The Contracting Molecular Cores e1 and e2 in W51

THE ASTROPHYSICAL JOURNAL, 472 : 742È754, 1996 December 1 ( 1996. The American Astronomical Society. All rights reserved. Printed in U.S.A.

THE CONTRACTING MOLECULAR CORES e1 AND e2 IN W51 PAUL T. P. HO1 AND LISA M. YOUNG2 Received 1996 January 16 ; accepted 1996 June 12

ABSTRACT We report on observations of the star-forming region W51 in the NH (J, K) \ (1, 1) and (2, 2) tran3 sitions at 1.3 cm. These observations have much greater sensitivity than our previous observations and they allow a kinematic analysis of the motions of the gas in two small star-forming cores. We present evidence for radial contraction of the gas in W51 e2 onto a young star and possibly also for similar contraction near W51 e1. The H II region/molecular core W51 e2 seems to be in a stage where the central star has already arrived on the main sequence, whereas the cooler gas on scales of 0.1È0.2 pc is still contracting onto the mass concentration at the center. In contrast to the interpretation of Rudolph et al., we propose that the contraction extends only over a few tenths of a parsec near e2 instead of over the entire W51 molecular cloud complex. Subject headings : ISM : individual (W51) È ISM : kinematics and dynamics È radio lines : ISM 1.

INTRODUCTION

Large Array (VLA) in the D conÐguration (maximum baseline 1.0 km) for a period of 9 hr on 1990 January 5. We observed the NH (J, K) \ (1, 1) and (2, 2) inversion tran3 and 23.722633 GHz, respectively, each sitions at 23.694495 with a bandwidth of 6.25 MHz. This bandwidth covers the main quadrupole hyperÐne component and all four of the satellite components. We used 64 spectral channels, each 97 kHz wide, giving a velocity resolution of 1.24 km s~1. During these observations the phase center of the array was located at R.A. (1950) \ 19h21m24s. 0, decl. (1950) \ 14¡25@00A, and the bandpass was centered at a velocity of ]60 km s~1 with respect to the local standard of rest. The data were calibrated using the standard Ñux calibrator 3C 286 (assumed Ñux of 2.4 Jy at 1.3 cm), and the bandpass calibrators 3C 84 and 3C 273. Periodic observations of a phase calibrator 1923]413 were used to construct a phase closure solution which was then applied to the rest of the data. A preliminary image of wideband data (inner 75% of the bandpass) was used to self-calibrate the data in phase. Each channel was then mapped, once using a uniform weighting of the visibilities (beam of 2A. 6 ] 2A. 6) and once using a natural weighting (beam of 3A. 8 ] 3A. 4). A map of the continuum, constructed by averaging together seven o†-line channels close to the end of the bandpass, was then subtracted from each channel. The resultant ““ line only ÏÏ maps were then deconvolved using the CLEAN algorithm developed by Clark (1980). The achieved rms noise in the o†-line channels is D5È6 mJy beam~1, which is D1.5 times theoretical expectations.

During the past decade, radio interferometric techiques have greatly elucidated the dynamics of regions of star formation by providing angular resolutions on the order of arcseconds. In particular, observations of neutral gas have shown the presence of infall and spin-up motions, rotating disks, and expanding molecular shells, all at the later stages in the formation of stars (cf. Ho & Haschick 1986 ; Torrelles et al. 1989, 1993 ; Sargent & Beckwith 1991 ; Carral & Welch 1992 ; Kawabe et al. 1993). For example, observations of the NH (J, K) \ (3, 3) inversion transition in the star-forming 3 G10.6[0.4 (Keto, Ho, & Haschick 1987, 1988) have region shown redshifted absorption that corresponds to infalling gas, as well as a large velocity gradient across the source which indicates rotation or spin-up. Temperature and density proÐles as a function of radius further indicate that the region is collapsing from the inside out. In this paper we present a new analysis of the kinematics of the gas in two small condensations in the core of the W51 molecular cloud, located at a distance of D7 kpc (Genzel et al. 1981). Previous studies of W51 have found 3 ] 106 L in _ a region 30@ in size (Thronson & Harper 1979 ; Ja†e, Becklin, & Hildebrand 1984), a number of IR objects (Genzel et al. 1981 ; Bally et al. 1987), H O masers (Schneps 2 et al. 1981), and shocked H emission (Beckwith & Zucker2 man 1982). The two small H II regions of interest here, W51 e1 and W51 e2, were found by Scott (1978). Ho, Genzel, & Das (1983) discovered warm NH condensations 3 in both emiscentered on these two H II regions and seen sion and absorption. However, because of poor sensitivity and poor spectral resolution, the hyperÐne structures could not be resolved and the line proÐles and kinematics could not be determined. The present observations were made with receivers which were more sensitive by a factor of D5, and with 4 times higher spectral resolution to help elucidate the kinematics of the region. 2.

