979 research outputs found
Diamonds in HD 97048
We present adaptive optics high angular resolution (\sim0\farcs1)
spectroscopic observations in the 3 m region of the Herbig Ae/Be star HD
97048. For the first time, we spatially resolve the emission in the diamond
features at 3.43 and 3.53 m and in the adjacent continuum. Using both the
intensity profiles along the slit and reconstructed two-dimensional images of
the object, we derive full-width at half-maximum sizes consistent with the
predictions for a circumstellar disk seen pole-on. The diamond emission
originates in the inner region ( AU) of the disk.Comment: ApJLetter, in pres
Exploring Brown Dwarf Disks
We discuss the spectral energy distribution of three very low mass objects in
Chamaeleon I for which ground-based spectroscopy and photometry as well as ISO
measurements in the mid-infrared are available (Comeron et al. 2000; Persi et
al. 2000). One of these stars (Cha Halpha1) is a bona-fide brown dwarf, with
mass 0.04-0.05 Msun. We show that the observed emission is very well described
by models of circumstellar disks identical to those associated to T Tauri
stars, scaled down to keep the ratio of the disk-to-star mass constant and to
the appropriate stellar parameters. This result provides a first indication
that the formation mechanism of T Tauri stars (via core contraction and
formation of an accretion disk) extends to objects in the brown dwarf mass
range.Comment: 4 pages, 4 figures, A&A Letters in pres
X-Shooter study of accretion in -Ophiucus: very low-mass stars and brown dwarfs
We present new VLT/X-Shooter optical and NIR spectra of a sample of 17
candidate young low-mass stars and BDs in the rho-Ophiucus cluster. We derived
SpT and Av for all the targets, and then we determined their physical
parameters. All the objects but one have M*<0.6 Msun, and 8 have mass below or
close to the hydrogen-burning limit. Using the intensity of various emission
lines present in their spectra, we determined the Lacc and Macc for all the
objects. When compared with previous works targeting the same sample, we find
that, in general, these objects are not as strongly accreting as previously
reported, and we suggest that the reason is our more accurate estimate of the
photospheric parameters. We also compare our findings with recent works in
other slightly older star-forming regions to investigate possible differences
in the accretion properties, but we find that the accretion properties for our
targets have the same dependence on the stellar and substellar parameters as in
the other regions. This leads us to conclude that we do not find evidence for a
different dependence of Macc with M* when comparing low-mass stars and BDs.
Moreover, we find a similar small (1 dex) scatter in the Macc-M* relation as in
some of our recent works in other star-forming regions, and no significant
differences in Macc due to different ages or properties of the regions. The
latter result suffers, however, from low statistics and sample selection biases
in the current studies. The small scatter in the Macc-M* correlation confirms
that Macc in the literature based on uncertain photospheric parameters and
single accretion indicators, such as the Ha width, can lead to a scatter that
is unphysically large. Our studies show that only broadband spectroscopic
surveys coupled with a detailed analysis of the photospheric and accretion
properties allows us to properly study the evolution of disk accretion rates.Comment: accepted for publication in Astronomy & Astrophysics. Abstract
shortened to fit arXiv constraint
The effect of local optically thick regions in the long-wave emission of young circumstellar disks
Multi-wavelength observations of protoplanetary disks in the sub-millimeter
continuum have measured spectral indices values which are significantly lower
than what is found in the diffuse interstellar medium. Under the assumption
that mm-wave emission of disks is mostly optically thin, these data have been
generally interpreted as evidence for the presence of mm/cm-sized pebbles in
the disk outer regions. In this work we investigate the effect of possible
local optically thick regions on the mm-wave emission of protoplanetary disks
without mm/cm-sized grains. A significant local increase of the optical depth
in the disk can be caused by the concentration of solid particles, as predicted
to result from a variety of proposed physical mechanisms. We calculate the
filling factors and implied overdensities these optically thick regions would
need to significantly affect the millimeter fluxes of disks, and we discuss
their plausibility. We find that optically thick regions characterized by
relatively small filling factors can reproduce the mm-data of young disks
without requesting emission from mm/cm-sized pebbles. However, these optically
thick regions require dust overdensities much larger than what predicted by any
of the physical processes proposed in the literature to drive the concentration
of solids. We find that only for the most massive disks it is possible and
plausible to imagine that the presence of optically thick regions in the disk
is responsible for the low measured values of the mm spectral index. For the
majority of the disk population, optically thin emission from a population of
large mm-sized grains remains the most plausible explanation. The results of
this analysis further strengthen the scenario for which the measured low
spectral indices of protoplanetary disks at mm wavelengths are due to the
presence of large mm/cm-sized pebbles in the disk outer regions.Comment: 13 pages, 2 figures, A&A in pres
Brown dwarf disks with ALMA
We present ALMA continuum and spectral line data at 0.89 mm and 3.2 mm for
three disks surrounding young brown dwarfs and very low mass stars in the
Taurus star forming region. Dust thermal emission is detected and spatially
resolved for all the three disks, while CO(J=3-2) emission is seen in two
disks. We analyze the continuum visibilities and constrain the disks physical
structure in dust. The results of our analysis show that the disks are
relatively large, the smallest one with an outer radius of about 70 AU. The
inferred disk radii, radial profiles of the dust surface density and disk to
central object mass ratios lie within the ranges found for disks around more
massive young stars. We derive from our observations the wavelength dependence
of the millimeter dust opacity. In all the three disks data are consistent with
the presence of grains with at least millimeter sizes, as also found for disks
around young stars, and confirm that the early stages of the solid growth
toward planetesimals occur also around very low mass objects. We discuss the
implications of our findings on models of solids evolution in protoplanetary
disks, on the main mechanisms proposed for the formation of brown dwarfs and
very low mass stars, as well as on the potential of finding rocky and giant
planets around very low mass objects.Comment: 15 pages, 10 figures, accepted for publication in Ap
Trapping dust particles in the outer regions of protoplanetary disks
Aims. We attempt to explain grain growth to mm sized particles and their retention in the outer regions of protoplanetary disks, as observed at sub-mm and mm wavelengths, by investigating whether strong inhomogeneities in the gas density profiles can decelerate excessive radial drift and help the dust particles to grow.
