1,991 research outputs found
Structure and Evolution of the Envelopes of Deeply Embedded Massive Young Stars
The physical structure of the envelopes around a sample of fourteen massive
(1000-100,000 solar L) young stars is investigated on 100- 100,000 AU scales
using maps and spectra in submillimeter continuum and lines of C17O, CS and
H2CO. The total column densities and the temperature profiles are obtained by
fitting self-consistent dust models to submillimeter photometry. Both the
molecular line and dust emission data indicate density gradients ~r^{-alpha},
with alpha=1.0-1.5, significantly flatter than the alpha=2.0 generally found
for low-mass objects. This flattening may indicate that in massive young
stellar objects, nonthermal pressure is more important for the support against
gravitational collapse, while thermal pressure dominates for low-mass sources.
We find alpha=2 for two hot core-type sources, but regard this as an upper
limit since in these objects, the CS abundance may be enhanced in the warm gas
close to the star.Comment: To be published in The Astrophysical Journal. 54 pages including 14
figures Revised version with references adde
The Impact of the Massive Young Star GL 2591 on its Circumstellar Material: Temperature, Density and Velocity Structure
The temperature, density and kinematics of the gas and dust surrounding the
luminous young stellar object GL~2591 are investigated on scales as small as
100 AU, probed by 4.7 micron absorption spectroscopy, to over 60,000 AU, probed
by single-dish submillimeter spectroscopy. These two scales are connected by
interferometric 86-226 GHz images of size 60,000 AU and resolution 2000 AU in
continuum and molecular lines. The data are used to constrain the physical
structure of the envelope and investigate the influence of the young star on
its immediate surroundings.Comment: To be published in The Astrophysical Journal, Vol. 522 No. 2 (1999
Sep 10). 45 pages including 11 figure
The c2d Spitzer spectroscopy survey of ices around low-mass young stellar objects, III: CH4
CH4 is proposed to be the starting point of a rich organic chemistry. Solid
CH4 abundances have previously been determined mostly toward high mass star
forming regions. Spitzer/IRS now provides a unique opportunity to probe solid
CH4 toward low mass star forming regions as well. Infrared spectra from the
Spitzer Space Telescope are presented to determine the solid CH4 abundance
toward a large sample of low mass young stellar objects. 25 out of 52 ice
sources in the (cores to disks) legacy have an absorption feature at 7.7
um, attributed to the bending mode of solid CH4. The solid CH4 / H2O abundances
are 2-8%, except for three sources with abundances as high as 11-13%. These
latter sources have relatively large uncertainties due to small total ice
column densities. Toward sources with H2O column densities above 2E18 cm-2, the
CH4 abundances (20 out of 25) are nearly constant at 4.7+/-1.6%. Correlation
plots with solid H2O, CH3OH, CO2 and CO column densities and abundances
relative to H2O reveal a closer relationship of solid CH4 with CO2 and H2O than
with solid CO and CH3OH. The inferred solid CH4 abundances are consistent with
models where CH4 is formed through sequential hydrogenation of C on grain
surfaces. Finally the equal or higher abundances toward low mass young stellar
objects compared with high mass objects and the correlation studies support
this formation pathway as well, but not the two competing theories: formation
from CH3OH and formation in gas phase with subsequent freeze-out.Comment: 27 pages, 7 figures, accepted by Ap
H2O and OH gas in the terrestrial planet-forming zones of protoplanetary disks
We present detections of numerous 10-20 micron H2O emission lines from two
protoplanetary disks around the T Tauri stars AS 205A and DR Tau, obtained
using the InfraRed Spectrograph on the Spitzer Space Telescope. Follow-up 3-5
micron Keck-NIRSPEC data confirm the presence of abundant water and spectrally
resolve the lines. We also detect the P4.5 (2.934 micron) and P9.5 (3.179
micron) doublets of OH and 12CO/13CO v=1-0 emission in both sources. Line
shapes and LTE models suggest that the emission from all three molecules
originates between ~0.5 and 5 AU, and so will provide a new window for
understanding the chemical environment during terrestrial planet formation. LTE
models also imply significant columns of H2O and OH in the inner disk
atmospheres, suggesting physical transport of volatile ices either vertically
or radially; while the significant radial extent of the emission stresses the
importance of a more complete understanding of non-thermal excitation
processes.Comment: 9 pages, 3 figures, 1 table, aastex, to appear in the Astrophysical
Journa
Modeling Spitzer observations of VV Ser. I. The circumstellar disk of a UX Orionis star
We present mid-infrared Spitzer-IRS spectra of the well-known UX Orionis star
VV Ser. We combine the Spitzer data with interferometric and spectroscopic data
from the literature covering UV to submillimeter wavelengths. The full set of
data are modeled by a two-dimensional axisymmetric Monte Carlo radiative
transfer code. The model is used to test the prediction of (Dullemond et al.
