2,434 research outputs found
VLA Imaging of the Disk Surrounding the Nearby Young Star TW Hya
The TW Hya system is perhaps the closest analog to the early solar nebula. We
have used the Very Large Array to image TW Hya at wavelengths of 7mm and 3.6 cm
with resolutions 0.1 arcseconds (about 5 AU) and 1.0 arcseconds (about 50 AU),
respectively. The 7mm emission is extended and appears dominated by a dusty
disk of radius larger than 50 AU surrounding the star. The 3.6 cm emission is
unresolved and likely arises from an ionized wind or gyrosynchrotron activity.
The dust spectrum and spatially resolved 7mm images of the TW Hya disk are
fitted by a simple model with temperature and surface density described by
radial power laws, and . These
properties are consistent with an irradiated gaseous accretion disk of mass
with an accretion rate and viscosity parameter . The estimates of
mass and mass accretion rates are uncertain as the gas-to-dust ratio in the TW
Hya disk may have evolved from the standard interstellar value.Comment: 13 pages, 3 figures, accepted by ApJ Letter
Spitzer IRS Spectra of Luminous 8 micron Sources in the Large Magellanic Cloud: Testing color-based classifications
We present archival Spitzer IRS spectra of 19 luminous 8 micron selected
sources in the Large Magellanic Cloud (LMC). The object classes derived from
these spectra and from an additional 24 spectra in the literature are compared
with classifications based on 2MASS/MSX (J, H, K, and 8 micron) colors in order
to test the "JHK8" classification scheme (Kastner et al. 2008). The IRS spectra
confirm the classifications of 22 of the 31 sources that can be classified
under the JHK8 system. The spectroscopic classification of 12 objects that were
unclassifiable in the JHK8 scheme allow us to characterize regions of the
color-color diagrams that previously lacked spectroscopic verification,
enabling refinements to the JHK8 classification system. The results of these
new classifications are consistent with previous results concerning the
identification of the most infrared-luminous objects in the LMC. In particular,
while the IRS spectra reveal several new examples of asymptotic giant branch
(AGB) stars with O-rich envelopes, such objects are still far outnumbered by
carbon stars (C-rich AGB stars). We show that Spitzer IRAC/MIPS color-color
diagrams provide improved discrimination between red supergiants and
oxygen-rich and carbon-rich asymptotic giant branch stars relative to those
based on 2MASS/MSX colors. These diagrams will enable the most luminous IR
sources in Local Group galaxies to be classified with high confidence based on
their Spitzer colors. Such characterizations of stellar populations will
continue to be possible during Spitzer's warm mission, through the use of IRAC
[3.6]-[4.5] and 2MASS colors.Comment: 31 pages, 10 figures, to be published in A
Molecules in the Circumstellar Disk Orbiting BP Piscium
BP Psc is a puzzling late-type, emission-line field star with large infrared
excess. The star is encircled and enshrouded by a nearly edge-on, dust
circumstellar disk, and displays an extensive jet system similar to those
associated with pre-main sequence (pre-MS) stars. We conducted a mm-wave
molecular line survey of BP Psc with the 30 m telescope of the Institut de
Radio Astronomie Millimetrique (IRAM). We detected lines of 12CO and 13CO and,
possibly, very weak emission from HCO+ and CN; HCN, H2CO, and SiO are not
detected. The CO line profiles of BP Psc are well fit by a model invoking a
disk in Keplerian rotation. The mimumum disk gas mass, inferred from the 12CO
line intensity and 13CO/12CO line ratio, is ~0.1 Jupiter masses. The weakness
of HCO+ and CN (relative to 13CO) stands in sharp contrast to the strong HCO+
and CN emission that characterizes most low-mass, pre-main sequence stars that
have been the subjects of molecular emission-line surveys, and is suggestive of
a very low level of X-ray-induced molecular ionization within the BP Psc disk.
