355 research outputs found
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
ALMA observations of the vibrationally-excited rotational CO transition towards five AGB stars
We report the serendipitous detection with ALMA of the vibrationally-excited
pure-rotational CO transition towards five asymptotic giant branch
(AGB) stars, Cet, R Aqr, R Scl, W Aql, and Gru. The observed lines
are formed in the poorly-understood region located between the stellar surface
and the region where the wind starts, the so-called warm molecular layer. We
successfully reproduce the observed lines profiles using a simple model. We
constrain the extents, densities, and kinematics of the region where the lines
are produced. R Aqr and R Scl show inverse P-Cygni line profiles which indicate
infall of material onto the stars. The line profiles of Cet and R Scl show
variability. The serendipitous detection towards these five sources shows that
vibrationally-excited rotational lines can be observed towards a large number
of nearby AGB stars using ALMA. This opens a new possibility for the study of
the innermost regions of AGB circumstellar envelopes.Comment: 6 pages, 2 figures, 2 tables, 2016MNRAS.463L..74
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
Micron-sized forsterite grains in the pre-planetary nebula of IRAS 17150-3224 - Searching for clues on the mysterious evolution of massive AGB stars
We study the grain properties and location of the forsterite crystals in the
circumstellar environment of the pre-planetary nebula (PPN) IRAS 17150-3224 in
order to learn more about the as yet poorly understood evolutionary phase prior
to the PPN. We use the best-fit model for IRAS 17150-3224 of Meixner et al.
(2002) and add forsterite to this model. We investigate different spatial
distributions and grain sizes of the forsterite crystals in the circumstellar
environment. We compare the spectral bands of forsterite in the mid-infrared
and at 69 micrometre in radiative transport models to those in ISO-SWS and
Herschel/PACS observations. We can reproduce the non-detection of the
mid-infrared bands and the detection of the 69 micrometre feature with models
where the forsterite is distributed in the whole outflow, in the superwind
region, or in the AGB-wind region emitted previous to the superwind, but we
cannot discriminate between these three models. To reproduce the observed
spectral bands with these three models, the forsterite crystals need to be
dominated by a grain size population of 2 micrometre up to 6 micrometre. We
hypothesise that the large forsterite crystals were formed after the superwind
phase of IRAS 17150-3224, where the star developed an as yet unknown hyperwind
with an extremely high mass-loss rate (10^-3 Msol/yr). The high densities of
such a hyperwind could be responsible for the efficient grain growth of both
amorphous and crystalline dust in the outflow. Several mechanisms are discussed
that might explain the lower-limit of 2 micrometre found for the forsterite
grains, but none are satisfactory. Among the mechanisms explored is a possible
selection effect due to radiation pressure based on photon scattering on
micron-sized grains.Comment: Accepted by A&
The problematically short superwind of OH/IR stars - Probing the outflow with the 69 {\mu}m spectral band of forsterite
Spectra of OH/IR stars show prominent spectral bands of crystalline olivine
(MgFeSiO). To learn more about the timescale of the
outflows of OH/IR stars, we study the spectral band of crystalline olivine at
69 {\mu}m. The 69 {\mu}m band is of interest because its width and peak
wavelength position are sensitive to the grain temperature and to the exact
composition of the crystalline olivine. With Herschel/PACS, we observed the 69
{\mu}m band in the outflow of 14 OH/IR stars. By comparing the crystalline
olivine features of our sample with those of model spectra, we determined the
size of the outflow and its crystalline olivine abundance.
The temperature indicated by the observed 69 {\mu}m bands can only be
reproduced by models with a geometrically compact superwind
( 2500 AU = 1400 R).This means that the superwind
started less than 1200 years ago (assuming an outflow velocity of 10 km/s). The
small amount of mass lost in one superwind and the high progenitor mass of the
OH/IR stars introduce a mass loss and thus evolutionary problem for these
objects, which has not yet been understood.Comment: Accepted by A&
Dusty wind of W Hya. Multi-wavelength modelling of the present-day and recent mass-loss
Low- and intermediate-mass stars go through a period of intense mass-loss at
the end of their lives in a phase known as the asymptotic giant branch (AGB).
During the AGB a significant fraction of their initial mass is expelled in a
stellar wind. This process controls the final stages of their evolution and
contributes to the chemical evolution of galaxies. However, the wind-driving
mechanism of AGB stars is not yet well understood, especially so for
oxygen-rich sources. Characterizing both the present-day mass-loss and wind
structure and the evolution of the mass-loss rate of such stars is paramount to
advancing our understanding of this processes. We modelled the dust envelope of
W Hya using an advanced radiative transfer code. The dust model was analysed in
the light of a previously calculated gas-phase wind model and compared to
measurements available in the literature, such as infrared spectra, infrared
images, and optical scattered light fractions. We find that the dust spectrum
of W Hya can partly be explained by a gravitationally bound dust shell that
probably is responsible for most of the amorphous AlO emission. The
composition of the large (\,0.3\,m) grains needed to explain the
scattered light cannot be constrained, but probably is dominated by silicates.
Silicate emission in the thermal infrared was found to originate from beyond 40
AU from the star and we find that they need to have substantial near-infrared
opacities to be visible at such large distances. The increase in near-infrared
opacity of the dust at these distances roughly coincides with a sudden increase
in expansion velocity as deduced from the gas-phase CO lines. Finally, the
recent mass loss of W Hya is confirmed to be highly variable and we identify a
strong peak in the mass-loss rate that occurred about 3500 years ago and lasted
for a few hundred years.Comment: 15 pages, 13 figure
The CoRoT B-type binary HD50230: a prototypical hybrid pulsator with g-mode period and p-mode frequency spacings
B-type stars are promising targets for asteroseismic modelling, since their
frequency spectrum is relatively simple.
We deduce and summarise observational constraints for the hybrid pulsator,
HD50230, earlier reported to have deviations from a uniform period spacing of
its gravity modes. The combination of spectra and a high-quality light curve
measured by the CoRoT satellite allow a combined approach to fix the position
of HD50230 in the HR diagram.
To describe the observed pulsations, classical Fourier analysis was combined
with short-time Fourier transformations and frequency spacing analysis
techniques. Visual spectra were used to constrain the projected rotation rate
of the star and the fundamental parameters of the target. In a first
approximation, the combined information was used to interpret multiplets and
spacings to infer the true surface rotation rate and a rough estimate of the
inclination angle.
We identify HD50230 as a spectroscopic binary and characterise the two
components. We detect the simultaneous presence of high-order g modes and
low-order p and g-modes in the CoRoT light curve, but were unable to link them
to line profile variations in the spectroscopic time series. We extract the
relevant information from the frequency spectrum, which can be used for seismic
modelling, and explore possible interpretations of the pressure mode spectrum.Comment: 26 pages, 12+6 figures, accepted for publication in Astronomy and
Astrophysic
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