2003) that disks around UX Orionis stars must have a self-shadowed shape, and
that these disks are seen nearly edge-on, looking just over the edge of a
puffed-up inner rim, formed roughly at the dust sublimation radius. We find
that a single, relatively simple model is consistent with all the available
observational constraints spanning 4 orders of magnitude in wavelength and
spatial scales, providing strong support for this interpretation of UX Orionis
stars. The grains in the upper layers of the puffed-up inner rim must be small
(0.01-0.4 micron) to reproduce the colors (R_V ~ 3.6) of the extinction events,
while the shape and strength of the mid-infrared silicate emission features
indicate that grains in the outer disk (> 1-2 AU) are somewhat larger (0.3-3.0
micron). From the model fit, the location of the puffed-up inner rim is
estimated to be at a dust temperature of 1500 K or at 0.7-0.8 AU for small
grains. This is almost twice the rim radius estimated from near-infrared
interferometry. A best fitting model for the inner rim in which large grains in
the disk mid-plane reach to within 0.25 AU of the star, while small grains in
the disk surface create a puffed-up inner rim at ~0.7-0.8 AU, is able to
reproduce all the data, including the near-infrared visibilities. [Abstract
abridged]Comment: 12 pages, accepted for publication in Ap
The Masses of Transition Circumstellar Disks: Observational Support for Photoevaporation Models
We report deep Sub-Millimeter Array observations of 26 pre-main-sequence
(PMS) stars with evolved inner disks. These observations measure the mass of
the outer disk (r ~20-100 AU) across every stage of the dissipation of the
inner disk (r < 10 AU) as determined by the IR spectral energy distributions
(SEDs). We find that only targets with high mid-IR excesses are detected and
have disk masses in the 1-5 M_Jup range, while most of our objects remain
undetected to sensitivity levels of M_DISK ~0.2-1.5 M_Jup. To put these results
in a more general context, we collected publicly available data to construct
the optical to millimeter wavelength SEDs of over 120 additional PMS stars. We
find that the near-IR and mid-IR emission remain optically thick in objects
whose disk masses span 2 orders of magnitude (~0.5-50 M_Jup). Taken together,
these results imply that, in general, inner disks start to dissipate only after
the outer disk has been significantly depleted of mass. This provides strong
support for photoevaporation being one of the dominant processes driving disk
evolution.Comment: Accepted for publication by ApJL, 4 pages and 3 figure
The Double Dust Envelopes of R Coronae Borealis Stars
The study of extended, cold dust envelopes surrounding R Coronae Borealis (RCB) stars began with their discovery by the Infrared Astronomical Satellite. RCB stars are carbon-rich supergiants characterized by their extreme hydrogen deficiency and their irregular and spectacular declines in brightness (up to 9 mag). We have analyzed new and archival Spitzer Space Telescope and Herschel Space Observatory data of the envelopes of seven RCB stars to examine the morphology and investigate the origin of these dusty shells. Herschel, in particular, has revealed the first-ever bow shock associated with an RCB star with its observations of SU Tauri. These data have allowed the assembly of the most comprehensive spectral energy distributions (SEDs) of these stars with multiwavelength data from the ultraviolet to the submillimeter. Radiative transfer modeling of the SEDs implies that the RCB stars in this sample are surrounded by an inner warm (up to 1200 K) and an outer cold (up to 200 K) envelope. The outer shells are suggested to contain up to 10-3 M o of dust and have existed for up to 105 years depending on the expansion rate of the dust. This age limit indicates that these structures have most likely been formed during the RCB phase
