2,613 research outputs found
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
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
Sulphur-bearing molecules in AGB stars I: The occurrence of hydrogen sulfide
Through a survey of (sub-)millimetre emission lines of various
sulphur-bearing molecules, we aim to determine which molecules are the primary
carriers of sulphur in different types of AGB stars. In this paper, the first
in a series, we investigate the occurrence of HS in AGB circumstellar
envelopes and determine its abundance, where possible. We have surveyed 20 AGB
stars with a range of mass-loss rates and of different chemical types using the
APEX telescope to search for rotational transition lines of five key
sulphur-bearing molecules: CS, SiS, SO, SO and HS. Here we present our
results for HS, including detections, non-detections and detailed radiative
transfer modelling of the detected lines. We compare results based on different
descriptions of the molecular excitation of HS and different abundance
distributions, including those derived from chemical modelling results. We
detected HS towards five AGB stars, all of which have high mass-loss rates
of yr and are oxygen-rich. HS
was not detected towards the carbon or S-type stars that fall in a similar
mass-loss range. For the stars in our sample with detections, we find peak
o-HS abundances relative to H between and . Overall, we conclude that HS can play a significant role in
oxygen-rich AGB stars with higher mass-loss rates, but is unlikely to play a
key role in stars of other chemical types or the lower mass-loss rate
oxygen-rich stars. For two sources, V1300 Aql and GX Mon, HS is most likely
the dominant sulphur-bearing molecule in the circumstellar envelope.Comment: 14 pages, 7 figures, accepted in A&
ISOCAM observations of the L1551 star formation region
The results of a deep mid-IR ISOCAM survey of the L1551 dark molecular cloud
are presented. The aim of this survey is a search for new YSO (Young Stellar
Object) candidates, using two broad-band filters centred at 6.7 and 14.3
micron. Although two regions close to the centre of L1551 had to be avoided due
to saturation problems, 96 sources were detected in total (76 sources at 6.7
micron and 44 sources at 14.3 micron). Using the 24 sources detected in both
filters, 14 were found to have intrinsic mid-IR excess at 14.3 micron and were
therefore classified as YSO candidates. Using additional observations in B, V,
I, J, H and K obtained from the ground, most candidates detected at these
wavelengths were confirmed to have mid-IR excess at 6.7 micron as well, and
three additional YSO candidates were found. Prior to this survey only three
YSOs were known in the observed region (avoiding L1551 IRS5/NE and HL/XZ Tau).
This survey reveals 15 new YSO candidates, although several of these are
uncertain due to their extended nature either in the mid-IR or in the
optical/near-IR observations. Two of the sources with mid-IR excess are
previously known YSOs, one is a brown dwarf MHO 5 and the other is the well
known T Tauri star HH30, consisting of an outflow and an optically thick disk
seen edge on.Comment: 14 Pages, 8 Figure
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
First detection of methanol towards a post-AGB object, HD101584
The circumstellar environments of objects on the asymptotic giant branch and
beyond are rich in molecular species. Nevertheless, methanol has never been
detected in such an object, and is therefore often taken as a clear signpost
for a young stellar object. However, we report the first detection of CH3OH in
a post-AGB object, HD101584, using ALMA. Its emission, together with emissions
from CO, SiO, SO, CS, and H2CO, comes from two extreme velocity spots on either
side of the object where a high-velocity outflow appears to interact with the
surrounding medium. We have derived molecular abundances, and propose that the
detected molecular species are the effect of a post-shock chemistry where
circumstellar grains play a role. We further provide evidence that HD101584 was
a low-mass, M-type AGB star
Search for aluminium monoxide in the winds of oxygen-rich AGB stars
Aluminium monoxide, AlO, is likely efficiently depleted from the gas around
oxygen-rich evolved stars to form alumina clusters and dust seeds. Its presence
in the extended atmospheres of evolved stars has been derived from optical
spectroscopy. More recently, AlO gas was also detected at long wavelengths
around the supergiant VY CMa and the oxygen-rich asymptotic giant branch (AGB)
star o Cet (Mira A). In search of AlO, we mined data obtained with APEX, the
IRAM 30m telescope, Herschel/HIFI, SMA, and ALMA, which were primarily aimed at
studying other molecular species. We report here on observations of AlO towards
a sample of eight oxygen-rich AGB stars in different rotational transitions, up
to seven for some stars. We present definite detections of one rotational
transition of AlO for o Cet and R Aqr, and tentative detections of one
transition for R Dor and o Cet, and two for IK Tau and W Hya. The presented
spectra of WX Psc, R Cas, and TX Cam show no signature of AlO. For o Cet, R
Aqr, and IK Tau, we find that the AlO(N=9-8) emission likely traces the inner
parts of the wind, out to only a few tens of AU, where the gas has not yet
reached its terminal velocity. The conclusive detections of AlO emission in the
case of o Cet and R Aqr confirm the presence of AlO gas in outflows of AGB
stars. The tentative detections further support this. Since most of the
observations presented in this study were obtained with stronger emission from
other species than AlO in mind, observations with higher sensitivity in
combination with high angular resolution will improve our understanding of the
presence and behaviour of AlO. From the current data sets we cannot firmly
conclude whether there is a direct correlation between the wind properties and
the detection rate of AlO emission. We hope that this study can serve as a
stimulus to perform sample studies in search of AlO in oxygen-rich outflows.Comment: 9 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic
The Star Clusters in the Irregular Galaxy NGC 4449
We examine the star clusters in the irregular galaxy NGC 4449. We use a
near-infrared spectrum and broad-band images taken with the HST to place a
limit of 8--15 Myrs on the age of the bright central ojbect in NGC 4449. Its
luminosity and size suggest that it is comparable to young super star clusters.
However, there is a peculiar nucleated-bar structure at the center of this star
cluster, and we suggest that this structure is debris from the interaction that
has produced the counter-rotating gas systems and extended gas streamers in the
galaxy.
From the images we identify 60 other candidate compact star clusters in NGC
4449. Fourteen of these could be background elliptical galaxies or old globular
star clusters. Of the star clusters, three, in addition to the central object,
are potentially super star clusters, and many others are comparable to the
populous clusters found in the LMC. The star clusters span a large range in
ages with no obvious peak in cluster formation that might be attributed to the
interaction that the galaxy has experienced.Comment: To be published in PASP, Feb. 2001; also attainable from
ftp.lowell.edu, cd pub/dah/n4449pape
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&
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