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Interactions between a Bright YSO and the MSX Infrared-Dark Cloud G79.3+0.3: An Early Stage of Triggered Star Formation?

Millimeter and mid-infrared observations have been made of the dense clumps of dust and gas and of young stellar objects (YSOs) associated with the bright, compact submillimeter source G79.3+0.3 P1 in the relatively nearby MSX infrared-dark cloud G79.3+0.3. The Gemini mid-infrared observations reported here indicate the presence of three YSOs within the cloud. BIMA 3 mm continuum observations show that the brightest of the YSOs is likely to be a Herbig Ae/Be star. High-angular-resolution molecular-line observations suggest that a wind from this star may be triggering collapse in the adjacent molecular cloud. The submillimeter source G79.3+0.3 P1 itself does not contain infrared sources and may represent an earlier stage of star formation.


Introduction
have identified a large population of cold dust clouds in images of the Galactic Plane made with the MSX satellite (Price et al. 2001). These clouds appear as dark patches of absorbing material against a background of mid-infrared emission bands and emission from warm, small dust grains. Egan et al. defined the infrared-dark clouds (IRDCs) as having one to several magnitudes of extinction at 8 µm and no obvious emission in any of the 8 to 25 µm bands observed by the MSX satellite. They concluded that the IRDCs possess hundreds of magnitudes of visual extinction and contain large column densities of cold dust. Observations of a selection of these clouds in H 2 CO by Carey et al. (1998) revealed that much of the gas inside the IRDCs has T K ≈ 10-20 K and n(H 2 ) 10 6 cm −3 , with H 2 column densities ranging up to 10 23 cm −2 . Most of these objects are quite distant, with kinematic distances between 1 and 8.5 kpc.
The IRDCs appear to be a very interesting population of molecular clouds. On theoretical grounds, we expect that molecular clouds on the verge of star formation will contain extremely cold and dense condensations, which will show up as IRDCs if they are favourably located in front of a region of extended mid-IR emission. The selection criteria for the MSX IRDCs ) ensures that they do not yet contain high-mass main-sequence stars and HII regions.
Trying to relate the physical properties of this selection of dust clouds to those of betterstudied nearby molecular clouds, such as those in the Orion-Taurus region, is problematic. Most of the best studied clouds lie in the outer Galaxy where there is insufficient background emission for IRDCs to be identifiable. By contrast, the large distances of many MSX IRDCs make them difficult to detect at visible and near-infrared wavelengths, and difficult to pick out from the surrounding clutter of lower density clouds in molecular-line surveys.
A notable exception is the cloud G79.3+0.3, located in the Cygnus Rift at an estimated distance of 800 pc (Miller 1937;Ikhsanov 1961;Wendker et al. 1991), close enough to the Sun that it can be seen at visible wavelengths . Figure 1 and Figure 2 show our JCMT SCUBA 850 µm image of the G79.3+0.3 IRDC and the MSX 8 µm image of the same region, respectively. Unlike the other IRDCs we have observed, this cloud exhibits a variety of indicators of star formation. Most prominently, the HII region DR15 (G79.307+0.277) lies behind and slightly south of the IRDC, between P4 and P5 in the figure. It is not immediately obvious whether the IRDC is part of a larger complex including DR15 or is an unrelated foreground cloud. In visible and near-IR images of the region , there is an association of stars (to the west of DR15) whose northern boundary is defined by the southern edge of the IRDC. If DR15 is related to this association, it suggests that both DR15 and the association lie beyond the IRDC. In the 8 µm MSX image (Figure 2), the dark filament of the IRDC (whose dust content is traced by the SCUBA 850 µm emission in Figure 1) is interrupted between P1 and P3 by a bright patch of warm dust emission which is suggestive of deeply embedded hot stars. Because of this, the eastern and western parts of the IRDC were initially assigned different designations in the MSX catalog (G79. 34+0.33 and G79.27+0.38, labeled G79.34 and G79.27 in Figure 1), although the figure clearly shows them to comprise a single, connected filament. A Herbig-Haro jet that appears to be driven by a YSO at RA(2000) = 20 31 45.5, Dec(2000) = +40 18 44 (see Figure 2) has been discovered in front of the IRDC at visible wavelengths .
