2MASS Picture of the Week Archive Captions

Atlas Image mosaic, covering 16´ × 16´ on the sky of the open cluster NGC 2420. This cluster lies farther from the Galactic center than does the Sun, but is also far higher above the Galactic plane. It has been featured as an interesting object analyzed by the WIYN Open Cluster Study on the NOAO Press page. Freeland et al. (2002, in prep.), based on precise optical photometry, find that the cluster, surprisingly, has solar abundance, even at its position in the Milky Way. Grocholski & Sarajedini (2002, AJ, 123, 1603), based partly on 2MASS data, measure the distance and visual extinction to the cluster, and its age. The distance they find, based on the presence of "red clump" stars in the cluster, to be about 2.6 kpc (8575 light years) from us, with rather low visual extinction (0.15 mag), and an age of about 1.6 Gyr. This makes NGC 2420 a rather old open cluster. These parameters are evident from the 2MASS color-color and color-magnitude diagrams. The green track on the latter diagram is a solar-metallicity isochrone with the appropriate age; the red clump is the small group of stars at J-Ks~0.6, Ks~10.3 mag.


Atlas Image, covering 7´ × 7´ on the sky of Hickson Compact Group (HCG) 91, one of several small galaxy groups catalogued by Hickson (1982, ApJ, 255, 382). This group is dominated by the galaxy triple system, VV 700, the major member of which is the nearly face-on spiral galaxy NGC 7214 (seen toward the bottom of the 2MASS image). A small spiral appears to be a close companion of NGC 7214. The smaller, also face-on spiral in VV 700 near the center of the image is ESO 467-G013. Also part of HCG 91 to the north is the edge-on spiral ESO 467-G015. The group is at a redshift of 0.024 (recession velocity of about 7200 km/s). 2MASS is generally sensitive to galaxies and galaxy groups out to this distance and somewhat beyond. This galaxies in the group, in fact, are included in the Second Incremental Data Release Extended Source Catalog. More information about HCG 91 can be obtained at the NASA/IPAC Extragalactic Database (NED).


Atlas Image mosaic, covering 5´ × 5´ on the sky of the variable star QX Puppis, aka the Calabash Nebula, Rotten Egg Nebula, OH 231.8+4.2, in the open cluster Messier 46. This nebula is associated with an evolved star, which is probably in the proto- or pre-planetary nebula phase, although this is still controversial. (Examples of planetary nebulae can be found here and here.) The nebula is a source of molecular OH maser emission and has a bipolar morphology, although one pole is clearly more extended than the other. A connection still needs to be made between the end of the life of an asymptotic giant branch star and the formation of a planetary nebula. Bujarrabal et al. (2002, A&A 389, 271) and Latter et al. (2002, in preparation) may have made it through imaging of the Rotten Egg using the Hubble Space Telescope; the spectacular optical and near-infrared results show jets and an disk or torus obscuring the central star, as the Calabash evolves to a bipolar planetary nebula.


Atlas Image, covering 5´ × 5´ on the sky of NGC 6153. Against the glare of the infrared-bright K giant, HD 148705, to the northeast sits this purplish glowing neon lamp, that is a planetary nebula. Pottasch et al. (1984, ApJ, 312, L33) showed that from its IRAS infrared spectrum, NGC 6153 shows the highest known nebular neon abundance. In fact, Pottasch, Dennefield & Mo (1986, A&A, 312, 397) showed that it has higher abundances of almost all measurable elements than any other planetary nebula. Planetary nebulae are formed as low-mass stars, like our Sun, reach the end of their lives and lose their outer envelopes to the interstellar medium. Within the purple ring of NGC 6153 is the hot central star, originally the core of the dying star, which will eventually become a white dwarf and cool off over billions of years.


Atlas Image mosaic, covering 11´ × 11´ on the sky of the Bubble Nebula, NGC 7635. The Bubble has been blown by the ionizing photons and stellar winds from the massive star BD+60° 2522 (the bright blue star just south of the image center), along with other massive stars in the Sharpless 162 complex. The Nebula has also been imaged by the Hubble Space Telescope. To the star's west is a cloud of dust and molecular gas that must also be feeling the pressure and excitation of the stars. This long, cigar-shaped cloud is particularly bright in the Ks band of 2MASS.
































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