1,581 research outputs found
IRAS 18511+0146: a proto Herbig Ae/Be cluster?
Context: The evolution of a young protocluster depends on the relative
spatial distributions and dynamics of both stars and gas. Aims: We study the
distribution and properties of the gas and stars surrounding the luminous (10^4
L_sun) protocluster IRAS 18511+0146. Methods: IRAS 18511+0146 and the cluster
associated with it has been investigated using the sub-millimetre (JCMT-SCUBA),
infrared (Spitzer-MIPSGAL, Spitzer-GLIMPSE, Palomar) and radio (VLA) continuum
data. Cluster simulations have been carried out in order to understand the
properties of clusters as well as to compare with the observations. Results:
The central most obscured part of the protocluster coincident with the compact
sub-millimetre source found with SCUBA is responsible for at least 2/3 of the
total luminosity. A number of cluster members have been identified which are
bright in mid infrared and show rising (near to mid infrared) spectral energy
distributions suggesting that these are very young stellar sources. In the mid
infrared 8.0 micron image, a number of filamentary structures and clumps are
detected in the vicinity of IRAS 18511+0146. Conclusions: Based on the
luminosity and cluster size as well as on the evolutionary stages of the
cluster members, IRAS 18511+0146 is likely to be protocluster with the most
massive object being a precursor to a Herbig type star.Comment: Accepted by the Astronomy and Astrophysics (23 Pages, 5 Tables, 12
Figures
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&
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
First results from a VLBA proper motion survey of H2O masers in low-mass YSOs: the Serpens core and RNO15-FIR
This article reports first results of a long-term observational program aimed
to study the earliest evolution of jet/disk systems in low-mass YSOs by means
of VLBI observations of the 22.2 GHz water masers. We report here data for the
cluster of low-mass YSOs in the Serpens molecular core and for the single
object RNO~15-FIR. Towards Serpens SMM1, the most luminous sub-mm source of the
Serpens cluster, the water maser emission comes from two small (< 5 AU in size)
clusters of features separated by ~25 AU, having line of sight velocities
strongly red-shifted (by more than 10 km/s) with respect to the LSR velocity of
the molecular cloud. The two maser clusters are oriented on the sky along a
direction that is approximately perpendicular to the axis of the radio
continuum jet observed with the VLA towards SMM1. The spatial and velocity
distribution of the maser features lead us to favor the interpretation that the
maser emission is excited by interaction of the receding lobe of the jet with
dense gas in the accretion disk surrounding the YSO in SMM1. Towards
RNO~15-FIR, the few detected maser features have both positions and (absolute)
velocities aligned along a direction that is parallel to the axis of the
molecular outflow observed on much larger angular scales. In this case the
maser emission likely emerges from dense, shocked molecular clumps displaced
along the axis of the jet emerging from the YSO. The protostar in Serpens SMM1
is more massive than the one in RNO~15-FIR. We discuss the case where a high
mass ejection rate can generate jets sufficiently powerful to sweep away from
their course the densest portions of circumstellar gas. In this case, the
excitation conditions for water masers might preferably occur at the interface
between the jet and the accretion disk, rather than along the jet axis.Comment: 18 pages (postscript format); 9 figures; to be published into
Astronomy & Astrophysics, Main Journa
First results from the CALYPSO IRAM-PdBI survey - III. Monopolar jets driven by a proto-binary system in NGC1333-IRAS2A
Context: The earliest evolutionary stages of low-mass protostars are
characterised by hot and fast jets which remove angular momentum from the
circumstellar disk, thus allowing mass accretion onto the central object.
However, the launch mechanism is still being debated. Aims: We would like to
exploit high-angular (~ 0.8") resolution and high-sensitivity images to
investigate the origin of protostellar jets using typical molecular tracers of
shocked regions, such as SiO and SO. Methods: We mapped the inner 22" of the
NGC1333-IRAS2A protostar in SiO(5-4), SO(65-54), and the continuum emission at
1.4 mm using the IRAM Plateau de Bure interferometer in the framework of the
CALYPSO IRAM large program. Results: For the first time, we disentangle the
NGC1333-IRAS2A Class 0 object into a proto-binary system revealing two
protostars (MM1, MM2) separated by ~ 560 AU, each of them driving their own
jet, while past work considered a single protostar with a quadrupolar outflow.