3.

RESULTS

3.1. Continuum Emission Figure 1 shows a continuum map made by averaging together the line-free channels, which correspond to velocities 90È97 km s~1. A noticeable ““ bowl ÏÏ e†ect, caused by the lack of short spacing information, is present at a level of 1% of the peak. However, the lack of short spacing information a†ected primarily the continuum emission, but not the line emission, which is spatially more compact and considerably weaker. Our continuum map is very similar to the previous 1.3 cm map (Ho, Genzel, & Das 1983), and the 3.4 mm map (Rudolph et al. 1990). The relevant sources of interest (e1, e2, IRS 1, and IRS 2) are labeled.

OBSERVATIONS

These observations were made using the NRAO3 Very 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 ; ho=cfa.harvard.edu. 2 University of Illinois at Urbana-Champaign, Urbana, IL 61801 ; lyoung=aoc.nrao.edu. 3 The NRAO is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation.

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FIG. 1.ÈContinuum emission toward W51 as constructed from line-free channels. The contour levels are 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 100 ] (20 mJy beam~1 ; 6.4 K). The gray-scale image is the negative image of the continuum emission. The displayed gray scales range from 20 mJy beam~1 to [60 mJy beam~1. The synthesized beam is 2A. 6 ] 2A. 6. The peak Ñux density is 1.47 Jy. In this experiment, the lack of short spacings resulted in the inability to recover the extended continuum Ñuxes. This can be seen as extended negative Ñux at the level of [30 mJy beam~1. However, this did not a†ect the more compact line emission maps since the continuum contributions were removed.

3.2. L ine Emission Figure 2 presents the individual channel maps in both the (1, 1) and (2, 2) transitions. Figure 3a presents the integrated intensity in the (1, 1) and (2, 2) lines of NH toward W51, 3 superposed on a gray-scale map of the continuum emission. These maps were constructed by integrating the line-only maps over the central hyperÐne component. The two cores e1 and e2 are seen to be emitting strongly and surrounded by extended emission. Directly toward e2, the absorption seen in the (1, 1) line and the diminished emission seen in the (2, 2) line is due to the presence of absorption. The core toward e1 can be seen to be o†set from the continuum emission of e1. This may also be due to absorption e†ects. The region IRS 1 is seen in absorption but not in emission, and the strongest line emission in the region comes from the area of IRS 2. There are also regions of emission west of IRS 2, and southwest of IRS 2 near the coordinates 19h21m21s, ]14¡24@48A, which are not seen in the continuum. To check the extended nature of the Ðlament to the west, we made larger maps. Figures 4a and 4b show that the emission west of IRS 2 extends beyond the primary beam. Note that the di†use emission surrounding the cores e1 and e2, the absorption toward IRS 1, and the extended emission near IRS 2, had not been detected previously in NH because of 3

their low intensity. Figure 4c also shows a map of the second moment of (2, 2) intensity. The line width in the (2, 2) transition can be seen to be generally small, on the order of 2È4 km s~1 (FWHM), although it increases up to 7 km s~1 near the H II regions. This e†ect can also be seen in the (1, 1) transition, although less clearly because of the confusion from the inner satellite hyperÐne components. 3.3. Envelope Properties From the ratio of intensities for the main and satellite hyperÐne components (cf. Ho & Townes 1983), we estimate that typical optical depth of the (1, 1) line in the di†use envelope around e1 and e2 is approximately 2. In the (2, 2) line we cannot detect the satellites in the envelope, so the upper limit of the optical depth in the (2, 2) line is 6. The ratio of the line intensities in the (2, 2) line to the (1, 1) line varies from 0.6 to 1.1. After correcting for the optical depth of the (1, 1) line, and assuming that the (2, 2) line is optically thin, we derive rotational temperatures of 25È35 K. There is a detectable velocity gradient in the di†use envelope around e1 and e2 ; the emission north of the cores is blueshifted by about 3È4 km s~1 with respect to the emission south of the cores. Farther to the north, the emission becomes redshifted again back to a velocity of 60 km s~1.

