3,131 research outputs found
Circumstellar environment of the M-type AGB star R Dor. APEX spectral scan at GHz
Our current insights into the circumstellar chemistry of asymptotic giant
branch (AGB) stars are largely based on studies of carbon-rich stars and stars
with high mass-loss rates. In order to expand the current molecular inventory
of evolved stars we present a spectral scan of the nearby, oxygen-rich star R
Dor, a star with a low mass-loss rate (/yr). We
carried out a spectral scan in the frequency ranges 159.0-321.5GHz and
338.5-368.5 GHz (wavelength range 0.8-1.9mm) using the SEPIA/Band-5 and SHeFI
instruments on the APEX telescope and we compare it to previous surveys,
including one of the oxygen-rich AGB star IK Tau, which has a high mass-loss
rate (/yr). The spectrum of R Dor is dominated by
emission lines of SO and the different isotopologues of SiO. We also detect
CO, HO, HCN, CN, PO, PN, SO, and tentatively TiO, AlO, and NaCl.
Sixteen out of approximately 320 spectral features remain unidentified. Among
these is a strong but previously unknown maser at 354.2 GHz, which we suggest
could pertain to HSiO, silanone. With the exception of one, none of these
unidentified lines are found in a similarly sensitive survey of IK Tau
performed with the IRAM 30m telescope. We present radiative transfer models for
five isotopologues of SiO (SiO, SiO, SiO, SiO,
SiO), providing constraints on their fractional abundance and radial
extent. We derive isotopic ratios for C, O, Si, and S and estimate that R Dor
likely had an initial mass in the range 1.3-1.6, in agreement with
earlier findings based on models of HO line emission. From the presence of
spectral features recurring in many of the measured thermal and maser emission
lines we tentatively identify up to five kinematical components in the outflow
of R Dor, indicating deviations from a smooth, spherical wind.Comment: 66 pages, 25 figures, Accepted for publication in Astronomy &
Astrophysics. Fully reduced FITS spectrum made available through CD
Sulphur molecules in the circumstellar envelopes of M-type AGB stars
The sulphur compounds SO and SO have not been widely studied in the
circumstellar envelopes of asymptotic giant branch (AGB) stars. By presenting
and modelling a large number of SO and SO lines in the low mass-loss rate
M-type AGB star R Dor, and modelling the available lines of those molecules in
a further four M-type AGB stars, we aim to determine their circumstellar
abundances and distributions. We use a detailed radiative transfer analysis
based on the accelerated lambda iteration method to model circumstellar SO and
SO line emission and molecular data files for both SO and SO that are
more extensive than those previously available. Using 17 SO lines and 98 SO2
lines to constrain our models for R Dor, we find an SO abundance of
6.7x10 and an SO abundance of 5x10 with both species having
high abundances close to the star. We also modelled SO and found an
abundance of 3.1x10, giving an SO/SO ratio of 21.6. We
derive similar results for the circumstellar SO and SO abundances and their
distributions for the low mass-loss rate object W Hya. For these stars, the
circumstellar SO and SO abundances are much higher than predicted by
chemical models and these two species may account for all available sulphur.
For the higher mass-loss rate stars, we find shell-like SO distributions with
peak abundances that decrease and peak abundance radii that increase with
increasing mass-loss rate. The positions of the peak SO abundance agree very
well with the photodissociation radii of HO. We find evidence that SO is
most likely through the photodissociation of HO and the subsequent reaction
between S and OH. The S-bearing parent molecule appears not to be HS. The
SO models suggest an origin close to the star for this species, also
disagreeing with current chemical models.Comment: 25 page
Molecular hydrogen in the disk of the Herbig Ae star HD97048
We present high-resolution spectroscopic mid-infrared observations of the
circumstellar disk around the Herbig Ae star HD97048 obtained with the VLT
Imager and Spectrometer for the mid-InfraRed (VISIR). We conducted observations
of mid-infrared pure rotational lines of molecular hydrogen (H2) as a tracer of
warm gas in the disk surface layers. In a previous paper, we reported the
detection of the S(1) pure rotational line of H2 at 17.035 microns and argued
it is arising from the inner regions of the disk around the star. We used VISIR
on the VLT for a more comprehensive study based on complementary observations
of the other mid-infrared molecular transitions, namely S(2) and S(4) at 12.278
microns and 8.025 microns respectively, to investigate the physical properties
of the molecular gas in the circumstellar disk around HD97048. We do not detect
neither the S(2) line nor the S(4) H2 line from the disk of HD97048, but we
derive upper limits on the integrated line fluxes which allows us to estimate
an upper limit on the gas excitation temperature, T_ex < 570 K. This limit on
the temperature is consistent with the assumptions previously used in the
analysis of the S(1) line, and allows us to set stronger contraints on the mass
of warm gas in the inner regions of the disk. Indeed, we estimate the mass of
warm gas to be lower than 0.1 M_Jup. We also discuss the probable physical
mechanisms which could be responsible of the excitation of H2 in the disk of
HD97048.Comment: accepted for publication in Ap
The surprisingly carbon-rich environment of the S-type star W Aql
W Aql is an asymptotic giant branch (AGB) star with an atmospheric elemental
abundance ratio C/O0.98 and reported circumstellar molecular
abundances intermediate between those of M-type (C/O1)
AGB stars. This intermediate status is considered typical for S-type stars,
although our understanding of the chemical content of their circumstellar
envelopes (CSEs) is currently rather limited. We performed observations in the
frequency range 159-268 GHz with the APEX telescope and make abundance
estimates through comparison to available spectra towards some well-studied AGB
stars and based on rotational diagram analysis in the case of SiC2. We conclude
that W Aql's CSE appears considerably closer to that of a C-type AGB star than