In this paper we focus our attention on the star-forming activity in the vicinity of its most prominent condensation, G79.3+0.3 P1. This region was selected for closer study because comparison of the SCUBA and MSX images discussed below showed the presence of a faint, point-like emission source MSX5C G079.3398+00.3415 in the MSX image that was nearly coincident with the brightest compact source of emission at 850 and 450 µm. This suggested the presence of a deeply embedded star interacting with the cold dust cloud, but the relatively coarse resolution of both the MSX and SCUBA images hampered a more detailed interpretation. To address these issues, we combined JCMT observations at 450 µm, BIMA interferometry at 3 mm, and Gemini North imaging at 10.75 µm, together with archival data from the MSX satellite and the 2MASS survey.

JCMT Observations
JCMT SCUBA observations were obtained at 850 and 450 µm where the resolution is 14 ′′ and 8 ′′ , respectively (Holland et al. 1999). "Scan mapping" was employed to produce Nyquist sampled images. Sky subtraction was provided by chopping to a nearby location. We used the Emerson-2 scan-mapping procedure (Emerson 1995) as adapted for SCUBA (Jenness et al. 1998a) with offsets of 20 ′′ , 30 ′′ , and 65 ′′ at position angles of 0 • and 90 • . The data reduction process for scan maps attempts to correct for the chopping, so that sources appear only once, as positive features. However, structures larger than the chop throw may not be reproduced correctly. The zero-level is uncertain unless the maps are large enough to include some blank sky in the image. Descriptions of SCUBA and its observing modes may be found in Holland et al. (1999) and Jenness et al. (2000).
The SCUBA observations were taken under fair weather conditions (τ 850 = 0.24-0.30) on 1999 April. The data were processed using the standard routines provided by the SURF package (Jenness & Lightfoot 1998b) for flat fielding, extinction correction, and sky subtraction. Uranus were used as a flux-density calibrator, using the calculated flux densities of Uranus produced by the FLUXES program, which makes use of the measurements reported by Griffin & Orton (1993). Our measured flux densities are good to ±15%. Pointing was performed on Uranus and G34.3, and should be better than 2 ′′ .
Observations of the HCO + (3 − 2) transition were made using RxA3 in a raster-scanning mode. The grid of sample points had a spacing of 10 ′′ × 10 ′′ , centered at RA = 20 32 03.4, Dec=+40 19 20 in J2000 coordinates and oriented with the Y-axis of the grid at a position angle −94. • 4 to follow the overall orientation of the G79.3+0.3 IRDC. The nominal beam size of the JCMT at this wavelength is 18. ′′ 4, so the grid of samples is approximately Nyquist spaced. With the raster-mapping technique, the beam is smeared somewhat in the direction of motion of the telescope during the sample, giving an effective beam size of 20. ′′ 5 × 18. ′′ 4, with the long axis aligned north-south. In this paper we report on a subset of the whole dataset, a square region 160 ′′ ×160 ′′ with a position angle for the vertical axis of 0 • centered at RA=20 32 22.24, Dec=+40 19 53 (J2000), covering the same area as the BIMA HCO + (1−0) observations discussed below.
The HCO + (3 − 2) observations were taken on the night of 2001 June 26 (UT) under weather conditions which are fairly poor for the JCMT but still quite usable at 267 GHz. The zenith opacity at 225 GHz ranged from 0.14 to 0.20 and the seeing was greater than 1 ′′ . The Starlink program SPECX was used to process the observations into a datacube.

Gemini Observations
Observations were carried out at the Gemini North Telescope on 2000 December 7 and 2001 July 10 using OSCIR, the U. Florida mid-IR imager/spectrometer. OSCIR employs Si:As blocked-impurity-band (BIB) detector array with 128×128 pixels, optimized for a wavelength coverage from 8 to 25 µm. On Gemini its plate scale is 0. ′′ 089 per pixel, which gives a FOV of 11. ′′ 4 ×11. ′′ 4. All observations were made using the standard chop/nod technique to remove the sky and telescope background emission, and employing a chop frequency of 3 Hz and a throw of 25 ′′ . Images were obtained in the N-band (λ 0 = 10.75 µm, ∆λ = 5.23 µm), with 60 seconds of on-source integration time per field. A 3×3 mosaic was obtained by combining five fields acquired in December (mainly the SE part of the mosaic), with six acquired in July.
