113 research outputs found
Molecular Excitation and Differential Gas-Phase Depletions in the IC 5146 Dark Cloud
We present a combined near-infrared and molecular-line study of 25' x 8' area
in the Northern streamer of the IC 5146 cloud. Using the technique pioneered by
Lada et al 1994, we construct a Gaussian smoothed map of the infrared
extinction with the same resolution as the molecular line observations in order
to examine correlations of integrated intensities and molecular abundances with
extinction for C17O, C34S, and N2H+. We find that over a visual extinction
range of 0 to 40 magnitudes, there is good evidence for the presence of
differential gas-phase depletions in the densest portions of IC 5146. Both CO
and CS exhibit a statistically significant (factor of ~3) abundance reduction
near Av ~ 12 magnitudes while, in direct contrast, at the highest extinctions,
Av > 10 magnitudes, N2H+ appears relatively undepleted. Moreover, for Av < 4
magnitudes there exists little or no N2H+. This pattern of depletions is
consistent with the predictions of chemical theory.Comment: 36 pages (13 figures), accepted by the Astrophysical Journa
MIPS: The Multiband Imaging Photometer for SIRTF
The Multiband Imaging Photometer for SIRTF (MIPS) is to be designed to reach as closely as possible the fundamental sensitivity and angular resolution limits for SIRTF over the 3 to 700μm spectral region. It will use high performance photoconductive detectors from 3 to 200μm with integrating JFET amplifiers. From 200 to 700μm, the MIPS will use a bolometer cooled by an adiabatic demagnetization refrigerator. Over much of its operating range, the MIPS will make possible observations at and beyond the conventional Rayleigh diffraction limit of angular resolution
Circular Polarization of Water Masers in the Circumstellar Envelopes of Late Type Stars
We present circular polarization measurements of circumstellar H_2O masers.
The circular polarization detected in the (6_{16}-5_{23}) rotational transition
of the H_{2}O maser can be attributed to Zeeman splitting in the intermediate
temperature and density regime. The magnetic fields are derived using a
general, LTE Zeeman analysis as well as a full radiative transfer method
(non-LTE), which includes a treatment of all hyperfine components
simultaneously as well as the effects of saturation and unequal populations of
the magnetic substates. The differences and relevances of these interpretations
are discussed extensively. The field strengths are compared with previous
detections of the magnetic field on the SiO and OH masers. We show that the
magnetic pressure dominates the thermal pressure by a factor of 20 or more.Comment: 15 pages, 15 figures; accepted for publication in A&A; (Abstract
Abridged
A multi-transition submillimeter water maser study of evolved stars - detection of a new line near 475 GHz
Context: Maser emission from the H2O molecule probes the warm, inner
circumstellar envelopes of oxygen-rich red giant and supergiant stars.
Multi-maser transition studies can be used to put constraints on the density
and temperature of the emission regions.
Aims: A number of known H2O maser lines were observed toward the long period
variables R Leo and W Hya and the red supergiant VY CMa. A search for a new,
not yet detected line near 475 GHz was conducted toward these stars.
Methods: The Atacama Pathfinder Experiment telescope was used for a
multi-transition observational study of submillimeter H2O lines.
Results: The 5_33-4_40 transition near 475 GHz was clearly detected toward VY
CMa and W Hya. Many other H2O lines were detected toward all three target
stars. Relative line intensity ratios and velocity widths were found to vary
significantly from star to star.
Conclusions: Maser action is observed in all but one line for which it was
theoretically predicted. In contrast, one of the strongest maser lines, in R
Leo by far the strongest, the 437 GHz 7_53-6_60 transition, is not predicted to
be inverted. Some other qualitative predictions of the model calculations are
at variance with our observations. Plausible reasons for this are discussed.
