2MASS Picture of the Week Archive Captions

Atlas Image, covering 8.3´ × 17.1´ on the sky, of the nebula RCW 87. Also known as IRAS 15015-5720 and misclassified originally as a planetary nebula (Acker et al. 1987, A&AS, 71, 163), the nebula is more clearly an HII region (ionized nebula) and young star cluster embedded in a dark dust cloud, situated less than 1° from the Galactic Plane. The paucity of stars around the nebula, even in the near-infrared, imply a high extinction toward this region. The nebula appears to have a distinct boundary. The near-infrared emission from the nebula is likely a combination of light from the bright young stars in the cluster reflected by dust and possibly 2.12µ emission from the molecular hydrogen gas from which the stars recently formed and still appear to be forming; along the northwest edge of the nebula appears to be what could be an embedded young stellar object. About 4.5´ to the northeast of the nebula's center is another smaller embedded nebula, which could be the site of ongoing star formation as well. Little is known about this probable recent star formation site, which deserves further study. 2MASS is revealing many poorly-studied or previously-unknown star formation regions, such as this one.


Atlas Image mosaic, covering 6.0´ × 6.0´ on the sky, of the source Cassiopeia A. Cas A, for short, has a faint, ghostly shell-like appearance in the 2MASS image, with several brighter filaments. Cas A was first discovered in the late '40s by radio astronomers as the brightest radio object in the sky at 1 GHz. It is now understood to be possibly the youngest supernova remnant in the Milky Way Galaxy! No clear records of the supernova's appearance in the optical sky exist historically, as is the case for the Crab Nebula (the Chinese 'guest star' of 1054 AD), but it has been linked to a 6th magnitude star, no longer visible near the constellation Cassiopeia, in a catalogue by Flamsteed from 1680 AD. The inference is that Cas A may have been a subluminous supernova, resulting from the explosion of a Wolf-Rayet star; this picture is motivated by the chemical composition of the various fast and slow moving knots in the remnant, which, along with analyses of the radio and X-ray emission, are consistent with the explosion of a very massive star with very little hydrogen in its envelope. Cas A is at a distance of ~3.4 kpc (11,000 ly); at only ~2° from the Galactic Plane, it is seen behind ~5-6 mag of visual extinction (Hurford & Fesen 1996, ApJ, 469, 246). Most of the energy from Cas A is, in fact, radiated in the infrared. The mid-infrared emission, as seen by the recent ISO satellite, is mostly thermal dust emission, with some line emission, and is associated primarily with the fast-moving knots, suggesting that dust is condensing out in the expanding supernova ejecta (Lagage et al. 1996, A&A, 315, L273). Image mosaic by S. Van Dyk (IPAC).


Atlas Image collage of Uranus, Neptune, and their moons. Uranus and Neptune were observed serendipitously and separately by the 2MASS Southern Facility, during routine operations on 1998 June 7 and June 11 UT, respectively. Both planets appear very blue, i.e., they are not nearly as bright in the Ks band as in the shorter wavelength bands, due to more reflection of sunlight at short wavelengths and to absorption of light by methane gas in their atmospheres. The moons, or satellites, of both planets have icy surfaces with no (or very little) atmospheres. They simply reflect sunlight by various amounts, depending on the albedo, or reflectivity, of each moon's surface. Umbriel has a surprisingly dark, icy surface. Miranda is the innermost and smallest of the five large Uranian moons. Triton is odd, in that it is in a highly-inclined retrograde orbit around Neptune, leading planetary scientists to infer that Triton was captured by Neptune's gravity. For more information about these and other planets, see NASA/JPL's Welcome to the Planets and NASA/GSFC's Planetary Fact Sheets. Moon identification in and further analysis of these 2MASS images by B. Nelson (IPAC).


Atlas Image mosaic, covering 6.7´ × 6.7´ on the sky, of the Herbig Ae/Be star LkH 198, at a distance of 600 to 900 pc. The Herbig Ae/Be stars are intermediate-mass pre-main sequence objects, showing emission lines in their optical spectra and appearing with associated nebulosity (they are more massive than their lower-mass counterparts, the T Tauri stars). LkH 198 is the bright object at the center of the image; 35´´ north of this star is another Ae/Be star, V376 Cas. Both stars can be seen in the near-infrared to be embedded in associated nebulous clouds. (Diffraction spike artifacts can be seen emanating from both bright objects; latent image artifacts, produced by the mode of the survey scanning, show a similar pattern of "red stars" both due north and due south of the two stars.) Both stars are thought to drive a low-velocity bipolar molecular outflow. LkH 198 also has an embedded infrared companion (unseen in the 2MASS image). Also seen in the 2MASS image is a blue elliptical loop associated with LkH 198 and a similar "sickle-shaped" nebula (weakly) seen to the west of V376 Cas; this extended emission is consistent with light scattering by small dust grains. A diagram of this complex region, showing the relationship of its various components, is provided by Koresko et al. (1997, ApJ, 485, 213; their Figure 7). Image mosaic by S. Van Dyk (IPAC).


Atlas Image mosaic, covering 15.3´ × 13.6´ on the sky, of the heavily reddened young open cluster Trumpler 27. This relatively unstudied cluster lies about 5° from the Galactic Center at a distance of 1.65 kpc from us (Bakker & The 1983, A&AS, 52, 27), behind a visual extinction of 4 mag or more. The brightest star in the 2MASS image is a M0 supergiant; the next brightest star, to the northwest, is thought to be a Cepheid variable, due to its proximity to the instability strip on the H-R Diagram. The bright reddish star to the northeast of the image center is a WC9 Wolf-Rayet star with a very large infrared excess, due to thermal reemission of the star's ultraviolet photons by dust grains local to the star. The cluster, however, is dominated by young, massive O- and B-type main sequence stars. An optical photometric estimate of the cluster's age is about 107 yr (Battinelli et al. 1994, A&AS, 104, 379).
































Return to the 2MASS Home Page