy et al.
(1995). Three continuum sources were detected with masses ranging from 0.007 to 0.016M fi .
Sources within a projected distance of less than 35 00 of the Trapezium appear to have
circumstellar masses less than 0.016M fi . However, the largest circumstellar structures
associated with the Orion YSOs are located at considerably greater projected distances
from the Trapezium. An example is the teardrop shaped, externally illuminated YSO
182­413 (HST 10) at a projected distance of 57 00 from ` 1 Orionis C. This object has an
ionization front with a major axis diameter of nearly 2 00 and is associated with a large
dark region seen in silhouette against background nebulosity in Hff and in the transitions
of various ions. It is interpreted as a nearly edge­on circumstellar disk (McCaughrean &
O'Dell 1996) with major axis diameter of nearly 0.4 00 . The largest of the circumstellar disks
seen in silhouette, 114­426, is located nearly 100 00 from ` 1 Orionis C and has a major axis
diameter of nearly 2 00 (McCaughrean et al. 1998). The large angular dimensions and high
opacity of these objects at visual and near­infrared wavelengths suggest that they may be
detectable at millimeter wavelengths.

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In this work, we present sensitive new observations of the circumstellar environments
of large objects located at projected distances of 60 00 and 100 00 from the Trapezium region.
The first field was centered on 182­413 (HST 10); it contains the pure silhouette, 183­405
(HST 16), and the bright objects 183­419, 184­427, and 174­414. The second field was
centered on the giant silhouette 114­426.
2. OBSERVATIONS
Between November 1995 and February 1996, the two fields centered on 182\Gamma413
(O'Dell & Wen 1994) and 114\Gamma426 (McCaughrean & O'Dell 1996) were observed with the
Owens Valley Radio Observatory millimeter wave array located near Big Pine, California.
Three configurations of the six 10.4 m telescopes provided baselines in the range 15­115 m.
Cryogenically cooled SIS receivers were used to observe the 13 CO (2\Gamma1) and C 18 O (2\Gamma1)
rotational lines and the 1.3 mm continuum. For the line observations, the digital correlator
was centered at v LSR = 5 km s \Gamma1 and configured to produce two ¸ 80 km/s wide bands
with 1.3 km/s resolution for both lines (after on­line Hanning smoothing). The continuum
observations used two 1 GHz wide bands of an analog correlator. Typical average system
temperatures were ¸ 1200 K.
Gain and phase were calibrated by monitoring the quasar 0528+134 every ¸ 20 minutes.
Observations of 3c273 and/or 3c454.3 were used for passband calibration. The flux density
scale was based on the fluxes of 0528+134 (3­4.7 Jy), 3c273 (¸ 30 Jy) and 3c454.3 (¸ 5 Jy),
determined from measurements of Uranus and Neptune. We estimate the uncertainties to
be less than 20%. All calibration and editing of the data was performed with the MMA
software package (Scoville et al. 1993). After calibration the (u; v) data sets were loaded
into the NRAO AIPS package for mapping and analysis.
With baselines in the range ¸ 10\Gamma88 k–, our data is partially sensitive to the extended
emission structures of the Orion nebula. Maps made from the whole (u; v) data set show
contamination from large scale emission both in the continuum and in the CO isotopic
transitions. Because we are primarily interested in the detection of compact sources ( !
¸ 2 00 )
associated with the dark disks, we also exploited the spatial filtering capability of the
interferometer, using only data obtained on the baselines longer than 35 k– (cf. Mundy et
al. 1995; Felli et al. 1993; Garay et al. 1987). This resolves out smooth structures ?
¸ 6 00 in
extent. Our cut­off of 35 k– is the optimum compromise between filtering capability and
sensitivity.
We used the AIPS IMAGR task to convert both data sets into weighted data

