2MASS Picture of the Week Archive Captions

Atlas Image, covering 5.8´ × 5.8´ on the sky, of the spiral galaxy NGC 3877, taken as part of routine operations on 1998 Dec 24 UT at the 2MASS Northern Facility on Mt. Hopkins, AZ. What is most interesting about this image is that the supernova SN 1998S was caught in the act in the near-infrared, about 49´´ southwest of the galaxy's nucleus, along the spiral arm! SN 1998S (Li, Filippenko, & Moran 1998, IAUC 6829) is of type II-"narrow" (Schlegel 1990, MNRAS, 244, 269). Supernovae of this subtype tend to show broad emission lines of hydrogen (and other elements) in their optical spectra (thus, the Type II classification), due to the rapid expansion of the hydrogen-rich supernova ejecta. But, atop the broad lines are narrow lines, likely due to emission from the interaction region of the SN shock with slower-moving very dense circumstellar matter lost by the progenitor star in the late stages of its evolution prior to explosion. SN 1998S emits strongly in the near-infrared, primarily due to the broad hydrogen recombination lines, but also due to the first and second overtone bands of CO (the carbon monoxide molecule) in the H and Ks bands (Gerardy et al. 1998, BAAS, 30, 1324). Gerardy et al. find from their near-IR spectra a CO overabundance of ~2 and a temperature in the CO-forming region of 4000 to 4500 K. Their spectra may indicate that molecule formation is common in Type II supernovae; this is important, since molecular emission can be a strong coolant and a first step toward dust formation. The SN had Ks magnitude 13.16, indicating a possible brightening in this and the other near-IR bands since observations by others on 1998 Nov 10 UT (Garnavich et al. 1998, IAUC 7058), which already indicated a large infrared excess had developed then.


Atlas Image, covering 6.8´ × 6.8´ on the sky, of a new globular cluster candidate seen near the Galactic Plane, only 10° away from the Galactic Center. This object was serendipitously discovered in the 2MASS data, and is primarily seen only in the H and Ks bands; hence, the rather reddish color for the cluster stars. The characteristics of the stars and the total extinction to the cluster have not yet been determined, however, further analysis, including follow-up non-2MASS observations, will be undertaken. To date there are 147 known globular clusters associated with our Milky Way Galaxy; if confirmed to be a globular, this new cluster would be number 148. Because these clusters are know to exist in larger numbers towards the center of our Galaxy it is likely that a number are hidden behind dust clouds found in that direction, many of which may be discovered by 2MASS. For more about this discovery, see the abstract for the poster presentation at the 194th Meeting of the American Astronomical Society.


Atlas Image Mosaic, covering 6.7´ × 6.7´ on the sky, of the barred spiral galaxy NGC 2903. These data are part of the online Spring 1999 Incremental data release.


Atlas Image Mosaic, covering 9.8´ × 9.8´ on the sky, of the H II region complex Sharpless 252. Optically, this complex is dominated by HD 42088, the blue star east of center in the mosaic, at the heart of the nebula NGC 2175. H II regions are areas of ionized gas, where electrons have been stripped away from once-neutral hydrogen (H) and other atoms, by the intense ultraviolet radiation from hot, young stars. In the near-infrared we see the young, massive embedded and infrared-bright stars, forming in the other parts of the complex: the bright clusters Sh 252A in the west, Sh 252C north of center in the mosaic, and Sh 252E to the east. The blue star HD 252325 excites the gas that is Sh 252B (between A and C). Faint, extended Ks-bright emission is seen throughout the center of the mosaic. These data are part of the online Spring 1999 Incremental data release.


Atlas Image, covering 6.3´ × 8.2´ on the sky, of the prototype pre-main sequence star T Tauri, which appears in the 2MASS Image Gallery of young stellar objects. T Tau is actually a 0´´.6-separation binary (or possible multiple) star system. However, neither of the fainter red "stars" immediately east of north and west of south of T Tau is the binary companion, but both are actually known Ks filter glint artifacts. (The reddish "stars" trailing to the south of T Tau are also latent image artifacts, produced by the mode of the survey scanning. The diffraction spikes around this bright object are characteristic of the 2MASS optics.) Hind's reflection nebula (NGC 1555) can also be clearly seen to the west, showing a complex of structure. Any extended emission from the inner Burnham's nebula (HH 255) is overwhelmed by the brightness of the central star in this rendition. T Tau, and many other young stellar objects in Taurus and elsewhere, are part of the Spring 1999 data release.
































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