The standard stars β Peg and Vega were used to provide flux calibration. Their flux densities were taken to be 352.13 Jy and 37.77 Jy, respectively, through the OSCIR N-band filter (Fisher 2001). Our 2001 July run was plagued by light cirrus and higher humidity; hence the flux of 2MASSI 2032220+402017, observed the previous December, was used to estimate the fluxes of the fainter sources detected in the July fields.
All data were taken with the telescope active optics in open-loop, using an elevationdependent lookup table, and with fast guiding provided by the secondary mirror "tip-tilt" corrections under control of the Peripheral Wave Front Sensor (PWFS) locked on a nearby guide star. Hence some residual astigmatism was present in the point-spread function. The detected sources should be considered unresolved since their measured FWHMs (at best 0. ′′ 47; the diffraction limit in N-band for Gemini is 0. ′′ 28) are similar to those measured for the imaged standard stars. There is a discrepancy of order 2 ′′ in the coordinates of the detected source between the December data and the July data. The adopted coordinates are those derived in July, since a guide star with better position accuracy was used (error < 0. ′′ 2). However, the accuracy of the model of the PWFS arm 2 for the distance between the OSCIR center and the PWFS is unknown. There is a 1. ′′ 5 offset between the position of 2MASSI 2032220+402017, with a nominal accuracy of 0. ′′ 2, and the apparent position of the OSCIR source. We do not believe this difference is real because of the possible systematic error mentioned above, so in this paper we assume that these two sources are the same.

YSOs and Infrared Stars
The original motivation for the N-band Gemini observations reported here was to determine whether the MSX 8 µm source was composed of a single star, a cluster of fainter stars, or a diffuse emission patch, and to determine what relationship if any it had to the P1 source in the SCUBA images. In fact, most of the mid-IR emission comes from a single point-like source coincident with the star 2MASSI 2032220+402017 (see Figure 3a).
In addition, two fainter sources are also found in the field, but far enough from the central source that they are unlikely to be directly associated with it. Figure 3b shows the locations of the three stars superimposed on a contour map of the SCUBA 450 µm emission. The angular resolution of the 450 µm map is 7 ′′ , sufficient to demonstrate that 2MASSI 2032220+402017 lies a full beamwidth away from the peak of the 450 µm emission. In fact, none of the three N-band sources lie within the top contours of the 450 µm emission from the P1 source. If YSOs are forming within the dust clumps constituting the P1 source, they are too faint and/or too deeply embedded to be detectable in these images.
From the N-band images it is impossible to rule out the presence of a significant component of extended emission around the 2MASS star. As mentioned above, the PSF had a FWHM of 0. ′′ 47, with roughly 60% of the signal contained in a circle of diameter 0. ′′ 8 centered on the star. An additional 40% of the signal comes from a region extending out as far as 2. ′′ 7 from the star. Because we were unable to take observations of a reliable PSF calibration star, it is possible that as much as 40% of the emission could be extended on a scale larger than 0. ′′ 8. Alternatively, the apparent extended emission could simply be due to the wings of the PSF.
The photometric properties of all three N-band sources are summarized in Table 1. The two fainter sources have been calibrated relative to 2MASSI 2032220+402017. In Table 1, the uncertainty in the flux-density estimates has been divided into internal errors (which indicate the significance of the detection) and systematic errors due largely to the unknown degree of possible extended emission around the 2MASS star. The flux density of 2MASSI 2032220+402017 was determined using a large aperture to capture all of the signal. The photometry of the weaker sources were measured in two ways. Firstly, assuming the apparent extended emission is due to the wings of the PSF, the photometry was done by convolving the images with a PSF derived from the bright source, effectively using a small aperture to maximize the signal-to-noise ratio. Secondly, assuming that the apparent emission is genuinely extended, the photometry was done using a 2. ′′ 7 aperture comparable to that used to calibrate the 2MASS star. The reported fluxes in Table 1 are the average of the two measured values.
On probabilistic grounds we can determine that the two weak N-band sources are YSOs, most likely associated with the IRDC, and are not simply background field stars seen through the cloud. The (N-K S ) colors of unreddened stars are quite small, so we can estimate the surface density of N-band stars by using the 2MASS survey to measure the brightness distribution at K S in nearby fields with low extinction. The two N-band stars have N magnitudes of 7.5 and 8.4. If the stars were merely background stars, the corresponding K magnitudes would be similar, hundreds of times brighter than we would expect to find in the small area of sky we observed.