We reveal (i) a clumpy, fast (up to |V-VLSR| > 50 km/s), and blueshifted jet
emerging from the brightest MM1 source, and (ii) a slower redshifted jet,
driven by MM2. Silicon monoxide emission is a powerful tracer of
high-excitation (Tkin > 100 K; n(H2) > 10^5 cm-3) jets close to the launching
region. At the highest velocities, SO appears to mimic SiO tracing the jets,
whereas at velocities close to the systemic one, SO is dominated by extended
emission, tracing the cavity opened by the jet. Conclusions: Both jets are
intrinsically monopolar, and intermittent in time. The dynamical time of the
SiO clumps is < 30-90 yr, indicating that one-sided ejections from protostars
can take place on these timescales.Comment: Astronomy & Astrophysics Letter, in pres
Relative Evolutionary Time Scale of Hot Molecular Cores with Respect to Ultra Compact HII Regions
Using the Owens Valley and Nobeyama Radio Observatory interferometers, we
carried out an unbiased search for hot molecular cores and ultracompact UC HII
regions toward the high-mass star forming region G19.61--0.23. In addition, we
performed 1.2 mm imaging with SIMBA, and retrieved 3.5 and 2 cm images from the
VLA archive data base. The newly obtained 3 mm image brings information on a
cluster of high-mass (proto)stars located in the innermost and densest part of
the parsec scale clump detected in the 1.2 mm continuum. We identify a total of
10 high-mass young stellar objects: one hot core (HC) and 9 UC HII regions,
whose physical parameters are obtained from model fits to their continuum
spectra. The ratio between the current and expected final radii of the UC \HII
regions ranges from 0.3 to 0.9, which leaves the possibility that all O-B stars
formed simultaneously. Under the opposite assumption -- namely that star
formation occurred randomly -- we estimate that HC lifetime is less than
1/3 of that of UCHII regions on the basis of the source number ratio
between them.Comment: 13 pages, 2 figs, including a color fi
On the shape of the mass-function of dense clumps in the Hi-GAL fields. II. Using Bayesian inference to study the clump mass function
Context. Stars form in dense, dusty clumps of molecular clouds, but little is
known about their origin, their evolution and their detailed physical
properties. In particular, the relationship between the mass distribution of
these clumps (also known as the "clump mass function", or CMF) and the stellar
initial mass function (IMF), is still poorly understood. Aims. In order to
better understand how the CMF evolve toward the IMF, and to discern the "true"
shape of the CMF, large samples of bona-fide pre- and proto-stellar clumps are
required. Two such datasets obtained from the Herschel infrared GALactic Plane
Survey (Hi-GAL) have been described in paper I. Robust statistical methods are
needed in order to infer the parameters describing the models used to fit the
CMF, and to compare the competing models themselves. Methods. In this paper we
apply Bayesian inference to the analysis of the CMF of the two regions
discussed in Paper I. First, we determine the Bayesian posterior probability
distribution for each of the fitted parameters. Then, we carry out a
quantitative comparison of the models used to fit the CMF. Results. We have
compared the results from several methods implementing Bayesian inference, and
we have also analyzed the impact of the choice of priors and the influence of
various constraints on the statistical conclusions for the preferred values of
the parameters. We find that both parameter estimation and model comparison
depend on the choice of parameter priors. Conclusions. Our results confirm our
earlier conclusion that the CMFs of the two Hi-GAL regions studied here have
very similar shapes but different mass scales. Furthermore, the lognormal model
appears to better describe the CMF measured in the two Hi-GAL regions studied
here. However, this preliminary conclusion is dependent on the choice of
parameters priors.Comment: Submitted for publication to A&A on November 12, 2013. This paper
contains 11 pages and 7 figure
Glycolaldehyde in Perseus young solar analogs
Aims: In this paper we focus on the occurrence of glycolaldehyde (HCOCH2OH)
in young solar analogs by performing the first homogeneous and unbiased study
of this molecule in the Class 0 protostars of the nearby Perseus star forming
region. Methods: We obtained sub-arcsec angular resolution maps at 1.3mm and
1.4mm of glycolaldehyde emission lines using the IRAM Plateau de Bure (PdB)
interferometer in the framework of the CALYPSO IRAM large program. Results:
Glycolaldehyde has been detected towards 3 Class 0 and 1 Class I protostars out
of the 13 continuum sources targeted in Perseus: NGC1333-IRAS2A1,
NGC1333-IRAS4A2, NGC1333-IRAS4B1, and SVS13-A. The NGC1333 star forming region
looks particularly glycolaldehyde rich, with a rate of occurrence up to 60%.
The glycolaldehyde spatial distribution overlaps with the continuum one,
tracing the inner 100 au around the protostar. A large number of lines (up to
18), with upper-level energies Eu from 37 K up to 375 K has been detected. We
derived column densities > 10^15 cm^-2 and rotational temperatures Trot between
115 K and 236 K, imaging for the first time hot-corinos around NGC1333-IRAS4B1
and SVS13-A. Conclusions: In multiple systems glycolaldehyde emission is
detected only in one component. The case of the SVS13-A+B and IRAS4-A1+A2
systems support that the detection of glycolaldehyde (at least in the present
Perseus sample) indicates older protostars (i.e. SVS13-A and IRAS4-A2), evolved
enough to develop the hot-corino region (i.e. 100 K in the inner 100 au).
However, only two systems do not allow us to firmly conclude whether the
primary factor leading to the detection of glycolaldehyde emission is the
environments hosting the protostars, evolution (e.g. low value of Lsubmm/Lint),
or accretion luminosity (high Lint).Comment: A&A, in pres
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