2MASS Picture of the Week Archive Captions

Three-color composite Atlas Image Mosaic, covering 0.83° × 0.97° on the sky, of the intermediate-age Galactic supernova remnant IC 443, which is at a distance of about 1.5 kpc. What can be seen over this large area are two regions of near-infrared emission from the remnant. The bright bluish arc to the northeast appears to be line emission from excited iron in these remnant filaments, bright in the J band (Rho et al., in preparation). Along the south, from east to west, is the interaction of the remnant with the nearby molecular cloud. IC 443 offers a unique laboratory for studying such interactions. The supernova shock is exciting 2.12-micron H2 molecular line emission, very bright in the Ks band (Richter, Graham, & Wright 1995, ApJ, 454, 277). Some H2 emission is also seen weakly to the north. The emission from the partially dissociative "J-type" shock-cloud interaction is complex and clumpy. The overall structure of the remnant, as seen by 2MASS, follows both the radio and X-ray emission contours, showing that the radiation in all these wavelength regimes seems to arise from the same regions in the remnant. Analysis of this large-area 2MASS mosaic will provide important insight into the interaction of supernova remnants with their immediate environment, and therefore into the probable nature of the supernova remnant's stellar progenitor. (See also Burton 1987, QJRAS, 28, 269; Burton et al. 1988, MNRAS, 231, 617; Rho et al. 2001, ApJ, 547, 885.) These data are part of the 2MASS Spring 1999 Incremental data release. Image mosaic by E. Kopan (IPAC).


Messier 8: The Lagoon Nebula. The red emission in the optical image is from hydrogen, after recombination, in this famous H II region. Note the absence of the nebular hydrogen emission in the 2MASS image. The Hourglass Nebula (see the 2MASS Image Gallery) is clearly visible in both images, toward the center.
Image mosaic by E. Kopan (IPAC).


Extremely Young Stellar Objects in L1551. The Lynds 1551 (L1551) dark cloud region contains several very young low-mass stellar objects in the process of forming stars. At the earliest phases of formation, nascent stars are deeply embedded in dense dust cores and molecular gas. Their emission is reprocessed into long-wavelength far-IR and sub-mm radiation. Even at near-IR wavelengths, the young stars are not detectable as point sources, but associated nebular and reflected emission can be detected, as in this 2MASS composite three-band image. The early evolution of young stars is accompanied by at least one episode of jet-like mass loss (Edwards et al. 1986, ApJ, 307, L65). The bright yellow shrouded source toward the center of the image is the well-known IRS 5; the reddish object to the northeast is L1551-NE. Both sources are almost certainly deeply-embedded forming stars. The blue-greenish emission fanning out to the southwest of IRS 5 is a gas outflow from that source. IRS 5 is evolved enough that its jet-like outflow has possibly excavated a cavity with large opening angle in the ambient medium (Hodapp & Ladd 1995, ApJ, 453, 715). The oblate shape of IRS 5 may be due to a circumstellar disk, which is perpendicular to the outflow (Strom et al. 1985, AJ, 90, 2575). L1551-NE is likely younger and also has fan-shaped nebulosity extending to the southwest; but much of the nebulosity is likely reflected light, possibly tracing out an outflow-blown cavity.
The two bright stars to the north are the unusual T Tauri stars HL and XZ Tau. HL Tau has a circumstellar accretion disk (Close et al. 1997, ApJ, 478, 766). Both stars are associated with HH30 and other Herbig-Haro objects. The star to the southwest of those two stars is LkH358. All of these objects are about 105 years old and at a distance of about 140 pc (456 light years).


NGC 7419, a heavily-reddened Cepheus open star cluster. The cluster in the near-IR is highlighted by the five bright red supergiants. Beauchamp, Moffat, & Drissen (1994, ApJS, 93, 187) derived an age of 14 ± 2 million yr and a distance of 2.3 kpc from theoretical evolutionary tracks, and found that the cluster is dynamically relaxed. Beauchamp et al. also found that there is at most one blue supergiant member, which contradicts theoretical expectations. The brightest star in the 2MASS image is the M7.5 OH/IR star MY Cephei. The yellowish bright star to the southeast is the carbon star MZ Cephei, which has Ks=3.7, J-H~1.7, H-K~1.6. The red "stars" just to the northeast of the brightest stars are filter glint artifacts, and the "fuzzy" bluish "stars" due north of these bright stars are latent image ("ghost") artifacts, produced by the mode of the survey scanning.


The Crab Nebula, or Messier 1, is one of the most spectacular and intensively studied objects in the sky. It is the remnant of a supernova in AD 1054, observed as a "guest star" by the Chinese in today's constellation Taurus. It is among the brightest remnants across a broad wavelength spectrum. The Crab Nebula is probably the best-known synchrotron emission nebula. The synchrotron is what is primarily seen in the 2MASS image. In addition,the central power-law source, the Crab pulsar, is photo-exciting line emission. The gas in the nebula has not yet mixed with the interstellar medium, and so study of the line-emitting gas gives us insight on the progenitor star. The blue-green [Fe II] 1.644 µm emission is excited in the optically thick filaments in the Crab Nebula by the power-law photoionization source within; H2 seen as K-band emission in the filaments could not normally survive in these nebular conditions and must have formed early in the remnant's expansion when densities were higher (Graham, Wright, & Longmore 1990, ApJ, 352, 172).
































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