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FIG. 2a FIG. 2.ÈContour maps for 18 velocity channels in the (a) NH (1, 1) and (b) NH (2, 2) lines. The emission velocity of each channel is shown. Contour 3 6, 7, 8, 9, 10, 15, 203 ] (10 mJy beam~1 ; 1.7 K). The synthesized beam for the NH maps is levels are [10, [9, [8, [7, [6, [5, [4, [3, [2, [1, 1, 2, 3, 4, 5, 3 3A. 8 ] 3A. 4. These maps have not been corrected for the primary beam attenuation.

3.4. Core Properties In the more intense cores around e1 and e2, the line opacities are higher, although the blending of emission and absorption makes the analysis difficult for the (1, 1) line. In the core e1, the optical depth of the (2, 2) line in emission is about 4.5. In e2, the optical depth of the (2, 2) line in emission is somewhat higher, D5È10. The optical depth of the (2, 2) line in absorption in e2 is apparently inÐnite ; this appearance is probably caused by spatial beam smoothing of the unresolved absorption with the surrounding emission, which preferentially weakens the central hyperÐne component relative to the satellite hyperÐne components. These optical depths are higher than any other yet seen in NH (2, 2) lines, even in Orion, suggesting an unusually large3 column density. 3.5. Kinematics (W 51-e2 Core) The kinematics are the most striking results in this study. This is best seen in position-velocity plots of the (2, 2) line through the e2 and e1 cores as shown in Figure 5. In the

core e2 we see all the ammonia hyperÐne components in both emission and absorption against the H II region in the core. Note that the absorption is redshifted with respect to the emission in a classic Inverse P Cygni proÐle (Figs. 5a and 5e). The di†erence in velocity between the peak of the redshifted line and the peak of the blueshifted line is about 7 km s~1. Since the absorbing gas must be in front of the H II region and the emitting gas must be behind the H II region, we infer that both the absorbing and emitting gas must be moving toward the H II region. This is exactly analogous to the previous example of contraction around the ultracompact H II region G10.6[0.4 (cf. Ho & Haschick 1986). Note also the C shape of the emission line immediately around e2 in the position-velocity plot. With respect to the envelope emission around e2 at 56 km s~1, the emission velocity is more blueshifted toward e2 and returns to the systemic velocity at 5A away. A similar backward-C shape structure is also seen in the redshifted velocities, after taking into account of the fact that the material directly in front of the H II region appears in absorption. The C and backward-

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FIG. 2b

C structures are well resolved with an extent of almost four synthesized beams. The magnitude of the C structures, or the change in central velocity of the emission or absorption line from the edges of the structure to its center, is about 4 km s~1. This velocity pattern is highly suggestive of a radial collapse onto the H II region in the center, with the C-shape structures arising from projection of the radial motion onto the line of sight. Away from the H II region, the position-velocity plots describe an O shape structure with about the same intensity in the redshifted and blueshifted velocities (Figs. 5a and 5e). We note also that the same C-shape structures are found independent of the direction of the position-velocity cuts. These Ðndings are consistent with the expectations for a spherically symmetric radially collapsing core. 3.6. Kinematics (W 51-e1 Core) We may also have detected absorption in the core e1. The position-velocity plots toward e1 describe an O structure as is found in position-velocity plots o†set from the H II region in e2 (Figs. 5a and 5d). Although the gas is not seen in

absorption, the redshifted half of the line is weaker than the blueshifted half. Thus even though the continuum emission is weaker toward e1 than e2, there may be a small amount of absorption on the redshifted side which reduces its intensity. This velocity structure may also be interpreted as contraction, whose magnitude of infall is comparable to that observed toward e2. We note that in Figures 3a and 3b, the NH emission peak toward e1 is o†set about 4A northeast of 3 H II region. Although this could be due to absorption the e1 toward e1, there is a much fainter H II region e4, detected by Gaume, Johnston, & Wilson (1993), which is well centered with the NH emission. In particular, since the OH masers 3 centered on e4, the NH core and its likely are much better 3 contraction, may also be centered on e4 instead of e1. 3.7. Spin-up Motions What about spin-up ? We expect that because of the partial conservation of angular momentum, as the gas contracts, it will rotate faster. This was the case observed for G10.6[0.4. The observed velocity gradient for the envelope around the e1 and e2 cores is on the order of 4È5 km s~1

