Thermal Infrared Imaging of Ultracompact H II Regions in W49A

THE ASTROPHYSICAL JOURNAL, 540 : 316È331, 2000 September 1 ( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.

THERMAL INFRARED IMAGING OF ULTRACOMPACT H II REGIONS IN W49A NATHAN SMITH Astronomy Department, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455 ; nathans=astro.umn.edu

JAMES M. JACKSON,1 KATHLEEN E. KRAEMER,1,2 LYNNE K. DEUTSCH, AND ALBERTO BOLATTO Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215

JOSEPH L. HORA AND GIOVANNI FAZIO Smithsonian Astrophysical Observatory, 60 Garden Street MS/65, Cambridge, MA 02138

WILLIAM F. HOFFMANN Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721

AND ADITYA DAYAL Infrared Processing and Analysis Center, MS 100-22, Caltech, Pasadena, CA 91125 Received 2000 February 10 ; accepted 2000 April 7

ABSTRACT Several compact radio continuum sources in W49A show detectable 8È20 km emission in MIRAC2 images obtained at the IRTF. In general, the infrared morphologies of these sources closely resemble the radio continuum emission. Spectral energy distributions indicate an infrared continuum excess above the level expected from free-free emission, consistent with thermal emission from dust grains heated to a few hundred K. The bright radio continuum sources concentrated at the western end of the ring of ultracompact H II regions are not detected in the mid-infrared, while those at other positions in the ring are detected. This could be due to a localized region of high extinction along the line of sight. In addition, there are a few new infrared sources with no radio continuum counterparts. Finally, several infrared sources show strong 12.8 km [Ne II] emission, yielding neon abundances that are typically a few percent of the cosmic abundance of neon but are high considering the expected Ne``/Ne` ratios for the range of spectral types of the ionizing sources. We conclude that the [Ne II] emission must come from shells around the ultracompact H II regions, where the neon is able to survive as Ne` rather than Ne`` because the radiation Ðeld has been softened by absorption of hard UV photons within the H II regions. Subject headings : H II regions È ISM : individual (W49A) È stars : formation 1.

UC H II regions are powered by young massive stars surrounded by ionized gas and dust cocoons and are still in the process of clearing away the dense material that remains from the cloud core out of which they formed. The existence of such ““ cocoon stars ÏÏ was Ðrst postulated by Davidson & Harwit (1967). Wood & Churchwell (1989) studied the radio continuum emission from 75 UC H II regions and identiÐed Ðve distinct morphologies : spherical or unresolved, cometary, core-halo, shell, and irregular. Each of these morphologies can be seen in high-resolution radio continuum maps of W49A (e.g., Welch et al. 1987 ; D97). About a dozen of these compact sources at the core of W49A are concentrated in a remarkable ringlike structure. Wood & Churchwell also identiÐed a problem with the statistics of UC H II regions : there are too many UC H II regions in the Galaxy given their expected lifetime of D104 yr needed to clear away the circumstellar cocoons. The collection of UC H II regions in W49A presents a further problem, because the sound crossing time of the cloud core is larger than the expected UC H II region lifetime (Dreher et al. 1984). This indicates that the UC H II region lifetime is signiÐcantly longer than 103È104 yr as derived by Davidson & Harwit (1967). Calculations by Garcia-Segura & Franco (1996) suggest that pressure conÐnement by the surrounding medium may help explain this UC H II region discrepancy, because it can stall the expansion of the UC H II region. Because of the large number of individual sources in W49, it is worthwhile to review the nomenclature. At low

INTRODUCTION

The molecular cloud core and star formation complex W49A is host to one of the highest concentrations of young massive stars known in the Galaxy. Its strong molecular, far-infrared (IR), and radio continuum emission (e.g., Scoville & Solomon 1973 ; Ward-Thompson & Robson 1990 ; Westerhout 1958), combined with its location on the far side of the Galaxy at a distance of 11.4 kpc (Gwinn, Moran, & Reid 1992), also make W49A one of the most luminous Galactic star formation complexes, with L Z 107 L _ (Becklin, Neugebauer, & Wynn-Williams 1973 ; WardThompson & Robson 1990). To account for the ionizing Ñux associated with all the radio continuum sources in W49A, DePree, Mehringer, & Goss (1997, hereafter D97) require D40 stars with spectral types between O4 and O9. This is an unusually high concentration of O-type stars, reminiscent of NGC 3603 and the Carina Nebula in our Galaxy, 30 Doradus in the Large Magellanic Cloud, and NGC 604 in M33. However, W49A di†ers from these ““ mini-starbursts ÏÏ in that its luminous sources are still in the earliest ultracompact (UC) H II region phase of their evolution. 1 Visiting Astronomer at the IRTF, operated by the University of Hawaii under contract from NASA. 2 Current address : Air Force Research Laboratory/VSBC, 29 Randolph Road, Hanscom AFB, MA 01731-3010.

316

INFRARED IMAGING OF W49A resolution, the radio continuum source W49 is resolved into two sources : W49A is the star formation complex discussed here, and W49B is a supernova remnant about 12@ away, probably at the same distance as W49A (Mezger, Schraml, & Terzian 1967). With spatial resolution of D1@, the cometary shaped UC H II region W49 South is resolved from the rest of the sources in W49A (e.g., Harvey, Campbell, & Ho†mann 1977). At high spatial resolution (D1A) W49A breaks up into several dozen compact sources designated with one or two capital letters (i.e., W49A/G ; we adopt the notation used by Dreher et al. 1984, D97, and others). Some of these sources show substructure and are labeled with additional numbers (i.e., W49A/G1 or W49A/G2). W49 is located on the far side of the Galaxy, and so extinction along the line of sight (A B 300 mag ; Westbrook v at wavelengths shorter et al. 1976) precludes its observation than a few microns. Even at mid-IR wavelengths, the local and Galactic extinction is considerable ; Gillett et al. (1975) Ðnd the 9.7 km optical depth toward W49 South to be D4, and Westbrook et al. (1976) estimate that the dust column density is a factor of 6 higher toward the core of W49A. The e†ect of extinction is less severe at longer wavelengths, and the molecular line, recombination line, and continuum emission from W49A have been studied extensively in the far-IR to radio. In this paper we present observations of the mid-IR (8È20 km) emission from several compact sources with spatial resolution comparable to that achieved by radio observations, allowing us to investigate the nature of the dust emission from individual sources. Far-IR observations (e.g., Harvey et al. 1977 ; Rieke et al. 1973) published to date lack the spatial resolution needed to investigate how the thermal dust emission varies from one source to another. 2.

