240 research outputs found
How to Detect the Signatures of Self-Gravitating Circumstellar Discs with the Atacama Large Millimetre/sub-millimetre Array
In this paper we present simulated Atacama Large Millimetre/sub-millimetre
Array (ALMA) observations of self-gravitating circumstellar discs with
different properties in size, mass and inclination, located in four of the most
extensively studied and surveyed star-forming regions. Starting from a Smoothed
Particle Hydrodynamics (SPH) simulation and representative dust opacities, we
have initially constructed maps of the expected emission at sub-mm wavelengths
of a large sample of discs with different properties. We have then simulated
realistic observations of discs as they may appear with ALMA using the Common
Astronomy Software Application ALMA simulator. We find that, with a proper
combination of antenna configuration and integration time, the spiral structure
characteristic of self-gravitating discs is readily detectable by ALMA over a
wide range of wavelengths at distances comparable to TW Hydrae (pc), Taurus - Auriga and Ophiucus (pc) star-forming regions.
However, for discs located in Orion complex (pc) only the largest
discs in our sample (outer radius of 100 au) show a spatially resolved
structure while the smaller ones (outer radius of 25 au) are characterized by a
spiral structure that is not conclusively detectable with ALMA.Comment: 12 pages, 10 figure
Near-IR imaging of T Cha: evidence for scattered-light disk structures at solar system scales
T Chamaeleontis is a young star surrounded by a transitional disk, and a
plausible candidate for ongoing planet formation. Recently, a substellar
companion candidate was reported within the disk gap of this star. However, its
existence remains controversial, with the counter-hypothesis that light from a
high inclination disk may also be consistent with the observed data. The aim of
this work is to investigate the origin of the observed closure phase signal to
determine if it is best explained by a compact companion. We observed T Cha in
the L and K s filters with sparse aperture masking, with 7 datasets covering a
period of 3 years. A consistent closure phase signal is recovered in all L and
K s datasets. Data were fit with a companion model and an inclined
circumstellar disk model based on known disk parameters: both were shown to
provide an adequate fit. However, the absence of expected relative motion for
an orbiting body over the 3-year time baseline spanned by the observations
rules out the companion model. Applying image reconstruction techniques to each
dataset reveals a stationary structure consistent with forward scattering from
the near edge of an inclined disk.Comment: 6 pages, 3 figures, accepted for publication in MNRAS Letter
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
Molecular Line Emission from Accretion Disks Around YSOs
In this work we model the expected molecular emission from protoplanetary
disks, modifying different physical parameters, such as dust grain size, mass
accretion rate, viscosity, and disk radius, to obtain observational signatures
in these sources. Having in mind possible future observations, we study
correlations between physical parameters and observational characteristics. Our
aim is to determine the kind of observations that will allow us to extract
information about the physical parameters of disks. We also present prospects
for molecular line observations of protoplanetary disks, using millimeter and
submillimeter interferometers (e.g., SMA or ALMA), based on our results.Comment: 6 pages, 2 figures. Proceeding of the workshop "Magnetic fields and
star formation: theory versus observation", Madrid (Spain), April 21 - 25,
200
Dense gas and exciting sources of the molecular outflow in the AFGL 437 star-forming region
We present Very Large Array (VLA) high resolution observations of the
NH3(1,1) and NH3(2,2) molecular transitions towards the high mass star forming
region AFGL 437. Our aim was to investigate if the poorly collimated CO
molecular outflow previously detected in the region is the result of a
projection effect, with no intrinsic bipolarity, as suggested by Gomez et al.
We complemented our observations with radio continuum archived data from the
VLA at 2 and 3.6 cm, and with unpublished public data at 450 {\mu}m taken with
Submillimetre Common-User Bolometer Array at the James Clerk Maxwell Telescope.
