2MASS Picture of the Week Archive Captions

Atlas Image mosaic, covering 17.0´ × 17.0´ on the sky, of the open star cluster IC 5146. This cluster was identified by Walker (1959, ApJ, 130, 57) as extremely young. The 2MASS image peers through the dusty nebulosity seen around this cluster in the optical, revealing many more stars. Forte & Orsatti (1984, ApJS, 56, 211) identified ~110 members from optical plates; they found a reddening to the stars of ~3-4 visual magnitudes, and concluded that the objects most embedded in the nebulosity are protostellar, with ages no more than 1-3 Myr. The extinction is likely at least 10 visual magnitudes or more, with a distance to the cluster of at least 460 pc (1500 light years; Kramer et al. 1998, A&A, 342, 257), if not 1 kpc or more (Forte & Orsatti 1984). Click here for a 2MASS color-color diagram of IC 5146. Image mosaic by S. Van Dyk (IPAC).


Atlas Image mosaic, covering 6.0´ × 6.0´ on the sky, of Hubble's Variable Nebula, consisting of the infrared source R Monocerotis and the nebula NGC 2261. R Mon is thought to be a relatively young pre-main sequence star, and NGC 2261 is one lobe of a bipolar outflow from R Mon (Aspin et al. 1988, A&A, 197, 242). The core and nebula are both highly variable, on the timescales of a few months or more. The high infrared excess from R Mon, as seen in the 2MASS image, is thought to arise from a dust disk around the young star that is reradiating its optical light into the infrared (Imhoff & Mendoza 1974, RevMAA, 1, 25). Aspin et al. (1988) provided the first high-resolution near-IR images of R Mon, showing an interesting eastern jet from the star at 3.6 µm, which is probably the inner hot regions of the circumstellar disk. (The "star" trailing to the south is a Ks-bright known persistence artifact; also seen are the prominent diffraction spike artifacts.) Image mosaic by S. Van Dyk (IPAC).


Atlas Image mosaic, covering 11.0´ × 11.0´ on the sky, of the nearby spiral galaxy NGC 2403. This galaxy is of late Hubble type (ScIII, or Scd), and is a member of the Messier 81 (M81) group, at a distance of 3.2 Mpc (10.4 million light years; Freedman & Madore 1988, ApJ, 332, L63). It has many similarities to the galaxy Messier 33 in the Local Group (Garnett et al. 1997, ApJ, 489, 63). Both galaxies contain a number of giant HII regions and young, massive stars (Drissen et al. 1999, AJ, 117, 1249). In the near-infrared 2MASS image, these optically bright regions of recent star formation are highlighted by clusters of luminous red supergiant stars. Image mosaic by S. Van Dyk (IPAC).


Atlas Image mosaic, covering 57.4´ × 48.2´ on the sky, of NGC 3603 (left) and NGC 3576 (right). (The mosaic has been binned to 2´´ pixels.) Both nebulae (referred to together as RCW 57) are among the largest HII (ionized hydrogen) regions in the Galaxy. Both contain large numbers of hot, young, massive stars. Though only about 30´ apart on the sky, NGC 3603 is about twice as far away from us as NGC 3576 (which is at a distance of about 3000 pc, or 9800 light years). Both HII regions are ideal laboratories for studying star formation over a broad mass scale in a number of wavelength regimes. Both regions are partially obscured by foreground dust in the Galaxy, and both possess a considerable amount of internal dust (as can particularly be seen for NGC 3576, much like Messier 17), so studying them in the near-infrared, which penetrates the dust, gives us a more complete census of both young low-mass and high-mass stars and a more complete picture of the spatial distribution of recent star formation. In the optical, NGC 3576 shows bright extended loop-like nebulosity to the northeast, which is not seen in the 2MASS image. Image mosaic by E. Kopan (IPAC).


Atlas Image mosaic, covering 28.4´ × 36.6´ on the sky, of NGC 3603. (The mosaic has been binned to 2´´ pixels.) This nebula is the largest HII (ionized hydrogen) region in the Milky Way Galaxy and is considered a giant HII region, in the same class of objects as 30 Doradus in the Large Magellanic Cloud. (Many spiral galaxies have large numbers of these giant HII regions; the Milky Way has very few.) NGC 3603 is along the Carina spiral arm of the Galaxy, at a distance of about 6000-7000 pc (20000-23000 light years) from us. The nebula is ionized by an enormous cluster of hot, young, massive stars (seen shining brightly toward the center of the 2MASS image); this bright core of stars resembles the truly spectacular stellar cluster, R136, which is the source of ionization for 30 Dor. Brandl et al. (1999, A&A, 352, L69) recently imaged the central cluster in the near-infrared with ESO's Very Large Telescope; their star-for-star analysis reveals pre-main sequence stars and low-mass main sequence stars, with ages of less than 1 million years, which shows for the first time that sub-solar mass stars do form along with the most massive stars in such giant starbursts.
































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