OBSERVATIONS AND DATA REDUCTION

W49 was observed on the nights of 1997 September 12È14 using the 3 m NASA/IRTF and the mid-IR array camera MIRAC2 (Ho†mann et al. 1998). MIRAC2 utilizes a 128 ] 128 Si :As hybrid BIB array. Images were obtained at 8.0, 8.6, 9.1, 9.7, 9.8, 10.5, 10.7, 11.3, 12.3, 12.8, 13.2, and 20.6 km. The 20.6 km Ðlter has a bandwidth of *j B 2 km, and images at all other wavelengths were obtained using a circular variable Ðlter (CVF) with *j/j B 1.8%. The observations are summarized in Table 1. The 3 p background values given in Table 1 were determined for each image by averaging the 3 p values measured at several di†erent positions that appeared to have no emission sources. Sky subtraction was achieved with standard chop-nod observations. The individual frames were corrected for gain variations, bad pixels, and air mass di†erences and were then combined into the Ðnal images using standard IR shiftand-add methods. The images were calibrated using the IR standard star c Aql. Adopted values for the zero magnitude Ñux and Ñux densities of c Aql are given in Table 2. The measured FWHM of the standard star was within 0A. 1 of that expected for di†raction limited observations on a 3 m telescope at all wavelengths (at the distance of 11.4 kpc, 1@@ B 0.5 pc). Since W49A is a star formation complex with a spatial extent much larger than the MIRAC2 Ðeld of view, three separate positions were observed : (1) the cometary UC H II region W49 South, (2) sources S, R, and Q (SRQ), and (3) the ring of UC H II regions at the center of W49A. The Ðnal images are displayed in Figure 1 (W49 South), Figure 2 (SRQ), and Figure 3 (the ring of UC H II regions).

317 TABLE 1 MID-INFRARED OBSERVATIONS

Source Name W49 South . . . . . .

SRQ . . . . . . . . . . . . .

Ring . . . . . . . . . . . . .

j (km)

Exposure Timea

3 pb

8.0 8.6 9.1 9.7 9.8 10.5 10.7 11.3 12.3 12.8 13.2 20.6 10.5 10.7 11.3 12.3 12.8 13.2 20.6 12.3 12.8 13.2 20.6

620 510 720 1280 501 301 242 617 358 378 436 681 209 227 242 242 226 241 194 281 362 377 537

0.09 0.07 0.10 0.12 0.08 0.10 0.12 0.10 0.20 0.15 0.17 0.45 0.08 0.08 0.13 0.20 0.19 0.20 0.15 0.08 0.11 0.18 0.10

a Total on source integration time in seconds. b 3 p level in Jy arcsec~2 above the background.

Although SRQ was observed at 10.7 km, we do not display the resulting image in Figure 2 because the 10.7 km image is virtually identical to the 10.5 km image. After calibrating the images, photometry was performed on all distinguishable sources and is given in Table 2. We assume an uncertainty in these Ñux measurements of ^10%. For most sources, Ñux densities are given in Jy, but some extended sources have sizes and shapes that make their total Ñux densities difficult to include in a circular aperture. For these sources (labeled ““ di†use ÏÏ in Table 2) the surface brightness was measured near the peak in Jy arcsec~2 as described in the notes to Table 2. The names for individual sources in Table 2 correspond to features labeled in Figures 1È3. The photometry in Table 2 was used to create spectral energy distributions (SEDs) of individual sources. For sources with radio continuum counterparts, the expected contribution from free-free emission in the SEDs is shown as a dashed line. This line is calculated by extrapolating from the 3.6 cm Ñuxes given by D97 and assuming optically thin free-free emission with S P l~0.1. The optically thin assumption is justiÐed, since lD97 give values for q that are less than unity for all sources, 3.6cmq except R where \ 1.3. For R the free-free level at IR 3.6cm wavelengths is estimated by extrapolating from the 1.3 cm Ñux given by D97 where q \ 0.1 for R. Although every 1.3cm other source is thin at 3.6 cm, some show slowly rising spectra (S P l0.6) indicating a possible contribution from an ionizedl wind. In this case our estimates for the free-free contribution in the IR may be too small by as much as a factor of 2. Several CVF settings were chosen to isolate speciÐc spectral features. The wide silicate absorption feature covers the wavelength range 8È12 km. Some known emission features included in our Ðlter set are from PAHs at 8.6 and 11.3 km,

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TABLE 2 PHOTOMETRY OF INDIVIDUAL SOURCES IN W49A

SOURCE NAME

APERTUREa RADIUS (arcsec)

Fb . . . . . . . . . . . . . . . . . . . . . 0 c Aqlc . . . . . . . . . . . . . . . . . W49 South . . . . . . . . . . . S....................... R ...................... Q ...................... A, B, C, C1, D, Ed . . . F ...................... G ...................... H ...................... I ....................... J ....................... J1 . . . . . . . . . . . . . . . . . . . . . . J2 . . . . . . . . . . . . . . . . . . . . . . K ...................... L ...................... M ..................... DD . . . . . . . . . . . . . . . . . . . . DD South . . . . . . . . . . . . EE . . . . . . . . . . . . . . . . . . . . EE East . . . . . . . . . . . . . . HH . . . . . . . . . . . . . . . . . . . . HH West . . . . . . . . . . . . . BB East . . . . . . . . . . . . . .

7 5 2 Di†use

S (Jy) AT EACH WAVELENGTH (km) l 8.0

8.6

9.1

9.7

9.8

10.5

10.7

11.3

12.3

12.8

13.2

20.6

59.17 112.7

51.53 98.20

46.24 89.75

40.90 83.89

40.10 82.26

35.11 72.01

33.85 70.08

30.46 63.65

25.85 54.00

23.92 50.45

22.54 47.96

9.44 20.10

30.5

26.1

18.7

13.4

17.6

35.5 3.91 0.89 0.11

39.9 4.60 1.31 0.15

65.2 6.92 3.28 0.27

134.7 13.2 10.7 0.58 \0.08 1.68 13.9 \0.08 0.56 2.64 \0.08 0.13 \0.08 0.31 0.47 0.35 8.54 0.25 0.20 0.18 \0.08 0.30

179.3 26.9 15.0 0.90 \0.11 1.59 24.9 \0.11 1.16 5.35 \0.11 0.35 \0.11 0.43 1.14 0.53 15.2 0.24 0.17 0.25 0.24 0.45

159.7 18.6 15.7 0.73 \0.18 1.49 30.7 \0.18 2.10 4.54 \0.18 \0.18 \0.18 0.37 1.03 0.52 14.5 0.23 0.10 0.20 0.18 0.33

388.9 74.3 13.6 1.30 \0.10 5.11 36.1 0.18 3.43 9.07 0.73 0.95 0.31 0.73 4.97 0.90 80.0 0.26 0.25 0.65 0.25 0.98

3 4 1 3 3 2 2 1 Di†use 3 Di†use 6 Di†use Di†use Di†use Di†use Di†use

a For sources labeled here as ““ di†use,ÏÏ values in the table are I in Jy arcsec~2 rather than S in Jy. Values for these sources were determined by taking the l l median of the surface brightness in a 1A square aperture centered on the source peak. b Adopted values for the zero magnitude Ñux in Jy for each Ðlter (from Cohen et al. 1992). c Adopted values for the calibration star in Jy. d These sources were not detected at any j. Here we give 3 p upper limits.

