612 research outputs found
Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tauri as seen by Herschel/HIFI
During their asymptotic giant branch evolution, low-mass stars lose a significant fraction of their mass through an intense wind, enriching the interstellar medium with products of nucleosynthesis. We observed the nearby oxygen-rich asymptotic giant branch star IK Tau using the highresolution HIFI spectrometer onboard Herschel. We report on the first detection of H^(16)_2O and the rarer isotopologues H^(17)_2O and H^(18)_2O in both the
ortho and para states. We deduce a total water content (relative to molecular hydrogen) of 6.6 × 10^(−5), and an ortho-to-para ratio of 3:1. These results are consistent with the formation of H_2O in thermodynamical chemical equilibrium at photospheric temperatures, and does not require pulsationally induced non-equilibrium chemistry, vaporization of icy bodies or grain surface reactions. High-excitation lines of ^(12)CO, ^(13)CO, ^(28)SiO, ^(29)SiO, ^(30)SiO, HCN, and SO have also been detected. From the observed line widths, the acceleration region in the inner wind zone can be characterized, and we show that the wind acceleration is slower than hitherto anticipated
Eyes in the sky: Interactions between AGB winds and the interstellar magnetic field
We aim to examine the role of the interstellar magnetic field in shaping the
extended morphologies of slow dusty winds of Asymptotic Giant-branch (AGB)
stars in an effort to pin-point the origin of so-called eye shaped CSE of three
carbon-rich AGB stars. In addition, we seek to understand if this pre-planetary
nebula (PN) shaping can be responsible for asymmetries observed in PNe.
Hydrodynamical simulations are used to study the effect of typical interstellar
magnetic fields on the free-expanding spherical stellar winds as they sweep up
the local interstellar medium (ISM). The simulations show that typical Galactic
interstellar magnetic fields of 5 to 10 muG, are sufficient to alter the
spherical expanding shells of AGB stars to appear as the characteristic eye
shape revealed by far-infrared observations. The typical sizes of the simulated
eyes are in accordance with the observed physical sizes. However, the eye
shapes are of transient nature. Depending on the stellar and interstellar
conditions they develop after 20,000 to 200,000yrs and last for about 50,000 to
500,000 yrs, assuming that the star is at rest relative to the local
interstellar medium. Once formed the eye shape will develop lateral outflows
parallel to the magnetic field. The "explosion" of a PN in the center of the
eye-shaped dust shell gives rise to an asymmetrical nebula with prominent
inward pointing Rayleigh-Taylor instabilities.
Interstellar magnetic fields can clearly affect the shaping of wind-ISM
interaction shells. The occurrence of the eyes is most strongly influenced by
stellar space motion and ISM density. Observability of this transient phase is
favoured for lines-of-sight perpendicular to the interstellar magnetic field
direction. The simulations indicate that shaping of the pre-PN envelope can
strongly affect the shape and size of PNe.Comment: Accepted for publication in A&A. Final version will contain animated
result
Estimating Stellar Parameters from Spectra using a Hierarchical Bayesian Approach
A method is developed for fitting theoretically predicted astronomical
spectra to an observed spectrum. Using a hierarchical Bayesian principle, the
method takes both systematic and statistical measurement errors into account,
which has not been done before in the astronomical literature. The goal is to
estimate fundamental stellar parameters and their associated uncertainties. The
non-availability of a convenient deterministic relation between stellar
parameters and the observed spectrum, combined with the computational
complexities this entails, necessitate the curtailment of the continuous
Bayesian model to a reduced model based on a grid of synthetic spectra. A
criterion for model selection based on the so-called predictive squared error
loss function is proposed, together with a measure for the goodness-of-fit
between observed and synthetic spectra. The proposed method is applied to the
infrared 2.38--2.60 \mic ISO-SWS data (Infrared Space Observatory - Short
Wavelength Spectrometer) of the star Bootis, yielding estimates for
the stellar parameters: effective temperature \Teff = 4230 83 K, gravity
g = 1.50 0.15 dex, and metallicity [Fe/H] = dex.Comment: 15 pages, 8 figures, 5 tables. Accepted for publication in MNRA
Confirmation of circumstellar phosphine
Phosphine (PH3) was tentatively identified a few years ago in the carbon star
envelopes IRC+10216 and CRL2688 from observations of an emission line at 266.9
GHz attributable to the J=1-0 rotational transition. We report the detection of
the J=2-1 rotational transition of PH3 in IRC+10216 using the HIFI instrument
on board Herschel, which definitively confirms the identification of PH3.
