2MASS Picture of the Week Archive Captions

Atlas Image mosaic, covering 13.5´ × 13.5´ on the sky, of the southern HII region RCW 36. This region is associated with a molecular cloud in which the star cluster, seen in the 2MASS image, is embedded. RCW 36 was previously imaged in the near infrared (at 2 µm) by Braz & Epchtein (1982, A&A, 111, 91), where they resolved five sources. Many more sources are seen within the dust and nebulosity of the region. To see a 2MASS color-color diagram for these young stars (the inner 2´), click here. The cluster appears to be behind at least 10 magnitudes of visual extinction. Image mosaic by S. Van Dyk (IPAC).


Atlas Image, covering 6.0´ × 6.0´ on the sky, of the star AG Car. This star is one of the prototypical luminous blue variable stars, or LBVs, which are massive, evolved supergiant stars, in life-ending transition from an O star, with a mass 40 solar masses, to a Wolf-Rayet star. AG Car, at a distance of 6 kpc (19560 light years), has over 1 million times the luminosity of the Sun, and changes in spectral type from early A to late O; the star has varied greatly in luminosity over the last two decades. More importantly, it shows a dusty circumstellar nebula, visible in the 2MASS image as the blue ring around the star. Smith et al. (1997, MNRAS, 290, 265), from a compositional study of the nebula, conclude that AG Car probably experienced an earlier, brief red supergiant phase, when it ejected its hydrogen-rich outer layers to form the ~1 pc-diameter ring nebula; the ring matter has subsequently been swept up by the present hot supergiant wind. (The purplish "star" to the north of AG Car's diffraction spike is a known persistence artifact; a similar red artifact can be seen to the north of the very red star, which is to the southwest of AG Car.)


Breaking ranks with "tradition," we feature a diagram which details the 2MASS galaxy morphology "trident" sequence. The galaxy images represent typical face-on orientations for each Hubble type. Angular sizes range from 50´´ to 100´´ (for comparison, the 2MASS "beam" size is ~3´´). The integrated brightness ranges between 10.0 and 11.0 mag at 2.2 µm (flux densities of 30 to 70 mJy, respectively). Classifications are primarily derived from the Third Reference Catalog of of Bright Galaxies (RC3; de Vaucouleurs et al. 1991, Berlin: Springer). The image and associated paper are recently published in Jarrett (2000, PASP, 112, 1008).


Atlas Image Mosaic, covering 9.0´ × 9.0´ on the sky, of the Serpens Nebula star formation region. This region, at a distance of only 310 pc (1011 light years; De Lara et al. 1991, A&A, 243, 139), is a reflection nebula around a cluster of very young stars embedded in the natal molecular cloud. It is also collection of molecular outflows from these young stellar objects; Hurt & Barsony (1996, ApJ, 460, L45) found examples of the short-lived earliest protostellar stage. The Serpens molecular cloud is one of the most spectacular protostellar nurseries in the Milky Way Galaxy, with a very high stellar density (Eiroa & Casali 1992, A&A, 262, 468). Giovannetti et al. (1998, A&A, 330, 990) found from near-infrared imaging of the nebula that two recent bursts of star formation have occurred, one 105 years ago, and the other is around 3 × 106 years ago.


Atlas Image, covering 5.5´ × 5.5´ on the sky, of the barred irregular galaxy NGC 4449. This galaxy, at a distance of 3.9 Mpc (12.7 million light years) is about 1.4 times more luminous than the Large Magellanic Cloud and is forming stars about twice the rate, as seen in the many optically blue stars and ionized hydrogen in the main body of the galaxy (e.g., Hunter, Walker, & Wilcots 2000, AJ, 119, 668). In the 2MASS near-infrared image the bar is evident, as well as several knots of emission from stellar associations, where large concentrations of massive, evolved red supergiants exist. The galaxy is unusual, in the amount of extended neutral hydrogen that envelopes the optical- and near-infrared-bright emission.































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