477 research outputs found
Physical Properties of Galactic Planck Cold Cores revealed by the Hi-GAL survey
Previous studies of the initial conditions of massive star formation have
mainly targeted Infrared-Dark Clouds (IRDCs) toward the inner Galaxy. This is
due to the fact that IRDCs were first detected in absorption against the bright
mid-IR background, requiring a favourable location to be observed. By
selection, IRDCs represent only a fraction of the Galactic clouds capable of
forming massive stars and star clusters. Due to their low dust temperatures,
IRDCs are bright in the far-IR and millimeter and thus, observations at these
wavelengths have the potential to provide a complete sample of star-forming
massive clouds across the Galaxy. Our aim is to identify the clouds at the
initial conditions of massive star formation across the Galaxy and compare
their physical properties as a function of their Galactic location. We have
examined the physical properties of a homogeneous galactic cold core sample
obtained with the Planck satellite across the Galactic Plane. With the use of
Herschel Hi-GAL observations, we have characterized the internal structure of
them. By using background-subtracted Herschel images, we have derived the H2
column density and dust temperature maps for 48 Planck clumps. Their basic
physical parameters have been calculated and analyzed as a function of location
within the Galaxy. These properties have also been compared with the empirical
relation for massive star formation derived by Kauffmann & Pillai (2010). Most
of the Planck clumps contain signs of star formation. About 25% of them are
massive enough to form high mass stars. Planck clumps toward the Galactic
center region show higher peak column densities and higher average dust
temperatures than those of the clumps in the outer Galaxy. Although we only
have seven clumps without associated YSOs, the Hi-GAL data show no apparent
differences in the properties of Planck cold clumps with and without star
formation.Comment: 22 pages, 11 figures, accepted for publication in A&
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
ALMA Observations of the Young Substellar Binary System 2M1207
We present ALMA observations of the 2M1207 system, a young binary made of a
brown dwarf with a planetary-mass companion at a projected separation of about
40 au. We detect emission from dust continuum at 0.89 mm and from the rotational transition of CO from a very compact disk around the young brown
dwarf. The small radius found for this brown dwarf disk may be due to
truncation from the tidal interaction with the planetary-mass companion. Under
the assumption of optically thin dust emission, we estimated a dust mass of 0.1
for the 2M1207A disk, and a 3 upper limit of for dust surrounding 2M1207b, which is the tightest upper
limit obtained so far for the mass of dust particles surrounding a young
planetary-mass companion. We discuss the impact of this and other
non-detections of young planetary-mass companions for models of planet
formation, which predict the presence of circum-planetary material surrounding
these objects.Comment: 10 pages, 6 figures, accepted for publication in A
An HST Imaging Survey of Low-Mass Stars in the Chamaeleon I Star Forming region
We present new HST/WFPC2 observations of 20 fields centered around T Tauri
stars in the Chamaeleon I star forming region. Images have been obtained in the
F631N ([OI]6300A), F656N (Ha) and F673N ([SII]6716A+6731A) narrow-band filters,
plus the Johnson V-band equivalent F547M filter. We detect 31 T Tauri stars
falling within our fields. We discuss the optical morphology of 10 sources
showing evidence of either binarity, circumstellar material, or mass loss. We
supplement our photometry with a compilation of optical, infrared and
sub-millimeter data from the literature, together with new sub-mm data for
three objects, to build the Spectral Energy Distributions (SED) of 19 single
sources. Using an SED model fitting tool, we self-consistently estimate a
number of stellar and disk parameters, while mass accretion rates are directly
derived from our Ha photometry. We find that bolometric luminosities derived
from dereddened optical data tend to be underestimated in systems with high
alpha(2-24} IR spectral index, suggesting that disks seen nearly edge-on may
occasionally be interpreted as low luminosity (and therefore more evolved)
sources. On the other hand, the same alpha(2-24) spectral index, a tracer of
the amount of dust in the warmer layers of the circumstellar disks, and the
mass accretion rate appear to decay with the isocronal stellar age, suggesting
that the observed age spread (~0.5-5 Myr) within the cluster is real. Our
sample contains a few outliers that may have dissipated their circumstellar
disks on shorter time-scale.Comment: to appear on Astronomical Journal, accepted April 16, 2012 (AJ-10740
Accretion in protoplanetary disks: the imprint of core properties
In this Letter we present a theoretical scenario to explain the steep
correlation between disk accretion rates and stellar masses observed in
pre-main sequence stars. We show that the correlations and spread observed in
the two best studied regions, rho-Ophiuchus and Taurus, can be reproduced by a
simple model of single star formation from a rotating collapsing core and the
viscous evolution of the circumstellar disk. In this model, the rate of
rotation of the parent core sets the value of the `centrifugal radius' within
which the infalling matter is loaded onto the surface of the disk. As a
consequence, the disk accretion rate measured long after the dispersal of the
parental core bears the imprint of the initial conditions of star formation.
The observed trend results naturally if, at the onset of the collapse, cores of
all masses rotate with the same distribution of angular velocities, measured in
units of the break-up rotation rate.Comment: Accepted for publication in Astrophysical Journal Letter
ALMA Observations of Ï-Oph 102: Grain Growth and Molecular Gas in the Disk around a Young Brown Dwarf
We present ALMA continuum and spectral line observations of the young brown dwarf Ï-Oph 102 at about 0.89 mm and 3.2 mm. We detect dust emission from the disk at these wavelengths and derive an upper limit on the radius of the dusty disk of ~40 AU. The derived variation of the dust opacity with frequency in the millimeter (mm) provides evidence for the presence of mm-sized grains in the disk's outer regions. This result demonstrates that mm-sized grains are found even in the low-density environments of brown dwarf disks and challenges our current understanding of dust evolution in disks. The CO map at 345 GHz clearly reveals molecular gas emission at the location of the brown dwarf, indicating a gas-rich disk as typically found for disks surrounding young pre-main-sequence stars. We derive a disk mass of ~0.3%-1% of the mass of the central brown dwarf, similar to the typical values found for disks around more massive young stars
The kinematic relationship between disk and jet in the DG Tauri system
We present high angular resolution millimeter wavelength continuum and
13CO(2-1) observations of the circumstellar disk surrounding the TTauri star DG
Tauri. We show that the velocity pattern in the inner regions of the disk is
consistent with Keplerian rotation about a central 0.67 Msun star. The disk
rotation is also consistent with the toroidal velocity pattern in the initial
channel of the optical jet, as inferred from HST spectra of the first
de-projected 100 AU from the source. Our observations support the tight
relationship between disk and jet kinematics postulated by the popular
magneto-centrifugal models for jet formation and collimation.Comment: 4 pages, A&A Letter accepte
The Early Evolution of Massive Stars: Radio Recombination Line Spectra
Velocity shifts and differential broadening of radio recombination lines are
used to estimate the densities and velocities of the ionized gas in several
hypercompact and ultracompact HII regions. These small HII regions are thought
to be at their earliest evolutionary phase and associated with the youngest
massive stars. The observations suggest that these HII regions are
characterized by high densities, supersonic flows and steep density gradients,
consistent with accretion and outflows that would be associated with the
formation of massive stars.Comment: ApJ in pres
- âŠ