Radiative transfer calculations indicate that infrared pumping to excited
vibrational states plays an important role in the excitation of PH3 in the
envelope of IRC+10216, and that the observed lines are consistent with
phosphine being formed anywhere between the star and 100 R* from the star, with
an abundance of 1e-8 relative to H2. The detection of PH3 challenges chemical
models, none of which offers a satisfactory formation scenario. Although PH3
locks just 2 % of the total available phosphorus in IRC+10216, it is together
with HCP, one of the major gas phase carriers of phosphorus in the inner
circumstellar layers, suggesting that it could be also an important phosphorus
species in other astronomical environments. This is the first unambiguous
detection of PH3 outside the solar system, and a further step towards a better
understanding of the chemistry of phosphorus in space.Comment: Accepted for publication in ApJ Letter
Structure of the outer layers of cool standard stars
Context: Among late-type red giants, an interesting change occurs in the
structure of the outer atmospheric layers as one moves to later spectral types
in the Hertzsprung-Russell diagram: a chromosphere is always present, but the
coronal emission diminishes and a cool massive wind steps in.
Aims: Where most studies have focussed on short-wavelength observations, this
article explores the influence of the chromosphere and the wind on
long-wavelength photometric measurements.
Methods: The observational spectral energy distributions are compared with
the theoretical predictions of the MARCS atmosphere models for a sample of 9 K-
and M-giants. The discrepancies found are explained using basic models for flux
emission originating from a chromosphere or an ionized wind.
Results: For 7 out of 9 sample stars, a clear flux excess is detected at
(sub)millimeter and/or centimeter wavelengths. The precise start of the excess
depends upon the star under consideration. The flux at wavelengths shorter than
about 1 mm is most likely dominated by an optically thick chromosphere, where
an optically thick ionized wind is the main flux contributor at longer
wavelengths.
Conclusions: Although the optical to mid-infrared spectrum of the studied K-
and M-giants is well represented by a radiative equilibrium atmospheric model,
the presence of a chromosphere and/or ionized stellar wind at higher altitudes
dominates the spectrum in the (sub)millimeter and centimeter wavelength ranges.
The presence of a flux excess also has implications on the role of these stars
as fiducial spectrophotometric calibrators in the (sub)millimeter and
centimeter wavelength range.Comment: 13 pages, 6 figures, 7 pages of online material, submitted to A&
PACS and SPIRE range spectroscopy of cool, evolved stars
Context: At the end of their lives AGB stars are prolific producers of dust
and gas. The details of this mass-loss process are still not understood very
well. Herschel PACS and SPIRE spectra offer a unique way of investigating
properties of AGB stars in general and the mass-loss process in particular.
Methods: The HIPE software with the latest calibration is used to process the
available PACS and SPIRE spectra of 40 evolved stars. The spectra are convolved
with the response curves of the PACS and SPIRE bolometers and compared to the
fluxes measured in imaging data of these sources. Custom software is used to
identify lines in the spectra, and to determine the central wavelengths and
line intensities. Standard molecular line databases are used to associate the
observed lines. Because of the limited spectral resolution of the spectrometers
several known lines are typically potential counterparts to any observed line.
