4,270 research outputs found
Classifying the secondary component of the binary star W Aquilae
AIMS: The object W Aql is an asymptotic giant branch (AGB) star with a faint
companion. By determining more carefully the properties of the companion, we
hope to better constrain the properties of the AGB star. METHODS: We present
new spectral observations of the binary star W Aql at minimum and maximum
brightness and new photometric observations of W Aql at minimum brightness.
RESULTS: The composite spectrum near minimum light is predominantly from the
companion at wavelengths < 6000 . This spectrum can be
classified as F8 to G0, and the brightness of the companion is that of a dwarf
star. Therefore, it can be concluded that the companion is a main sequence
star. From this, we are able to constrain the mass of the AGB component to 1.04
- 3 and the mass of the W Aql system to 2.1 - 4.1 . Our
photometric results are broadly consistent with this classification and suggest
that the main sequence component suffers from approximately 2 mag of extinction
in the V band primarily due to the dust surrounding the AGB component.Comment: 5 pages, 1 figure, research not
Jet Power in Pre-Planetary Nebulae: Observations vs. Theory
High velocity jets are among the most prominent features of a wide class of
planetary nebulae, but their origins are not understood. Several different
types of physical model have been suggested to power the jets, but there is no
consensus or preferred scenario. We compare current theoretical ideas on jet
formation with observations, using the best studied pre-planetary nebulae in
millimeter CO, where the dynamical properties are best defined. In addition to
the mass, velocity, momentum, and energy of the jets, the mass and energetics
of the equatorial mass-loss that typically accompanies jet formation prove to
be important diagnostics. Our integrated approach provides estimates for some
key physical quantities - such as the binding energy of the envelope when the
jets are launched - and allows testing of model features using correlations
between parameters. Even with a relatively small sample of well-observed
objects, we find that some specific scenarios for powering jets can be ruled
out or rendered implausible, and others are promising at a quantitative level.Comment: 4 pages, 1 figure, to appear in "Planetary Nebulae: an Eye to the
Future", IAU Symposium 283, eds. A Manchado, L. Stanghellini, D. Schoenberne
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
The Herschel/PACS view of disks around low-mass stars in Chamaleon-I
Circumstellar disks are expected to be the birthplaces of planets. The
potential for forming one or more planets of various masses is essentially
driven by the initial mass of the disks. We present and analyze Herschel/PACS
observations of disk-bearing M-type stars that belong to the young ~2 Myr old
Chamaleon-I star forming region. We used the radiative transfer code RADMC to
successfully model the SED of 17 M-type stars detected at PACS wavelengths. We
first discuss the relatively low detection rates of M5 and later spectral type
stars with respect to the PACS sensitivity, and argue their disks masses, or
flaring indices, are likely to be low. For M0 to M3 stars, we find a relatively
broad range of disk masses, scale heights, and flaring indices. Via a
parametrization of dust stratification, we can reproduce the peak fluxes of the
10 m emission feature observed with Spitzer/IRS, and find that disks
around M-type stars may display signs of dust sedimentation. The Herschel/PACS
observations of low-mass stars in Cha-I provide new constraints on their disk
properties, overall suggesting that disk parameters for early M-type stars are
comparable to those for more massive stars (e.g., comparable scale height and
flaring angles). However, regions of the disks emitting at about 100 m may
still be in the optically thick regime, preventing direct determination of disk
masses. Thus the modeled disk masses should be considered as lower limits.
Still, we are able to extend the wavelength coverage of SED models and start
characterizing effects such as dust sedimentation, an effort leading the way
towards ALMA observations of these low-mass stars
CO in OH/IR stars close to the Galactic centre
Aims: A pilot project has been carried out to measure circumstellar CO
emission from three OH/IR stars close to the Galactic centre. The intention was
to find out whether it would be possible to conduct a large-scale survey for
mass-loss rates using, for example, the Atacama Large Millimeter Array (ALMA).
Such a survey would increase our understanding of the evolution of the Galactic
bulge.
Methods: Two millimetre-wave instruments were used: the Nobeyama Millimeter
Array at 115 GHz and the Submillimeter Array at 230 GHz. An interferometer is
necessary as a `spatial filter' in this region of space because of the
confusion with interstellar CO emission.
Results: Towards two of the stars, CO emission was detected with positions
and radial velocities coinciding within the statistical errors with the
corresponding data of the associated OH sources. However, for one of the stars
the line profile is not what one expects for an unresolved expanding
circumstellar envelope. We believe that this CO envelope is partially resolved
and that this star therefore is a foreground star not belonging to the bulge.
Conclusions: The results of the observations have shown that it is possible
to detect line profiles of circumstellar CO from late-type stars both within
and in the direction of the Galactic bulge. ALMA will be able to detect CO
emission in short integrations with sensitivity sufficient to estimate
mass-loss rates from a large number of such stars.Comment: 5 pages, 3 figure
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
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
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