This 1°.18 × 0°.72 Atlas Image mosaic shows one of the most luminous star-forming complexes in the Galaxy, W51. The environment here is rich in newly-forming massive stars,compact and extended H II regions, and molecular clouds, including the K_s-bright, arc-like region, G49.5-0.4, to the northeast. The extended emission in K_s (2.17 µm) is thermal emission from ionized gas associated with the H II regions (Goldader & Wynn-Williams 1994, ApJ, 433, 164). W51 is along the Sagittarius arm in our Galaxy at about 7.5 kpc. The visual extinction in this region is very high, from A_V ~ 25 to 1000! Hot, main-sequence OB stars can be detected in these near-IR colors, which are consistent with the distance and extintion to W51 and are likely members. A number of embedded massive stars probably comprise the famous IRS sources (Wynn-Williams, Becklin, & Neugebauer 1974, ApJ, 187, 473) in G49.5-0.4. The supernova remnant W51C is not detected in this image.

The Wolf-Rayet ring nebula M1-67. Several Wolf-Rayet (W-R) stars, which represent the final evolutionary stages of very massive stars, have surrounding them thick shells of matter called "ring nebulae." The W-R star 124 (van der Hucht et al. 1981, SpSciRev, 28, 227) has a relatively young ejection nebula (M1-67). In this 2MASS image, the bright K_s-band emission seen around the bright W-R star (near the center of the image) is likely to be molecular H₂. The molecular gas excitation is due to either ultraviolet flourescence, as photons from the star are intercepted by the ring, or to shocks interacting with surrounding gas. The nebula was recently imaged in H-alpha by the Hubble Space Telescope (Grosdidier et al. 1998, ApJ, 506, L127). Grosdidier et al. find unprecented structure never seen before, including what appear to be hot, dense clumps in the wind coming from the star.

The face-on spiral galaxy NGC 5247.

NGC 416 (Lindsay 83) is one of several populous star clusters in the Small Magellanic Cloud (SMC), which are analogous to globular clusters in our own Galaxy, but significantly younger. This cluster was recently observed with the Hubble Space Telescope by Mighell, Sarajedini, & French (1998, ApJ, 494, L189). From their deep photometric observations, they derive an age for the cluster of 6.6±0.5 Gyr. For comparison, the Galactic globular cluster 47 Tuc is 13 Gyr old. NGC 416, however, is intermediate in age, relative to other populous (or, globular) clusters in the SMC. The metallicity, or heavy element content, is less for NGC 416 and the other SMC clusters, relative to Galactic clusters, such as 47 Tuc, due to the very different star formation history of the SMC, as compared to the Milky Way.
(The very red star seen in the SMC field, to the northeast of NGC 416 and toward the edge of the image, is likely to be a dust-obscured asymptotic giant-branch or carbon star in that galaxy.)

The embedded young star cluster IRAS 20050+2720, the bright agglomeration of stars near the center of the 2MASS three-color image. This cluster has been recently studied by Chen et al. (1997, ApJ, 475, 163). The cluster appears to consist of several subclusterings, three of which were studied by Chen et al.; there appear to be several other subclusterings not included in their near-IR imaging. The IRAS source itself is the brightest, reddest southern subclustering ("Subcluster A"). Some reflection nebulosity is associated with the cluster stars. Chen et al. find an extinction to the cluster of A_V ~ 10, although the extinction is clearly higher for subcluster A. (It is also clear from the 2MASS image that significant extinction is distributed throughout this field.) More than half of the point sources in the cluster appear to show an infrared excess beyond this extinction, indicating that we may be seeing thermal dust emission from circumstellar disks around classical T Tauri stars. These sources are therefore likely pre-main sequence objects. Chen et al. find that the slope and turnover of the K-band luminosity function imply an overall age for this cluster of ~1 Myr, although the subclusters are of somewhat different ages. The IRAS source "subcluster A" is also associated with dense molecular gas as traced by millimeter radio observations and coincides with the center of a multipolar outflow with a dynamical age of only 10³ to 10⁴ yr.

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