2MASS Picture of the Week Archive Captions

Atlas Image mosaic, covering 20´ × 20´ on the sky of Sharpless 311. This nebula is an HII (or ionized hydrogen) region. Within the region is the bright, hot emission-line star HD 64315. Associated with Sharpless 311 is the open star cluster NGC 2467, which is the assumed heart of the much larger Puppis I OB stellar association. Feinstein & Vasquez (1989, A&AS, 77, 321), based on optical photometry of the cluster, emphasize that the stars in NGC 2467 are not all of the same distance, that it is not a true open cluster; they conclude that one star group has a distance consistent with Puppis I, and another is beyond even the more distant Puppis II association. Previous near-infrared studies of NGC 2467 has consisted only of integrated photometry (Persi et al. 1987, A&AS, 70, 437). 2MASS clearly resolves several groupings of stars more heavily obscured in the optical. The 2MASS color-color and color-magnitude diagrams indicate a number of populations in the field. The cyan, blue, and green lines (stellar tracks) on the latter diagram show several young stellar groups at different distances and experiencing different amounts of reddening; the green track represents the two obvious subclusters in the east and northeast, at ~6300 kpc and extinction of 5.5 visual magnitudes (beyond Pup II). (The largest grouping on the latter diagram are foreground stars.) Other, more reddened stars are also in this field near the Galactic plane. Image mosaic by S. Van Dyk (IPAC).


Atlas Image mosaic, covering 48´ × 48´ on the sky of the famous Double Cluster, h+ Persei, aka NGC 869 and NGC 884, respectively. From wide-field optical photometry of the stars in these two open clusters, Keller et al. (2001, AJ, 122, 248) recently found that both clusters share a common distance of about 2.2 kpc (or 7300 light years) from us. The central regions of both clusters have an average extinction of nearly 1.7 visual magnitudes (the reddening is highly variable across the field). With this information Keller et al. find that the clusters also share a common age of about 12.6 Myr. This determination of age, distance, and extinction for the two clusters (similar values were also recently found by Marco & Bernabeu 2001, A&A, 372, 477) is consistent with what is seen in the 2MASS near-infrared color-color and color-magnitude diagrams. (The green line on the 2MASS color-magnitude diagram is the 12.6 Myr solar-metallicity isochrone; the two other features are stars in the field not associated with the double cluster.) Purplish known persistence artifacts trail (in decreasing brightness) either due north or due south (or both!) from the brightest stars in the 2MASS image. Image mosaic by S. Van Dyk (IPAC).


Atlas Image, covering 8´ × 8´ on the sky of NGC 3079. This nearly edge-on SBc peculiar-type galaxy, at a distance of about 18 Mpc (58.7 million light years) exhibits several rather unusual properties: It is quite dusty, as can even be seen in the 2MASS image; it has a relatively perturbed morphology; it possesses an active nucleus (AGN); and, it also shows a superbubble of hot gas emerging from its disk. The superbubble, seen in the X-rays, radio, and in optical line emission, is driven by a vigorous wind, thought to arise from the AGN (Cecil et al. 2001, ApJ, 555, 338). Previous near-infrared high-resolution imaging of the galactic central region by Israel et al. (1998, A&A, 336, 433) show that the nucleus is obscured by about six visual magnitudes and confirm that it is surrounded by dense molecular gas. Image mosaic by S. Van Dyk (IPAC).


Atlas Image, covering 5´ × 5´ on the sky of NGC 2022. This is an example of a planetary nebula, having a small ring-like, or elliptical, appearance, as we see it. This nebula was previously imaged in the near-infrared by Latter et al. (1995, ApJS, 100, 159). From infrared spectra Hora, Latter, & Deutsch (1999, ApJS, 124, 195) find that the light from NGC 2022 is dominated by emission lines of neutral hydrogen. Planetary nebulae are formed as stars, like our Sun, reach the end of their lives and lose their outer envelopes to the interstellar medium. The hot central star (which can be faintly seen in the 2MASS image toward the center of the reddish ring), 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 15´ × 15´ on the sky of Gum 25, aka RCW 40. This is a bright southern HII, or ionized hydrogen, region, with a cluster of hot, young, massive stars that are providing the ionizing ultraviolet photons. Based on integrated photometry of the region and a distance of 1.7 kpc, Copetti (2000, A&AS, 147, 93) estimates that the ionizing flux is at least ~1.58×1048 s-1, which is a factor of ~1300 smaller than that from the giant HII region NGC 3603. The 2MASS color-color and color-magnitude diagrams show that the young massive stars in Gum 25 are consistent with this distance and an extinction of about 3.6 visual magnitudes. The nebula is clearly dusty, as can be seen in the 2MASS image. A comparison with an optical image shows that much of the ionizing star cluster is obscured by dust. Only in the 2MASS image is the stellar populations in the cluster adequately revealed. Image mosaic by S. Van Dyk (IPAC).

































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