HOT CORES IN W49N AND THE TIMESCALE FOR HOT CORE EVOLUTION D. J. Wilner. C. G. De Pree. W. J. Welch. and W. M. Goss

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1 The Astrophysical Journal, 550:L81 L85, 2001 March The American Astronomical Society. All rights reserved. Printed in U.S.A. HOT CORES IN W49N AND THE TIMESCALE FOR HOT CORE EVOLUTION D. J. Wilner Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA C. G. De Pree Department of Physics and Astronomy, Agnes Scott College, 141 East College Avenue, Decatur, GA W. J. Welch Radio Astronomy Laboratory and Department of Astronomy, University of California at Berkeley, Berkeley, CA and W. M. Goss National Radio Astronomy Observatory, P.O. Box O, Socorro, NM Received 2001 January 3; accepted 2001 February 8; published 2001 March 12 ABSTRACT We present subarcsecond resolution observations of the star-forming region W49N made with the Berkeley- Illinois-Maryland Association array at 1.4 mm wavelength in the continuum and CH 3 CN J p lines. The continuum image, at a resolution of 0.18 (2000 AU), shows many of the previously identified ultracompact H ii regions and at least one new source whose steep spectrum to short wavelengths indicates dust emission. This compact dust concentration also shows the strongest CH 3 CN emission in the region. Additional peaks of CH 3 CN emission likely mark hot cores produced by other deeply embedded young massive stars that may be precursors to O-type stars like those that power the ultracompact H ii regions. The number ratio of hot cores to ultracompact H ii regions found in W49N provides a measure of the relative timescales of these early evolutionary phases. Although the samples are small, the source counts suggest that the hot core lifetime is most likely shorter than, but comparable to, the ultracompact H ii region lifetime. Subject headings: H ii regions ISM: individual (W49N) stars: formation 1. INTRODUCTION The star-forming region W49 is among the most luminous in the Galaxy ( 10 7 L, ) and is located in one of the most massive giant molecular clouds ( 10 6 M, ). Located at a distance of 11.4 kpc, the W49N cloud core contains at least a dozen ultracompact H ii regions powered by OB-type stars arranged in a 2 pc diameter ring (Dreher et al. 1984; Dickel & Goss 1990; De Pree et al. 2000). The finding of so many compact sources within W49N opened the issue of the ultracompact H ii region lifetime problem since the large population of sources seems to be at odds with the dynamical timescales for free expansion (Dreher et al. 1984; Wood & Churchwell 1989). Although W49N is located on the far side of the Galaxy, its extreme environment allows observational tests of many aspects of massive star formation. So far, the subarcsecond view of W49N has been limited to observations of radio continuum emission that traces ionized gas. A more complete census of the star formation activity, including younger embedded massive objects, may be obtained through surveys of dust continuum emission and localized emission from high-excitation molecular lines. The maturing performance of millimeter interferometers operating in the 1 mm atmospheric window make possible such observations. Evidence that star formation is still in progress within W49N comes from strong H 2 O maser emission, outflowing with a velocity spread of nearly 500 km s 1 (e.g., Gwinn, Moran, & Reid 1992), high-velocity CO emission (Scoville et al. 1986; González-Alfonso et al. 1995), and strong millimeter continuum emission attributed to dust condensations (Sievers et al. 1991). In this Letter, we present images of W49N in the continuum at 1.4 mm and in spectral lines of CH 3 CN (methyl cyanide) that reveal previously unseen hot cores, the warm environments of luminous young stars thought to be in an L81 evolutionary phase prior to ultracompact H ii region development (Kurtz et al. 2000). 2. OBSERVATIONS We observed W49N with the Berkeley-Illinois-Maryland Association (BIMA) array 1 during the fall and winter seasons of 1999 in three configurations of the nine antennas equipped with receivers for operation at 1.4 mm wavelength. Table 1 lists the main observing parameters. System temperatures ranged from 250 to 800 K (single sideband). Complex gains were derived from frequent observations of nearby calibrators, and several iterations of self-calibration were used to improve the images. The absolute flux density scale was set with observations of Uranus and is accurate to 20%. The correlator was configured to provide about 700 MHz bandwidth in each sideband of the first local oscillator free of strong spectral lines, and a 50 MHz window of 64 channels spanning the CH 3 CN JK p 12 K 11K, K p 0, 1, and 2 lines. Note that the K p 1 and K p 2 lines are offset in frequency from the K p 0 line by 4 and 17 MHz (5.8 and 23.1 km s 1 ), respectively. The calibration procedures were performed with the suite of routines in the MIRIAD software package. A continuum image with maximum resolution was made by combining the visibilities from each sideband in a multifrequency synthesis image with an effective frequency of GHz (wavelength 1.4 mm), restored with a circular beam of 0.18 (FWHM), which corresponds to about 2000 AU at the source distance. The dynamic range of this image was limited to about 70 by residual calibration errors. Spectral line images were made with a visibility weighting scheme that resulted in a larger synthesized beam, 0.93 # 0.77 (FWHM) position angle (P.A.) 27, for im- 1 The BIMA array is operated by the Berkeley-Illinois-Maryland Association under funding from the National Science Foundation.