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FIG. 3.ÈIntegrated intensity maps for the (left) NH (1, 1) and (right) NH (2, 2) lines. Only the central quadrupole hyperÐne component is included in the 3 3 image of the continuum emission as shown in Fig. 1. The contour levels are integration. These NH maps are superposed on the same negative gray-scale 3 [3, [2, [1, 1, 2, 3, 4, 5, 10, 20, 30, 40, 50, 100 ] 20 mJy beam~1 km s~1. The synthesized beam for the NH maps is 3A. 8 ] 3A. 4. [30, [20, [10, [5, [4, 3 region. Note the presence of absorption in the (1, 1) and the depression in emission in the (2, 2) toward e2. This is due to absorption against the H II

pc~1 (Figs. 5a and 5h). This could represent spin-up motions from a more di†use state. For the cores, only higher angular resolution will reveal whether the observed broad lines are to be attributed to radial as well as spin-up motions. 4.

DISCUSSION

4.1. W orking Model for W 51-e2 Our working model is a spherically symmetric, radial gravitational collapse onto the H II region in the core e2. The key observational facts are : (a) the redshifted absorption and blueshifted emission seen in the core ; (b) the curvy C-shape of the emission line on the position-velocity cuts which persists and becomes O-shape and symmetrical away from the H II region ; (c) the circular symmetry (in the plane of the sky) of the position-velocity diagrams. 4.2. Difficulties There are some obvious objections to this model : (1) the absorption and emission in e2 may arise from unrelated clouds ; (2) the absorption in e2 may be merely an extension of the core e1 in front of e2. The possibility of a chance projection of another cloud against the H II region is a standard objection, also raised in the case of G10.6[0.4 (cf. Guilloteau et al. 1988). We can, however, make some counterarguments. Against the Ðrst objection, that the absorption and emission may arise from unrelated clouds, we argue instead that the redshifted and blueshifted gas seen in e1 and e2 must be associated with the central parts of the cores, near the H II region in the case of e2. In the position-velocity plots, the C and the backward-C structures appear continuous and smooth in space and velocity, implying that they are part of a complete structure (Fig. 5a). Most important is the

Ðnding that away from the H II region, a complete O structure is recovered when the absorption reverts back to emission. In addition, these excursions in velocity from the systematic values in e2 are well centered spatially with the H II region and are only seen within a few arcseconds of it. Moreover, as in the case of G10.6[0.4, we Ðnd that the optical depth in the absorption feature is higher than that of the emission features in either e1 or e2, implying that the absorbing gas arise in the very hot, dense gas closer to the H II region. This implication can be tested, as with G10.6[0.4, by measuring the temperature of the absorbing gas. This test cannot be done with the current data because of the difficulty of deducing the optical depth of the (1, 1) line due to blending of the hyperÐne components. Such a test may be possible with the (3, 3) line in future observations. Thus, the redshifted and blueshifted gas must both arise in the central, condensed part of the core in a coherent structure. Against the second objection, that the absorption toward e2 may be just an extension of the material associated with e1, we point out that the optical depth in fact decreases between e2 and e1, as can be seen in Figure 5a. The relative intensity of the hyperÐne satellites can be seen to decrease between the two cores in the position-velocity plots, suggesting two well localized density peaks. Hence the more optically thick absorption in front of e2 is more likely to be associated with the emitting gas in e2, rather than with the gas in e1. 4.3. Implied Central Mass Assuming that the molecular gas in/around the core e2 is a spherically symmetric distribution of gas collapsing radially, we can use position-velocity diagrams like the ones in Figure 5 to measure the velocity and extent of the infall.

