212 research outputs found
Young massive stars in the ISOGAL survey I. VLA observations of the ISOGAL l=+45 field
We present VLA radio continuum observations at 3.6 and 6 cm of a ~0.65
sq.deg. field in the galactic plane at l=+45deg . These observations are meant
to be used in a comparison with ISO observations at 7 and 15 um of the same
region. In this paper we compare the radio results with other radio surveys and
with the IRAS-PSC.
At 3.6 and/or 6 cm we detect a total of 34 discrete sources, 13 of which are
found in five separate extended complexes. These are all multiple or single
extended thermal ultra-compact HII (UCHII) regions. While for each of these
complexes an IRAS counterpart could be reliably found, no IRAS counterpart
could be reliably identified for any of the remaining 21 sources. Of these 21
compact sources, six are candidate UCHII regions, and the other 15 are most
probably background extragalactic non-thermal sources.
The five IRAS sources associated with the radio continuum complexes all
satisfy the Wood & Churchwell (1989; WC89) color criteria for UCHII. None of
the other 38 IRAS point sources present in our surveyed field show the same
colors. This fraction of WC89 type to total IRAS sources is consistent with
what is found over the entire galactic plane. The fact that, when observed with
a compact VLA configuration, the IRAS sources with "UCHII colors" are found to
be associated with arcminute-scale extended sources, rather than with compact
or unresolved radio sources, may have important implications on the estimated
lifetime of UCHII regions.Comment: 15 pages, 22 eps figures, A&A Supp. in press, higher resolution
figures available at http://www.arcetri.astro.it/~lt/preprints/preprints.htm
Dust trapping by spiral arms in gravitationally unstable protostellar discs
In this paper we discuss the influence of gravitational instabilities in
massive protostellar discs on the dynamics of dust grains. Starting from a
Smoothed Particle Hydrodynamics (SPH) simulation, we have computed the
evolution of the dust in a quasi-static gas density structure typical of
self-gravitating disc. For different grain size distributions we have
investigated the capability of spiral arms to trap particles. We have run 3D
radiative transfer simulations in order to construct maps of the expected
emission at (sub-)millimetre and near-infrared wavelengths. Finally, we have
simulated realistic observations of our disc models at (sub-)millimetre and
near-infrared wavelengths as they may appear with the Atacama Large
Millimetre/sub-millimetre Array (ALMA) and the High-Contrast Coronographic
Imager for Adaptive Optics (HiCIAO) in order to investigate whether there are
observational signatures of the spiral structure. We find that the pressure
inhomogeites induced by gravitational instabilities produce a non-negligible
dynamical effect on centimetre sized particles leading to significant
overdensities in spiral arms. We also find that the spiral structure is readily
detectable by ALMA over a wide range of (sub-)millimetre wavelengths and by
HiCIAO in near-infrared scattered light for non-face-on discs located in the
Ophiucus star-forming region. In addition, we find clear spatial spectral index
variations across the disc, revealing that the dust trapping produces a
migration of large grains that can be potentially investigated through
multi-wavelenghts observations in the (sub-)millimetric. Therefore, the spiral
arms observed to date in protoplanetary disc might be interpreted as density
waves induced by the development of gravitational instabilities.Comment: 14 pages, 12 figures. Accepted for publication in MNRA
Spatially resolved PAH emission in the inner disks of Herbig Ae/Be stars
We present adaptive optics high angular resolution (\sim0.1\arcsec)
spectroscopic observations in the 3 \um region of eight well known Herbig Ae/Be
stars with circumstellar disks. We detect the aromatic emission feature at 3.3
\um for four out of six of our objects with flared disks (HD 169142, HD 97048,
HD 100453, HD 100546), some weaker additional features at 3.4 and 3.46 m
and nanodiamond features at 3.43 and 3.53 \um in two of our flared object (HD
100546 and HD 97048 respectively). We also detect hydrogen recombination line
at 3.74 \um in practically all objects. The emission in the polycyclic aromatic
hydrocarbons (PAHs) feature at 3.3 \um, additional and nanodiamond features in
the 3.4-3.5 \um region is, for the first time, spatially resolved in all the
sources where the features are detected. The full-width at half-maximum sizes
that we derive are typical of emission arising in a circumstellar disk. On the
other hand, the continuum emission is unresolved, with the exception of HD
97048 where it is marginally resolved. We compare the observed spatial
distribution of the 3.3 m PAH feature and the adjacent continuum to the
predictions of a disk model that includes transiently heated small grains and
PAHs in addition to large grains in thermal equilibrium \cite[]{habart2004a}.
