126 research outputs found
A Herschel PACS survey of the dust and gas in Upper Scorpius disks
We present results of far-infrared photometric observations with Herschel
PACS of a sample of Upper Scorpius stars, with a detection rate of previously
known disk-bearing K and M stars at 70, 100, and 160 micron of 71%, 56%, and
50%, respectively. We fit power-law disk models to the spectral energy
distributions of K & M stars with infrared excesses, and have found that while
many disks extend in to the sublimation radius, the dust has settled to lower
scale heights than in disks of the less evolved Taurus-Auriga population, and
have much reduced dust masses. We also conducted Herschel PACS observations for
far-infrared line emission and JCMT observations for millimeter CO lines. Among
B and A stars, 0 of 5 debris disk hosts exhibit gas line emission, and among K
and M stars, only 2 of 14 dusty disk hosts are detected. The OI 63 micron and
CII 157 micron lines are detected toward [PZ99] J160421.7-213028 and [PBB2002]
J161420.3-190648, which were found in millimeter photometry to host two of the
most massive dust disks remaining in the region. Comparison of the OI line
emission and 63 micron continuum to that of Taurus sources suggests the
emission in the former source is dominated by the disk, while in the other
there is a significant contribution from a jet. The low dust masses found by
disk modeling and low number of gas line detections suggest that few stars in
Upper Scorpius retain sufficient quantities of material for giant planet
formation. By the age of Upper Scorpius, giant planet formation is essentially
complete.Comment: 48 pages, 14 figures, accepted A&
Radiative transfer in protoplanetary disks
We present a new 3D continuum radiative transfer code, MCFOST, based on a
Monte-Carlo method. The reliability and efficiency of the code is tested by
comparison with five different radiative transfer codes previously tested by
Pascucci et al., 2004, using a 2D disk configuration. When tested against the
same disk configuration, no significant difference is found between the
temperature and SED calculated with MCFOST and with the other codes. The
computed values are well within the range of values computed by the other
codes. The code-to-code differences are small, they rarely exceed 10% and are
usually much smaller.Comment: 20 pages, 11 figures, GRETA conference : "Radiative transfer and
Applications to Very Large Telescopes
Continuum and line modelling of discs around young stars. I. 300000 disc models for Herschel/GASPS
We have combined the thermo-chemical disc code ProDiMo with the Monte Carlo
radiative transfer code MCFOST to calculate a grid of ~300000 circumstellar
disc models, systematically varying 11 stellar, disc and dust parameters
including the total disc mass, several disc shape parameters and the
dust-to-gas ratio. For each model, dust continuum and line radiative transfer
calculations are carried out for 29 far IR, sub-mm and mm lines of [OI], [CII],
12CO and o/p-H2O under 5 inclinations. The grid allows to study the influence
of the input parameters on the observables, to make statistical predictions for
different types of circumstellar discs, and to find systematic trends and
correlations between the parameters, the continuum fluxes, and the line fluxes.
The model grid, comprising the calculated disc temperatures and chemical
structures, the computed SEDs, line fluxes and profiles, will be used in
particular for the data interpretation of the Herschel open time key programme
GASPS. The calculated line fluxes show a strong dependence on the assumed UV
excess of the central star, and on the disc flaring. The fraction of models
predicting [OI] and [CII] fine-structure lines fluxes above Herschel/PACS and
Spica/SAFARI detection limits are calculated as function of disc mass. The
possibility of deriving the disc gas mass from line observations is discussed.Comment: accepted by MNRAS. 5 pages, 4 figures, 3 table
Dust and Gas in the disc of HL Tauri: Surface density, dust settling, and dust-to-gas ratio
The recent ALMA observations of the disc surrounding HL Tau reveal a very
complex dust spatial distribution. We present a radiative transfer model
accounting for the observed gaps and bright rings as well as radial changes of
the emissivity index. We find that the dust density is depleted by at least a
factor 10 in the main gaps compared to the surrounding rings. Ring masses range
from 10-100 M in dust, and, we find that each of the deepest gaps is
consistent with the removal of up to 40 M of dust. If this material
has accumulated into rocky bodies, these would be close to the point of runaway
gas accretion. Our model indicates that the outermost ring is depleted in
millimetre grains compared to the central rings. This suggests faster grain
growth in the central regions and/or radial migration of the larger grains. The
morphology of the gaps observed by ALMA - well separated and showing a high
degree of contrast with the bright rings over all azimuths - indicates that the
millimetre dust disc is geometrically thin (scale height 1 au at 100
au) and that a large amount of settling of large grains has already occurred.