FIG. 4.ÈLarger maps of (a) the integrated emission of NH (1, 1), (b) the integrated emission of NH (2, 2), and (c) second moment of the NH (2, 2). The contour maps in (a) and (b) are superposed on the 3 that the emission to the west is well 3extended beyond the primary beam. The3 contour levels for (a) and (b) are the same as in Fig. 3. The positive gray-scale image of the continuum emission and show contour map in (c) is superposed on its own gray scale image. The contour levels for (c) are 1, 2, 3, 4, 5 km s~1. The gray scale levels cover the same range.

748 FIG. 5b

FIG. 5.ÈPosition-velocity diagrams in NH (2, 2) line : (a) declination cuts starting at R.A.(1950) \ 19h21m26s. 55, at 1A steps in R.A., left to right, and top to bottom ; this panel is centered on sources e1 3 and e2 ; (b) same as (a) but starting at R.A. (1950) \ 19h21m24s. 48 ; this panel is centered on source IRS 1 ; the contours in this panel are dashed to indicate absorption (c) same as (a) but starting at R.A. (1950) \ 19h21m22s. 49 ; this panel is centered on source IRS 2 ; (d) R.A. cuts starting at decl. (1950) \ ]14¡24@29A, at 1A steps in declination, left to right, and top to bottom ; this panel is centered on source e1 ; (e) same as (d) but starting at decl. (1950) \ ]14¡24@38A ; this panel is centered on source e2 ; ( f ) same as (d) but starting at decl. (1950) \ ]14¡24@39A ; this panel is centered on source IRS 1 ; (g) same as (d) but starting at decl. (1950) \ ]14¡25@10A ; this panel is centered on source IRS 2 ; (h) cuts at P.A. \ 33¡.8 starting at R.A. (1950) \ 19h21m25s. 80, which are along the Ðlament running northwest of e2 ; the positions are at the projected declinations ; (i) R.A. cuts starting at decl. (1950) \ ]14¡25@9A, in the NH (1, 1) line ; this panel is centered on the Ðlament west of IRS 2 and shows that it is relatively optically 3 thin compared to the core near IRS 2. The contour levels are [10, [9, [8, [7, [6, [5, [4, [3, [2, [1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20 ] (10 mJy beam~1). Note that the cores associated with e1, e2, and IRS 2 are distinguished by their high optical depths, as evidenced by strong hyperÐne satellite emission. Note also the C and backward C structures in the cores, which may be consistent with radial motions.

FIG. 5a

749 FIG. 5c

FIG. 5d

750 FIG. 5e

FIG. 5f

751 FIG. 5g

FIG. 5h

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FIG. 5i

Adopting an infall velocity of 3.7 ^ 0.6 km s~1 and a radius of 0.10 ^ 0.05 pc, we estimate that there must be 160 ^ 90 M of binding mass within a radius of 0.10 pc from the H II _ region. This mass is a lower limit because it is based on a free-fall velocity neglecting thermal, turbulent, and magnetic support. Some fraction of the enclosed mass will be the stellar mass associated with the H II region. A gas mass of 200 M in 0.1 pc corresponds to an average density of _ cm~3, and an infall velocity of 4 km s~1 at 0.1 pc n D 106 2 H implies a time of about 104 yr for unimpeded contraction. 4.4. Extra Gas In the position-velocity plots made with uniform weighting for higher resolution, there is some indication that the emission toward e2 may show a complete circle. That is, in addition to the Ðve hyperÐne emission components, there is an additional feature redshifted with respect to the central absorption which in some cases is connected to the blueshifted emission (see also Fig. 5a). This is at Ðrst surprising because if the ammonia gas were entirely in a spherical cloud, collapsing radially, then there would not be any emission from the front of the shell because the front of the shell would be all in absorption against the H II region. However, we note that the H II region is still very small compared to the synthesized beam, while the contraction zone is larger than the synthesized beam, as seen in the position-velocity plots. Thus, there must be redshifted emission within the synthesized beam which is not in front of the H II region. That the line of sight to the H II region subtends only a small portion of the contraction zone is conÐrmed by the constant velocity of the absorption feature. Note that the excess redshifted emission is at a higher velocity than

