1,460 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
CARMA interferometric observations of 2MASS J044427+2512: the first spatially resolved observations of thermal emission of a brown dwarf disk
We present CARMA 1.3 mm continuum data of the disk surrounding the young
brown dwarf 2MASS J044427+2512 in the Taurus molecular cloud. The high angular
resolution of the CARMA observations (0.16 arcsec) allows us to spatially
resolve for the first time the thermal emission from dust around a brown dwarf.
We analyze the interferometric visibilities and constrain the disk outer radius
adopting disk models with power-law radial profiles of the dust surface
density. In the case of a power-law index equal to or lower than 1, we obtain a
disk radius in the range of about 15 - 30 AU, while larger disks are inferred
for steeper radial profiles. By combining this information on the disk spatial
extent with the sub-mm spectral index of this source we find conclusive
evidence for mm-sized grains, or larger, in this brown dwarf disk. We discuss
the implications of our results on the models of dust evolution in
proto-planetary disks and brown dwarf formation.Comment: 14 pages, 3 figures, Accepted for publication in ApJ Letter
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
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
X-Shooter spectroscopy of young stellar objects: V - Slow winds in T Tauri stars
Disks around T Tauri stars are known to lose mass, as best shown by the
profiles of forbidden emission lines of low ionization species. At least two
separate kinematic components have been identified, one characterised by
velocity shifts of tens to hundreds km/s (HVC) and one with much lower velocity
of few km/s (LVC). The HVC are convincingly associated to the emission of jets,
but the origin of the LVC is still unknown. In this paper we analyze the
forbidden line spectrum of a sample of 44 mostly low mass young stars in Lupus
and -Ori observed with the X-Shooter ESO spectrometer. We detect
forbidden line emission of [OI], [OII], [SII], [NI], and [NII], and
characterize the line profiles as LVC, blue-shifted HVC and red-shifted HVC. We
focus our study on the LVC. We show that there is a good correlation between
line luminosity and both L and the accretion luminosity (or the
mass-accretion rate) over a large interval of values (L L; L L;
M/yr). The lines show the presence of a slow
wind ( cm), warm (T K), mostly neutral. We estimate the mass of the emitting gas and
provide a value for the maximum volume it occupies. Both quantities increase
steeply with the stellar mass, from M and
AU for M M, to
M and AU for M M, respectively.
These results provide quite stringent constraints to wind models in low mass
young stars, that need to be explored further
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
Photoevaporation of Circumstellar Disks due to External FUV Radiation in Stellar Aggregates
When stars form in small groups (N = 100 - 500 members), their circumstellar
disks are exposed to little EUV radiation but a great deal of FUV radiation
from massive stars in the group. This paper calculates mass loss rates for
circumstellar disks exposed to external FUV radiation. Previous work treated
large disks and/or intense radiation fields in which the disk radius exceeds
the critical radius (supercritical disks) where the sound speed in the FUV
heated layer exceeds the escape speed. This paper shows that significant mass
loss still takes place for subcritical systems. Some of the gas extends beyond
the disk edge (above the disk surface) to larger distances where the
temperature is higher, the escape speed is lower, and an outflow develops. The
evaporation rate is a sensitive function of the stellar mass and disk radius,
which determine the escape speed, and the external FUV flux, which determines
the temperature structure of the flow. Disks around red dwarfs are readily
evaporated and shrink to disk radii of 15 AU on short time scales (10 Myr) when
exposed to moderate FUV fields with = 3000. Although disks around solar
type stars are more durable, these disks shrink to 15 AU in 10 Myr for intense
FUV radiation fields with = 30,000; such fields exist in the central 0.7
pc of a cluster with N = 4000 stars. If our solar system formed in the presence
of such strong FUV radiation fields, this mechanism could explain why Neptune
and Uranus in our solar system are gas poor, whereas Jupiter and Saturn are gas
rich. This mechanism for photoevaporation can also limit the production of
Kuiper belt objects and can suppress giant planet formation in sufficiently
large clusters, such as the Hyades, especially for disks associated with low
mass stars.Comment: 49 pages including 12 figures; accepted to Ap
Star formation in clusters: early sub-clustering in the Serpens core
We present high resolution interferometric and single dish observations of
molecular gas in the Serpens cluster-forming core. Star formation does not
appear to be homogeneous throughout the core, but is localised in spatially-
and kinematically-separated sub-clusters. The stellar (or proto-stellar)
density in each of the sub-clusters is much higher than the mean for the entire
Serpens cluster. This is the first observational evidence for the hierarchical
fragmentation of proto-cluster cores suggested by cluster formation models.Comment: 11 pages, 3 Figures, ApJ Letters in pres
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