460 research outputs found
Halting Planet Migration in the Evacuated Centers of Protoplanetary Disks
Precise Doppler searches for extrasolar planets find a surfeit of planets
with orbital periods of 3-4 days, and no planets with orbital periods less than
3 days. The circumstellar distance, R_0, where small grains in a protoplanetary
disk reach sublimation temperature (~1500 K) corresponds to a period of ~6
days. Interior to R_0, turbulent accretion due to magneto-rotational
instability may evacuate the disk center. We suggest that planets with orbital
periods of 3-4 days are so common because migrating planets halt once this
evacuated region contains the sites of their exterior 2:1 Lindblad resonances.Comment: 9 pages, 1 figure, to appear in ApJ letter
Disks around Hot Stars in the Trifid Nebula
We report on mid-IR observations of the central region in the Trifid nebula,
carried out with ISOCAM in several broad-band infrared filters and in the low
resolution spectroscopic mode provided by the circular variable filter.
Analysis of the emission indicates the presence of a hot dust component (500 to
1000 K) and a warm dust component at lower temperatures (150-200 K) around
several members of the cluster exciting the HII region, and other stars
undetected at optical wavelengths. Complementary VLA observations suggest that
the mid-IR emission could arise from a dust cocoon or a circumstellar disk,
evaporated under the ionization of the central source and the exciting star of
the nebula. In several sources the silicate band is seen in
emission. One young stellar source shows indications of crystalline silicates
in the circumstellar dust.Comment: 4 pages with 1 figur
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
ISO spectroscopy of circumstellar dust in 14 Herbig Ae/Be systems: towards an understanding of dust processing
We present Infrared Space Observatory (ISO) spectra of fourteen isolated
Herbig Ae/Be (HAEBE) stars, to study the characteristics of their circumstellar
dust. These spectra show large star-to-star differences, in the emission
features of both carbon-rich and oxygen-rich dust grains. The IR spectra were
combined with photometric data ranging from the UV through the optical into the
sub-mm region. We defined two key groups, based upon the spectral shape of the
infrared region. The derived results can be summarized as follows: (1) the
continuum of the IR to sub-mm region of all stars can be reconstructed by the
sum of a power-law and a cool component, which can be represented by a black
body. Possible locations for these components are an optically thick,
geometrically thin disc (power-law component) and an optically thin flared
region (black body); (2) all stars have a substantial amount of cold dust
around them, independent of the amount of mid-IR excess they show; (3) also the
near-IR excess is unrelated to the mid-IR excess, indicating different
composition/location of the emitting material; (4) remarkably, some sources
lack the silicate bands; (5) apart from amorphous silicates, we find evidence
for crystalline silicates in several stars, some of which are new detections;
(6) PAH bands are present in at least 50% of our sample, and their appearance
is slightly different from PAHs in the ISM; (7) PAH bands are, with one
exception, not present in sources which only show a power-law continuum in the
IR; their presence is unrelated to the presence of the silicate bands; (8) the
dust in HAEBE stars shows strong evidence for coagulation; this dust processing
is unrelated to any of the central star properties (such as age, spectral type
and activity).Comment: 15 pages, accepted by A&
Imaging Polarimetric Observations of a New Circumstellar Disk System
Few circumstellar disks have been directly observed. Here we use sensitive
differential polarimetric techniques to overcome atmospheric speckle noise in
order to image the circumstellar material around HD 169142. The detected
envelope or disk is considerably smaller than expectations based on the
measured strength of the far-IR excess from this system
Silicate Emission in the TW Hydrae Association
The TW Hydrae Association is the nearest young stellar association. Among its
members are HD 98800, HR 4796A, and TW Hydrae itself, the nearest known
classical T Tauri star. We have observed these three stars spectroscopically
between 3 and 13 microns. In TW Hya the spectrum shows a silicate emission
feature that is similar to many other young stars with protostellar disks. The
11.2 micron feature indicative of significant amounts of crystalline olivine is
not as strong as in some young stars and solar system comets. In HR 4796A, the
thermal emission in the silicate feature is very weak, suggesting little in the
way of (small silicate) grains near the star. The silicate band of HD 98800
(observed by us but also reported by Sylvester and Skinner (1996)) is
intermediate in strength between TW Hya and HR 4796.Comment: 22 pages, 11 figures, LaTeX2e and AAS LaTeX macros v5.0. Accepted for
publication in A
A Keplerian Disk around the Herbig Ae star HD169142
We present Submillimeter Array observations of the Herbig Ae star HD169142 in
1.3 millimeter continuum emission and 12CO J=2-1 line emission at 1.5 arcsecond
resolution that reveal a circumstellar disk. The continuum emission is centered