Ammonia emission was found mainly in three clumps located at the south and east
of the position of the compact infrared cluster of AFGL 437, where the CO
outflow seemed to have its origin. One of the NH3(1,1) clumps coincides with
the maximum of NH3(2,2) and with a local peak of emission at 450 {\mu}m. A near
infrared source (s11) is also found at that position. Our continuum map at 2 cm
shows extended elongated emission associated with the infrared source AFGL
437W. This elongated morphology and its spectral index between 3.6 and 2 cm
(\simeq 0.4) suggest the presence of a jet in AFGL 437W. We suggest that
several molecular bipolar outflows may exist in the region. The observed CO
outflow would be the superposition of those individual outflows, which would
explain its low degree of collimation observed at larger scales.Comment: 9 pages, 5 figures, 3 tables, accepted by MNRA
G345.45+1.50: An expanding ring-like structure with massive star formation
Ring-like structures in the ISM are commonly associated with high-mass stars.
Kinematic studies of large structures in GMCs toward these ring-like structures
may help us to understand how massive stars form. The origin and properties of
the ring-like structure G345.45+1.50 is investigated through observations of
the 13CO(3-2) line. The aim of the observations is to determine the kinematics
in the region and to compare physical characteristics estimated from gas
emission with those previously determined using dust continuum emission. The
13CO(3-2) line was mapped toward the whole ring using the APEX telescope. The
ring is found to be expanding with a velocity of 1.0 km/s, containing a total
mass of 6.9e3 Msun, which agrees well with that determined using 1.2 mm dust
continuum emission. An expansion timescale of 3e6 yr and a total energy of 7e46
erg are estimated. The origin of the ring might have been a supernova
explosion, since a 35.5 cm source, J165920-400424, is located at the center of
the ring without an infrared counterpart. The ring is fragmented, and 104
clumps were identified with diameters of between 0.3 and 1.6 pc, masses of
between 2.3 and 7.5e2 Msun, and densities of between 1.0e2 and 1.0e4 cm^-3. At
least 18% of the clumps are forming stars, as is shown in infrared images.
Assuming that the clumps can be modeled as Bonnor-Ebert spheres, 13 clumps are
collapsing, and the rest of them are in hydrostatic equilibrium with an
external pressure with a median value of 4e4 K cm^-3. In the region, the
molecular outflow IRAS 16562-3959 is identified, with a velocity range of 38.4
km/s, total mass of 13 Msun, and kinematic energy of 7e45 erg. Finally, five
filamentary structures were found at the edge of the ring with an average size
of 3 pc, a width of 0.6 pc, a mass of 2e2 Msun, and a column density of 6e21
cm^-2
High resolution observations of the outer disk around T Cha: the view from ALMA
T Cha is a young star surrounded by a transitional disk with signatures of
planet formation. We have obtained high-resolution and high-sensitivity ALMA
observations of T Cha in the --, --, and
-- emission lines to reveal the spatial distribution of the
gaseous disk around the star. In order to study the dust within the disk we
have also obtained continuum images at 850m from the line-free channels.
We have spatially resolved the outer disk around T Cha. Using the CO(3-2)
emission we derive a radius of 230 AU. We also report the detection of
the CO(3-2) and the CS(7-8) molecular emissions, which show smaller
radii than the CO(3-2) detection. The continuum observations at 850m allow
the spatial resolution of the dusty disk, which shows two emission bumps
separated by 40AU, consistent with the presence of a dust gap in the
inner regions of the disk, and an outer radius of 80AU. Therefore, T Cha
is surrounded by a compact dusty disk and a larger and more diffuse gaseous
disk, as previously observed in other young stars. The continuum intensity
profiles are different at both sides of the disk suggesting possible dust
asymmetries. We derive an inclination of i(deg)=675, and a position angle
of PA (deg)= 1136, for both the gas and dust disks. The comparison of the
ALMA data with radiative transfer models shows that the gas and dust components
can only be simultaneously reproduced when we include a tapered edge
prescription for the surface density profile. The best model suggests that most
of the disk mass is placed within a radius of 50AU. Finally, we derive a
dynamical mass for the central object of =1.50.2M,
comparable to the one estimated with evolutionary models for an age of
10Myr.Comment: 5 pages, 5 figures, accepted for publication in A&A Letter
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