Methods. We use coagulation/fragmentation and disk-structure models, to simulate the evolution of dust in a bumpy surface density profile, which we mimic with a sinusoidal disturbance. For different values of the amplitude and length scale of the bumps, we investigate the ability of this model to produce and retain large particles on million-year timescales. In addition, we compare the pressure inhomogeneities considered in this work with the pressure profiles that come from magnetorotational instability. Using the Common Astronomy Software Applications ALMA simulator, we study whether there are observational signatures of these pressure inhomogeneities that can be seen with ALMA.
Results. We present the conditions required to trap dust particles and the corresponding calculations predicting the spectral slope in the mm-wavelength range, to compare with current observations. Finally, we present simulated images using different antenna configurations of ALMA at different frequencies, to show that the ring structures will be detectable at the distances of either the Taurus Auriga or Ophiucus star-forming regions
On the gas content of transitional disks: a VLT/X-Shooter study of accretion and winds
Transitional disks (TDs) are thought to be a late evolutionary stage of
protoplanetary disks with dust depleted inner regions. The mechanism
responsible for this depletion is still under debate. To constrain the models
it is mandatory to have a good understanding of the properties of the gas
content of the inner disk. Using X-Shooter broad band -UV to NIR- medium
resolution spectroscopy we derive the stellar, accretion, and wind properties
of a sample of 22 TDs. The analysis of these properties allows us to put strong
constraints on the gas content in a region very close to the star (<0.2 AU)
which is not accessible with any other observational technique. We fit the
spectra with a self-consistent procedure to derive simultaneously SpT,Av,and
mass accretion rates (Macc) of the targets. From forbidden emission lines we
derive the wind properties of the targets. Comparing our findings to values for
cTTs, we find that Macc and wind properties of 80% of the TDs in our sample,
which is strongly biased towards strongly accreting objects, are comparable to
those of cTTs. Thus, there are (at least) some TDs with Macc compatible with
those of cTTs, irrespective of the size of the dust inner hole.Only in 2 cases
Macc are much lower, while the wind properties are similar. We do not see any
strong trend of Macc with the size of the dust depleted cavity, nor with the
presence of a dusty optically thick disk close to the star. In the TDs in our
sample there is a gas rich inner disk with density similar to that of cTTs
disks. At least for some TDs, the process responsible of the inner disk
clearing should allow for a transfer of gas from the outer disk to the inner
region. This should proceed at a rate that does not depend on the physical
mechanism producing the gap seen in the dust emission and results in a gas
density in the inner disk similar to that of unperturbed disks around stars of
similar mass.Comment: Accepted on Astronomy & Astrophysics. Abstract shortened to fit arXiv
constraint
Grain growth in the envelopes and disks of Class I protostars
We present new 3 mm ATCA data of two Class I Young Stellar Objects in the
Ophiucus star forming region: Elias29 and WL12. For our analysis we compare
them with archival 1.1 mm SMA data. In the (u,v) plane the two sources present
a similar behavior: a nearly constant non-zero emission at long baselines,
which suggests the presence of an unresolved component and an increase of the
fluxes at short baselines, related to the presence of an extended envelope. Our
data analysis leads to unusually low values of the spectral index , which may indicate that mm-sized dust grains have already formed
both in the envelopes and in the disk-like structures at such early stages. To
explore the possible scenarios for the interpretation of the sources we perform
a radiative transfer modeling using a Monte Carlo code, in order to take into
account possible deviations from the Rayleigh-Jeans and optically thin regimes.
Comparison between the model outputs and the observations indicates that dust
grains may form aggregates up to millimeter size already in the inner regions
of the envelopes of Class I YSOs. Moreover, we conclude that the embedded
disk-like structures in our two Class Is are probably very compact, in
particular in the case of WL12, with outer radii down to tens of AU.Comment: 12 pages, 8 figures, Accepted for publication in A&
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