[S IV] at 10.5 km, and [Ne II] at 12.8 km. The 12.3, 13.2, and 20.6 km Ðlters sample featureless thermal continuum emission from warm dust grains. The broad 7.7 and 12.7 km PAH features could contribute to some Ðlters as well (8.0, 12.3, 12.8 km). However, the mid-IR spectra presented by Gillett et al. (1975) do not show evidence for strong PAH features in the W49 sources observed. Emission from [Ar III] is weak or absent in the spectra obtained by Gillett et al., so it is unlikely that the 9.1 km CVF Ðlter will be signiÐcantly contaminated by [Ar III] at 8.99 km. Except for silicate absorption and 12.8 km [Ne II] emission, W49 sources observed by Gillett et al. show smooth continuum spectra in the mid-IR. In addition to examining the integrated SEDs of sources from the photometry in Table 2, we investigate the spatial distribution of the emission features listed above by subtracting the continuum emission. The continuum emission used for subtraction was determined by using a combination of images obtained in nearby continuum Ðlters. For example, a map of [Ne II] emission would utilize an average of the 12.3 and 13.2 km images subtracted from a 12.8 km image. We did not create maps for every spectral feature in every source due to the high background noise in many of the images. The emission maps are discussed later in the text describing the individual sources. With an estimate of the continuum level from the SEDs and the Ñux density in the 10.5 and 12.8 km Ðlters, we have estimated the strengths of the [S IV] and [Ne II] emission lines (see Table 3). As in Table 2, the values for several sources are

given in Jy arcsec~2 rather than Jy. Only sources W49 South, S, R, and Q were observed at 10.5 km, but all sources were observed at 12.8 km (although not all sources show signiÐcant [Ne II] emission in their SEDs). We have used 3.6 cm continuum Ñuxes from D97 and expressions given by Petrosian (1970) to estimate the abundance ratios n(S```)/n(H) and n(Ne`)/n(H). The [Ne II] abundances derived in Table 3 are particularly interesting with regard to expectations from the assumption of ionization equilibrium. For the sources listed in Table 3, D97 derive ionizing spectral types ranging from O4 to O8.5, and electron densities in the range 2.4 ] 103 ¹ n ¹ 9 ] 104 cm~3. Using the approe priate form of the Saha equation [with s(Ne`) \ 41 eV, and assuming log (2u`/u) B 0] given by log

Ne`` 3 20,6640 B log T [ [ log n ] 15.38 , (1) e Ne` 2 T

where T is the temperature of the radiation Ðeld corresponding to the spectral types of the ionizing sources, gives values of log Ne``/Ne` in the range 10.9È14.8 ; i.e., in these UC H II regions we should expect that all the neon is in ionization states Ne`` or higher if the gas is in equilibrium with the radiation Ðeld of the central ionizing star. The fact that we detect relatively strong [Ne II] is surprising. Ne` abundances will only approach the cosmic neon abundance with values of log (Ne``/Ne`) B 0, which would require either regions of extraordinarily high density (n Z 108 e cm~3), or regions that are shielded from the hard radiation

No. 1, 2000

INFRARED IMAGING OF W49A

319

FIG. 1.ÈImages of W49 South at the indicated wavelengths. Lowest contour is at 3 p above the background, and 3 p contour levels in Jy arcsec~2 are given in the lower right portion of each panel. Thereafter, contour levels increase by a factor of 2 above the preceding contour level. Negative gray scale is logarithmic to show the full dynamic range.

Ðelds of the central stars (the latter is more plausible considering the densities required). Thus, we might expect to observe [Ne II] emission in shells surrounding the UC H II regions for sources that are spatially resolved. Color temperature maps were made for W49 South by taking the ratio of the 12.3 and 20.6 km images, and the

optical depth maps at each wavelength used the resulting 12.3È20.6 km temperature map. Many of the other sources in W49A are pointlike or have insufficient signal to noise in their extended emission, so we only present color temperature and opacity maps for W49 South in this paper. The emission from W49 South is signiÐcantly extended such

FIG. 2.ÈImages of W49 SRQ at the indicated wavelengths. Lowest contour is at 3 p above the background, and 3 p contour levels in Jy arcsec~2 are given in the lower right portion of each panel. Thereafter, contour levels increase by a factor of 2 above the preceding contour level. Negative gray scale is logarithmic to show the full dynamic range.

320

FIG. 3.ÈImages of the ring of UC H II regions and surrounding sources at the indicated wavelengths. Lowest contour is at 3 p above the background, and 3 p contour levels in Jy arcsec~2 are given in the lower left portion of each panel. Thereafter, contour levels increase by a factor of 2 above the preceding contour level. Negative gray scale is logarithmic to show the full dynamic range.

321

FIG. 3.ÈContinued


323

TABLE 3 [Ne II] EMISSION LINE STRENGTHS AND ABUNDANCES IN W49A Source W49 South . . . . . . S ................. R ................. Q ................ G ................ J.................. L ................. M ................ DD . . . . . . . . . . . . . . DD South . . . . . . HH . . . . . . . . . . . . . . HH West . . . . . . . . BB East . . . . . . . . .

S (3.6 cm)a l 4.6 0.63 0.094 0.079 4.7 0.47 0.08 0.2 0.256 ... 0.012 ... ...

S (12.8 km) l 179.3 26.9 14.98 0.90 24.9 5.35 0.43 1.14 0.53 15.2 0.25 0.24 0.45

S (cont) l 146.0 15.9 13.20 0.66 22.3 3.59 0.33 0.75 0.44 11.5 0.19 0.13 0.32

S ([Ne II]) l 33.3 11.0 1.78 0.24 2.6 1.76 0.10 0.39 0.09 3.7 0.06 0.11 0.13

n(Ne II)/n(H) 1.09 ] 10~5 2.62 ] 10~5 2.84 ] 10~5 4.56 ] 10~6 8.3 ] 10~7 5.6 ] 10~6 1.9 ] 10~6 2.9 ] 10~6 5.3 ] 10~7 ... 7.5 ] 10~6 ... ...

a From DePree et al. 1997.

that di†erences in the resolution between 12 and 20 km did not create artifacts in the resulting temperature map. 3.