the absorption velocity. This argues that either the contraction is not entirely symmetric, or else some rotation exists which shifts the maximum radial velocity away from the center of the H II region (cf. Keto et al. 1988). 4.5. Overall Collapse Our model needs to be placed in context with the overall collapse model proposed by Rudolph et al. (1990). These authors presented results toward the W51 region in several molecular lines and in 3.4 mm continuum with the Hat Creek Interferometer. They concluded that there is contraction in W51 but that it is an overall collapse of a 6@ region towards a central mass of 40,000 M near e2 with a smaller collapse towards about 1200 M _ in IRS 2. The key _ observational points which were important for their model are as follows : (1) The HCO] shows redshifted absorption and blueshifted emission toward e2 and IRS 2. In e2 the absorption appears from 59 to 74 km s~1. (2) Both blueshifted and redshifted gas are in absorption toward IRS 1. (3) The HCO] absorption is broader and deeper in the interferometer measurement with a smaller beam than it is in the single-dish measurement by Nyman (1983). Point 1 argues for a very large contraction velocity modeled to be in excess of 20 km s~1 at radii less than 1 pc. This results in an estimate of a very large binding mass. It also argues that the contraction is extensive, modeled to be over a 6@ area. Point 2 also argues that contraction is over a large area, and moreover that IRS 1 is behind this contracting cloud. Point 3 argues that the higher contraction velocities are centered closer to e2 as might be expected of an inside-out collapse (Shu 1977). The NH data presented in this paper show evidence only 3

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for a very small collapse region extending over about 10A in diameter centered on e2, with possibly other small regions of collapse in e1 and IRS 2. We argue that the collapse seen in the core e2 is well conÐned because we can see the radial projection of the gas velocity onto the line of sight as a curved structure in the position-velocity plots. The top and bottom of the C structures deÐne the spatial limits of the contraction zone. Beyond these points, the detected gas remains at ambient velocities, as can be seen in Figure 5. Such a behavior will not be observed if the contraction originates from a region substantially farther out as estimated by Rudolph et al. (1990), i.e., a contraction over a 6@ region will not show a substantial change in radial velocity over 10A. Unfortunately, the 10A region of contraction around e2 is not well resolved by the 6A beam of the HCO] experiment. Moreover, the detected range of absorption velocities toward e2 is much smaller in the NH data than is 3 seen in HCO], extending only from about 55 to 60 km s~1. The absorption features seen in NH toward e2 are not seen toward IRS 1 or IRS 2 as is the case3 for HCO]. What are seen toward IRS 1 and IRS 2 in NH are very narrow NH 3 absorption lines with velocities between 62 and 68 km s~13 (Figs. 5b, 5c, 5f ). Note that the satellite hyperÐne components are much fainter for these absorption features. From their spectra in (1, 1) and (2, 2) transitions, we can therefore deduce that this material in absorption toward IRS 1 and IRS 2 have low optical depths and low temperatures and are well separated in velocities from all the material seen in emission (Figs. 3, 5b, 5c, 5f ). Since the NH data to not appear to be consistent with 3 the model of Rudolph et al. (1990), but rather with a much smaller region of contraction, we suggest an alternative interpretation for the HCO] data. There is evidence that the same absorption features seen in NH in e2 are also 3 observed in HCO]. Reexamining the HCO] spectrum and maps toward e2, one can see depression in the emission between 56 and 58 km s~1, the same range that is seen in absorption in NH . This is also seen in the comparison of 3 the HCO] spectrum from the interferometer versus the single dish. While the blueshifted sides agree well, the redshifted side of the interferometer spectrum can be seen to be weaker. Such behavior has been seen before in the C18O and CS millimeter wave line observations toward G10.6[0.4 (Ho, Terebey, & Turner 1994 ; Omodaka et al. 1992). At millimeter wavelengths, the brightness temperatures of the H II regions are some 25 times weaker than at 1 cm wavelength. Hence absorptions that are very evident in the NH lines may be very weak in millimeter 3 wave lines. Addressing the key observational results of HCO], point (1) in ° 4.5 concerning a broad absorption line in HCO] is probably due to a superposition of a contraction toward e2 which is well localized and centered at 55 km s~1 and an intervening cool foreground cloud absorbing at 62È68 km s~1. Furthermore, this cool foreground cloud appears stationary, with no detectable velocity gradients (Figs. 5c, 5f ). Reexamining the HCO] data, one can see now why, as a stationary screen, the absorptions toward IRS 1, IRS 2, and e2, in the range of velocities between 60 and 70 km s~1, remain very similar and unchanged in radial velocity. This is the reason why the contraction model centered on e2, put forth by Rudolph et al. (1990), could not Ðt the absorption toward IRS 2. This range of velocities also never appears in emission, suggesting that it is indeed very

