128 research outputs found
A Spatially Resolved Inner Hole in the Disk around GM Aurigae
We present 0.3 arcsec resolution observations of the disk around GM Aurigae
with the Submillimeter Array (SMA) at a wavelength of 860 um and with the
Plateau de Bure Interferometer at a wavelength of 1.3 mm. These observations
probe the distribution of disk material on spatial scales commensurate with the
size of the inner hole predicted by models of the spectral energy distribution.
The data clearly indicate a sharp decrease in millimeter optical depth at the
disk center, consistent with a deficit of material at distances less than ~20
AU from the star. We refine the accretion disk model of Calvet et al. (2005)
based on the unresolved spectral energy distribution (SED) and demonstrate that
it reproduces well the spatially resolved millimeter continuum data at both
available wavelengths. We also present complementary SMA observations of CO
J=3-2 and J=2-1 emission from the disk at 2" resolution. The observed CO
morphology is consistent with the continuum model prediction, with two
significant deviations: (1) the emission displays a larger CO J=3-2/J=2-1 line
ratio than predicted, which may indicate additional heating of gas in the upper
disk layers; and (2) the position angle of the kinematic rotation pattern
differs by 11 +/- 2 degrees from that measured at smaller scales from the dust
continuum, which may indicate the presence of a warp. We note that
photoevaporation, grain growth, and binarity are unlikely mechanisms for
inducing the observed sharp decrease in opacity or surface density at the disk
center. The inner hole plausibly results from the dynamical influence of a
planet on the disk material. Warping induced by a planet could also potentially
explain the difference in position angle between the continuum and CO data
sets.Comment: 12 pages, 6 figures, accepted for publication in Ap
Empirical Constraints on Turbulence in Protoplanetary Accretion Disks
We present arcsecond-scale Submillimeter Array observations of the CO(3-2)
line emission from the disks around the young stars HD 163296 and TW Hya at a
spectral resolution of 44 m/s. These observations probe below the ~100 m/s
turbulent linewidth inferred from lower-resolution observations, and allow us
to place constraints on the turbulent linewidth in the disk atmospheres. We
reproduce the observed CO(3-2) emission using two physical models of disk
structure: (1) a power-law temperature distribution with a tapered density
distribution following a simple functional form for an evolving accretion disk,
and (2) the radiative transfer models developed by D'Alessio et al. that can
reproduce the dust emission probed by the spectral energy distribution. Both
types of models yield a low upper limit on the turbulent linewidth (Doppler
b-parameter) in the TW Hya system (<40 m/s), and a tentative (3-sigma)
detection of a ~300 m/s turbulent linewidth in the upper layers of the HD
163296 disk. These correspond to roughly <10% and 40% of the sound speed at
size scales commensurate with the resolution of the data. The derived
linewidths imply a turbulent viscosity coefficient, alpha, of order 0.01 and
provide observational support for theoretical predictions of subsonic
turbulence in protoplanetary accretion disks.Comment: 18 pages, 9 figures, accepted for publication in Ap
A New Probe of the Planet-Forming Region in T Tauri Disks
We present new observations of the FUV (1100-2200 Angstrom) radiation field
and the near- to mid-IR (3--13.5 micron) spectral energy distribution (SED) of
a sample of T Tauri stars selected on the basis of bright molecular disks (GM
Aur, DM Tau, LkCa15). In each source we find evidence for Ly alpha induced H2
fluorescence and an additional source of FUV continuum emission below 1700
Angstroms. Comparison of the FUV spectra to a model of H2 excitation suggests
that the strong continuum emission is due to electron impact excitation of H2.