W49 SOUTH

3.1. Morphology W49 South is the cometary UC H II region located a few arcminutes to the southeast of the W49A molecular cloud core. Figure 1 reveals that W49 South displays a parabolic shape opening to the east at all observed mid-IR wavelengths. The morphology in these mid-IR images is remarkably similar to that in other cometary UC H II regions, such as in 8È13 km images of G5.89È0.39 (Ball et al. 1992). In general, W49 South is more extended at longer wavelengths. At 13 and 20 km, the extended emission reaches D5A to the north and west, and D10A to the southeast and east of the central peak. At all observed wavelengths the location of the maximum surface brightness is near the apex of the parabolic arc, and the emission drops o† quickly to the west of this peak. A roughly circular halo of extended emission in the 8.0, 8.6, and 11.3 to 20.6 km images suggests the presence of a nearly complete shell with a projected radius of D4A. The peak emission is located on the western rim of this shell. This halo or shell is not detected in the 9 and 10 km images due to lower surface brightness caused by silicate absorption, but it is seen at 11.3 km, even though this is still within the silicate absorption feature. This extended structure may be partially due to the 11.3 km PAH feature, but we do not detect signiÐcant 11.3 km emission above the continuum in the SED of W49 South (° 3.3). The 20.6 km image shows more clumpy structure than other mid-IR wavelengths, and a northern emission protrusion is detected, which extends beyond the parabolic boundary of the UC H II region as delineated in radio images. Aside from this protrusion at 20.6 km, the spatial distribution of mid-IR emission in W49 South is very similar in shape and total extent to the radio continuum emission (i.e., D97 ; Welch et al. 1987), indicating that warm dust is coincident with the ionized gas on spatial scales as small as D1017 cm (D1A at d \ 11.4 kpc). 3.2. Dust T emperature and Opacity The color temperature map in Figure 4a reveals at least two temperature peaks in W49 South, suggesting a pair of self-luminous sources. The color temperature maps are useful for showing relative dust temperature distribution,

but absolute temperature values are uncertain due to unknown contributions from extinction and grain emissivity as a function of wavelength. The hotter temperature peak is o†set from the emission peak by D1A, and the second peak is roughly coincident with the radio continuum source W49 South-1 (D97). Comparing Figures 1 and 4, higher temperatures are anticorrelated with relatively strong mid-IR and radio continuum emission. The warmer grains in the extended regions of W49 South, which presumably occupy the ionized cavity of the UC H II region (see below), are located roughly 5A from both the emission and temperature peaks. We therefore infer that they are roughly 5 ] 1017 cm from the central engine. If W49 South is powered by a main-sequence O4 star (D97) with L B 106 L , we should expect grains which emit like _ blackbodies to have temperatures on the order of 50È100 K, since it can be shown that

A B

L 1@4 K, T \ 1.5 ] 109 * BB D2

(2)

where L is the luminosity of the heating source in L and _ cm. D is the*emitting grainÏs distance from that source in Indeed, the SED of W49 South (Fig. 5a and ° 3.3) indicates that the far-IR emission is well Ðtted by a 50 K blackbody. However, the observed mid-IR color temperatures greater than 100 K (for average values in a 1A aperture) indicate a population of superheated grains in the interior of the UC H II region cavity. These superheated grains surviving in the vicinity of ionized gas are probably small (a [ 0.1 km) iron or graphite grains. If these grains really do occupy the ionized cavity of the UC H II region, then additional heating from resonantly trapped Lya photons or collisions with the local ionized gas may also be important in explaining their large superheat (e.g., Natta & Panagia 1976). Figures 4b, 4c, and 4d show optical depth maps at 12.3, 13.2, and 20.6 km. The regions of relatively high optical depth in the extended di†use material are anticorrelated with the higher temperature dust, indicating that the hotter dust does occupy the regions of relatively low density in the interior cavity of the UC H II region, as conjectured above. The morphology of the optical depth maps at the di†erent wavelengths is very similar ; all show enhanced density in roughly the same positions around the core, including the clump at the northern rim of the UC H II region that was seen in the 20.6 km emission image. All three emission

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FIG. 4.ÈEmission features in W49 South. (a) Dust grain color temperature from a ratio of the 12.3È20.6 km images. Contours are at 150, 160, 170, 175, 180, 190, 195, and 200 K. (b, c, d ) Dust emission optical depth at the indicated wavelengths. Contours are at 0.0002, 0.0005, 0.001, 0.0015, 0.002, 0.0025, 0.003, 0.0035, and 0.004 in (b) and (c), and 0.001, 0.002, 0.003, 0.004, 0.006, 0.008, and 0.01 in (d ). (e, f ) Continuum-subtracted [S IV] and [Ne II] emission, respectively. For (e) and ( f ) the lowest contour is at 3 p above the background, and 3 p contour levels in Jy arcsec~2 are given in the lower right portion of each panel. Thereafter, contour levels increase by a factor of 2 above the preceding contour level.

optical depth maps show a similar elongated high optical depth structure in the center of the images along a position angle of D135¡. This corresponds to the western perimeter of the parabolic structure seen in emission images. This high-opacity ridge bisects the two temperature peaks and has a maximum value of q B 0.01 in the 20.6 km opacity map. The distribution of dust column density traced by the emission optical depth images appears to be more complex than what one might expect from a moving star bow shock model for cometary UC H II regions. For example, W49 South does not show the ““ long, narrow tails ÏÏ that are characteristic of bow shocks formed by supersonic O-type stars as described by Wood & Churchwell (1989). 3.3. Spectral Energy Distribution Using the results of the photometry listed in Table 2, we have constructed the SED of W49 South in Figure 5. Because W49 South is isolated and is [email protected] from the rest of W49A, it has been resolved and its Ñux has been measured in the far-IR (not possible with the many sources in the more crowded ring of UC H II regions). We have therefore augmented the photometry in Table 2 with far-IR and radio Ñuxes from the literature, as cited in the Ðgure caption. Figure 5a shows the observed mid-IR to radio SED for W49 South. Even without correcting for the considerable 8È13 km extinction toward W49 South (Gillett et al. 1975 indicate that the 8È13 km extinction is at least a factor of 10), Figure 5 shows that the observed Ñuxes exceed the level of expected free-free emission by more than an order of magnitude. This, and the similar morphology of the IR and

radio maps, indicate that the observed mid-IR emission is from heated dust grains in the vicinity of the ionized gas. The far-IR to millimeter Ñuxes are well Ðtted by a 50 K blackbody. However, the mid-IR Ñuxes exceed the 50 K blackbody emission, indicating a population of hotter grains like those discussed above. For comparison, Ñux distributions of a 150 K (Ðt to the 12.3, 13.2, and 20.6 km data) and a 210 K (Ðt to the 8.0, 12.3, and 13.2 km data) blackbody are shown in Figure 5 (although the actual grain temperature will depend on an adopted emissivity law and the amount of foreground extinction). This is consistent with the earlier result from the color temperature map in Figure 4a, indicating a range of dust emission temperatures around 150È200 K. Figure 5b shows the mid-IR SED of W49 South on an expanded wavelength scale. The most prominent feature in the mid-IR spectrum of W49 South is deep silicate absorption centered at 9.7 km. Our imaging photometry agrees well with the spectrum of the same source presented by Gillett et al. (1975), and the dotted line through the data points follows the shape of their model Ðt but is extrapolated to D20 km. Spatial variations in the amount of silicate absorption toward W49 South are investigated in Figure 6, where we show sections of images from Figure 1 at the indicated wavelengths across the silicate feature. The images in Figure 1 were displayed with a logarithmic scale to show the full dynamic range of the observations, but more detail in the central region of W49 South is seen in Figure 6 due to a higher contrast, linear display. The 8.0 and 12.3 km images are similar, displaying a parabolic