-- 5 --
cubes for each of the molecular lines maps. For the continuum map the two 1 GHz
wide side­bands were averaged together to increase the sensitivity. For all maps the
resulting synthesised beams are ¸ 2: 00 0 \Theta 1: 00 7. The noise level in the line observations is
¸ 200 mJy beam \Gamma1 channel \Gamma1 , corresponding to ¸ 1:5 K on a brightness temperature scale
(for a source that fills the synthesised beam). In the continuum maps, the noise level was
¸ 7 and ¸ 8 mJy beam \Gamma1 for the fields centered on 182\Gamma413 and 114\Gamma426 respectively. No
correction for primary beam attenuation was applied.
3. RESULTS
Compact molecular line emission was not detected in either the image made from the
full (u; v) data set or that from the data above the 35 k– cut­off. Figures 1 and 2 display the
continuum maps for each field obtained from the 35 k– cut­off data set. The positions of the
dark silhouette sources from O'Dell & Wen (1994) and from McCaughrean & O'Dell (1996)
are marked and labeled. No source is detected in the field centered on 182\Gamma413, above a 3oe
limit of ¸21 mJy. In the 114\Gamma426 field, a 26\Sigma8 mJy (¸ 3oe) peak is detected within 1.1 00 of
the dark silhouette. Precise position and parameters are listed in Table 1.
The mass of circumstellar dust can be estimated from the millimeter wavelength flux
by assuming optically thin thermal emission from a single temperature population of dust
grains. For an unresolved source, the flux can be expressed as F š = B š (T D )Ÿ š M=d 2 , where
TD is the temperature of the dust, B š the Plank function, d the distance of the source,
M the mass, and Ÿ š the dust mass opacity. The exact behavior of Ÿ š in the mm and
sub­mm wave range is poorly known and there are indications that it may be environment
dependent (Beckwith & Sargent 1991). Usually a power law dependence of the opacity with
the frequency is assumed. For consistency with the previous studies (Mundy et al. 1995) we
adopt Ÿ š = 0:1(š=1200 GHz) and TD = 50 K. The assumed value of Ÿ š imply a gas to dust
ratio of 100 by mass (Hildebrand 1983). The resulting mass of the source near 114\Gamma426 is
¸ 0:020 M fi . In the 182\Gamma413 field, the 3oe level of 21 mJy correspond to an upper limit on
the mass of ¸ 0:015 M fi .
The mass of circumstellar molecular gas can be estimated from the CO isotopic
lines. Our observations can be used to infer that the contrast between the CO lines
produced by our targets and the diffuse background emission differs by less than the
noise in our spectra. However, it is unlikely that the intrinsic brightness temperature
of the CO lines produced by the proplyds is the same as the background to within the
noise limit. Therefore, the upper bounds of 1.5 K in 1.3 km s \Gamma1 wide channels imply an
upper bound on the column density of 13 CO of N( 13 CO) ! 1:4 \Theta 10 15 cm \Gamma2 where we

-- 6 --
have assumed an excitation temperature T ex = 50 K and a limit on the surface brightness
of the 13 CO emission of I( 13 CO) ! 2 K km s \Gamma1 . For an assumed H 2 to 13 CO abundance
ratio of X 13
= N(H 2
)=N( 13 CO) = 7 \Theta 10 5 (Frerking, Langer, & Wilson 1982) this
implies that the column density of H 2
, N(H 2
) ! 10 21 (X 13 =7 \Theta 10 5 ) cm \Gamma2 and a mass limit
of M(H 2
) ! 3 \Theta 10 \Gamma4 (X 13
=7 \Theta 10 5 ) M fi in the 1.7 00 by 2 00 diameter synthesized beam. For
114­426 and 182­413 (HST 10), this upper bound combined with the lower bounds on
the masses of these objects set by Chen et al.(1998) and McCaughrean et al. (1998) of
2 \Theta 10 \Gamma3 M fi implies that 13 CO is depleted by about an order of magnitude or more with
respect to the interstellar 13 CO to dust ratio in the circumstellar environment of these
objects.
The object 182­413 (HST 10) has a bright teardrop shaped ionization front visible
in Hff and other nebular emission lines. The region between the ionization front and the
dark edge­on disk seen in silhouette has relatively low dust opacity and glows faintly in
several atomic and low ionization potential emission lines. The disk photo­evaporation
model of Johnstone, Hollenbach & Bally (1998) predicts that the column density of the
region bewteen the disk surface and the ionization front is that required to absorb the
soft UV radiation (wavelengths longer than 912 š A) that dominates heating in a photon
dominated region (PDR). Thus, the envelope is expected to have a hydrogen column
density of order N(H) ú 10 21 cm \Gamma2 , a corresponding extinction of about A v = 1 to 2
magnitudes, and is likely to be mostly atomic. The disk is dark in Hff and in ionized
transitions (seen in silhouette against the bright nebular background) but bright in the
6300 š A [Oi] line (Bally et el. 1998; Johnstone et al. 1998) and in the 2.12 ¯m H 2
line. Chen
et al. (1998) show that the H 2
emission is associated with the disk (probably originates at
the disk surface) and not with the envelope that lies between the disk and the ionization
front. The extinction towards the central star of 182­413 (HST 10) is very large with
A v ? 50 magnitudes (N(H) ?? 10 22 cm \Gamma2 ). Therefore, the silhouette is likely to be fully
self­shielded, predominantly molecular, and the most massive component of this system
after its central star. It is this region to which our dust and gas upper limits apply.
The giant silhouette 114­426 does not have an ionization front. However, as in the case
of 182­413 (HST 10), the central star is hidden in the visual wavelength images by a nearly
edge­on disk which has a very large column density and our gas and dust limits for this
source refer to the contents of the dark region seen in silhouette.