These results lend some support to the notion that BP Psc is an evolved star
whose circumstellar disk has its origins in a catastrophic interaction with a
close companion.Comment: 6 pages, 4 figures; to appear in Astronomy & Astrophysic
The Multiple Young Stellar Objects of HBC 515: An X-ray and Millimeter-wave Imaging Study in (Pre-main Sequence) Diversity
We present Chandra X-ray Observatory and Submillimeter Array (SMA) imaging of
HBC 515, a system consisting of multiple young stellar objects (YSOs). The five
members of HBC 515 represent a remarkably diverse array of YSOs, ranging from
the low-mass Class I/II protostar HBC 515B, through Class II and transition
disk objects (HBC 515D and C, respectively), to the "diskless", intermediate-
mass, pre-main sequence binary HBC 515A. Our Chandra/ACIS imaging establishes
that all five components are X-ray sources, with HBC 515A - a
subarcsecond-separation binary that is partially resolved by Chandra - being
the dominant X-ray source. We detect an X-ray flare associated with HBC 515B.
In the SMA imaging, HBC 515B is detected as a strong 1.3 mm continuum emission
source; a second, weaker mm continuum source is coincident with the position of
the transition disk object HBC 515C. These results strongly support the
protostellar nature of HBC 515B, and firmly establish HBC 515A as a member of
the rare class of relatively massive, X-ray luminous "weak-lined T Tauri stars"
that are binaries and have shed their disks at very early stages of pre-MS
evolution. The coexistence of two such disparate objects within a single,
presumably coeval multiple YSO system highlights the influence of pre- MS star
mass, binarity, and X-ray luminosity in regulating the lifetimes of
circumstellar, planet-forming disks and the timescales of star-disk
interactions.Comment: Accepted for publication in A&A; 11 pages, 5 figure
X-rays from HH210 in the Orion nebula
We report the detection during the Chandra Orion Ultradeep Project (COUP) of
two soft, constant, and faint X-ray sources associated with the Herbig-Haro
object HH210. HH210 is located at the tip of the NNE finger of the emission
line system bursting out of the BN-KL complex, northwest of the Trapezium
cluster in the OMC-1 molecular cloud. Using a recent Halpha image obtained with
the ACS imager on board HST, and taking into account the known proper motions
of HH210 emission knots, we show that the position of the brightest X-ray
source, COUP703, coincides with the emission knot 154-040a of HH210, which is
the emission knot of HH210 having the highest tangential velocity (425 km/s).
The second X-ray source, COUP704, is located on the complicated emission tail
of HH210 close to an emission line filament and has no obvious optical/infrared
counterpart. Spectral fitting indicates for both sources a plasma temperature
of ~0.8 MK and absorption-corrected X-ray luminosities of about 1E30 erg/s
(0.5-2.0 keV). These X-ray sources are well explained by a model invoking a
fast-moving, radiative bow shock in a neutral medium with a density of ~12000
cm^{-3}. The X-ray detection of COUP704 therefore reveals, in the complicated
HH210 region, an energetic shock not yet identified at other wavelengths.Comment: 5 pages, 3 figures; accepted for publication in A&A Letter
Refining Chandra/ACIS Subpixel Event Repositioning Using a Backside Illuminated CCD Model
Subpixel event repositioning (SER) techniques have been demonstrated to
significantly improve the already unprecedented spatial resolution of Chandra
X-ray imaging with the Advanced CCD Imaging Spectrometer (ACIS). Chandra CCD
SER techniques are based on the premise that the impact position of events can
be refined, based on the distribution of charge among affected CCD pixels. ACIS
SER models proposed thus far are restricted to corner split (3- and 4-pixel)
events, and assume that such events take place at the split pixel corners. To
improve the event counting statistics, we modified the ACIS SER algorithms to
include 2-pixel split events and single pixel events, using refined estimates
for photon impact locations. Furthermore, simulations that make use of a
high-fidelity backside illuminated (BI) CCD model demonstrate that mean photon
impact positions for split events are energy dependent leading to further
modification of subpixel event locations according to event type and energy,
for BI ACIS devices. Testing on Chandra CCD X-ray observations of the Orion
Nebula Cluster indicates that these modified SER algorithms further improve the
spatial resolution of Chandra/ACIS, to the extent that the spreading in the
spatial distribution of photons is dominated by the High Resolution Mirror
Assembly, rather than by ACIS pixelization.Comment: 23 pages, 8 figures, 2nd version, submitted to Ap
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