No. 1, 2000


FIG. 5.ÈSpectral energy distribution of W49 South. (a) mid-IR to radio SED. Blackbodies at the indicated temperatures are shown for comparison. Supplementary far-IR and radio data points are taken from the literature as follows : (53 km) Harvey et al. (1977) ; (350 km) Rieke et al. (1973) ; (3563,6928 km) Akabane et al. (1989) ; (2 cm) Dickel & Goss (1990) ; (3.6 cm) DePree et al. (1997) ; (6 cm) van Gorkom (1980). Expected free-free contribution at all wavelengths is extrapolated from 3.6 cm data assuming S P l~0.1. (b) SED of W49 South showing only the mid-IR photometry. l Dashed line represents free-free emission estimated as in (a), and the dotted line is a sketch of the continuum level following Gillett et al. (1975).

shape with intensity decreasing smoothly away from the central peak. Images at 9.1, 9.7, and 10.7 km (wavelengths which are more strongly a†ected by silicate absorption) show a dark clump or ridge of higher than average extinction which seems to divide the arc of emission. This e†ect is most prominent in the 9.7 km image. The silicate absorption map in Figure 6 reveals di†erential extinction in a ridge

325

oriented roughly southeast to northwest (spatially coincident with the ridge seen in the optical depth maps in Fig. 4), and a clump of high extinction at the position of the mid-IR emission peak. This indicates that there is a signiÐcant amount of nonuniform local extinction due to silicate dust in W49 South. The spatial distribution of silicate grains does not appear to be correlated with the spatial distribution of superheated grains in the color temperature map discussed above. The SED does not indicate signiÐcant PAH emission at 8.6 or 11.3 km, and di†erence images comparing the 11.3 and 13.2 km emission did not show appreciably di†erent morphology than in similar images comparing the 12.3 and 13.2 km emission, except for a slight excess emission in the central region of the 11.3 km image. The 11.3 km emission feature is usually attributed to neutral PAH emission (e.g., Allamandola, Hudgins, & Sanford 1999), so if there was 11.3 km emission in W49 South, we might expect it to exist in a shell exterior to the ionized cavity. Since the slight excess 11.3 km coincides spatially with the superheated grains in the ionized cavity of the UC H II region, this excess is probably due to di†erences in grain temperature rather than PAH emission. There is weak [S IV] at 10.5 km and a clear excess from [Ne II] emission at 12.8 km in the SED of W49 South. The Ñux from [S IV] above the continuum (note that by ““ continuum ÏÏ we mean the Ñux density associated with the smooth variation through the silicate feature, shown by the dotted line in Fig. 5b) is 3.7 Jy, indicating an abundance n(S```)/n(H) B 9.05 ] 10~8. This is roughly 0.5% of the cosmic abundance of sulfur. We have estimated the contribution above the continuum level from the [Ne II] line, and the derived neon abundance is given in Table 3. The spatial distribution of excess [S IV] and [Ne II] emission is investigated by subtracting continuum emission from the 10.5 and 12.8 km images in Figures 4e and 4f. Emission from sulfur is concentrated at the western edge of W49 South, coincident with the high-opacity ridge identiÐed in Figures 4b, 4c, and 4d. The peak of [S IV] emission is displaced to the south from both the temperature peaks and the IR continuum emission peaks. This concentration of [S IV] emission near the location of a density enhancement suggests that the western parabolic feature in W49 South may be an edge-on ionization front. In contrast to the localized [S IV] emission, the [Ne II] is more extended and identiÐes zones of

FIG. 6.ÈGray-scale images of the central region of W49 South showing the spatial dependence of silicate absorption. Images at the indicated mid-IR wavelengths are shown at a higher contrast setting than in Fig. 1. The panel at the far right is a map of di†erential silicate absorption made by subtracting the normalized continuum (average of 8.0 and 12.3 km) from the 9.7 km image ; brighter areas indicate more silicate absorption.

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lower excitation. Emission from the 12.8 km neon line emanates from the periphery of the shell-like structure in W49 South, in accord with our expectations described in ° 2. The [Ne II] emission appears to extend to the east beyond the edge of the Ðeld of view in Figure 4f. This may indicate a breakout of ionized gas from the UC H II region due to a density gradient in the surrounding medium. 4.

SOURCES SRQ

4.1. Morphology The three sources S, R, and Q (sometimes referred to collectively as W49 southwest), show a morphology in Figure 2 similar to the radio continuum (D97). All three generally show more extended structure at longer IR wavelengths. S is an irregular, or possibly core-halo type UC H II region, with a diameter growing from D5A at 10 km to D9A at 20 km. R is unresolved at all wavelengths but may be physically associated with extended emission to the west. Although the clumpy shell structure of source Q resembles its radio morphology, the peak emission at mid-IR wavelengths is coincident with a relative minimum at the center of the radio shell (the images at various wavelengths were aligned by matching the position of the unresolved source R). This suggests that the IR and radio shells are o†set in R.A. by about 0.2 s (D3A). In the radio continuum images presented by D97, Q looks like a somewhat distorted and clumpy shell, but in the mid-IR images, Q looks more like a nearly edge-on torus than a limb brightened shell ; this is especially apparent in the 12.8 and 20.6 km images in Figure 2. 4.2. Spectral Energy Distributions The SEDs of sources S, R, and Q are shown in Figures 7a, 7b, and 7c. All three sources show mid-IR SEDs dominated by thermal continuum emission from dust. All three sources also show a steep decrease in emission shortward of 12 km, presumably due to silicate absorption. Aside from this silicate absorption, the SEDs are remarkably Ñat compared to a single-temperature blackbody, indicating thermal emission from dust at a range of temperatures from a few to several 102 K. A color temperature map (not shown) constructed from the 13.2 and 20.6 km images in Figure 2 shows a color temperature gradient across source S, with grain temperature decreasing in the direction away from source R. While sources S and Q have SEDs that rise to longer wavelengths, it is noteworthy that R is the only object observed in W49A with an SED that falls from 13 to 20 km. This may indicate a lack of cooler dust or less absorption at shorter wavelengths compared to the other sources. The SEDs of sources S, R, and Q do not indicate a signiÐcant contribution from 11.3 km PAH emission. However, the continuum-subtracted 11.3 km image in Figure 8a shows some spatial variation in the residual PAH emission. The emission peaks of S and R are weaker relative to the extended emission, and the emission peak of S has shifted slightly toward the southwest as compared to the continuum emission. S shows extended PAH emission toward the northeast, but the more di†use emission is truncated toward the southwest. A similar characteristic can be seen in the continuum-subtracted [Ne II] image (see below). This is reminiscent of the morphology observed in the continuum emission from W49 South, and may indicate a density gra-

FIG. 7.ÈSpectral energy distributions of sources S (a), R (b), and Q (c) in W49A. Expected level of free-free emission is estimated from 3.6 cm Ñux as in Fig. 5. Note that the value plotted on the Y -axis for source Q is surface brightness.

dient in the ambient medium. There is also extended PAH emission D5A to the northwest of S, scarcely above the 3 p detection limit and not seen in the [Ne II] continuumsubtracted image. R shows extended emission only toward the west, and appears more clumpy than in continuum maps. The continuum-subtracted PAH emission from Q is very weak but appears to comprise a thin, clumpy shell with