The ultimate source of this excitation is likely X-ray irradiation which
creates hot photo-electrons mixed in the molecular layer. Analysis of the SED
of each object finds the presence of inner disk gaps with sizes of a few AU in
each of these young (~1 Myr) stellar systems. We propose that the presence of
strong H2 continuum emission and inner disk clearing are related by the
increased penetration power of high energy photons in gas rich regions with low
grain opacity.Comment: 5 pages, 3 figures, accepted by ApJ Letter
Disruption of Glucagon-Like Peptide 1 Signaling in Sim1 Neurons Reduces Physiological and Behavioral Reactivity to Acute and Chronic Stress
Organismal stress initiates a tightly orchestrated set of responses involving complex physiological and neurocognitive systems. Here, we present evidence for glucagon-like peptide 1 (GLP-1)-mediated paraventricular hypothalamic circuit coordinating the global stress response. The GLP-1 receptor (Glp1r) in mice was knocked down in neurons expressing single-minded 1, a transcription factor abundantly expressed in the paraventricular nucleus (PVN) of the hypothalamus. Mice with single-minded 1-mediated Glp1r knockdown had reduced hypothalamic-pituitary-adrenal axis responses to both acute and chronic stress and were protected against weight loss associated with chronic stress. In addition, regional Glp1r knockdown attenuated stress-induced cardiovascular responses accompanied by decreased sympathetic drive to the heart. Finally, Glp1r knockdown reduced anxiety-like behavior, implicating PVN GLP-1 signaling in behavioral stress reactivity. Collectively, these findings support a circuit whereby brainstem GLP-1 activates PVN signaling to mount an appropriate whole-organism response to stress. These results raise the possibility that dysfunction of this system may contribute to stress-related pathologies, and thereby provide a novel target for intervention
Resolving the CO Snow Line in the Disk around HD 163296
We report Submillimeter Array (SMA) observations of CO (J=2--1, 3--2 and
6--5) and its isotopologues (13CO J=2--1, C18O J=2--1 and C17O J=3--2) in the
disk around the Herbig Ae star HD 163296 at ~2" (250 AU) resolution, and
interpret these data in the framework of a model that constrains the radial and
vertical location of the line emission regions. First, we develop a physically
self-consistent accretion disk model with an exponentially tapered edge that
matches the spectral energy distribution and spatially resolved millimeter dust
continuum emission. Then, we refine the vertical structure of the model using
wide range of excitation conditions sampled by the CO lines, in particular the
rarely observed J=6--5 transition. By fitting 13CO data in this structure, we
further constrain the vertical distribution of CO to lie between a lower
boundary below which CO freezes out onto dust grains (T ~ 19 K) and an upper
boundary above which CO can be photodissociated (the hydrogen column density
from the disk surface is ~ 10^{21} cm-2). The freeze-out at 19 K leads to a
significant drop in the gas-phase CO column density beyond a radius of ~155 AU,
a "CO snow line" that we directly resolve. By fitting the abundances of all CO
isotopologues, we derive isotopic ratios of 12C/13C, 16O/18O and 18O/17O that
are consistent with quiescent interstellar gas-phase values. This detailed
model of the HD 163296 disk demonstrates the potential of a staged, parametric
technique for constructing unified gas and dust structure models and
constraining the distribution of molecular abundances using resolved
multi-transition, multi-isotope observations.Comment: 40 pages, 13 figures, accepted for publication in Ap
Stringent Limits on the Polarized Submillimeter Emission from Protoplanetary Disks
We present arcsecond-resolution Submillimeter Array (SMA) polarimetric
observations of the 880 um continuum emission from the protoplanetary disks
around two nearby stars, HD 163296 and TW Hydrae. Although previous
observations and theoretical work have suggested that a 2-3% polarization
fraction should be common for the millimeter continuum emission from such
disks, we detect no polarized continuum emission above a 3-sigma upper limit of
7 mJy in each arcsecond-scale beam, or <1% in integrated continuum emission. We
compare the SMA upper limits with the predictions from the exploratory Cho &
Lazarian (2007) model of polarized emission from T Tauri disks threaded by
toroidal magnetic fields, and rule out their fiducial model at the ~10-sigma
level. We explore some potential causes for this discrepancy, focusing on model
parameters that describe the shape, magnetic field alignment, and size
distribution of grains in the disk. We also investigate related effects like
the magnetic field strength and geometry, scattering off of large grains, and