to that of an M-type AGB star. In particular, we detect emission from C2H,
SiC2, SiN, and HC3N, molecules previously only detected towards the CSEs of
C-type stars. This conclusion, based on the chemistry of the gaseous component
of the CSE, is further supported by reports in the literature on the presence
of atmospheric molecular bands and spectral features of dust species typical
for C-type AGB stars. Although our observations mainly trace species in the
outer regions of the CSE, our conclusion matches closely that based on recent
chemical equilibrium models for the inner wind of S-type stars: the atmospheric
and circumstellar chemistry of S-type stars likely resembles that of C-type AGB
stars much more closely than that of M-type AGB stars. Further observational
investigation of the gaseous circumstellar chemistry of S-type stars is
required to characterise its dependence on the atmospheric C/O. Non-equilibrium
chemical models of the CSEs of AGB stars need to address the particular class
of S-type stars and the chemical variety that is induced by the range in
atmospheric C/O.Comment: 12 pages, 8 figures (+26 pages appendix) Accepted for publication in
Astronomy & Astrophysic
CO and HCN isotopologue ratios in the outflows of AGB stars
Isotopologue line intensity ratios of circumstellar molecules have been
widely used to trace the photospheric elemental isotopic ratios of evolved
stars. However, depending on the molecular species and the physical conditions
of the environment, the circumstellar isotopologue ratio may deviate
considerably from the stellar atmospheric value. In this paper, we aim to
examine how the CO and HCN abundance ratios vary radially due to chemical
reactions in the outflows of AGB stars and the effect of excitation and optical
depth on the resulting line intensity ratios. We find that the circumstellar
12CO/13CO can deviate from its atmospheric value by up to 25-94% and 6-60% for
C- and O-type CSEs, respectively. We show that variations of the intensity of
the ISRF and the gas kinetic temperature can significantly influence the CO
isotopologue ratio in the outer CSEs. On the contrary, the H12CN/H13CN ratio is
stable for all tested mass-loss rates. The RT modeling shows that the
integrated line intensity ratio of CO of different rotational transitions
varies significantly for stars with intermediate mass-loss rates due to
combined chemical and excitation effects. In contrast, the excitation
conditions for the both HCN isotopologues are the same. We demonstrate the
importance of using the isotopologue abundance profiles from chemical models as
inputs to RT models in the interpretation of isotopologue observations.
Previous studies of CO isotopologue ratios are based on multi-transition data
for individual sources and it is difficult to estimate the errors in the
reported values due to assumptions that are not entirely correct according to
this study. If anything, previous studies may have overestimated the
circumstellar 12CO/13CO abundance ratio. The use of the HCN as a tracer of C
isotope ratios is affected by fewer complicating problems, provided one
accounts corrections for high optical depths.Comment: 14 pages, 11 figure
Gender Differences in Vascular Interventions for Lower Limb Ischaemia
AbstractObjectives to investigate changes in intervention for lower limb ischaemia with regard to gender. Material and method type of procedure, the age and gender of all patients (8687) undergoing interventions (12 295) for lower limb ischaemia in Stockholm 1970–1994, were obtained from the National Board of Health and Welfare.Results interventions rose from 18 per million inhabitants in 1970 to 786 in 1994. The proportion of women treated grew from 34% to 48%. The number of embolectomies decreased from 80 to 69 per million. Infrainguinal reconstructions increased most from 38 in 1980–1984 to 186 per million inhabitants in 1990–1994, with the proportion of women increasing from 33 to 43%. Mean age increased from 63 to 71 years. Conclusions there was a marked increase in the number of procedures, the proportion of female patients and mean age
Where is the warm H2 ? A search for H2 emission from disks around Herbig Ae/Be stars
Mid-IR emission lines of H2 are useful probes to determine the mass of warm
gas present in the surface layers of disks. Numerous observations of Herbig
Ae/Be stars (HAeBes) have been performed, but only 2 detections of mid-IR H2
toward HD97048 and AB Aur have been reported. We aim at tracing the warm gas in
the disks of 5 HAeBes with gas-rich environments and physical characteristics
close to those of AB Aur and HD97048, to discuss whether the detections toward
these 2 objects are suggestive of peculiar conditions for the gas. We search
for the H2 S(1) emission line at 17.035 \mu\m with VISIR, and complemented by
CH molecule observations with UVES. We gather the H2 measurements from the
literature to put the new results in context and search for a correlation with
some disk properties. None of the 5 VISIR targets shows evidence for H2
emission. From the 3sigma upper limits on the integrated line fluxes we
constrain the amount of optically thin warm gas to be less than 1.4 M_Jup in
the disk surface layers. There are now 20 HAeBes observed with VISIR and TEXES
instruments to search for warm H2, but only two detections (HD97048 and AB Aur)
were made so far. We find that the two stars with detected warm H2 show at the
same time high 30/13 \mu\m flux ratios and large PAH line fluxes at 8.6 and
11.3 \mu\m compared to the bulk of observed HAeBes and have emission CO lines
detected at 4.7 \mu\m. We detect the CH 4300.3A absorption line toward both
HD97048 and AB Aur with UVES. The CH to H2 abundance ratios that this would
imply if it were to arise from the same component as well as the radial
velocity of the CH lines both suggest that CH arises from a surrounding
envelope, while the detected H2 would reside in the disk. The two detections of
the S(1) line in the disks of HD97048 and AB Aur suggest either peculiar
physical conditions or a particular stage of evolution.Comment: accepted for publication in A&A : 10 pages, 6 figure
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
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