The model predicts that, as observed, the 3.3 m PAH emission feature is
significantly broader than that of the adjacent continuum and that about 50% of
its integrated intensity comes from a radius 30 AU. We find that the
predicted brightness profiles reproduce very well the observed ones. This
proves beyond doubt that the energetic 3.3 m PAH emission feature takes
its origin in the inner disk regions.Comment: 7 figures, accepted to A&
Detectability of Glycine in Solar-type System Precursors
Glycine (NH2CH2COOH) is the simplest amino acid relevant for life. Its
detection in the interstellar medium is key to understand the formation
mechanisms of pre-biotic molecules and their subsequent delivery onto planetary
systems. Glycine has extensively been searched for toward hot molecular cores,
although these studies did not yield any firm detection. In contrast to hot
cores, low-mass star forming regions, and in particular their earliest stages
represented by cold pre-stellar cores, may be better suited for the detection
of glycine as well as more relevant for the study of pre-biotic chemistry in
young Solar System analogs. We present 1D spherically symmetric radiative
transfer calculations of the glycine emission expected to arise from the
low-mass pre-stellar core L1544. Water vapour has recently been reported toward
this core, indicating that a small fraction of the grain mantles in L1544
(~0.5%) has been injected into the gas phase. Assuming that glycine is
photo-desorbed together with water in L1544, and considering a solid abundance
of glycine on ices of ~1E-4 with respect to water, our calculations reveal that
several glycine lines between 67 GHz and 80 GHz have peak intensities larger
than 10 mK. These results show for the first time that glycine could reach
detectable levels in cold objects such as L1544. This opens up the possibility
to detect glycine, and other pre-biotic species, at the coldest and earliest
stages in the formation of Solar-type systems with near-future instrumentation
such as the Band 2 receivers of ALMA.Comment: 5 pages, 2 figures, 1 tables. Accepted for publication in ApJ Letter
Testing circumstellar disk lifetimes in young embedded clusters associated with the Vela Molecular Ridge
Context. The Vela Molecular Ridge hosts a number of young embedded star
clusters in the same evolutionary stage. Aims. The main aim of the present work
is testing whether the fraction of members with a circumstellar disk in a
sample of clusters in the cloud D of the Vela Molecular Ridge, is consistent
with relations derived for larger samples of star clusters with an age spread.
Besides, we want to constrain the age of the young embedded star clusters
associated with cloud D. Methods. We carried out L (3.78 microns) photometry on
images of six young embedded star clusters associated with cloud D of the Vela
Molecular Ridge, taken with ISAAC at the VLT. These data are complemented with
the available HKs photometry. The 6 clusters are roughly of the same size and
appear to be in the same evolutionary stage. The fraction of stars with a
circumstellar disk was measured in each cluster by counting the fraction of
sources displaying a NIR excess in colour-colour (HKsL) diagrams. Results. The
L photometry allowed us to identify the NIR counterparts of the IRAS sources
associated with the clusters. The fraction of stars with a circumstellar disk
appears to be constant within errors for the 6 clusters. There is a hint that
this is lower for the most massive stars. The age of the clusters is
constrained to ~1-2 Myr. Conclusions. The fraction of stars with a
circumstellar disk in the observed sample is consistent with the relations
derived from larger samples of star clusters and with other age estimates for
cloud D. The fraction may be lower for the most massive stars. Our results
agree with a scenario where all intermediate and low-mass stars form with a
disk, whose lifetime is shorter for higher mass stars.Comment: 13 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
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
Millimeter Imaging of MWC 758: Probing the Disk Structure and Kinematics
We investigate the structure and kinematics of the circumstellar disk around the Herbig Ae star MWC 758 using high-resolution observations of the ^(12)CO (3-2) and dust continuum emission at the wavelengths of 0.87 and 3.3 mm. We find that the dust emission peaks at an orbital radius of about 100 AU, while the CO intensity has a central peak coincident with the position of the star. The CO emission is in agreement with a disk in Keplerian rotation around a 2.0 M_⊙ star, confirming that MWC 758 is indeed an intermediate-mass star. By comparing the observation with theoretical disk models, we derive that the disk surface density Σ(r) steeply increases from 40 to 100 AU and decreases exponentially outward. Within 40 AU, the disk has to be optically thin in the continuum emission at millimeter wavelengths to explain the observed dust morphology, though our observations lack the angular resolution and sensitivity required to constrain the surface density on these spatial scales. The surface density distribution in MWC 758 disk is similar to that of "transition" disks, though no disk clearing has been previously inferred from the analysis of the spectral energy distribution (SED). Moreover, the asymmetries observed in the dust and CO emission suggest that the disk may be gravitationally perturbed by a low-mass companion orbiting within a radius of 30 AU. Our results emphasize that SEDs alone do not provide a complete picture of disk structure and that high-resolution millimeter-wave images are essential to reveal the structure of the cool disk mid-plane
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