2 L82 HOT CORES IN W49N Vol. 550 TABLE 1 BIMA 1.4 mmobservations of W49N Parameter Value Dates (configuration) Sep 15 (C) Baseline range m Pointing center (J2000) h 10 m 13 ṣ 41, Phase calibrators , Bandpass calibrators... 3C 273, Flux calibrator... Uranus Primary beam half-power beamwidth Synthesized beam half-power beamwidth Spectral line window channels, 50 MHz Species/transition... CH 3 CN J K p Rest frequency GHz LSR velocity range... 9 to58kms 1 rms (channel maps) mjy beam 1 (3.5 K) rms (continuum images) mjy beam 1 proved brightness temperature sensitivity. Since accurate absolute positions were lost in the self-calibration process, we have aligned the 1.4 mm images with the 7 mm image of De Pree et al. (2000) on the ultracompact H ii region peaks in order to obtain coordinates consistent with earlier work. 3. RESULTS AND DISCUSSION 3.1. Continuum Sources Figure 1 shows the central part of the high-resolution 1.4 mm continuum image together with close-up views of regions with detected compact sources in boxes labeled (i) to (v). Unlike previous millimeter and submillimeter images of W49N made with bolometers on single-dish telescopes (Sievers et al. 1991; Buckley & Ward-Thompson 1996), the BIMA image filters extended emission from the dense cloud core and highlights individual sources of high brightness contrast. The letter designations in Figure 1 follow the nomenclature first introduced by Dreher et al. (1984) for sources identified at centimeter wavelengths. The boxes are chosen to allow for direct comparisons with the 7 mm images of De Pree et al. (2000) that have 0.04 resolution and show the finest details of the ionized structures Ultracompact H ii Regions Most of the 1.4 mm sources are readily associated with previously described ultracompact H ii regions. Small parts of the shell sources C, D, and G1 are found to persist at 1.4 mm, and the emission is confined to the highest density edges. The strongest emission at 1.4 mm is associated with the most extreme ultracompact H ii regions, in particular sources A1 and A2, B1 and B2, and the group that comprises the G2 complex. The flux densities of these sources, measured in boxes that contain the groups, are all comparable to, or less than, their flux densities at 3.4 mm (De Pree et al. 2000). Given the uncertainties in flux density calibration, these values suggest Fig. 1. Upper left: BIMA 1.4 mm continuum image of the W49N core with 0.18 resolution (2000 AU). The boxes labeled with roman numerals indicate the regions displayed in the surrounding panels. Panels (i) (v): Close-up views of the 1.4 mm emission from the ultracompact H ii region complexes and the newly identified dust peak K2. The lowest contour and contour interval is 15 mjy beam 1. The scale bar in each panel represents 5000 AU.

3 No. 1, 2001 WILNER ET AL. L83 Taking the upper limit at 3.4 mm of about 10 mjy indicated by the data of De Pree et al. (2000) implies that the spectrum a of this source rises to short wavelengths ( Sn n ) with a power- law index a 1 3.0, which suggests an origin in dust emission. A forest of weak spectral lines may also contribute, but this is unlikely to be a large effect (Sutton et al. 1984). The source has an elongated halo more than 5000 AU in extent. A single source plausibly accounts for the peak emission, although multiple components may be present. The mass of gas and dust may be estimated from the 1.4 mm emission by 2 S1.4 mm D M p 4 M, ( mjy )( 11.4 kpc ) (1) ( ) 1 k 1.4 mm (10.56/T d ) 2 1 # (e 1), 0.01 cm g Fig. 2. Images of CH 3 CN J p integrated intensity superposed on the 3.6 cm continuum image of De Pree et al. (1997). These tracers show the hot cores in CH 3 CN (dark contours) and the ultracompact H ii regions at 3.6 cm (light contours). Upper panel: LSR velocity range 0 22 km s 1, containing a blend of the K p 0 and K p 1 lines. Contour levels are (2, 3, 4, ) # 1.75 Jy km s 1. For the 3.6 cm image, the beam size is 0.8 (FWHM), and the lowest contour level is 20 mjy beam 1 with steps increasing by factors of 2. Lower panel: LSR velocity range km s 1, isolating the K p 2 line. Contour levels are (2, 3, 4, ) # 0.85 Jy km s 1. The synthesized beam for the CH 3 CN images is 0.93 # 0.77 (FWHM), P.A. p 27. The letter labels correspond to the Table 2 listing. that the bulk of the ionized gas has low opacity at 1.4 mm. However, the most compact subcomponents may remain optically thick. In particular, the peak 1.4 mm brightness temperature of the G2a source is 420 K in the 0.18 beam; if the effective source size at 1.4 mm is close to the 0.04 indicated by 7 mm imaging, then the brightness temperature corrected for beam dilution still could be 8000 K or more. In that case, the emission measure would be greater than pc cm 6 and the plasma density nearly 10 7 cm 3. If the small source size is due to