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327

FIG. 8.ÈContinuum-subtracted emission in the 11.3 km PAH feature (a) and the 12.8 km [Ne II] line (b) for sources S, R, and Q. Lowest contour is at 3 p above the background, and 3 p contour levels in Jy arcsec~2 are given in the lower left portion of each panel. Thereafter, contour levels increase by a factor of 2 above the preceding contour level.

a slightly larger size than the mid-IR continuum shell structure. This may be a photodissociation region at the outer edge of the shell. This PAH shell in Figure 8a appears to extend all the way up (north) to the extended emission associated with source R. S and Q show strong 12.8 km [Ne II] emission, while this line is rather weak in R. The strength of these lines above the continuum and abundances derived from the [Ne II] emission are given in Table 3. The spatial variation of [Ne II] emission is presented in the continuum-subtracted neon map in Figure 8b. The [Ne II] morphology of S is similar to the PAH emission discussed above, in that it is extended more toward the northeast than toward the southwest. The [Ne II] emission also appears slightly more extended than the PAH emission in all directions, although this may be the result of higher signal to noise in the [Ne II] map. As in the PAH emission map, source R is extended only toward the west, although it appears smoother and has a larger extent (out to D6A from the point source) than in PAH emission. The continuum-subtracted [Ne II] emission from Q is roughly anticorrelated with the PAH emission in Figure 8a. The [Ne II] emission appears to comprise a thicker, partially Ðlled incomplete shell or torus, which is located interior to the thinner PAH shell. The low-level extended [Ne II] emission from Q appears to extend as far as and possibly further north than source R. The slope of the continuum at 10 km is steep, so the choice of a continuum level at 10.5 km is rather arbitrary because we do not have observations at shorter wavelengths. We are therefore not able to derive reliable [S IV] abundances for sources S, R, and Q. 5.

THE RING OF UC H II REGIONS

5.1. Morphology To those familiar with the radio continuum morphology of the ring of UC H II regions in W49A (see Welch et al.

1987), the most surprising result of this study might be that the ring is incomplete in the mid-IR. Inspection of Figure 3 reveals that while several radio continuum sources in the ring are detected at in the mid-IR, a few are missing at all observed IR wavelengths. SpeciÐcally, we do not detect sources A, B, B1, C, C1, D, or E (except for a questionable detection of C at 12.8 km), while we do detect sources F, G, H, I, J, L, and M (and possibly J1, J2, and K with lower conÐdence). We return to this problem in ° 5.4. For the detected mid-IR sources, the mid-IR morphology closely matches that observed in the radio continuum. In Figure 3, there are many features that are just above the 3 p contour, but that we do not consider as reliable detections. Many of these features in the lowest contour may indicate Ñuctuations in di†use background emission. In this and the next section, we only consider sources that can be identiÐed at more than one mid-IR wavelength, and in most cases have a radio continuum source associated with them (with a few exceptions that we discuss). In the mid-IR images of the ring of UC H II regions, sources F, I, J, and M are nearly pointlike sources, although I and J deÐnitely show some low-level extended structure out to a radius of D4A. Sources H, J1, J2, and K are questionable detections, but they are consistent with pointlike sources. L is an irregular or shell source, and G is discussed separately below. 5.2. Spectral Energy Distributions The SEDs for sources in the ring of UC H II regions are shown in Figure 9. We have not shown the SEDs for H, J1, J2, or K in this Ðgure because of their questionable detection, or because they were only detected at one wavelength. The results for the ring sources are very similar to the previously described sources in that they all show signiÐcant emission above the expected free-free Ñux, even without correcting for extinction. Again, this indicates that these are dusty UC H II regions and the similarity in radio and

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FIG. 9.ÈSpectral energy distributions of sources in the ring of UC H II regions. Note that the Y -axis value for source L is surface brightness. The contribution of free-free emission at mid-IR wavelengths is estimated as in Figs. 5 and 7.

mid-IR morphology indicates that the dust is in close proximity to the ionized gas. The mid-IR emission does not seem to be more extended than the ionized gas (except for G, see below), as one might expect if the mid-IR emission was from dust shells around ionized cavities. The Ñux from source F falls smoothly from 12.3 to 13.2 km with no evidence of [Ne II] emission and then rises from 13 to 20 km. However, the SED of F is also consistent with a smooth increase from 12 to 20 km, within the errors. Both G and I show a steep rise from 12 to 13 km and are Ñat from 13 to 20 km. The 12 km Ðlters are just at the long wavelength tail of the broad 9.7 km silicate absorption feature, so the sharp rise from 12 to 13 km may be attributed to very strong silicate absorption. G shows possible evidence for [Ne II] emission, but I does not. J, L, and M show a more gradual rise from 12 to 20 km, and all three show [Ne II] emission. The derived abundances of Ne` for J, L, and M are roughly 1% of the cosmic abundance, suggesting that most of the emitting Ne` is located in regions of higher density (following the argument outlined in ° 2) than the values of n derived from radio observations by e 13È20 km SEDs of the ring sources D97. The relatively Ñat indicate thermal emission from heated grains at a range of temperatures around a few 102 K. 5.3. Source G This source is the brightest mid-IR source in the ring of UC H II regions and is also the brightest in the radio continuum. It is by far the most powerful H O maser source known in the Galaxy, and proper motion 2measurements of its concentrated water maser spots with radio interferometry have established the size scale of the Milky Way (Gwinn et al. 1992). In the 20.6 km image, it shows extended structure out to a radius of D5A from the emission peak. This is more extended than the radio continuum emission ; G is the only object in our sample which has this property. This may simply be a result of the large intrinsic luminosity of G, allowing it to heat the outlying dust grains (at r Z 1017cm) enough that their thermal emission is detectable in the

images presented here. A 13.2/20.6 km color temperature map (not shown) indicates a temperature peak at the same position as the emission peak of source G, with lower temperatures in the surrounding extended emission. The mid-IR luminosity of G, integrated from 12 to 20 km, is roughly 2 ] 105 L , which corresponds to the _ zero age main-sequence luminosity of a single O6 star. D97 require multiple O6 stars to account for the ionized gas in this source, so it would seem that most of the stellar radiation is reprocessed by cooler grains at larger distances from the stars than the dust traced by the mid-IR emission. However, the true mid-IR luminosity could be signiÐcantly higher if extinction along the line of sight is important from 12 to 20 km ; the mid-IR luminosity is only D1% of the total far-IR luminosity coming from the ring of UC H II regions in W49A (2 ] 107 L ; Ward-Thompson & Robson _ 1990). G has the second lowest Ne` abundance (the lowest is DD) of the observed UC H II regions with detected [Ne II] emission. The abundance for G is more than 2 orders of magnitude lower than the cosmic abundance of neon, indicating a harder radiation Ðeld or lower electron density than in the other UC H II regions in W49 (the other UC H II regions with higher neon abundances require higher values of n , with the spectral types of their ionizing stars). However, egiven the extraordinary maser outÑow observed at the location of source G (see above), we might expect higher than average densities to be observed here. Perhaps the interface between regions of ionized and molecular gas is peculiar in source G, such that it allows considerable molecular material to exist in the vicinity of several O6 stars, while conspiring to produce a smaller column density of [Ne II]. If shocks were important in the ionization state of the gas, Ne` abundances might be misleading because the gas is not in radiative equilibrium. 5.4. T he Missing Sources There are two possible explanations for why sources A through E are not detected. Since all the sources that were