Assuming a standard dust settling model, we find that the observations are
consistent with a turbulent viscosity coefficient of a few . We
estimate the gas/dust ratio in this thin layer to be of the order of 5 if the
initial ratio is 100. The HCO and CO emission is consistent with gas in
Keplerian motion around a 1.7 star at radii from au.Comment: 12 pages, 8 figures, published in ApJ, same version as before but
with slightly extended discussion on temperature and masses to account for
literature published since initial submissio
HST Scattered Light Imaging and Modeling of the Edge-on Protoplanetary Disk ESO-H 569
We present new HST ACS observations and detailed models for a recently
discovered edge-on protoplanetary disk around ESO H 569 (a low-mass T
Tauri star in the Cha I star forming region). Using radiative transfer models
we probe the distribution of the grains and overall shape of the disk
(inclination, scale height, dust mass, flaring exponent and surface/volume
density exponent) by model fitting to multiwavelength (F606W and F814W) HST
observations together with a literature compiled spectral energy distribution.
A new tool set was developed for finding optimal fits of MCFOST radiative
transfer models using the MCMC code emcee to efficiently explore the high
dimensional parameter space. It is able to self-consistently and simultaneously
fit a wide variety of observables in order to place constraints on the physical
properties of a given disk, while also rigorously assessing the uncertainties
in those derived properties. We confirm that ESO H 569 is an optically
thick nearly edge-on protoplanetary disk. The shape of the disk is well
described by a flared disk model with an exponentially tapered outer edge,
consistent with models previously advocated on theoretical grounds and
supported by millimeter interferometry. The scattered light images and spectral
energy distribution are best fit by an unusually high total disk mass (gas+dust
assuming a ratio of 100:1) with a disk-to-star mass ratio of 0.16.Comment: Accepted for publication in Ap
An upper limit on the mass of the circumplanetary disk for DH Tau b
DH Tau is a young (1 Myr) classical T Tauri star. It is one of the few
young PMS stars known to be associated with a planetary mass companion, DH Tau
b, orbiting at large separation and detected by direct imaging. DH Tau b is
thought to be accreting based on copious H emission and exhibits
variable Paschen Beta emission. NOEMA observations at 230 GHz allow us to place
constraints on the disk dust mass for both DH Tau b and the primary in a regime
where the disks will appear optically thin. We estimate a disk dust mass for
the primary, DH Tau A of , which gives a disk-to-star
mass ratio of 0.014 (assuming the usual Gas-to-Dust mass ratio of 100 in the
disk). We find a conservative disk dust mass upper limit of 0.42
for DH Tau b, assuming that the disk temperature is dominated by irradiation
from DH Tau b itself. Given the environment of the circumplanetary disk,
variable illumination from the primary or the equilibrium temperature of the
surrounding cloud would lead to even lower disk mass estimates. A MCFOST
radiative transfer model including heating of the circumplanetary disk by DH
Tau b and DH Tau A suggests that a mass averaged disk temperature of 22 K is
more realistic, resulting in a dust disk mass upper limit of 0.09
for DH Tau b. We place DH Tau b in context with similar objects and discuss the
consequences for planet formation models.Comment: accepted for publication in A
Intricate visibility effects from resolved emission of young stellar objects: the case of MWC158 observed with the VLTI
In the course of our VLTI young stellar object PIONIER imaging program, we
have identified a strong visibility chromatic dependency that appeared in
certain sources. This effect, rising value of visibilities with decreasing
wavelengths over one base, is also present in previous published and archival
AMBER data. For Herbig AeBe stars, the H band is generally located at the
transition between the star and the disk predominance in flux for Herbig AeBe
stars. We believe that this phenomenon is responsible for the visibility rise
effect. We present a method to correct the visibilities from this effect in
order to allow "gray" image reconstruction software, like Mira, to be used. In
parallel we probe the interest of carrying an image reconstruction in each
spectral channel and then combine them to obtain the final broadband one. As an
illustration we apply these imaging methods to MWC158, a (possibly Herbig) B[e]