pressure confinement of the ionized gas (De Pree, Rodríguez, & Goss 1995), then extreme molecular hydrogen densities are required ( 10 9 cm 3 ), perhaps ameliorated by the surrounding turbulence (Xie et al. 1996) Dust Peaks Several features in the 1.4 mm image have no correspondence with the known ultracompact H ii regions. The most prominent one is the source most to the northeast in Figure 1, shown in panel (v) and designated K2. The 1.4 mm flux density in a 1 box centered on the peak is mjy (with the error estimate dominated by the flux density scale uncertainty). where S 1.4 mm is the flux density at 1.4 mm, D is the source distance, k 1.4 mm is the mass absorption coefficient (0.01 cm 2 g 1 ; Ossenkopf & Henning 1994), and B(T d) is the Planck function for dust temperature T d. Taking a bulk temperature of 50 K for the K2 source (see 3.2), the implied mass is about 200 M,. The dominant error lies in the mass opacity, which is uncertain by at least a factor of a few. No other similar, isolated, steep spectrum, continuum sources are found within the field of view. The 1.4 mm image also shows two new peaks in the crowded region around source G. One of these is located just to north of G2a and G2b, and the other one appears as a southwest extension of source G2a. The latter is especially interesting because De Pree et al. (2000) locate the center of expansion of the water maser flow at this spot. These sources lie in very close proximity to the strongest peak in the map, which is a location that can be prone to artifacts from the deconvolution process. We have checked that these sources persist in images of various subsets of the data, but their identification and detailed properties should be viewed with appropriate caution. The limits at these positions provided by subarcsecond images at longer wavelengths constrain the spectral indices to be steeper than 1.5, which is consistent either with dust emission or with optically thick hypercompact H ii regions excited by very young stars. If these features are dust peaks, then the mass associated with each one is 35 M (50 K/ATS)., d 3.2. Methyl Cyanide Emission The use of the CH 3 CN molecule in tracing centers of activity within massive star-forming regions has been shown very effective (e.g., Olmi, Cesaroni, & Walmsley 1993, Olmi et al. 1996) following the example set by the high abundance and excitation of this species in the Orion KL hot core (Loren, Mundy, & Erickson 1981). CH 3 CN and other complex nitrogen-bearing molecules are thought to form in dense environments after prompt localized heating initiates evaporation of icy grain mantles. Figure 2 shows the integrated CH 3 CN emission from the K p 0 and K p 1 lines of the J p transition, which are blended in velocity, and emission from the K p 2 line, overlaid on the 3.6 cm continuum map from De Pree, Mehringer, & Goss (1997), which has comparable angular resolution and highlights the many ultracompact H ii regions. These images of integrated line emission were made by clipping the channel maps at the 1.5 j level to avoid diluting the signal with noise. The lower energy levels of these transitions range from 58 to 87 K above the ground state. The high angular resolution clearly separates several individual sources. At least

4 L84 HOT CORES IN W49N Vol. 550 Image a J2000 (19 h 10 m ) (s) TABLE 2 W49N CH 3 CN Hot Cores d J2000 ( 9 06 ) (arcsec) Kp2 Sdv a (Jy km s 1 ) v LSR (km s 1 ) Dv FWHM (km s 1 ) MCN a b MCN b MCN c MCN d MCN e MCN f a Typical uncertainty 40%. b Coincident with continuum source K2. six compact CH 3 CN peaks can be identified in both images, as well as extended emission at a lower level in and around the B and G complexes. Table 2 lists the locations of the CH 3 CN peaks and includes LSR velocities, line widths, and fluxes derived from Gaussian fits to the K p 2 line. The modestly strong CH 3 CN emission associated with the B complex does not show a distinct peak and is not listed in Table 2. The velocities are generally commensurate with the surrounding material, as traced by, e.g., the recent 4 resolution images of CS J p 2 1 emission of Dickel et al. (1999). The line widths are typically 7 km s 1 (FWHM), which suggests substantial turbulence or unresolved systematic motions within these sources, as has been often found in the better studied examples (e.g., the Turner-Welch object; Wyrowski et al. 1997). The CH 3 CN J p lines appear different from the NH 3 (3, 3) line mapped at 4 resolution with the Very Large Array (Jackson & Kraemer 1994; which instead shows extended emission surrounding the entire ring of ultracompact H ii regions, without clear, discrete, compact peaks. These differences may be due to the higher resolution of the CH 3 CN observations, chemistry, excitation conditions, or a combination of these effects. The intensity