on the star position and resolved, and provides a mass estimate of about 0.02
solar masses for the disk. The CO images show patterns in position and velocity
that are well matched by a disk in Keplerian rotation with low inclination to
the line-of-sight. We use radiative transfer calculations based on a flared,
passive disk model to constrain the disk parameters by comparison to the
spectral line emission. The derived disk radius is 235 AU, and the inclination
is 13 degrees. The model also necessitates modest depletion of the CO
molecules, similar to that found in Keplerian disks around T Tauri stars.Comment: 10 pages, 2 figures, accepted by A
Signatures of Planets in Spatially Unresolved Disks
Main sequence stars are commonly surrounded by debris disks, composed of cold
dust continuously replenished by a reservoir of undetected dust-producing
planetesimals. In a planetary system with a belt of planetesimals (like the
Solar System's Kuiper Belt) and one or more interior giant planets, the
trapping of dust particles in the mean motion resonances with the planets can
create structure in the dust disk, as the particles accumulate at certain
semimajor axes. Sufficiently massive planets may also scatter and eject dust
particles out of a planetary system, creating a dust depleted region inside the
orbit of the planet. In anticipation of future observations of spatially
unresolved debris disks with the Spitzer Space Telescope, we are interested in
studying how the structure carved by planets affects the shape of the disk's
spectral energy distribution (SED), and consequently if the SED can be used to
infer the presence of planets. We numerically calculate the equilibrium spatial
density distributions and SEDs of dust disks originated by a belt of
planetesimals in the presence of interior giant planets in different planetary
configurations, and for a representative sample of chemical compositions. The
dynamical models are necessary to estimate the enhancement of particles near
the mean motion resonances with the planets, and to determine how many
particles drift inside the planet's orbit. Based on the SEDs and predicted
colors we discuss what types of planetary systems can be
distinguishable from one another and the main parameter degeneracies in the
model SEDs.Comment: 40 pages (pre-print form), including 16 figures. Published in ApJ
200
IRS Spectra of Solar-Type Stars: \break A Search for Asteroid Belt Analogs
We report the results of a spectroscopic search for debris disks surrounding
41 nearby solar type stars, including 8 planet-bearing stars, using the {\it
Spitzer Space Telescope}. With accurate relative photometry using the Infrared
Spectrometer (IRS) between 7-34 \micron we are able to look for excesses as
small as 2% of photospheric levels with particular sensitivity to weak
spectral features. For stars with no excess, the upper limit in a
band at 30-34 m corresponds to 75 times the brightness of our
zodiacal dust cloud. Comparable limits at 8.5-13 m correspond to
1,400 times the brightness of our zodiacal dust cloud. These limits correspond
to material located within the 1 to 5 AU region that, in our solar
system, originates from debris associated with the asteroid belt. We find
excess emission longward of 25 m from five stars of which four also
show excess emission at 70 m. This emitting dust must be located around
5-10 AU. One star has 70 micron emission but no IRS excess. In this case, the
emitting region must begin outside 10 AU; this star has a known radial velocity
planet. Only two stars of the five show emission shortward of 25 \micron
where spectral features reveal the presence of a population of small, hot dust
grains emitting in the 7-20 m band. The data presented here strengthen the
results of previous studies to show that excesses at 25 \micron and shorter
are rare: only 1 star out of 40 stars older than 1 Gyr or % shows an
excess. Asteroid belts 10-30 times more massive than our own appear are rare
among mature, solar-type stars
Crystalline Silicate Feature of the Vega-like star HD145263
We have observed the 8-13 m spectrum (R250) of the Vega-like star
candidate HD145263 using Subaru/COMICS. The spectrum of HD145263 shows the
broad trapezoidal silicate feature with the shoulders at 9.3 m and 11.44
m, indicating the presence of crystalline silicate grains. This detection
implies that crystalline silicate may also be commonly present around Vega-like
stars. The 11.44 m feature is slightly shifted to a longer wavelength
compared to the usual 11.2-3 m crystalline forsterite feature detected
toward Herbig Ae/Be stars and T Tauri stars. Although the peak shift due to the
effects of the grain size can not be ruled out, we suggest that Fe-bearing
crystalline olivine explains the observed peak wavelength fairly well.
Fe-bearing silicates are commonly found in meteorites and most interplanetary
dust particles, which originate from planetesimal-like asteroids. According to
studies of meteorites, Fe-bearing silicate must have been formed in asteroidal
planetesimals, supporting the scenario that dust grains around Vega-like stars
are of planetesimal origin, if the observed 11.44 m peak is due to
Fe-bearing silicates.Comment: accepted for Publication in ApJ
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