2,779 research outputs found
Non-equilibrium chemistry and dust formation in AGB stars as probed by SiO line emission
We have performed high spatial resolution observations of SiO line emission
for a sample of 11 AGB stars using the ATCA, VLA and SMA interferometers.
Detailed radiative transfer modelling suggests that there are steep chemical
gradients of SiO in their circumstellar envelopes. The emerging picture is one
where the radial SiO abundance distribution starts at an initial high
abundance, in the case of M-stars consistent with LTE chemistry, that
drastically decreases at a radius of ~1E15 cm. This is consistent with a
scenario where SiO freezes out onto dust grains. The region of the wind with
low abundance is much more extended, typically ~1E16 cm, and limited by
photodissociation. The surpisingly high SiO abundances found in carbon stars
requires non-equilibrium chemical processes.Comment: 2 pages, 1 figure. To be published in the proceedings of the
conference "Why Galaxies Care about AGB Stars", held in Vienna, August 7-11,
2006; F. Kerschbaum, C. Charbonnel, B. Wing eds, ASP Conf.Ser. in pres
Models for integrated and differential scattering optical properties of encapsulated light absorbing carbon aggregates
Optical properties of light absorbing carbon (LAC) aggregates encapsulated in a shell of sulfate are computed for realistic model geometries based on field measurements. Computations are performed for wavelengths from the UV-C to the mid-IR. Both climate- and remote sensing-relevant optical properties are considered. The results are compared to commonly used simplified model geometries, none of which gives a realistic representation of the distribution of the LAC mass within the host material and, as a consequence, fail to predict the optical properties accurately. A new core-gray shell model is introduced, which accurately reproduces the size- and wavelength dependence of the integrated and differential optical properties
ALMA view of the circumstellar environment of the post-common-envelope-evolution binary system HD101584
We study the circumstellar evolution of the binary HD101584, consisting of a
post-AGB star and a low-mass companion, which is most likely a
post-common-envelope-evolution system. We used ALMA observations of the 12CO,
13CO, and C18O J=2-1 lines and the 1.3mm continuum to determine the morphology,
kinematics, masses, and energetics of the circumstellar environment. The
circumstellar medium has a bipolar hour-glass structure, seen almost pole-on,
formed by an energetic jet, about 150 km/s. We conjecture that the
circumstellar morphology is related to an event that took place about 500 year
ago, possibly a capture event where the companion spiraled in towards the AGB
star. However, the kinetic energy of the accelerated gas exceeds the released
orbital energy, and, taking into account the expected energy transfer
efficiency of the process, the observed phenomenon does not match current
common-envelope scenarios. This suggests that another process must augment, or
even dominate, the ejection process. A significant amount of material resides
in an unresolved region, presumably in the equatorial plane of the binary
system.Comment: A&A Letter, accepte
Modelling mineral dust using stereophotogrammetry
Real, three-dimensional shape of a dust particle is derived from a pair of scanning-electron microscope images by means of stereophotogrammetry. The resulting shape is discretized, and preliminary discrete-dipole-approximation computations for the single dust particle reveal that scattering by such an irregular shape differs notably from scattering by a sphere or a Gaussian random sphere which both are frequently used shape models for dust particles
ALMA observations of the variable 12CO/13CO ratio around the asymptotic giant branch star R Sculptoris
[abridged] The 12CO/13CO ratio is often used as a measure of the 12C/13C
ratio in the circumstellar environment, carrying important information about
the stellar nucleosynthesis. External processes can change the 12CO and 13CO
abundances, and spatially resolved studies of the 12CO/13CO ratio are needed to
quantify the effect of these processes on the globally determined values.
Additionally, such studies provide important information on the conditions in
the circumstellar environment. The detached-shell source R Scl, displaying CO
emission from recent mass loss, in a binary-induced spiral structure as well as
in a clumpy shell produced during a thermal pulse, provides a unique laboratory
for studying the differences in CO isotope abundances throughout its recent
evolution. We observed both the 12CO(J=3-2) and the 13CO(J=3-2) line using
ALMA. We find significant variations in the 12CO/13CO intensity ratios and
consequently in the abundance ratios. The average CO isotope abundance ratio is
at least a factor three lower in the shell (~19) than that in the present-day
(60). Additionally, variations in the ratio of more
than an order of magnitude are found in the shell itself. We attribute these
variations to the competition between selective dissociation and isotope
fractionation in the shell, of which large parts cannot be warmer than ~35 K.