ratios of different K lines may be used to estimate kinetic temperatures, but in this case the observations of the K p 0, 1, and 2 lines do not allow for high-quality determinations because of the severe velocity blending of the K p 0 and K p 1 lines, the limited signal-to-noise ratio obtained at subarcsecond resolution, and the lack of leverage in excitation provided by the energy range of the three transitions. However, the brightnesses observed at the stronger peaks indicate minimum temperatures of 50 K, and the temperatures are probably substantially higher. Whether the hot cores visible in CH 3 CN emission are heated internally by embedded sources or externally by nearby luminous objects is not entirely clear. Even for the Orion KL hot core, which is more than 20 times closer than W49N, this question is hotly debated (e.g., Kaufman, Hollenbach, & Tielens 1998). The strongest CH 3 CN emission in W49N is coincident with the newly identified dust continuum peak (K2) in the northeast, and the 11 km s 1 LSR velocity matches that of the larger concentration of dense molecular gas in the area identified as CS-NE in the single-dish C 34 S J p 5 4 images of Serabyn, Güsten, & Schulz (1993). This CH 3 CN source is the only one that is well resolved spatially, with a Gaussian fit size (FWHM) of 1.5 (17,000 AU). The CH 3 CN line strength and extent are similar to the Orion KL hot core (Sutton et al. 1986; Wilner, Wright, & Plambeck 1994). The large size and high temperature of the emission region imply a luminosity greater than 10 5 L, for the embedded source or sources (or somewhat less if the geometry is highly nonspherical; see Cesaroni et al. 1998). The extended CH 3 CN emission coincident with the B complex of ultracompact H ii regions may result from heating by the ionizing stars, although additional embedded luminosity sources cannot be ruled out. Similarly, for the CH 3 CN peak nestled immediately south of the 1.4 mm components of the G complex, an internal luminosity source may not be required, as the stars that excite the surrounding ultracompact H ii regions may provide sufficient heating. By contrast, the other CH 3 CN emission peaks listed in Table 2 appear to be relatively isolated from ultracompact H ii regions, and they are each likely heated from within by nascent massive stars. Although there exists an enormous reservoir of dense molecular gas surrounding the CH 3 CN peaks, the masses in compact components at these positions indicated by the continuum flux density limits are less than 25 M (50 K/ATS) Timescale for Hot Core Evolution, d If hot cores evolve into ultracompact H ii regions, then the observed sample of these objects in W49N provides a statistical basis for estimating the relative timescales of these two early evolutionary phases. Previous studies at centimeter wavelengths catalog at least 12 individual ultracompact H ii regions in the region of interest, all with sufficient ionization to be powered by O-type stars. (see De Pree et al. 1997, 2000). The exact count is imprecise because some sources no doubt remain spatially unresolved from close neighbors, and the odd emission peak may be ionized from an outside source. These effects are unlikely to dominate the Poisson noise from the counting statistics, however. The 1.4 mm observations show six hot cores traced by CH 3 CN emission, and, again, the count is imprecise for similar reasons. The number ratio of hot cores to ultracompact H ii regions in the W49N region is 50% (6/12). Considering the counting noise, this ratio most probably falls between 25% and 100%. As long as star formation is continuous and uncorrelated, this number ratio tracks the relative lifetimes of the two kinds of objects. Since some fraction of the hot cores may not produce stars as massive as the O-type stars that power the observed ultracompact H ii regions, the lower end of the range is probably more appropriate. These numbers imply that the hot core phase lasts a time less than, but comparable to, the ultracompact H ii region lifetime, which is likely to be about 10 5 yr (see the review by Kurtz et al. 2000). If star formation began in a coherent burst as a result of a large-scale trigger such as global cloud collapse (Welch et al. 1987) or colliding clouds (Mufson & Liszt 1977; Serabyn et al. 1993), then this statistical conclusion is not necessarily valid. However, the derived timescale is nicely compatible with that from chemical considerations, as substantial conversion of grain mantle products to CH 3 CN in hot core conditions seems to require nearly 10 5 yr (Charnley, Tielens, & Millar 1992). Surveys of additional extreme regions of massive star formation can provide independent samples that will test the robustness of the number fraction found in W49N.

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