2MASS Picture of the Week Archive Captions

Atlas Image mosaic, covering 12.3´ × 24.2´ on the sky, of the star-forming region RCW 108. The infrared-bright nebula at the center of the region is a cluster of recently-formed stars still deeply embedded within their natal molecular cloud (Straw et al. 1987, ApJ, 314, 283), only 1.5° below the Galactic Plane. The cluster may have an age of only ~5×105 years. Large amounts of dust obscuration are evident throughout the image around the cluster, with at least 20 visual magnitudes of extinction toward the cluster itself. A number of other young embedded sources are seen within the cloud around the cluster. The molecular and dust cloud, at a distance of ~1.3 kpc (4238 light years), is likely part of the much larger Ara OB stellar association complex. It is possible that this young region was triggered into formation by the energy input into the interstellar medium by the hot winds from the young stars in the older nearby Ara OB1 association.


Atlas Image mosaic, covering 10.0´ × 10.0´ on the sky, of the Herbig Ae/Be star R Coronae Australis. Herbig Ae/Be stars are the intermediate-mass (2 to 8 solar masses) counterparts of T Tauri stars. The emission-line star highlights the Corona Australis molecular cloud complex and is embedded within it. The complex, at a distance of only ~130 pc (424 light years) is one of the nearest star-forming regions. The densest part of the molecular cloud core, containing about 50 solar masses of gas, has high visual extinction, about 35 magnitudes. The bright star just to the southeast of R CrA (at image center) is T CrA, and the two stars to the northeast, enshrouded in bluish reflection nebulosity near the image edge, are TY CrA (northern-most) and HD 176386 (southern-most). Stars are forming throughout the cloud, highlighted by several young stellar objects (YSOs), the most prominent ten or so of which surrounding R CrA have been dubbed the Coronet; starting clockwise from north the brighter reddish YSOs comprising the Coronet are IRS 6, IRS 5, IRS 2, Herbig-Haro (HH) 100 IR, and T Cr A itself. (The "sources" emanating due north and south of R CrA, with decreasing brightness are latent image artifacts, produced by the mode of the survey scanning; diffraction spike artifacts from R CrA are also seen in the image.) A number of other fainter YSOs and HH objects (IRS 10 through 15, HH 99, and HH 104) are also seen; Wilking et al. (1997, AJ, 114, 2029) recently produced a similar, yet deeper near-infrared map of this region and identified the various sources in the cloud. They find reflection or HH nebulae associated with most of the YSOs in the cloud, suggesting nearly coeval star formation in the cloud some 3 million years ago. They also find a shallower reddening vector slope than found for normal interstellar dust, implying the presence of larger than average dust grains throughout the cloud. Image mosaic by S. Van Dyk (IPAC).


Atlas Image, covering 5.0´ × 5.0´ on the sky, of the carbon star V713 Monocerotis. This star, also known as AFGL 935 and IRAS 06230-0930, is an example of asymptotic giant branch (AGB) stars which are surrounded by an expanding shell. The AGB stars have evolved from main sequence stars in the mass range 1.2-1.6 solar masses (Claussen et al. 1987), but a significant population come from 2.5-4 solar mass main sequence stars (Barnbaum, Kastner, & Zuckerman 1991, AJ, 102, 289). The bright red color of the star in the 2MASS image indicates a general infrared excess due to carbon-rich dust formation in the shell. (The fainter red "star" to the south is a latent image artifact, produced by the mode of the survey scanning; diffraction spike artifacts are also seen in the image.) The dust is subject to pressure from the star's radiation and is dragged outward with the atmospheric gas, and is the cause for the mass loss from the star. The evolved star experiences pulsations and is variable with a long period; the variability and the amount of mass loss appears correlated for the carbon stars. Although the dust formation mechanism is not known, it appears that the dust is induced by the stellar pulsations. LeBertre (1997, A&A, 324, 1059) finds that the infrared colors of carbon stars shed light on the processes of dust formation and mass loss. V713 Mon, at a distance of 2240 pc (from its 494-day pulsation period), has a shell expansion velocity of 13.7 km s-1 and a mass-loss rate of 2.8×10-6 solar masses per year. LeBertre finds a correlation between the mass-loss rate for this and other carbon stars and the stars' near-infrared colors. 2MASS is particularly well-suited for not only finding many carbon stars throughout the Galaxy, but can also assist in characterizing the nature of their evolution.


Atlas Image, covering 4.0´ × 4.0´ on the sky, of the Butterfly Nebula. The Butterfly, also known as M 2-9 and IRAS 17028-1004, is thought to be a young planetary nebula, which is the final stage of evolution for stars similar to our Sun. It has a distinctly bipolar structure, evident in the 2MASS image, with a bright central star. (The red "star" directly to the south of the nebula's central star is a known persistence artifact of the infrared-bright star; diffraction spike artifacts are also seen emanating from the bright star.) The star has an optical spectrum of late O- or early B-type, hot enough to ionize the gas in the two bipolar lobes, but it is clear from the star's high brightness in the Ks band that we are more likely seeing emission from a region of concentrated light scattering to the infrared, directly around the star. Within the lobes are a number of knots. The knots emit strongly in the [Fe II] lines, indicating high temperature shocks; in the lobes light is also primarily emitted by hydrogen recombination and continuum light scattering from the central star (Hora & Latter 1994, ApJ, 437, 281). The outer shell structure of the lobes is a well-defined photodissociation region, as H2 is radiatively excited and emits light at 2-2.5 µm. Studying fainter more extended lobes in the optical, Schwarz et al. (1997, A&A, 319, 267) find a distance of ~650 pc and a dynamical age for the nebula of ~1200 yr; they also argue that the central source is a hot, compact possible binary star system. The Hubble Space Telescope has also obtained an interesting view of this nebula.


Atlas Image Mosaic, covering 12.0´ × 12.0´ on the sky, of the infrared-luminous galaxy NGC 4945. Throughout the sky are a number of galaxies that are very luminous in the infrared (the luminosity LFar-IR>1010 L , based on IRAS measurements). These galaxies are now understood as experiencing vigorous star formation, from follow-up observations at optical and other wavelengths. Understanding this star formation at both high and low redshifts, and its relation to possible nuclear activity in these galaxies, is currently an important astronomical problem. The nuclear activity in galaxies is thought to be due to accretion by a supermassive central black hole. A prime example of a nearby luminous galaxy is the nearly edge-on barred NGC 4945. Prominent dust lanes can be seen in the 2MASS image, along the inner bar in the bottom half and along the outer spiral arm in the top half. This apparent asymmetry is a geometric effect, as the dust lanes lie along the leading edge of the bar and along the trailing edge of the spiral arms; at the point of galactic corotation, where the arms join the bar, the dust lanes switch sides. The nucleus of NGC 4945 is the brightest infrared source in the south, after the Magellanic Clouds. Spectra of the nucleus in the near-infrared show a mixture of Seyfert and starburst phenomena (Moorwood & Oliva 1994, ApJ, 429, 602), as well as evidence for activity at a range of wavelengths. Lipari, Tsvetanov, & Macchetto (1997, ApJS, 111, 369) recently made a comparison of the nucleus' optical and near-IR appearance, and find a distinct spatial offset; their conclusion is the active nucleus is dust-obscured and possibly fed by a nuclear starburst, which is driving a superwind, seen in emission at optical wavelengths. Image mosaic by R. Hurt (IPAC).
































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