2MASS Picture of the Week Archive Captions

Atlas Image mosaic, covering 6.7´ × 6.7´ on the sky, of the T-type methane brown dwarf, Gliese 570D (Gl 570D), companion to the Gl 570ABC triple star system. Gl 570D is widely separated from the main system and is indicated with an arrow. Its brightness is overwhelmed by Gl 570ABC, seen as the two bright stars in the 2MASS image (the Gl 570BC binary system is unresolved in the image). T dwarfs are so cool that the methane in the object's atmosphere dominates the absorption spectrum, which peaks in the near-infrared. The strong methane absorption, particularly at 2 µm, results in less light in the Ks band, relative to J and H, such that the dwarfs look quite blue in the near-infrared. Their appearance is very similar to the methane-rich gas planets in our own solar system, Uranus and Neptune. Their color and relative faintness make them distinct in the 2MASS database. What makes Gl 570D so particularly interesting is that it is significantly cooler and less luminous than any other known brown dwarf, including the prototype T dwarf, Gl 229B. Burgasser et al. (2000, ApJ, 531, L57) find that temperature and luminosity of Gl 570D is 750 K and 2.8 × 10-6 the luminosity of the Sun, respectively, and they infer a mass for the brown dwarf of ~50 Jupiter masses.


Atlas Image mosaic, covering 5.0´ × 5.0´ on the sky, of the molecular outflow source and star forming region W75N (also known as Cyg X FIR 33). W75N is embedded in a molecular cloud, at ~2 kpc distance from us, that is part of the larger complex of dense molecular clouds known as Cygnus X. Previous near-infrared imaging of W75N, made with a 7.8´´ circular beam, revealed a number of continuum sources and extended 2 µm-bright reflection nebula (Moore et al. 1988, MNRAS, 234, 95) or nebulae (Moore et al. 1991, MNRAS, 248, 377). Although the properties of the embedded luminous sources cannot be directly assessed, but must be gathered from the reflected nebular light, the bright continuum sources are likely ultracompact HII regions, containing one or more very young massive stars, and comprise a recently-formed cluster of stars in W75. The extinction in the nebulae toward the embedded sources is at least visual magnitude 15, but may be > 100 in the densest regions (Moore et al. 1991). 2MASS has covered the Cygnus X region and included much of it in the Second Incremental Data Release. Image mosaic by S. Van Dyk (IPAC).


Atlas Image mosaic, covering 6.0´ × 6.0´ on the sky, of the Hickson Compact Group 40 (HCG 40, aka Arp 321 and VV 116; Hickson 1982, ApJ, 255, 382). This isolated ensemble of seven galaxies (five of which are clearly seen in the 2MASS Image), at a redshift z=0.022, like the other HCGs, provides an interesting laboratory for studying the effects of close proximity and possible interaction on the evolution of galaxies. For instance, how these factors influence the presence of active galactic nuclei (e.g., Coziol et al. 1998, ApJ, 493, 563). HCG 40 is dominated by a giant elliptical galaxy (seen toward the center of the image). The giant elliptical and the two smaller spiral galaxies all show some levels of nuclear activity. The HCGs also offer the possibility to measure the amount of dark matter in the groups through their relative motion and the inferred mass-to-light ratios (M/L) for the groups. A kinematical study of the group suggest that it is a dynamical and compact septet of galaxies, covering only 749 kpc (Ribiero et al. 1998, ApJ, 497, 72). HCG 40's self-gravitating M/L~33, along with the ratios of other HCGs, imply a cosmological density parameter 0~0.2-0.4 (Ribiero et al.). The compactness of HCG 40 implies that it will evolve through strong galaxy merging on a timescale of a few group crossing times. Most of the HCGs have been observed by 2MASS so far; many of these, included HCG 40, are included in the Second Incremental Data Release. HCG 40 was previously imaged in the near-IR by Bushouse & Stanford (1992, ApJS, 79, 213). The near-IR light of galaxies is dominated by the major mass components and, therefore, can best be used to study the relationship between galaxy interaction, activity, and morphology. Image mosaic by S. Van Dyk (IPAC).


Atlas Image, covering 5.0´ × 5.0´ on the sky, of the planetary nebula NGC 3242. This nebula has an elliptical shape and several interesting structural features, including the inner bright elliptical ring, the two extensions to the ring, or ansae, that are placed roughly along the major axis of the elliptical emission, and the larger faint halo that envelops the inner structure (Hora, Latter, & Deutsch 1999, ApJS, 124, 195). The bright ring is pure continuum emission, while the faint halo is (dust) scattered light. Planetary nebulae are formed as low-mass stars, like our Sun, reach the end of their lives and lose their outer envelopes (for this nebula, what are now the ring and halo structures) to the interstellar medium. The bright source within the bright elliptical ring is the hot central star of the planetary nebula, originally the core of the dying star, which will eventually become a white dwarf and cool off over billions of years. These data are included in the Second Incremental Release!


Atlas Image mosaic, covering 10.0´ × 10.0´ on the sky, of the infrared source LDN 1641 S 4. This object, also known as GGD 7 (Gyulbudaghian, Glushkov, & Denisyuk 1978, ApJ, 224, L137), at a possible distance of about 500 pc, is part of a group of nebulae in Lynds dark cloud (LDN) 1641, in the Orion complex. The region was previously imaged in the near-infrared by Carballo, Eiroa, & Mampaso (1988, MNRAS, 232, 497); they identified five infrared sources (IRS 1-5) in this field. These sources are the five brightest clustered toward the center of the 2MASS image. Four of the sources (IRS 1-4) are clearly embedded in nebulosity. A dark dust lane runs between IRS 3 and 4. Carballo et al. suspected that IRS 1-3 were variable; comparison of their JHK photometry with the 2MASS values comfirms this, as well as possible variability for IRS 5. Carballo et al. concluded that IRS 2 and 3 are likely pre-main-sequence stars. Clearly, from the 2MASS image, there are many more stellar sources embedded in this dark cloud of young pre-stellar objects, some (the reddest ones) obscured by more than 20 visual magnitudes of extinction. Image mosaic by S. Van Dyk (IPAC). These data are included in the Second Incremental Release!
































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