607 research outputs found
The abundance of HCN in circumstellar envelopes of AGB stars of different chemical types
A multi-transition survey of HCN (sub-) millimeter line emission from a large
sample of AGB stars of different chemical type is presented. The data are
analysed and circumstellar HCN abundances are estimated. The sample stars span
a large range of properties such as mass-loss rate and photospheric C/O-ratio.
The analysis of the new data allows for more accurate estimates of the
circumstellar HCN abundances and puts new constraints on chemical models. In
order to constrain the circumstellar HCN abundance distribution a detailed
non-LTE excitation analysis, based on the Monte Carlo method, is performed.
Effects of line overlaps and radiative excitation from dust grains are
included. The median values for the derived abundances of HCN (with respect to
H2) are 3x10-5, 7x10-7 and 10-7 for carbon stars (25 stars), S-type AGB stars
(19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes
of the HCN envelopes are similar to those obtained in the case of SiO for the
same sample of sources and agree well with previous results from
interferometric observations, when these are available. We find that there is a
clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of
the star, in that carbon stars have about two orders of magnitude higher
abundances than M-type AGB stars, on average. The derived HCN abundances of the
S-type AGB stars have a larger spread and typically fall in between those of
the two other types, however, slightly closer to the values for the M-type AGB
stars. For the M-type stars, the estimated abundances are much higher than what
would be expected if HCN is formed in thermal equilibrium. However, the results
are also in contrast to predictions from recent non-LTE chemical models, where
very little difference is expected in the HCN abundances between the various
types of AGB stars.Comment: Accepted for publication in A&
Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae
S-type AGB stars have a C/O ratio which suggests that they are transition
objects between oxygen-rich M-type stars and carbon-rich C-type stars. As such,
their circumstellar compositions of gas and dust are thought to be sensitive to
their precise C/O ratio, and it is therefore of particular interest to examine
their circumstellar properties.
We present new Herschel HIFI and PACS sub-millimetre and far-infrared line
observations of several molecular species towards the S-type AGB star W Aql. We
use these observations, which probe a wide range of gas temperatures, to
constrain the circumstellar properties of W Aql, including mass-loss rate and
molecular abundances. We used radiative transfer codes to model the
circumstellar dust and molecular line emission to determine circumstellar
properties and molecular abundances. We assumed a spherically symmetric
envelope formed by a constant mass-loss rate driven by an accelerating wind.
Our model includes fully integrated H2O line cooling as part of the solution of
the energy balance. We detect circumstellar molecular lines from CO, H2O, SiO,
HCN, and, for the first time in an S-type AGB star, NH3. The radiative transfer
calculations result in an estimated mass-loss rate for W Aql of 4.0e-6 Msol
yr-1 based on the 12CO lines. The estimated 12CO/13CO ratio is 29, which is in
line with ratios previously derived for S-type AGB stars. We find an H2O
abundance of 1.5e-5, which is intermediate to the abundances expected for M and
C stars, and an ortho/para ratio for H2O that is consistent with formation at
warm temperatures. We find an HCN abundance of 3e-6, and, although no CN lines
are detected using HIFI, we are able to put some constraints on the abundance,
6e-6, and distribution of CN in W Aql's circumstellar envelope using
ground-based data. We find an SiO abundance of 3e-6, and an NH3 abundance of
1.7e-5, confined to a small envelope.Comment: 17 pages, 15 figure
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 detached dust and gas shells around the carbon star U Ant
Context: Geometrically thin, detached shells of gas have been found around a
handful of carbon stars. --Aims: Previous observations of scattered stellar
light in the circumstellar medium around the carbon star U Ant were taken
through filters centred on the resonance lines of K and Na. These observations
could not separate the scattering by dust and atoms. The aim of this paper is
to remedy this situation. --Methods: We have obtained polarization data on
stellar light scattered in the circumstellar medium around U Ant through
filters which contain no strong lines, making it possible to differentiate
between the two scattering agents. Kinematic, as well as spatial, information
on the gas shells were obtained through high-resolution echelle spectrograph
observations of the KI and NaD lines. --Results: We confirm the existence of
two detached shells around U Ant. The inner shell (at a radius of approx 43"
and a width of approx 2") consists mainly of gas, while the outer shell (at a
radius of approx 50" and a width of approx 7") appears to consist exclusively
of dust. Both shells appear to have an over-all spherical geometry. The gas
shell mass is estimated to be 2x10^-3 M(Sun), while the mass of the dust shell
is estimated to be 5x10^-5 M(Sun). The derived expansion velocity, from the KI
and NaD lines, of the gas shell, 19.5 km/s, agrees with that obtained from CO
radio line data. The inferred shell age is 2700 years. There is structure, e.g.
