2MASS Picture of the Week Archive Captions

Atlas Image mosaic, covering 10.0´ × 10.0´ on the sky, of the star cluster Lyngå 7. The true nature of this low-galactic-latitude cluster is unclear. The cluster was listed as an open cluster by Lyngå (1987, Lund Catalog of Open Cluster Data, 5th ed.), but Ortolani et al. (1993, A&A, 273, 415), based on a BVI optical color-magnitude diagram, conclude that Lyngå 7 would have to be among the oldest known open clusters. They conclude that it is likely a very young metal-rich globular cluster. Tavarez & Friel (1995, AJ, 110, 223), based on an integrated optical spectrum of the cluster, find that its location (galactocentric distance R0=4.4 kpc; 7.2 kpc from us), kinematics (6±15 km/s), and metallicity ([Fe/H]=-0.62±0.15 dex) make it consistent with the disk globular cluster population, with an age12 billion years. Tavarez & Friel find a reddening to the cluster of E(B-V)=0.73; here we show the 2MASS color-color diagram for the cluster. It is difficult to exactly determine the reddening from this diagram, but it does appear to be modest. Here we show the 2MASS color-magnitude diagram, with Padua theoretical isochrones (Bertelli et al. 1994, A&AS, 106, 275) overlaid. An age of ~12 billion years (green line) is consistent with the near-infrared data, however, with a lower reddening, E(B-V)~0.60. (The diagram also shows that ages of 10 billion years [red line] and 14.5 billion years [blue line] are also consistent with the data, since these age tracks become somewhat degenerate in the near-infrared colors; scatter in the datapoints, due to crowding, is also present; the other bluer stars on the diagram are foreground stellar contamination.) At least one likely dust-enshrouded AGB star, the reddish star (Ks=8.03, J-Ks=4.55, not shown on the diagram) is evident in the cluster (the other reddish "star" just south of the brightest star in the image is a known artifact). Lyngå 7 represents an important link between the oldest open clusters and the youngest globular clusters in the Galaxy. Image mosaic by S. Van Dyk (IPAC). These data are included in the Second Incremental Release!


Atlas Image mosaic, covering 13.0´ × 13.0´ on the sky, of the spiral galaxy NGC 7331 and its apparent neighbors. The recessional velocity of NGC 7331 is only 816 km/sec, while the velocities for the other galaxies in the 2MASS mosaic range from 6315 to 8800 km/sec for NGC 7335, 7336, 7337, and 7340 (in increasing eastward distance from NGC 7331). The highly-inclined NGC 7331, at a distance of 15.1 Mpc, determined using Cepheid variables by Hughes et al. (1998, ApJ, 501, 32), is of Hubble type Sb and has a weak LINER nucleus. "LINER" is low-ionization nuclear emission region and is a characteristic of low-luminosity active galactic nuclei, which are thought to harbor supermassive black holes. Evidence for a nuclear black hole in NGC 7331 was recently reinforced by a ROSAT X-ray detection of a bright source (Stockdale, Romanishin, & Cowan 1998, ApJ, 508, L33). The spiral arms, bulge, and extended disk are evident in the 2MASS image. The disk, in fact, contributes only in a minor way to the mass content of NGC 7331 (Bottema 1999, A&A, 348, 77). Molecular hydrogen is the dominant mass contributor to the interstellar medium in the galaxy's bulge and in a ringlike zone at a distance of ~3.5 kpc from the center; far-infrared emission from dust peaks inside the ring at 100 µm (warm dust), and in the ring at 85 µm (colder dust; Israel & Baas 1999, A&A, 351, 10; Bianchi et al. 1998, MNRAS, 298, L49). The overall far-infrared luminosity of NGC 7331 is similar to that of our Milky Way Galaxy. Image mosaic by E. Kopan (IPAC). These data are included in the Second Incremental Release!


Atlas Image mosaic, covering 8.0´ × 8.0´ on the sky, of the ultracompact HII region RAFGL 5173, also known as G192.16-03.82 and IRAS 05553+1631. This region, at a distance of about 2 kpc (6520 light years) from us, has a luminosity of about 3000 suns, which implies that the region is excited by a very young mid- to early-type B star with a mass of 5 to 10 times the Sun (Shepherd et al. 1998, ApJ, 507, 861). The region is at the center of a very extended pair of Herbig-Haro emission nebulae, HH 396/397. Herbig-Haro (HH) objects are shock-excited visible nebulae powered by outflows from young stellar objects; the emission from HH 396/397, one of the most spatially extended HH complexes, is not nearly as luminous in the near-infrared. Devine et al. (1999, AJ, 117, 2919) conclude from a recent combined optical/near-infrared study of this region that its dynamical age is about 105 years and that it is in a relatively late state of the evolution of a moderate-mass young stellar object, which will indeed become an early B-type star. Image mosaic by S. Van Dyk (IPAC). These data are included in the Second Incremental Release!


Atlas Image mosaic, covering 5.0´ × 5.0´ on the sky, of the planetary nebula NGC 6369. The nebula, noted for its peculiar fishtail-like outer loop morphology in the light of optical emission lines (also seen faintly in this near-infrared image, outside the bright ring of the nebula). This planetary nebula is associated with fast low-ionization emission regions, or FLIERs, which are thought to be discrete, collimated, and highly supersonic ejection events from the very hot central star (Hajian 1997, ApJ, 487, 304), which can be seen at the center of the ring. 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. The pinkish color in the bright ring of the nebula may arise from emission by atomic and molecular hydrogen primarily in the 2.17 µm wavelength (Ks) band, or dust scattering in a combination of all three bands. These data are included in the Second Incremental Release!


Atlas Image mosaic, covering 6.7´ × 6.7´ on the sky, of comet 52P/Harrington-Abell, which was serendipitously observed by 2MASS on 1998 Oct 8 UT and is among twenty comets included in the 2MASS Second Incremental Data Release. Comets, of course, are within our Solar System and orbit the Sun, and therefore do not move sidereally (that is, as the stars do). Astronomers, however, have determined the ephemerides, or orbital parameters, for large numbers of known comets, so their positions in the sky along their orbits can be accurately predicted. Occasionally, comets, asteroids, and even planets can "accidentally" wander into the Survey. Comets reflect sunlight, yet near-infrared photometry can provide useful compositional and structural information about these objects. This comet was seen optically to experience a significant outburst in 1998 July (IAUC 6975) and became unusually bright by August (IAUC 6994), exceeding visual magnitude 11 by 1999 February (IAUC 7113). Its coma diameter was 1-2´ and tail length of about 2´ in 1998 (IAUC 6975). In 2001 December the comet was seen to have split into two (IAUC 7769), following a brightening by more than two magnitudes between 2001 August and September (IAUC 7773).
































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