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detected show strong mid-IR excess emission from dust above the level expected from free-free emission, one possibility is that sources A through E simply do not contain signiÐcant amounts of dust. If the 3.6 cm Ñuxes of these sources are extrapolated to mid-IR wavelengths (ignoring extinction), then free-free emission from A, C, and E would be above the 3 p detection limits. However, extinction along the line of sight would probably render even the brightest of these undetectable in our observations. It is difficult to imagine why these sources would be depleted of dust, since molecular emission is stronger here than in other parts of the ring (Dickel & Goss 1990), and we should expect dust to be associated with this molecular gas. The spectral types of the exciting stars of A through E derived by D97 are later than several of the other ring sources that were detected, so depletion of dust by extremely hard radiation Ðelds seems unlikely. Could these be older O-type stars that have already cleared away their dust cocoons ? Probably not, since A through E appear far more compact than the rest of the radio sources, and A has a bipolar outÑow (D97), suggesting that these are in fact very young sources. A second, more probable explanation for the nondetection of A through E is di†erential extinction across the ring of UC H II regions. Sources A through E are all in close proximity to one another, at the western edge of the ring. Therefore, an extremely high column density cloud along the line of sight could explain their absence in Figure 3. Jackson & Kraemer (1994) found a close association of NH emission and absorption with the ring of UC H II 3 and found that the NH (3, 3) was anticorrelated regions with the more di†use continuum3 emission. Their Figure 1 shows that the NH was concentrated toward the western portion of the ring.3 Figure 5b from Serabyn, Gusten, & Schultz (1993) shows SO contours located over the western 2 half of the ring, and three separate clouds of C34S 5È4 clouds distributed across the ring. The C34S cloud at 12 km s~1 covers sources A through E and the western edge of G. This molecular gas may be associated with the dust needed to block the mid-IR emission from sources A through E. If true, this would argue that the 12 km s~1 C34S cloud is in front of the ring, and the others are behind (assuming that the radio continuum sources in the ring are at nearly the same distance along the line of sight). Dickel & Goss (1990) have derived column densities of H gas toward each of the 2 the ring. In general, bright radio continuum sources in higher molecular hydrogen column densities are seen toward the western part of the ring, and the path lengths and column densities toward sources A, B, C, and D are several times higher than toward sources in the eastern part of the ring. Based on the high column densities of molecular gas concentrated on the western part of the ring, we favor the interpretation that sources A through E are not detected in the present observations because of higher line-of-sight extinction than toward the other sources in the ring. This might suggest that the bright radio continuum sources A through E may be more compact than the other sources in the ring because they are embedded in more dense material and have not expanded as rapidly as the more di†use sources in the eastern part of the ring. It is difficult to determine quantitatively the column density of dust needed to block out the missing ring sources at 20 km because we do not know their intrinsic mid-IR Ñuxes a priori. As an example estimate, consider source A

329

for which D97 require the presence of an O6.5 mainsequence star. If all the D2 ] 105 L from the central _ engine is processed by grains at 50 K, the extinction-free Ñux density at 20 km should be D0.5 Jy for a distance to W49 of 11.4 kpc. This is a conservative lower limit if there is signiÐcant mid-IR emission from grains warmer than 50 K, as observed in the other sources in W49A. For line-of-sight dust extinction to render source A undetectable in the present observations (with 3 p B 0.1 Jy ; Table 1), the emitted Ñux must be reduced by at least a factor of 5, corresponding to q º 1.6. Assuming that the obscuring dust can be approximated as standard ““ Astronomical Silicate ÏÏ grains as in Draine & Lee (1984) with o \ 3.3 g cm~3 and Q \ 9a/j at 20 km, the required line-of-sight column abs mass of obscuring dust can be estimated from M

dust

ºN

grains

q 4 4 ]m \ na3o \ j qo , grains Q na2 3 27 cm abs (3)

where N is the column number density of grains, grains m \ (4/3)na3o is the mass of an individual grain, a is the grainsradius, and j is the observation wavelength in centigrain cm0.002). For q º 1.6 the column mass meters (here j \ cm density of obscuring dust required to render source A undetectable is M º 1.56 ] 10~3 g cm~2. Then, assuming a dust standard interstellar gas-to-dust mass ratio of D100, this corresponds to a required molecular hydrogen column density of N º 9.4 ] 1022 cm~2. The observed value of 2 this quantityH toward source A is N \ 2.5 ] 1024 cm~2 H2 there is a sufficient (Dickel & Goss 1990), indicating that line-of-sight column density such that extinction by dust is a plausible explanation for the mid-IR nondetection of the radio continuum sources at the western end of the ring in W49A. 6.

DIFFUSE SOURCES

6.1. DD and DD South DD and DD South are located to the northwest of the ring of UC H II regions in Figure 3. DD is a shell source (based on its morphology in radio continuum images ; D97) with a diameter of roughly 10A at all observed IR wavelengths. Its morphology is best recognized at 12.8 and 20.6 km, where it looks very similar to the broken shell revealed in radio continuum emission. About 10A south of DD, in a region of common extended low-level emission, is a very bright di†use source we designate DD South. It has no identiÐed counterpart in any published radio continuum observations (although there is some low-level emission at its position in the low-resolution image presented in Figure 1a by D97) and has not been detected before in the IR. This source is clearly detected at all four mid-IR wavelengths observed here and is the brightest source in Figure 3 at 20 km. In fact, at 20 km it is about twice as bright as G. It might seem surprising that this source was not detected earlier by Becklin et al. (1973), but they were searching for IR emission associated with the OH/H O maser sources G and W49 South and note that 2 ““ Measurements were conÐned to two sources which were discovered in the immediate vicinity of the maser sources ; mapping of the infrared emission from the whole radio source, 4 arcmin in diameter, was not attempted ÏÏ. DD South has an irregular or core-halo morphology (similar to

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source S) and has a diameter of D12A. However, a 13.2/20.6 km color temperature map (not shown) does not reveal any obvious temperature peak ; the temperature distribution is fairly constant across the extent of DD South. There is some suggestion from the images in Figure 3 (especially at 12.8 km) that at higher resolution and signal-to-noise, DD South may reveal itself to be a shell source very much like DD. The SEDs of DD and DD South are very similar (see Figs. 10a and 10b), rising continuously from 12 to 20 km, except for emission at 12.8 km from [Ne II]. It is surprising that [Ne II] is detected in DD South, since radio maps show only very weak continuum emission at the same location. The continuum IR emission in both sources is due to thermal emission from dust grains, because the mid-IR Ñux of DD is well above that expected for free-free emission, and there is only very weak radio continuum at the position of DD South. The shape of the IR continuum in both sources is Ñatter than expected from a single blackbody, indicating a range of temperatures around a few 102 K for the emitting dust. Interestingly, dust temperatures of 100È300 K and a lack of radio continuum emission, as seen in DD South, are observed properties of hot molecular cores in the Galaxy (Osorio, Lizano, & DÏAlessio 1999). 6.2. EE and EE East EE is a faint, irregular source not particularly distinct from the rest of the background in the four panels of Figure 3. We discuss it here because it is at the same position at all four observed mid-IR wavelengths and is coincident with the radio continuum source EE (D97). Its mid-IR SED is Ñat from 12 to 20 km in Figure 10c, but well above the level expected for free-free emission. EE East is detected at three of the four mid-IR wavelengths, and coincides with a distinct lack of emission in the 3.6 cm continuum map presented by D97. This may be a clump of dust and neutral gas that does not emit strongly in the radio continuum. Neither source shows evidence for [Ne II] emission above the thermal IR continuum level.