From Molecular Cores to Planet-forming Disks: An SIRTF Legacy Program
Crucial steps in the formation of stars and planets can be studied only at mid‐ to far‐infrared wavelengths, where the Space Infrared Telescope (SIRTF) provides an unprecedented improvement in sensitivity. We will use all three SIRTF instruments (Infrared Array Camera [IRAC], Multiband Imaging Photometer for SIRTF [MIPS], and Infrared Spectrograph [IRS]) to observe sources that span the evolutionary sequence from molecular cores to protoplanetary disks, encompassing a wide range of cloud masses, stellar masses, and star‐forming environments. In addition to targeting about 150 known compact cores, we will survey with IRAC and MIPS (3.6–70 μm) the entire areas of five of the nearest large molecular clouds for new candidate protostars and substellar objects as faint as 0.001 solar luminosities. We will also observe with IRAC and MIPS about 190 systems likely to be in the early stages of planetary system formation (ages up to about 10 Myr), probing the evolution of the circumstellar dust, the raw material for planetary cores. Candidate planet‐forming disks as small as 0.1 lunar masses will be detectable. Spectroscopy with IRS of new objects found in the surveys and of a select group of known objects will add vital information on the changing chemical and physical conditions in the disks and envelopes. The resulting data products will include catalogs of thousands of previously unknown sources, multiwavelength maps of about 20 deg^2 of molecular clouds, photometry of about 190 known young stars, spectra of at least 170 sources, ancillary data from ground‐based telescopes, and new tools for analysis and modeling. These products will constitute the foundations for many follow‐up studies with ground‐based telescopes, as well as with SIRTF itself and other space missions such as SIM, JWST, Herschel, and TPF/Darwin
Ices in the edge-on disk CRBR 2422.8-3423: Spitzer spectroscopy and Monte Carlo radiative transfer modeling
We present 5.2-37.2 micron spectroscopy of the edge-on circumstellar disk
CRBR 2422.8-3423 obtained using the InfraRed Spectrograph (IRS) of the Spitzer
Space Telescope. The IRS spectrum is combined with ground-based 3-5 micron
spectroscopy to obtain a complete inventory of solid state material present
along the line of sight toward the source. We model the object with a 2D
axisymmetric (effectively 3D) Monte Carlo radiative transfer code. It is found
that the model disk, assuming a standard flaring structure, is too warm to
contain the very large observed column density of pure CO ice, but is possibly
responsible for up to 50% of the water, CO2 and minor ice species. In
particular the 6.85 micron band, tentatively due to NH4+, exhibits a prominent
red wing, indicating a significant contribution from warm ice in the disk. It
is argued that the pure CO ice is located in the dense core Oph-F in front of
the source seen in the submillimeter imaging, with the CO gas in the core
highly depleted. The model is used to predict which circumstances are most
favourable for direct observations of ices in edge-on circumstellar disks. Ice
bands will in general be deepest for inclinations similar to the disk opening
angle, i.e. ~70 degrees. Due to the high optical depths of typical disk
mid-planes, ice absorption bands will often probe warmer ice located in the
upper layers of nearly edge-on disks. The ratios between different ice bands
are found to vary by up to an order of magnitude depending on disk inclination
due to radiative transfer effects caused by the 2D structure of the disk.
Ratios between ice bands of the same species can therefore be used to constrain
the location of the ices in a circumstellar disk. [Abstract abridged]Comment: 49 pages, accepted for publication in Ap
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