1,282 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 gas temperature in the surface layers of protoplanetary disks
Models for the structure of protoplanetary disks have so far been based on
the assumption that the gas and the dust temperature are equal. The gas
temperature, an essential ingredient in the equations of hydrostatic
equilibrium of the disk, is then determined from a continuum radiative transfer
calculation, in which the continuum opacity is provided by the dust. It has
been long debated whether this assumption still holds in the surface layers of
the disk, where the dust infrared emission features are produced. In this paper
we compute the temperature of the gas in the surface layers of the disk in a
self-consistent manner. The gas temperature is determined from a
heating-cooling balance equation in which processes such as photoelectric
heating, dissociative heating, dust-gas thermal heat exchange and line cooling
are included. The abundances of the dominant cooling species such as CO, C, C+
and O are determined from a chemical network based on the atomic species H, He,
C, O, S, Mg, Si, Fe (Kamp & Bertoldi 2000). The underlying disk models to our
calculations are the models of Dullemond, van Zadelhoff & Natta (2002). We find
that in general the dust and gas temperature are equal to withing 10% for A_V
>~ 0.1, which is above the location of the `super-heated surface layer' in
which the dust emission features are produced (e.g. Chiang & Goldreich 1997).
High above the disk surface the gas temperature exceeds the dust temperature
and can can become -- in the presence of polycyclic aromatic hydrocarbons -- as
high as 600 K at a radius of 100 AU. This is a region where CO has fully
dissociated, but a significant fraction of hydrogen is still in molecular form.
The densities are still high enough for non-negligible H_2 emission to be
produced.....(see paper for full abstract)Comment: 28 pages, 8 figures, accepted for publication in Ap
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
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
X-Shooter spectroscopy of FU Tau A
We have analyzed a broad-band optical and near-infrared spectrum of FU Tau A,
a presumed young brown dwarf in the Taurus star forming region that has
intrigued both theorists and observers by its over-luminosity in the HR diagram
with respect to standard pre-main sequence evolutionary models. The new data,
obtained with the X-Shooter spectrograph at the Very Large Telescope, include
an unprecedented wealth of information on stellar parameters and simultaneously
observed accretion and outflow indicators for FU Tau A. We present the first
measurements of gravity (log g = 3.5 +- 0.5), radial velocity (RV = 22.5 +- 2.9
km/s), rotational velocity (v sin(i) = 20 +- 5 km/s) and lithium equivalent
width (W_Li = 430 +- 20 mAA) for FUTau A. From the rotational velocity and the
published period we infer a disk inclination of i ~ 50^deg. The lithium content
is much lower than theoretically expected for such a young very low mass
object, adding another puzzling feature to this object's properties. We
determine the mass accretion rate of FU Tau A from comparison of the
luminosities of 24 emission lines to empirical calibrations from the literature
and find a mean of log (dM/dt)_acc [M_sun/yr] = -9.9 +- 0.2. The accretion rate
determined independently from modeling of the excess emission in the Balmer and
Paschen continua is consistent with this value. The corresponding accretion
luminosity is too small to make a significant contribution to the bolometric
luminosity. The existence of an outflow in FU Tau A is demonstrated through the
first detection of forbidden emission lines from which we obtain an estimate
for the mass loss rate, log (dM/dt)_out [M_sun/yr] < -10.4. The mass outflow
and inflow rates can be combined to yield (dM/dt)_out / (dM/dt)_acc ~ 0.3, a
value that is in agreement with jet launching models.Comment: 12 pages, accepted for publication in A&
An extensive VLT/X-Shooter library of photospheric templates of pre-main sequence stars
Studies of the formation and evolution of young stars and their disks rely on
the knowledge of the stellar parameters of the young stars. The derivation of
these parameters is commonly based on comparison with photospheric template
spectra. Furthermore, chromospheric emission in young active stars impacts the
measurement of mass accretion rates, a key quantity to study disk evolution.
Here we derive stellar properties of low-mass pre-main sequence stars without
disks, which represent ideal photospheric templates for studies of young stars.
We also use these spectra to constrain the impact of chromospheric emission on
the measurements of mass accretion rates. The spectra in reduced,
flux-calibrated, and corrected for telluric absorption form are made available
to the community. We derive the spectral type for our targets by analyzing the
photospheric molecular features present in their VLT/X-Shooter spectra by means
of spectral indices and comparison of the relative strength of photospheric
absorption features. We also measure effective temperature, gravity, projected
rotational velocity, and radial velocity from our spectra by fitting them with
synthetic spectra with the ROTFIT tool. The targets have negligible extinction
and spectral type from G5 to M8. We perform synthetic photometry on the spectra
to derive the typical colors of young stars in different filters. We measure
the luminosity of the emission lines present in the spectra and estimate the
noise due to chromospheric emission in the measurements of accretion luminosity
in accreting stars. We provide a calibration of the photospheric colors of
young PMS stars as a function of their spectral type in a set of standard
broad-band optical and near-infrared filters. For stars with masses of ~
1.5Msun and ages of ~1-5 Myr, the chromospheric noise converts to a limit of
measurable mass accretion rates of ~ 3x10^-10 Msun/yr.Comment: Accepted for publication on Astronomy & Astrophysics. The spectra of
the photospheric templates will be uploaded to Vizier, but are already
available on request. Abstract shortened for arxiv constraints. Language
edited versio
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