It is interesting that there appears to be a population of heavily reddened stars visible in the 2MASS K S -band image of G79.3+0.3 P1 (which sometimes are not even visible at H or J). We conclude that all three N-band sources are part of a much larger cluster of embedded and highly reddened young stars that has formed in the general area around G79.3+0.3 P1.
With the available data we can only place a lower limit on the reddening toward 2MASSI 2032220+402017. It is normal for the near-and mid-infrared emission from YSOs to be dominated by their circumstellar disks. The intrinsic H-K colors of these objects are larger than those of stars [e.g., see Malfait et al. (1998)], but are still small compared to the apparent reddening of 2MASSI 2032220+402017. Since the 2MASS star is close to the most heavily extincted part of the IRDC, we will assume that most of the reddening is interstellar. 2MASSI 2032220+402017 has an apparent brightness K S from the 2MASS Point Source Catalog of K S = 12.0 but is not detected at H-band (H > 17.3). From these measurements we can estimate A K > 8 mag using the extinction law from Martin & Whittet (1990), assuming that the intrinsic H-K color of 2MASSI 2032220+402017 is small. With this estimate for A K , the unreddened apparent magnitude K S < 4 mag.
At an assumed distance of 800 pc, the absolute magnitude of 2MASSI 2032220+402017 at K S would be brighter than -5.5 mag. Although this ostensibly is the absolute magnitude of an early-O star, the absence of a prominent H II region indicates that the star must be considerably fainter and cooler with a large IR excess. This object must be an extremely luminous pre-main sequence star with the energy output of an early-B star. It thus seems likely that 2MASSI 2032220+402017 is a Herbig Ae/Be star with a large IR excess (Malfait et al. 1998) due a circumstellar disk and/or dust in a strong ionized wind.
As discussed by Fuente et al. (2002), pre-main-sequence stars that will ultimately have spectral types earlier than B5 pass through a stage lasting several hundred thousand years during which they are surrounded by a massive circumstellar disk but are still too cool to produce a significant UV flux. In this stage, a stellar wind from the pre-main-sequence star will be the primary mechanism dispersing the surrounding cloud.

Continuum Emission Sources Containing Dust and Hot Gas
To investigate further the nature of 2MASSI 2032220+402017, BIMA observations with an angular resolution of 5 ′′ were made of the 3 mm continuum emission. At this resolution, the source breaks up into three components as shown in Figure 3c. The observed properties of the three components measured from the BIMA 3 mm image are given in Table 2. Two components, labeled A and B, accord well with the morphology of our SCUBA 450 µm map. Their measured flux densities are consistent with the emission arising from dust, as discussed below. The third 3 mm component, labeled C, coincides within the accuracy of the astrometry with 2MASSI 2032220+402017, but is not evident in the 450 µm SCUBA image. It must have a significantly smaller spectral index, consistent with the 3 mm emission being dominated by thermal bremsstrahlung from the ionized wind of a Herbig Ae/Be star (Skinner et al. 1993).
We estimated the dust temperatures and total masses of components A and B following Hildebrand (1983). Assuming that the dust opacity per unit total mass column density (dust and gas) varies as κ ν = κ 0 (ν/ν 0 ) β , the dust temperatures were estimated as functions of β from the ratios of the flux densities at 450 µm and 3 mm. The masses were calculated using where M tot is the total mass, D is the distance, and B ν (T ) is the Planck function for a dust temperature T . We adopted the value of κ 0 = 0.005 cm 2 gm −1 at 1.3 mm from Motte et al. (1998), and references therein, as an appropriate value for pre-stellar dense cores and clumps. Table 3 gives the derived dust temperatures and total masses as functions of β for the two sources.
If the flux densities for components A and B had been measured separately at a third frequency, it would be possible to estimate β for each component. Unfortunately, the beam size of the JCMT at 850 µm is so large that components A, B and C all blend together, and the contribution to the combined flux density from component C is unknown. Nevertheless, we can still derive bounds on the allowed range of β. The estimated temperatures in both components become unphysically large as β drops below 1.5. Also, if we assume that β is the same in components A and B, we can invert Equation 1 to predict their combined flux density at 850 µm as a function of β. For β > 2.0, the predicted value of the combined flux density becomes greater than 2.0 Jy, the observed flux density of the whole source at 850 µm. Thus, β is bounded in the range 1.5 ≤ β ≤ 2.0.