star intensively observed with PIONIER. Finally, we compare our result with a
parametric model fitted onto the data.Comment: 14 pages, 7 figure
Herschel Observations of the T Cha Transition Disk: Constraining the Outer Disk Properties
T Cha is a nearby (d similar to 100 pc) transition disk known to have an optically thin gap separating optically thick inner and outer disk components. Huelamo et al. recently reported the presence of a low-mass object candidate within the gap of the T Cha disk, giving credence to the suspected planetary origin of this gap. Here we present the Herschel photometry (70, 160, 250, 350, and 500 mu m) of T Cha from the "Dust, Ice, and Gas in Time" Key Program, which bridges the wavelength range between existing Spitzer and millimeter data and provide important constraints on the outer disk properties of this extraordinary system. We model the entire optical to millimeter wavelength spectral energy distribution (SED) of T Cha (19 data points between 0.36 and 3300 mu m without any major gaps in wavelength coverage). T Cha shows a steep spectral slope in the far-IR, which we find clearly favors models with outer disks containing little or no dust beyond similar to 40 AU. The full SED can be modeled equally well with either an outer disk that is very compact (only a few AU wide) or a much larger one that has a very steep surface density profile. That is, T Cha's outer disk seems to be either very small or very tenuous. Both scenarios suggest a highly unusual outer disk and have important but different implications for the nature of T Cha. Spatially resolved images are needed to distinguish between the two scenarios.DIGIT Herschel Open Time Key ProgramNASAAlexander von Humboldt FoundationEuropean CommissionAgence Nationale pour la Recherche of France PERG06-GA-2009-256513, ANR-07-BLAN-0221, ANR-2010-JCJC-0504-01CNRS/INSU, FranceAstronom
Algorithmes flous : Application Ă la quantification du tissu adipeux en IRM abdominale
Le syndrome métabolique, associé à l'obésité est une pathologie en forte progression dans nos sociétés occidentales. Un marqueur jouant un rÎle prépondérant dans le diagnostic de cette pathologie est le tour de taille, dépendant de la graisse viscérale et sous-cutanée. Nous proposons dans cet article une méthode automatique et fiable de segmentation des graisses viscérale et sous-cutanée utilisant succéssivement un algorithme de classification floue suivi d'un algorithme de connectivité nous permettant de nous affranchir des problÚmes d'inhomogénéités du tissu adipeux et du champs magnétique
Discovery of an 86 AU Radius Debris Ring Around HD 181327
HST/NICMOS PSF-subtracted coronagraphic observations of HD 181327 have
revealed the presence of a ring-like disk of circumstellar debris seen in 1.1
micron light scattered by the disk grains, surrounded by a di use outer region
of lower surface brightness. The annular disk appears to be inclined by 31.7
+/- 1.6 deg from face on with the disk major axis PA at 107 +/-2 deg . The
total 1.1 micron flux density of the light scattered by the disk (at 1.2" < r <
5.0") of 9.6 mJy +/- 0.8 mJy is 0.17% +/- 0.015% of the starlight. Seventy
percent of the light from the scattering grains appears to be confined in a 36
AU wide annulus centered on the peak of the radial surface brightness (SB)
profile 86.3 +/- 3.9 AU from the star, well beyond the characteristic radius of
thermal emission estimated from IRAS and Spitzer flux densities assuming
blackbody grains (~ 22 AU). The light scattered by the ring appears bilaterally
symmetric, exhibits directionally preferential scattering well represented by a
Henyey-Greenstein scattering phase function with g = 0.30 +/- 0.03, and has an
azimuthally medianed SB at the 86.3 AU radius of peak SB of 1.00 +/- 0.07 mJy
arcsec^-2. No photocentric offset is seen in the ring relative to the position
of the central star. A low surface brightness diffuse halo is seen in the
NICMOS image to a distance of ~ 4" Deeper 0.6 micron HST/ACS PSF-subtracted
coronagraphic observations reveal a faint outer nebulosity, asymmetrically
brighter to the North of the star. We discuss models of the disk and properties
of its grains, from which we infer a maximum vertical scale height of 4 - 8 AU
at the 87.6 AU radius of maximum surface density, and a total maximum dust mass
of collisionally replenished grains with minimum grain sizes of ~ 1 micron of ~
4 M(moon).Comment: 45 pages, 15 figures, accepted for publication in Ap
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