Based on our findings for W Hya and VY CMa, we find evidence that the H2O
masers in the AGB star W Hya arise from the regular circumstellar outflow,
while shock excitation in a high velocity flow seems to be required to excite
masers far from the red supergiant VY CMa.Comment: 9 pages, 4 figures, Astronomy and Astrophyics (in press
Mips: The Multiband Imaging Photometer For SIRTF
The Multiband Imaging Photometer for SIRTF (MIPS) is to be designed to reach as closely as possible the fundamental sensitivity and angular resolution limits for SIRTF over the 3 to 700μm spectral region. It will use high performance photoconductive detectors from 3 to 200μm with integrating JFET amplifiers. From 200 to 700μm, the MIPS will use a bolometer cooled by an adiabatic demagnetization refrigerator. Over much of its operating range, the MIPS will make possible observations at and beyond the conventional Rayleigh diffraction limit of angular resolution
MIPS: The Multiband Imaging Photometer for SIRTF
The Multiband Imaging Photometer for SIRTF (MIPS) is to be designed to reach as closely as possible the fundamental sensitivity and angular resolution limits for SIRTF over the 3 to 700μm spectral region. It will use high performance photoconductive detectors from 3 to 200μm with integrating JFET amplifiers. From 200 to 700μm, the MIPS will use a bolometer cooled by an adiabatic demagnetization refrigerator. Over much of its operating range, the MIPS will make possible observations at and beyond the conventional Rayleigh diffraction limit of angular resolution
Spatially extended K I 7699 emission in the nebula of VY CMa: Kinematics and geometry
Long-slit echelle spectra reveal bright extended emission from the K I
lambda7699 resonance line in the reflection nebula surrounding the extreme red
supergiant VY Canis Majoris. The central star has long been known for its
unusually-bright K I emission lines, but this is the first report of intrinsic
emission from K I in the nebula. The extended emission is not just a reflected
spectrum of the star, but is due to resonant scattering by K atoms in the outer
nebula itself, and is therefore a valuable probe of the kinematics and geometry
of VY CMa's circumstellar environment. Dramatic velocity structure is seen in
the long-slit spectra, and most lines of sight intersect multiple distinct
velocity components. A faint ``halo'' at large distances from the star does
appear to show a reflected spectrum, however, and suggests a systemic velocity
of +40 km/s with respect to the Sun. The most striking feature is blueshifted
emission from the filled interior of a large shell seen in images; the
kinematic structure is reminiscent of a Hubble flow, and provides strong
evidence for asymmetric and episodic mass loss due to localized eruptions on
the star's surface.Comment: 5 pages, 3 figures. Accepted MNRAS. Updated version with minor
change
Disk Evolution in OB Associations - Deep Spitzer/IRAC Observations of IC 1795
We present a deep Spitzer/IRAC survey of the OB association IC 1795 carried
out to investigate the evolution of protoplanetary disks in regions of massive
star formation. Combining Spitzer/IRAC data with Chandra/ACIS observations, we
find 289 cluster members. An additional 340 sources with an infrared excess,
but without X-ray counterpart, are classified as cluster member candidates.
Both surveys are complete down to stellar masses of about 1 Msun. We present
pre-main sequence isochrones computed for the first time in the Spitzer/IRAC
colors. The age of the cluster, determined via the location of the Class III
sources in the [3.6]-[4.5]/[3.6] color-magnitude diagram, is in the range of 3
- 5 Myr. As theoretically expected, we do not find any systematic variation in
the spatial distribution of disks within 0.6 pc of either O-type star in the
association. However, the disk fraction in IC 1795 does depend on the stellar
mass: sources with masses >2 Msun have a disk fraction of ~20%, while lower
mass objects (2-0.8 Msun) have a disk fraction of ~50%. This implies that disks
around massive stars have a shorter dissipation timescale.Comment: Accepted for publication in Ap
Searching for planetary-mass T-dwarfs in the core of Serpens
We searched for isolated planetary-mass T-dwarfs in the 3Myr old Serpens Core
cluster. We performed a deep imaging survey of the central part of this cluster
using the WIRCam camera at the CFHT. Observations were performed through the
narrow-band CH4_off and CH4_on filters, to identify young T-dwarfs from their
1.6micr methane absorption bands, and the broad-band JHK filters, to better
characterize the selected candidates. We complemented our WIRCam photometry
with optical imaging data from MegaCam at CFHT and Suprime-Cam at the Subaru
telescope and mid-IR flux measurements from the Spitzer c2d Legacy Survey. We
report four faint T-dwarf candidates in the direction of the Serpens Core with
CH4_on-CH4_off above 0.2 mag, estimated visual extinction in the range 1-9 mag
and spectral type in the range T1-T5 based on their dereddened CH4_on-CH4_off
colors. Comparisons with T-dwarf spectral models and optical to mid-IR
color-color and color-magnitude diagrams, indicate that two of our candidates
(ID1 and 2) are background contaminants (most likely heavily reddened
low-redshift quasars). The properties of the other two candidates (ID3 and 4)
are consistent with them being young members of the Serpens Core cluster,
although our analysis can not be considered conclusive. In particular, ID3 may
also be a foreground T-dwarf. It is detected by the Spitzer c2d survey but only
flux upper limits are available above 5.8 microns and, hence, we can not assess
the presence of a possible disk around this object. However, it presents some
similarities with other young T-dwarf candidates (SOri70 in the Sigma Ori
cluster and CFHTJ0344+3206 in the direction of IC348). If ID3 and 4 belong to
Serpens, they would have a mass of a few Jupiter masses and would be amongst
the youngest, lowest mass objects detected in a star-forming region so far.Comment: 11 Pages, 7 Figures, Accepted for publication on A&
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