Vol. 540

6.3. HH, HH W est, and BB East HH and HH West are irregular, faint sources to the south of the ring of UC H II regions, with morphology very similar to EE and EE East. The SED of HH in Figure 10d rises smoothly from 12 to 20 km, and HH is the only mid-IR source in W49A with 12 and 13 km Ñuxes consistent with extrapolated free-free emission. However, considering the large extinction toward W49, this is almost certainly thermal emission from dust ; the 20 km Ñux from HH is well above the expected free-free continuum level. The SED of HH West in Figure 10e shows a steep rise from an upper limit at 12.3È13 km and then is fairly Ñat out to 20 km. HH West has no radio counterpart, so we cannot estimate the expected free-free level. BB East is another IR source with no radio continuum counterpart ; we have used the designation BB East because of its proximity to source BB in the low-resolution image presented in Figure 1 by D97. Its detection is not as problematic as HH West or EE East, because like DD South it is a bright IR source. However, its position does shift to the south by about 4A at 20.6 km (this may be complicated by the fact that at 20.6 km, BB East is at the edge of the map). The SED of BB East is nearly identical to HH, except brighter. HH, HH West, and BB East all show fairly strong [Ne II] emission, but Ne` abundances are not derived in Table 3 for HH West and BB East because their radio continuum Ñux has not been detected. 7.

CONCLUSION

Properties of the mid-IR emission from the individual UC H II regions in W49A have been discussed in the previous sections, and in this Ðnal section we summarize the more general aspects of the observed mid-IR emission. The SEDs are rather Ñat in general, and none of the sources can be Ðtted with a single-temperature blackbody. For W49 South, the mid-IR Ñux is higher than that expected by extrapolating the far-IR emission from cool grains. This is probably true for most of the other sources in W49A as well, because the concentration of sources at low resolution

FIG. 10.ÈSpectral energy distributions of di†use sources around the ring of UC H II regions in Fig. 3. All sources except DD South show surface brightness on the Y -axis. The contribution of free-free emission at mid-IR wavelengths is estimated as in Figs. 5 and 7 for the sources with radio counterparts.

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has a very large far-IR luminosity from cool grains, similar to that for W49 South. The mid-IR emission comes from grains as hot as a few hundred K, but observations at j [ 8 km are needed to determine the contribution from hotter grains. There appear to be two main types of behavior in the 12È20 km SEDs of the UC H II regions in W49A : (1) Those that rise gradually or are nearly Ñat from 12 to 20 km (W49 South, S, F, J, L, EE, DD South, HH, and BB East), and (2) Those that show a steep rise to 13 km and are then rather Ñat from 13 to 20 km (R, G, I, and HH West). A few sources (Q, M, and DD) appear to be in between these two types of SEDs. There does not appear to be any correlation between the SED characteristics and morphological type, so the difference is probably due to di†erent amounts of silicate absorption along the line of sight, causing patchy absorption at j \ 13 km. The fact that the SEDs are relatively Ñat is interesting, because it indicates that the IR emission does not come from a quasi-spherical thin dust shell centered on the heating source, for which we would expect to observe a single grain temperature. Rather, it appears that the dust is located at a range of di†erent distances from the central engines. The mid-IR morphologies of W49A and most of its individual components are similar to the radio continuum morphologies when observed at comparable resolution. The SEDs can be explained by emission from heated grains, which indicates that these are dusty UC H II regions. The heated dust is coupled to the ionized gas on size scales as small as 1017cm, corresponding roughly to the spatial resolution of the current observations, assuming a distance to W49 of 11.4 kpc (Gwinn et al. 1992). This indicates that collisions between ionized gas and grains, combined with poor emissivities for small grains, may be responsible for raising the temperature of the grains far above their blackbody temperatures. We do not detect extended dust shells outside the ionized cavities of the UC H II regions. These outer shells, presumably composed of cooler dust, may be observable at high resolution with far-IR/submillimeter interferometers. Alternatively, observations at higher spatial resolution at both mid-IR and radio wavelengths may show some stratiÐcation between ionized gas and dust. Several of the prominent radio continuum sources in the ring of UC H II regions in W49A are not detected at

331

thermal IR wavelengths. We attribute this nondetection to selective extinction by a high column density of dust grains along the line of sight to the IR sources. The column density derived from published molecular observations is sufficient to provide the required extinction, given a standard gas-todust ratio. The SEDs of individual sources in W49A do not show evidence for PAH emission associated with known hydrocarbon emission features, although continuum-subtracted images show possible PAH emission from extended regions. Spatially resolved mid-IR spectroscopy with higher spectral resolution than that used by Gillett et al. (1975) would be useful in future investigations of possible PAH emission in W49A. Slightly more than half the W49 sources observed show signiÐcant [Ne II] emission in their SEDs. Abundances derived from this emission are generally a few percent of the cosmic abundance of neon (D10~4 ; Brown & Gould 1970). Considering ionization balance for the spectral types of the ionizing stars indicates that in the relatively hard radiation Ðelds of these UC H II regions, the dominant ionization state of neon should be Ne``, rather than Ne`. This same conclusion was reached for W49 South by Gillett et al. (1975). However, the neon abundances derived in Table 3 indicate that in the regions which emit detectable [Ne II], a signiÐcant fraction of the gas exists in the lower ionization state. The spatial distribution of [Ne II] is generally at least as extended as the di†use 20.6 km emission, and the extended [Ne II] emission is brighter relative to the central peak than the thermal dust emission. This enhancement of the [Ne II] emission in the more extended parts of the UC H II regions may be explained by a softer radiation Ðeld, probably combined with a high-density shell far from the ionizing central star(s), such that a signiÐcant fraction of the Ne`` has been able to recombine to Ne`. Softer radiation Ðelds at large distances from the ionizing sources might result if the harder photons are absorbed by dust grains that are suspected to reside within the UC H II region cavities. If the radiation Ðelds are not softened with increasing distance from the stars, the observed Ne` abundances would require implausible values of n that are D9 e n observed orders of magnitude larger than the values for e inside the UC H II region cavities.

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