Other observers have measured values for β in similar clouds with temperatures below 20 K that are larger than 1.5, ranging up to 2.5 [see, for example Ristorcelli (2002) and references therein]. On this basis, we believe that the combined mass of components A and B is likely to be about 15 M ⊙ , although it may be as low as 6 M ⊙ , or as high as 30 M ⊙ .

Molecular Gas in Relation to the YSOs
The integrated BIMA HCO + (1 − 0) emission in a 1.5 km/s velocity range centered on −0.8 km/s is shown in Figure 3d. The bulk of the small-scale emission detected by the BIMA interferometer is confined to a roughly circular blob south and east of the 2MASS star. The star itself appears to lie in a small "bay" devoid of emission. The morphology of the emission resembles that of the 13 CO (1-0) emission mapped by Fuente et al. (2002) around Herbig Ae/Be stars that are just beginning to disperse their placental clouds.
A comparison of HCO + (1 − 0) spectra from the BIMA interferometer data with singledish spectra of HCO + (3 − 2) in Figure 4 illustrates the relative virtues of interferometric and single-dish observations. Figure 4a directly compares the integrated emission in the blue wing where both datasets show prominent emission. The similarity of the maps is clear, but the much higher resolution of the BIMA data allows us to distinguish the A and B components of the G79.3+0.3 P1 from the larger blob of emission to the southeast. Figure 4b makes a similar comparison but with the HCO + (3 − 2) in the velocity range [−0.25, 2.25]. This range contains most of the HCO + (3 − 2) emission from the cloud in the single-dish data, but is almost devoid of HCO + (1 − 0) emission in the interferometer data. Figures 4c and 4d show spectra from the two HCO + datasets at the points where the integrated emission peaks in the blue wing (labelled "W" in Figure 4a) and in the main part of the line (labelled "M" in Figure 4b). The upper spectra show the single-dish data from the JCMT, showing the presence of significant emission in the velocity range [−0.25, 2.25]. The lower spectra show the BIMA HCO + (1 − 0) data, and were constructed by convolving the high-angular-resolution BIMA datacube with a Gaussian so that the effective beam size was comparable to the JCMT beam size. Both of these spectra exhibit the absence of detected emission in this velocity range in the BIMA data. Presumably, the emission is sufficiently smooth on small angular scales that it was resolved out by the interferometer.
On a larger scale, the JCMT observations show that there is significant emission in the blue wing, i.e. the velocity range −1.75 to −0.25 km/s, only from the small region near G79.3+0.3 P1 that is shown in Figure 4a. The BIMA data are therefore tracing the structure within a small parcel of gas with anomalous velocities.
We note that the absence of a counterpart to 2MASSI 2032220+402017 in the 2MASS J and H images suggests that it lies in or beyond the IRDC. Moreover, if the star were embedded in the IRDC we would expect the wind from the star to excite red and blue wings of roughly comparable strength. Although there is also a small amount of emission at velocities near +2.0 km/s that seems to be associated with the blob, the dominance of the blue wing indicates that the blob is a parcel of gas being driven into the far side of the IRDC. The red wing of the line in the BIMA data peaks at the same location as the blue wing, indicating that it arises in the same parcel of gas, and that the velocity dispersion in this parcel of gas is appreciably larger than in the rest of the cloud. Alternatively, the red wing might arise from an unrelated weak outflow.
There is some HCO + (1 − 0) emission associated with the A and B components of the 3 mm emission. In fact, the B component appears to sit on the edge of the brightest part of the HCO + blob. Even in these high-density cores, the emission is largely in the blue wing and not in the velocity range that contains most of the mass in the cloud. The presence of emission in the blue line wing indicates that the same energy source that has disturbed the back side of the IRDC to the southeast of G79.3+0.3 P1 is also affecting the back sides of both the A and B components. Unfortunately, the weakness of the emission in the A and B components precludes the detection of extended line wings that might be taken as evidence of collapse and/or outflow.