in the form of arcs, inside the gas shell, but it is not clear whether these
are due to additional shells. --Conclusions: Our results support the hypothesis
that the observed geometrically thin, detached shells around carbon stars are
the results of brief periods of intense mass loss, probably associated with
thermal pulses, and subsequent wind-wind interactions
The ATLAS tile calorimeter digitizer
The ATLAS Tile Calorimeter digitizer system samples photomultiplier signals from the scintillating tiles of the hadronic calorimeter. For each channel a pair of 10-bit ADCs digitize high and low gain signals at 40.08 MHz to provide the necessary 16-bit dynamic range. The sampled data is temporarily stored in digital pipelines for up to 6.375 \muÂs, awaiting a level-1 accept. For each accept received, the corresponding sampled pulse is transferred to a derandomizer buffer for subsequent readout to the data acquisition system (DAQ). The main functionality of the digitizer is implemented in radiation tolerant ASICs, using a fault tolerant architecture to minimize the consequences of radiation induced faults
CO observations of water-maser post-AGB stars and detection of a high-velocity outflow in IRAS 15452-5459
Many aspects of the evolutionary phase in which Asymptotic Giant Branch stars
(AGB stars) are in transition to become Planetary Nebulae (PNe) are still
poorly understood. An important question is how the circumstellar envelopes of
AGB stars switch from spherical symmetry to the axially symmetric structures
frequently observed in PNe. In many cases there is clear evidence that the
shaping of the circumstellar envelopes of PNe is linked to the formation of
jets/collimated winds and their interaction with the remnant AGB envelope.
Because of the short evolutionary time, objects in this phase are rare, but
their identification provides valuable probes for testing evolutionary models.
We have observed (sub)millimeter CO rotational transitions with the APEX
telescope in a small sample of stars hosting high-velocity OH and water masers.
These targets are supposed to have recently left the AGB, as indicated by the
presence of winds traced by masers, with velocities larger than observed during
that phase. We have carried out observations of several CO lines, ranging from
J=2-1 up to J=7-6. In IRAS 15452-5459 we detect a fast molecular outflow in the
central region of the nebula and estimate a mass-loss rate between 1.2x10^{-4}
Msun yr^{-1} (assuming optically thin emission) and 4.9x10^{-4} Msun yr^{-1}
(optically thick emission). We model the SED of this target taking advantage of
our continuum measurement at 345 GHz to constrain the emission at long
wavelengths. For a distance of 2.5 kpc, we estimate a luminosity of 8000 Lsun
and a dust mass of 0.01 Msun. Through the flux in the [CII] line (158 um), we
calculate a total mass of about 12 Msun for the circumstellar envelope, but the
line is likely affected by interstellar contamination.Comment: 12 pages, 9 figures, accepted for publication on A&
Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles - II. CO line survey of evolved stars: derivation of mass-loss rate formulae
We aim to (1) set up simple and general analytical expressions to estimate
mass-loss rates of evolved stars, and (2) from those calculate estimates for
the mass-loss rates of asymptotic giant branch (AGB), red supergiant (RSG), and
yellow hypergiant stars in our galactic sample. Rotationally excited lines of
CO are a very robust diagnostic in the study of circumstellar envelopes (CSEs).
When sampling different layers of the CSE, observations of these molecular
lines lead to detailed profiles of kinetic temperature, expansion velocity, and
density. A state-of-the-art, nonlocal thermal equilibrium, and co-moving frame
radiative transfer code that predicts CO line intensities in the CSEs of
late-type stars is used in deriving relations between stellar and
molecular-line parameters, on the one hand, and mass-loss rate, on the other.
We present analytical expressions for estimating the mass-loss rates of evolved
stellar objects for 8 rotational transitions of the CO molecule, apply them to
our extensive CO data set covering 47 stars, and compare our results to those
of previous studies. Our expressions account for line saturation and resolving
of the envelope, thereby allowing accurate determination of very high mass-loss
rates. We argue that, for estimates based on a single rotational line, the
CO(2-1) transition provides the most reliable mass-loss rate. The mass-loss
rates calculated for the AGB stars range from 4x10^-8 Msun/yr up to 8x10^-5
Msun/yr. For RSGs they reach values between 2x10^-7 Msun/yr and 3x10^-4
Msun/yr. The estimates for the set of CO transitions allow time variability to
be identified in the mass-loss rate. Possible mass-loss-rate variability is
traced for 7 of the sample stars. We find a clear relation between the
pulsation periods of the AGB stars and their derived mass-loss rates, with a
levelling off at approx. 3x10^-5 Msun/yr for periods exceeding 850 days.Comment: Accepted for publication by Astronomy and Astrophysics, 24 pages + 28
pages appendix, 20 figure
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