However, we also find that the 12CO/13CO ratio in the present-day mass loss is
significantly higher than the 12C/13C ratio determined in the stellar
photosphere from molecular tracers (~19). The origin of this discrepancy is
still unclear, but we speculate that it is due to an embedded source of
UV-radiation that is primarily photo-dissociating 13CO. This radiation source
could be the hitherto hidden companion. Alternatively, the UV-radiation could
originate from an active chromosphere of R Scl itself....Comment: 6 pages, 5 figures, online data available at
http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=J/A+A/556/L
HD101584: Circumstellar characteristics and evolutionary status
We have performed a study of the characteristics of the circumstellar
environment of the binary object HD101584, that provides information on a
likely evolutionary scenario. We have obtained and analysed ALMA observations,
complemented with observations using APEX, of a large number of molecular
lines. An analysis of the spectral energy distribution has also been performed.
Emissions from 12 molecular species (not counting isotopologues) have been
observed, and most of them mapped with angular resolutions in the range 0.1" to
0.6". Four circumstellar components are identified: i) a central compact source
of size 0.15", ii) an expanding equatorial density enhancement (a flattened
density distribution in the plane of the orbit) of size 3", iii) a bipolar
high-velocity outflow (150 km/s), and iv) an hourglass structure. The outflow
is directed almost along the line of sight. There is evidence of a second
bipolar outflow. The mass of the circumstellar gas is 0.5[D/1 kpc]^2 Msun,
about half of it lies in the equatorial density enhancement. The dust mass is
0.01[D/1 kpc]^2 Msun, and a substantial fraction of this is in the form of
large-sized, up to 1 mm, grains. The estimated kinetic age of the outflow is
770[D/1 kpc] yr. The kinetic energy and the scalar momentum of the accelerated
gas are estimated to be 7x10^(45)[D/1 kpc]^2 erg and 10^(39)[D/1 kpc]^2 g cm/s,
respectively. We provide good evidence that the binary system HD101584 is in a
post-common-envelope-evolution phase, that ended before a stellar merger.
Isotope ratios combined with stellar mass estimates suggest that the primary
star's evolution was terminated already on the first red giant branch (RGB).
Most of the energy required to drive the outflowing gas was probably released
when material fell towards the companion.Comment: Accepted for publication in A&
The GOAL study: a prospective examination of the impact of factor V Leiden and ABO(H) blood groups on haemorrhagic and thrombotic pregnancy outcomes
Factor V Leiden (FVL) and ABO(H) blood groups are the common influences on haemostasis and retrospective studies have linked FVL with pregnancy complications. However, only one sizeable prospective examination has taken place. As a result, neither the impact of FVL in unselected subjects, any interaction with ABO(H) in pregnancy, nor the utility of screening for FVL is defined. A prospective study of 4250 unselected pregnancies was carried out. A venous thromboembolism (VTE) rate of 1·23/1000 was observed, but no significant association between FVL and pre-eclampsia, intra-uterine growth restriction or pregnancy loss was seen. No influence of FVL and/or ABO(H) on ante-natal bleeding or intra-partum or postpartum haemorrhage was observed. However, FVL was associated with birth-weights >90th centile [odds ratio (OR) 1·81; 95% confidence interval (CI<sub>95</sub>) 1·04–3·31] and neonatal death (OR 14·79; CI<sub>95</sub> 2·71–80·74). No association with ABO(H) alone, or any interaction between ABO(H) and FVL was observed. We neither confirmed the protective effect of FVL on pregnancy-related blood loss reported in previous smaller studies, nor did we find the increased risk of some vascular complications reported in retrospective studies
Single scattering by realistic, inhomogeneous mineral dust particles with stereogrammetric shapes
Light scattering by single, inhomogeneous mineral dust particles was
simulated based on shapes and compositions derived directly from measurements
of real dust particles instead of using a mathematical shape model. We
demonstrate the use of the stereogrammetric shape retrieval method in the context
of single-scattering modelling of mineral dust for four different dust types
– all of them inhomogeneous – ranging from compact, equidimensional shapes
to very elongated and aggregate shapes. The three-dimensional particle shapes
were derived from stereo pairs of scanning-electron microscope images, and
inhomogeneous composition was determined by mineralogical interpretation of
localized elemental information based on energy-dispersive spectroscopy.