753

cool. Although this range of velocities does not appear in absorption in NH against e2, this does not argue against a 3 uniform screen. This is because of the relative faintness of e2 at 1.3 cm as compared to IRS 1 and IRS 2 (Fig. 1). Toward e2, the HCO] shows absorption at lower velocities because of the localized contraction centered at 55 km s~1. Toward IRS 1, the absorption of the so called blueshifted velocities in HCO] (point 2 in ° 4.5) is not due to contraction on a large scale but rather arises in the outer parts of the molecular core around e2 (Fig. 3). The evidence for this interpretation can be found by comparing the HCO] spectra toward e2 and IRS 1, where one can see that the entire emission proÐle toward e2 is in absorption toward IRS 1. There is also no need to suggest that there is an independent contraction toward IRS 2 centered at 65 km s~1. What is seen here in absorption in HCO] is likely due to the same cool foreground screen in front of e2. We note that the molecular gas toward IRS 2 is not centered on the main compact H II region but is displaced toward the south and centered on the fainter compact source D2. Slighly to the east of D2 and south of the main H II region, the line width in NH is very broad and the optical depth is very high although3 it is displaced from any known radio continuum source (Figs. 5c, 5g). If this is a region of contraction, it is centered at 60 km s~1. Finally, with regards to point 3 in ° 4.5, absorption that is broader and deeper in the interferometer beam as compared to the single-dish beam is due to the additional localized contraction directly on e2. There is also evidence for swept-up material due to a wind or outÑow in the vicinity of IRS 2 (Zhang & Ho 1995). This swept-up material appears to be part of the extended Ðlament west of IRS 2 (Fig. 5i). We therefore conclude that there is no overall collapse involving 40,000 M centered on e2. NH observations pre3 sented in this paper_show that there is a contraction around e2, but that it involves a much smaller amount of mass immediately around e2, on the order of 200 M . This case _ is therefore very analogous to the case of G10.6[0.4. 5.

SUMMARY

The actual observation of the contraction phase in star formation is very difficult. There are not many cases which are not disputed. The best examples so far may be G10.6[0.4 (Ho & Haschick 1986), B335 (Zhou et al. 1993), and HL Tau (Hayashi, Ohashi, & Miyama 1993 Even in these cases, the possibility remains that chance superposition of unrelated clouds, chemistry, and outÑows may inÑuence the observed kinematics. In this paper, we present another good candidate for collapse. The candidate source W51 e2 is not a surprising candidate because it is already known to be a very young object from its compact radio continuum and infrared emission. Because of the gregarious nature of OB cluster formation, we would anticipate a priori that such sites may be prime targets to discover stars in various stages of contraction and formation, therefore including the youngest examples. In this case, the key observational facts which constitute the ““ smoking gun ÏÏ are (1) redshifted absorption against the H II region, (2) velocity shifts both toward the red and the blue which are centered on the H II region, (3) the magnitude of the velocity shifts are peaked toward the H II region in such a manner to be consistent with projection of a radial collapse, (4) this contraction as evidenced by line broadening centered on the H II region and by position-velocity plots, can be explained

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by the self gravitation of D200 M of gas which is consis_ tent with the integrated NH emission, and (5) the interpre3 tation of the blueshifted and redshifted gas as the back and front portions of a spherical collapse is supported by their roughly equal intensity away from the H II region. By identifying the behavior of the gas in emission toward W51 e2 as due to contraction, we infer that a similar collapse is also occurring toward W51 e1. These two associated cores give support to our attempts to extend this technique to regions where a central H II region has not formed either due to

youth or a low stellar mass. What is next ? The resolved ringlike structures in position-velocity plots, an increased column density (opacity) and a higher temperture in the velocity component that is moving faster, and measured properties that are consistent with gravitational binding, are the key signatures that should be pursued in studies of contraction. P. T. P. H. is supported in part by NASA grant NAGW3121.

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