-- 7 --
3.1. Constrains on the Survival Time of the Circumstellar Material
A constraint on the photo­ionization age of the nebula can be derived from the large
number of YSOs with circumstellar matter surrounding the Trapezium, the mm­wavelength
detection statistics, and the estimated mass­loss rates. Within 20 00 of the Trapezium,
about 80% of stars are proplyds surrounded by extended but very compact circumstellar
material. Since most young stars near ` 1 Orionis C are in the proplyd phase, even the least
massive ones must retain their envelopes for extended periods. From published millimeter
wavelength interferometry, upper bounds on the masses of the material surrounding these
YSOs is 0.016M fi (Mundy, Looney, & Lada 1995; Lada et al. 1996). Our search for 230 GHz
13 CO and continuum emission from the much larger objects that contain large silhouettes
such as 182­413 (HST 10), 183­405 (HST 16), and 114­426 with the OVRO interferometer
suggests upper bounds less than about 0.02 M fi even for these relatively large objects.
This is consistent with the lower bounds (¸ 2 \Theta 10 \Gamma3 M fi ) estimated from the optical and
near­infrared extinction (Mc Caughrean & O'Dell 1996; Chen et al. 1998). We conclude
that the maximum mass of dusty circumstellar matter surrounding these YSOs is about
10 \Gamma2 M fi .
Adopting 0.01 M fi as an upper limit to the typical mass of the circumstellar
environment of a proplyd and a characteristic mass loss rate for the photo­ablation flow of
10 \Gamma7 M fi yr \Gamma1 (Bally et al. 1998; Johnstone, Hollenbach, & Bally 1998) leads to an estimate
for the maximum lifetime of the circumstellar environment of the proplyds of less than
10 5 years. Near ` 1 Orionis C the mass loss rate can be 10 \Gamma6:2 M fi yr \Gamma1 and the expected
photo­ablation survival time scale is only 1:6 \Theta 10 4 years. Assuming that the least massive
disks are at least 10 times less massive than the most massive detected objects (Beckwith et
al. 1990), the survival time for the lowest mass disks must be less than 10 4 years. However,
Prosser et al. (1994), Herbig & Terndrup (1986), and Hillenbrand (1997) find that the ages
of low mass stars in the Orion Nebula Cluster (ONC) range from about 3 \Theta 10 5 to over
10 6 years, with only a few as young as 10 5 years. Thus, most members of the ONC are
significantly older than their current estimated proplyd survival times.
There are two possible explanations for the apparently short lifetimes of the proplyds.
First, the photo­ablation mass loss rate estimates may be in serious error. O'Dell (1998)
argues that exponential profiles can be used to infer a static envelope with no mass loss.
In this scenario, a large confining pressure precisely counterbalances the thermal pressure
gradient of the ionized envelope. Alternatively, the photo­ablation rate estimates may be
correct, but photo­evaporation started only very recently. This implies that most of the
Orion Nebula Cluster stars were born between 3 \Theta 10 5 to 10 6 years before ` 1 Orionis C
started to ionize the nebula and the circumstellar environments of the proplyds (Bally et