To help identifications the relative contributions in line intensity of the
potential counterpart lines are listed for three characteristic temperatures
based on LTE calculations and assuming optically thin emission. Result: The
following data products are released: the reduced spectra, the lines that are
measured in the spectra with wavelength, intensity, potential identifications,
and the continuum spectra, i.e. the full spectra with all identified lines
removed. As simple examples of how this data can be used in future studies we
have fitted the continuum spectra with three power laws and find that the few
OH/IR stars seem to have significantly steeper slopes than the other oxygen-
and carbon-rich objects in the sample. As another example we constructed
rotational diagrams for CO and fitted a two-component model to derive
rotational temperatures.Comment: A&A accepte
ALMA data suggest the presence of a spiral structure in the inner wind of CW Leo
(abbreviated) We aim to study the inner wind of the well-known AGB star CW
Leo. Different diagnostics probing different geometrical scales have pointed
toward a non-homogeneous mass-loss process: dust clumps are observed at
milli-arcsec scale, a bipolar structure is seen at arcsecond-scale and
multi-concentric shells are detected beyond 1". We present the first ALMA Cycle
0 band 9 data around 650 GHz. The full-resolution data have a spatial
resolution of 0".42x0".24, allowing us to study the morpho-kinematical
structure within ~6". Results: We have detected 25 molecular lines. The
emission of all but one line is spatially resolved. The dust and molecular
lines are centered around the continuum peak position. The dust emission has an
asymmetric distribution with a central peak flux density of ~2 Jy. The
molecular emission lines trace different regions in the wind acceleration
region and suggest that the wind velocity increases rapidly from about 5 R*
almost reaching the terminal velocity at ~11 R*. The channel maps for the
brighter lines show a complex structure; specifically for the 13CO J=6-5 line
different arcs are detected within the first few arcseconds. The curved
structure present in the PV map of the 13CO J=6-5 line can be explained by a
spiral structure in the inner wind, probably induced by a binary companion.
From modeling the ALMA data, we deduce that the potential orbital axis for the
binary system lies at a position angle of ~10-20 deg to the North-East and that
the spiral structure is seen almost edge-on. We infer an orbital period of 55
yr and a binary separation of 25 au (or ~8.2 R*). We tentatively estimate that
the companion is an unevolved low-mass main-sequence star. The ALMA data hence
provide us for the first time with the crucial kinematical link between the
dust clumps seen at milli-arcsecond scale and the almost concentric arcs seen
at arcsecond scale.Comment: 22 pages, 18 Figures, Astronomy & Astrophysic
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&
Circumstellar molecular composition of the oxygen-rich AGB star IK Tau: I. Observations and LTE chemical abundance analysis
The aim of this paper is to study the molecular composition in the
circumstellar envelope around the oxygen-rich star IK Tau. We observed IK Tau
in several (sub)millimeter bands using the APEX telescope during three
observing periods. To determine the spatial distribution of the
emission, mapping observations were performed. To
constrain the physical conditions in the circumstellar envelope, multiple
rotational CO emission lines were modeled using a non local thermodynamic
equilibrium radiative transfer code. The rotational temperatures and the
abundances of the other molecules were obtained assuming local thermodynamic
equilibrium. An oxygen-rich Asymptotic Giant Branch star has been surveyed in
the submillimeter wavelength range. Thirty four transitions of twelve molecular
species, including maser lines, were detected. The kinetic temperature of the
envelope was determined and the molecular abundance fractions of the molecules
were estimated. The deduced molecular abundances were compared with
observations and modeling from the literature and agree within a factor of 10,
except for SO, which is found to be almost a factor 100 stronger than
predicted by chemical models. From this study, we found that IK Tau is a good
laboratory to study the conditions in circumstellar envelopes around
oxygen-rich stars with (sub)millimeter-wavelength molecular lines. We could
also expect from this study that the molecules in the circumstellar envelope
can be explained more faithful by non-LTE analysis with lower and higher
transition lines than by simple LTE analysis with only lower transition lines.
In particular, the observed CO line profiles could be well reproduced by a
simple expanding envelope model with a power law structure.Comment: 13 pages, 14 figures, 8 tables *Accepted for publication in Astronomy
and Astrophysic
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