Atlas Image, covering 8.3´ × 17.1´ on the sky,
of the nebula RCW 87. Also known as IRAS 15015-5720 and
misclassified originally as a planetary nebula (Acker et al. 1987, A&AS, 71,
163), the nebula is more clearly an HII region (ionized nebula) and
young star cluster embedded in a dark dust cloud, situated less than 1°
from the Galactic Plane. The paucity of stars around the nebula, even in the
near-infrared, imply a high extinction toward this region. The nebula appears
to have a distinct boundary. The near-infrared emission from the nebula is
likely a combination of light from the bright young stars in the cluster
reflected by dust and possibly 2.12µ emission from the molecular hydrogen
gas from which the stars recently formed and still appear to be forming; along
the northwest edge of the nebula appears to be what could be an embedded young
stellar object. About 4.5´ to the northeast of the nebula's center
is another smaller embedded nebula, which could be the site of ongoing star
formation as well. Little is known about this probable recent star formation
site, which deserves further study. 2MASS is revealing many poorly-studied
or previously-unknown star formation regions, such as this one.
Atlas Image mosaic,
covering 6.0´ × 6.0´ on the sky,
of the source Cassiopeia A. Cas A, for short, has a faint, ghostly
shell-like
appearance in the 2MASS image, with several brighter filaments.
Cas A was first discovered in the late '40s by radio astronomers as the
brightest radio object in the sky at 1 GHz. It is now understood to be
possibly the youngest supernova remnant in the Milky Way Galaxy! No
clear records of the supernova's appearance in the optical sky exist
historically, as is the case for the Crab Nebula (the Chinese 'guest star' of
1054 AD), but it has been linked to a 6th magnitude star, no longer visible
near the constellation Cassiopeia, in a catalogue by Flamsteed from 1680 AD.
The inference is that Cas A
may have been a subluminous supernova, resulting from the explosion of a
Wolf-Rayet
star; this picture is motivated by the chemical composition of the
various fast and slow moving knots in the remnant, which, along with analyses
of the radio and X-ray emission, are consistent with
the explosion of a very massive star with very little hydrogen in its
envelope. Cas A is at a distance of ~3.4 kpc (11,000 ly); at only ~2°
from the Galactic Plane, it is seen behind ~5-6 mag of visual
extinction (Hurford & Fesen 1996, ApJ, 469, 246).
Most of the energy from Cas A is, in fact, radiated in the infrared.
The mid-infrared emission, as seen by the recent ISO satellite, is mostly
thermal dust emission, with some line emission, and is associated primarily
with the fast-moving knots, suggesting that dust is condensing out in the
expanding supernova ejecta (Lagage et al. 1996, A&A, 315, L273).
Image mosaic by S. Van Dyk (IPAC).
Atlas Image collage of Uranus, Neptune, and their moons. Uranus and
Neptune were observed serendipitously and separately by the 2MASS Southern
Facility, during routine
operations on 1998 June 7 and June 11 UT, respectively. Both planets appear
very blue, i.e., they are not nearly as bright in the Ks band as in
the shorter wavelength bands, due to more reflection of sunlight at short
wavelengths and to absorption of light by methane gas in their atmospheres.
The moons, or satellites, of both planets have icy surfaces with no (or very
little) atmospheres. They
simply reflect sunlight by various amounts, depending on the albedo, or
reflectivity, of each moon's surface. Umbriel has a surprisingly dark, icy
surface. Miranda is the innermost and smallest of the five large Uranian moons.
Triton is odd, in that it is in a highly-inclined retrograde orbit around
Neptune, leading planetary scientists to infer that Triton was captured by
Neptune's gravity. For more information about these and other planets, see
NASA/JPL's Welcome to the Planets
and NASA/GSFC's
Planetary Fact Sheets.
Moon identification in and further analysis of these 2MASS images by
B. Nelson (IPAC).
Atlas Image mosaic,
covering 6.7´ × 6.7´ on the sky, of the Herbig Ae/Be star
LkH 198, at a distance of 600 to 900 pc.
The Herbig Ae/Be stars are intermediate-mass pre-main sequence objects,
showing emission lines in their optical spectra and appearing with associated
nebulosity (they are more massive than their lower-mass counterparts, the
T Tauri stars).
LkH 198 is the bright object at the center of the image;
35´´ north of this star is another
Ae/Be star, V376 Cas. Both stars can be seen in the near-infrared to
be embedded in associated nebulous clouds. (Diffraction spike artifacts can
be seen emanating from both bright objects; latent image artifacts, produced by the mode of the survey scanning, show a
similar pattern of "red stars" both due north and due south of the two stars.)
Both stars are thought to drive a low-velocity bipolar molecular outflow.
LkH 198 also has an embedded infrared companion (unseen
in the 2MASS image). Also seen in the 2MASS image is a blue elliptical loop
associated with LkH 198 and a similar "sickle-shaped"
nebula (weakly) seen to the west of V376 Cas; this extended emission is
consistent with light scattering by small dust grains. A diagram of this
complex region, showing the relationship of its various components, is
provided by Koresko et al. (1997, ApJ, 485, 213; their Figure 7).
Image mosaic by S. Van Dyk (IPAC).
Atlas Image mosaic,
covering 15.3´ × 13.6´ on the sky, of the heavily
reddened young open cluster Trumpler 27. This relatively unstudied cluster
lies about 5° from the Galactic Center at a distance of 1.65 kpc from us
(Bakker & The 1983, A&AS, 52, 27), behind a visual extinction of 4 mag or more.
The brightest star in the 2MASS image is a M0 supergiant; the next brightest
star, to the northwest, is thought to be a Cepheid variable, due to its
proximity to the instability strip on the H-R Diagram. The bright reddish star
to the northeast of the image center is a WC9 Wolf-Rayet star with a very large
infrared excess, due to thermal reemission of the star's ultraviolet photons
by dust grains local to the star. The cluster, however, is dominated by young,
massive O- and B-type main sequence stars. An optical photometric estimate
of the cluster's age is about 107 yr (Battinelli et al. 1994,
A&AS, 104, 379).
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