-- 8 --
al. 1998; Johnstone et al. 1998).
Either ` 1 Orionis C reached the main sequence very recently, or ` 1 Orionis C spent the
first part of its life so deeply embedded in a dense environment that its Lyman continuum
radiation was trapped (Hollenbach et al. 1994), or the proplyds were previously protected
from ` 1 Orionis C by intervening material. Since the birth a high mass star is expected to
halt star formation in its vicinity, the first possibility is a plausible scenario. However, many
high mass stars spend a substantial fraction of their lives (a few hundred thousand years)
in an ultra­compact Hii region phase during which much of the stellar Lyman continuum
radiation remains trapped close to the star (Wood & Churchwell 1989a, 1989b; Hollenbach
et al. 1994). In this scenario, the confining medium has only recently been destroyed,
possibly by photo­evaporation or by entrainment in the stellar wind, in which case, the
wind may be mass­loaded (cf. Williams, Hartquist, & Dyson,1995). It is also possible that
most of the proplyds lie on the far side of the Hii region and that the advancing ionization
front at the rear of the Orion Nebula has only uncovered these objects recently.
It is not unreasonable to assume that the distribution of the initial circumstellar
masses of the YSO follows a power law. It is likely to be similar to the distribution of
stellar masses, i.e. to follow a power law like the initial mass function. Then the detection
of several objects with circumstellar masses ¸ 10 \Gamma2 M fi and the upper bounds found for
about 20 other objects, can be used to determine the photo­ionization age of the nebula
statistically. Assume that the distribution of disk masses is a power law of the form
N(m;m + dm) = km \Gammafi dm with fi = 2.35, following the Salpeter IMF. The number of
disks detected above mass, M is R 1
M dN . If the mass spectrum has a lower mass cutoff
at m, the number of non­detections is proportional to R M
m dN . The ratio of detections to
nondetections, f , is then the ratio of these integrals, and the minimum mass of a disk in
the sample is expected to be roughly given by m = (M 1\Gammafi + [M 1\Gammafi =f ]) (1=1\Gammafi) . Since
3 out of 20 disks are detected, and the minimum detected mass, M is 0:007 M fi , the
minimum disk mass that we expect in our sample is m = 0.002 M fi . More than 80% of the
YSOs near Trapezium still have disks, implying that insufficient time has elapsed since the
onset of photoionization to evaporate disks with such masses. Thus, the photo­ionization
age of the proplyds, and perhaps of the bulk of the Orion Nebula, is not much more than
Ü P I ú m= —
M ú 2 \Theta 10 4 years.
Star formation in the Orion Nebula may be representative of the environments in
which most stars are born. For example, within 500 parsecs of the Sun, at least 90% of
all stars younger than 10 or 20 million years formed within the boundaries of four major
OB associations; Scorpius­Centaurus, Perseus OB2, Orion OB1, and the Lacerta OB1
associations (Blaauw 1991). These OB associations each produce about 10 4 stars during

-- 9 --
10 Myr, whereas in isolated dark clouds such as Taurus fewer than 10 3 stars form in a
comparable time interval. Indeed, most star formation in the OB associations occurs in
short duration `micro­bursts' during which the gravitational collapse of a 10 3 to 10 4 M fi
cloud core produces clusters of 50 to 1000 stars in less than 1 million years in a region
smaller than one parsec (Lada 1992; Lada, Alves, & Lada, 1996; Bally, Devine, & Reipurth
1996). The birth of one or more massive stars which heats, dissociates, and ionizes the
remaining gas may terminate star formation in such a region. Low mass stars formed in
OB associations are likely to be exposed to the harsh UV radiation, and in some cases, the
stellar winds produced by massive stars, for a time ranging from 0.1 to 10 million years.
Can planets form in such harsh environments? If so, extra­Solar planets may be
common; if not, extra­Solar planets may be rare. The circumstellar environments of the
young stars embedded in the Orion Nebula appear to be rapidly evaporating. They have
either already formed dense compact objects that can resist photo­erosion, in which case
the solid phase in these systems may continue to evolve into debris disks that can accrete
to form planets, or they may fail entierly to produce planets since their circumstellar
gas supply is rapidly dissipating. Thus, for planets to exist around most stars, most of
which are born in Orion­like environments, proto­planets must form in less than 10 6 years.
Either proto­planet formation is prompt or planetary systems are relatively rare. Future
observations of Orion's proplyds may tell.
Acknowledgements: 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 acknowledged. The Owens Valley millimeter­wave array is supported by NSF
grant AST­96­13717. Research at Owens Valley on young star and disk systems is also
supported by the Norris Planetary Origins Project.
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This preprint was prepared with the AAS L A T E X macros v4.0.

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Table 1. Detected source parameters
Name ff ffi Flux Mass
(1950.0) (1950.0) (mJy) (M fi )
114\Gamma426 5:32:43.87 \Gamma5:26:22.1 26\Sigma8 ¸0.020

-- 13 --
Fig. 1.--- Grey­scale and contour 1.3 mm continuum image of the field centered on 182\Gamma413.
Crosses mark the positions of the objects from O'Dell & Wen (1994) and McCaughrean &
O'Dell (1996) in the field. The dashed circle marks the OVRO primary beam (FWHP).
Contour values are \Gamma22, \Gamma15, 15, 22 mJy/beam, negative contours are dashed. The black
ellipse in the lower right corner shows the FWHP synthesised beam.

-- 14 --
Fig. 2.--- As in Figure 1 but for the field centered on 114\Gamma426. Contour values are \Gamma26,
\Gamma17, 17, 26 mJy/beam.