Follow-up BIMA observations of CS (5−4), another high-density tracer, show a compact region of enhanced emission on the rim of the HCO + cloud closest to the 2MASS star (see the white contours in Figure 3d). The absence of short spacings in the CS (5 − 4) observations compared to the HCO + (1 − 0) observations make them even less sensitive to the largescale structure of the emission. They have, however, higher angular resolution than the HCO + (1 − 0) observations and are useful for picking out fine-scale structure. We note that the B component of the 3 mm emission lies within the region of CS emission.

Conclusions
Our Gemini N-band observations demonstrate that the MSX 8 µm source MSX5C G079.3398+00.3415 associated with G79.3+0.3 P1 is dominated by a single compact object that we identify with the star 2MASSI 2032220+402017. This object is a luminous YSO with a strong IR excess that will likely become an early-B star when it reaches the main sequence. Two nearby, fainter YSOs were discovered in our N-band images, and a cluster of faint, heavily reddened stars are visible in the 2MASS K-band images of the region. This concentration of stars and YSOs indicates that the IRDC is actively forming low-mass stars, with the 2MASS star as the most massive star that has formed from it to date.
There are a variety of unusual features in the region around G79.3+0.3 P1 that suggest 2MASSI 2032220+402017 is interacting with the foreground IRDC. Most directly, the BIMA HCO + (1 − 0) line has a strongly enhanced blue wing in the velocity range −1.5 to 0 km/s, as well as a slight enhancement on the red wing, in a region immediately to the southeast of the 2MASS star. The emission in the blue wing shows considerable structure on small angular scales. The unusual nature of this structure is highlighted by the fact that the rest of the cloud shows little emission in the blue wing and very modest small-scale structure in the velocity range [−0.25, +2.25] km/s that traces most of the mass of the cloud. The presence of small-scale structure close to the 2MASS star is confirmed by CS (5 − 4) observations with even higher angular resolution.
The simplest interpretation of these data is that the 2MASS star is a Herbig Ae/Be star that is forming on the far side of the IRDC. This star must have a warm disk that is the source of the N-band emission, but the disk cannot produce the observed reddening at K. Hence the star must lie behind a lot of extinction from the IRDC. The strong wind that would be expected from such a star is probably impacting the back of the IRDC, exciting the blue wing of the HCO + line. Although it appears to be massive, the 2MASS star is still too young to have disrupted the IRDC. We speculate that in another million years or so this region may resemble a smaller version of DR15 to the south, appearing as an HII region partially obscured by foreground dust left over from the IRDC.
It is, at the very least, a striking coincidence that the two condensations of dust and gas that comprise the G79.3+0.3 P1 SCUBA source (the A and B components in the BIMA data) lie immediately adjacent in the plane of the sky to the most massive YSO in the cloud, and are apparently being impacted by the same wind that is exciting the blue wing in the HCO + (1 − 0) line in the rest of the IRDC. The combined masses of the two components are comparable to the mass of the Herbig Ae/Be star. These two objects are the obvious candidates for the next generation of stars to form in the vicinity of the 2MASS star. Further observations may reveal whether the wind from the 2MASS star has played a role in forming the A and B components, or is just now impacting these two pre-existing clumps, possibly driving them into collapse.

Acknowledgments
We thank H. J. Wendker for assistance with the distance to the Cygnus Rift. We would also like to thank the anonymous referee for a careful review which helped us to clarify the presentation in the paper. The JCMT is operated by the JAC on behalf of the Particle Physics and Astronomy Research Council of the UK, the Netherlands Organisation for Scientific Research, and the National Research Council of Canada. This paper is based partly on observations obtained at the Gemini Observatory, which is operated by AURA, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the NSF (USA), PPARC (UK), the NRC (Canada), CONICYT (Chile), the ARC (Australia), CNPq (Brazil) and CONICET (Argentina    c) lower left BIMA 3 mm continuum contour map. d) lower right BIMA HCO + (1-0) and CS (5−4) contour maps. The HCO + signal has been averaged over a velocity range 1.5 km/s centered on −0.8 km/s; the contour levels can be converted to K km/s by multiplying by a factor 12.0. The CS signal has been averaged over a velocity range 1.0 km/s centered on +0.25 km/s; the contour levels can be converted to K km/s in this case by multiplying by a factor 2.25.