the efficiency of grain alignment, including recent advances in grain alignment
theory, which are not considered in the fiducial model. We discuss the impact
each parameter would have on the data and determine that the suppression of
polarized emission plausibly arises from rounding of large grains, reduced
efficiency of grain alignment with the magnetic field, and/or some degree of
magnetic field tangling (perhaps due to turbulence). A poloidal magnetic field
geometry could also reduce the polarization signal, particularly for a face-on
viewing geometry like the TW Hya disk. The data provided here offer the most
stringent limits to date on the polarized millimeter-wavelength emission from
disks around young stars.Comment: 15 pages, 6 figures, accepted for publication in Ap
Dust Settling in Magnetorotationally-Driven Turbulent Discs II: The Pervasiveness of the Streaming Instability and its Consequences
We present a series of simulations of turbulent stratified protostellar discs
with the goal of characterizing the settling of dust throughout a minimum-mass
solar nebula. We compare the evolution of both compact spherical grains, as
well as highly fractal grains. Our simulations use a shearing-box formulation
to study the evolution of dust grains locally within the disc, and collectively
our simulations span the entire extent of a typical accretion disc. The dust is
stirred by gas that undergoes MRI-driven turbulence. This establishes a steady
state scale height for the dust that is different for dust of different sizes.
This sedimentation of dust is an important first step in planet formation and
we predict that ALMA should be able to observationally verify its existence.
When significant sedimentation occurs, the dust will participate in a streaming
instability that significantly enhances the dust density. We show that the
streaming instability is pervasive in the outer disc. We characterize the scale
heights of dust whose size ranges from a few microns to a few centimeters. We
find that for spherical grains, a power-law relationship develops for the scale
height with grain size, with a slope that is slightly steeper than -1/2. The
sedimentation is strongest in the outer disc and increases for large grains.
The results presented here show that direct measurements of grain settling can
be made by ALMA and we present favorable conditions for observability. The
streaming instability should also be directly observable and we provide
conditions for directly observing it. We calculate collision rates and growth
rates for the dust grains in our simulations of various sizes colliding with
other grains, and find that these rates are significantly enhanced through the
density enhancement arising from the streaming instability.Comment: 39 pages, 13 figures, submitted to MNRAS. Abstract is abridge
Resolved Images of Large Cavities in Protoplanetary Transition Disks
Circumstellar disks are thought to experience a rapid "transition" phase in
their evolution that can have a considerable impact on the formation and early
development of planetary systems. We present new and archival high angular
resolution (0.3" = 40-75 AU) Submillimeter Array (SMA) observations of the 880
micron dust continuum emission from 12 such transition disks in nearby
star-forming regions. In each case, we directly resolve a dust-depleted disk
cavity around the central star. Using radiative transfer calculations, we
interpret these dust disk structures in a homogeneous, parametric model
framework by reproducing their SMA visibilities and SEDs. The cavities in these
disks are large (R_cav = 15-73 AU) and substantially depleted of small
(~um-sized) dust grains, although their mass contents are still uncertain. The
structures of the remnant material at larger radii are comparable to normal
disks. We demonstrate that these large cavities are common among the
millimeter-bright disk population, comprising at least 20% of the disks in the
bright half of the millimeter luminosity (disk mass) distribution. Utilizing
these results, we assess some of the physical mechanisms proposed to account
for transition disk structures. As has been shown before, photoevaporation
models do not produce the large cavity sizes, accretion rates, and disk masses
representative of this sample. It would be difficult to achieve a sufficient
decrease of the dust optical depths in these cavities by particle growth alone:
substantial growth (to meter sizes or beyond) must occur in large (tens of AU)
regions of low turbulence without also producing an abundance of small
particles. Given those challenges, we suggest instead that the observations are
most commensurate with dynamical clearing due to tidal interactions with
low-mass companions --young brown dwarfs or giant planets on long-period
orbits.Comment: ApJ, in pres
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