Scattering computations were performed for particles of equal-volume
diameters, from 0.08 μm up to 2.8 μm at 550 nm wavelength, using the
discrete-dipole approximation. Particle-to-particle variation in scattering
by mineral dust was found to be quite considerable and was not well
reproduced by simplified shapes of homogeneous spheres, spheroids, or
Gaussian random spheres. Effective-medium approximation results revealed that
particle inhomogeneity should be accounted for even for small amounts of
absorbing media (here up to 2% of the volume), especially when considering
scattering by inhomogeneous particles at size parameters 3<<i>x</i><8. When
integrated over a log-normal size distribution, the linear depolarization
ratio and single-scattering albedo were also found to be sensitive to
inhomogeneity. The methodology applied is work-intensive and the
light-scattering method used quite limited in terms of size parameter
coverage. It would therefore be desirable to find a sufficiently accurate but
simpler approach with fewer limitations for single-scattering modelling of
dust. For validation of such a method, the approach presented here could be
used for producing reference data when applied to a suitable set of target
particles
The circumstellar envelope around the S-type AGB star W Aql Effects of an eccentric binary orbit
The CO(J=3-2) emission from the CSE of the binary S-type AGB star W Aql has
been observed at subarcsecond resolution using ALMA. The aim of this paper is
to investigate the wind properties of the AGB star and to analyse how the known
companion has shaped the CSE. The average mass-loss rate during the creation of
the detected CSE is estimated through modelling, using the ALMA brightness
distribution and previously published single-dish measurements as observational
constraints. The ALMA observations are presented and compared to the results
from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with
the same properties as the W Aql system and with two different orbital
eccentricities. Three-dimensional radiative transfer modelling is performed and
the response of the interferometer is modelled and discussed. The estimated
average mass-loss rate of W~Aql agrees with previous results. The size of the
emitting region is consistent with photodissociation models. The CO(J=3-2)
emission is dominated by a smooth component overlayed with two weak arc
patterns with different separations. The larger pattern is predicted by the
binary interaction model with separations of 10" and therefore likely due to
the known companion. It is consistent with a binary orbit with low
eccentricity. The smaller separation pattern is asymmetric and coincides with
the dust distribution, but the separation timescale (200 yrs) is not consistent
with any known process of the system. The separation of the known companions of
the system is large enough to not have a very strong effect on the
circumstellar morphology. The density contrast across the envelope of a binary
with an even larger separation will not be easily detectable, even with ALMA,
unless the orbit is strongly asymmetric or the AGB star has a much larger
mass-loss rate.Comment: 10 pages, 8 figure
G359.87+0.18: An FR II Radio Galaxy 15 Arcminutes from Sgr A*. Implications for the Scattering Region in the Galactic Center
G359.87+0.18 is an enigmatic object located 15' from Sgr A*. It has been
variously classified as an extragalactic source, Galactic jet source, and young
supernova remnant. We present new observations of G359.87+0.18 between 0.33 and
15 GHz and use these to argue that this source is an Faranoff-Riley II radio
galaxy. We are able to place a crude limit on its redshift of z > 0.1. The
source has a spectral index \alpha < -1 (S \propto \nu^\alpha), suggestive of a
radio galaxy with a redshift z >~ 2.
The scattering diameters of Sgr A* and several nearby OH masers (~ 1" at 1
GHz) indicate that a region of enhanced scattering is along the line of sight
to the Galactic center. If the region covers the Galactic center uniformly, the
implied diameter for a background source is at least 600" at 0.33 GHz, in
contrast with the observed 20" diameter of G359.87+0.18. Using the scattering
diameter of a nearby OH maser OH 359.762+0.120 and the widths of two, nearby,
non-thermal threads, G0.08+0.15 and G359.79+0.17, we show that a uniform
scattering region should cover G359.87+0.18. We therefore conclude that the
Galactic center scattering region is inhomogeneous on a scale of 5' (~ 10 pc at
a distance of 8.5 kpc). This scale is comparable to the size scale of molecular
clouds in the Galactic center. The close agreement between these two lengths
scales is an indication that the scattering region is linked intimately to the
Galactic center molecular clouds.Comment: Accepted for publication in the ApJ, vol. 515, LaTeX2e manuscript
using aaspp4 macro, 19 pages, 8 figures in 11 PostScript file
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