367 research outputs found
Monitoring evolved stars for binarity with the HERMES spectrograph
Binarity is often invoked to explain peculiarities that can not be explained
by the standard theory of stellar evolution. Detecting orbital motion via the
Doppler effect is the best method to test binarity when direct imaging is not
possible. However, when the orbital period exceeds the duration of a typical
observing run, monitoring often becomes problematic. Placing a high-throughput
spectrograph on a small semi- robotic telescope allowed us to carry out a
radial-velocity survey of various types of peculiar evolved stars. In this
review we highlight some findings after the first four years of observations.
Thus, we detect eccentric binaries among hot subdwarfs, barium, S stars, and
post- AGB stars with disks, which are not predicted by the standard binary
interaction theory. In disk objects, in addition, we find signs of the on-
going mass transfer to the companion, and an intriguing line splitting, which
we attribute to the scattered light of the primary.Comment: To appear in the proceedings of the conference "Setting a new
standard in the analysis of binary stars", A. Tkachenko (ed.), European
Astron. Soc. Publ. Se
IRAS08281-4850 and IRAS14325-6428: two A-type post-AGB stars with s-process enrichment
One of the puzzling findings in the study of the chemical evolution of
(post-)AGB stars is why very similar stars (in terms of metallicity, spectral
type, infrared properties, etc...) show a very different photospheric
composition. We aim at extending the still limited sample of s-process enriched
post-AGB stars, in order to obtain a statistically large enough sample that
allows us to formulate conclusions concerning the 3rd dredge-up occurrence. We
selected two post-AGB stars on the basis of IR colours indicative of a past
history of heavy mass loss: IRAS08281-4850 and IRAS14325-6428. They are cool
sources in the locus of the Planetary Nebulae (PNe) in the IRAS colour-colour
diagram. Abundances of both objects were derived for the first time on the
basis of high-quality UVES and EMMI spectra, using a critically compiled line
list with accurate log(gf) values, together with the latest Kurucz model
atmospheres. Both objects have very similar spectroscopically defined effective
temperatures of 7750-8000K. They are strongly carbon and s-process enriched,
with a C/O ratio of 1.9 and 1.6, and an [ls/Fe] of +1.7 and +1.2, for IRAS08281
and IRAS14325 resp. Moreover, the spectral energy distributions (SEDs) point to
heavy mass-loss during the preceding AGB phase. IRAS08281 and IRAS14325 are
prototypical post-AGB objects in the sense that they show strong post 3rd
dredge-up chemical enrichments. The neutron irradiation has been extremely
efficient, despite the only mild sub-solar metallicity. This is not conform
with the recent chemical models. The existence of very similar post-AGB stars
without any enrichment emphasizes our poor knowledge of the details of the AGB
nucleosynthesis and dredge-up phenomena. We call for a very systematic chemical
study of all cool sources in the PN region of the IRAS colour-colour diagram.Comment: 8 pages, 6 figures, accepted by A&
An evolutionary catalogue of Galactic post-AGB and related objects
With the ongoing AKARI infrared sky survey, of much greater sensitivity than
IRAS, a wealth of post-AGB objects may be discovered. It is thus time to
organize our present knowledge of known post-AGB stars in th galaxy with a view
to using it to search for new post-AGB objects among AKARI sources. We searched
the literature available on the NASA Astrophysics Data System up to 1 October
2006, and defined criteria for classifying sources into three categories: "very
likely", "possible" and "disqualified" post-AGB objects. The category of "very
likely" post-AGB objects is made up of several classes. We have created an
evolutionary, on-line catalogue of Galactic post-AGB objects, to be referred to
as "The Torun catalogue of Galactic post-AGB and related objects". The present
version of the catalogue contains 326 "very likely", 107 "possible" and 64
"disqualified" objects. For the very likely post-AGB objects, the catalogue
gives the available optical and infrared photometry, infrared spectroscopy and
spectral types, and links to finding charts and bibliography.Comment: 9 pages, 6 figures, accepted for publication in A&
An Inner Gaseous Disk around the Herbig Be Star MWC 147
We present high-spectral-resolution, optical spectra of the Herbig Be star
MWC 147, in which we spectrally resolve several emission lines, including the
[O I] lines at 6300 and 6363\deg. Their highly symmetric, double-peaked line
profiles indicate that the emission originates in a rotating circumstellar
disk. We deconvolve the Doppler-broadened [O I] emission lines to obtain a
measure of emission as a function of distance from the central star. The
resulting radial surface brightness profiles are in agreement with a disk
structure consisting of a flat, inner, gaseous disk and a flared, outer, dust
disk. The transition between these components at 2 to 3 AU corresponds to the
estimated dust sublimation radius. The width of the double-peaked Mg II line at
4481\deg suggests that the inner disk extends to at least 0.10 AU, close to the
corotation radius.Comment: accepted for ApJ Letters (Oct. 2010
The Red Rectangle: Its Shaping Mechanism and its Source of Ultraviolet Photons
The proto-planetary Red Rectangle nebula is powered by HD 44179, a
spectroscopic binary (P = 318 d), in which a luminous post-AGB component is the
primary source of both luminosity and current mass loss. Here, we present the
results of a seven-year, eight-orbit spectroscopic monitoring program of HD
44179, designed to uncover new information about the source of the
Lyman/far-ultraviolet continuum in the system as well as the driving mechanism
for the bipolar outflow producing the current nebula. Our observations of the
H-alpha line profile around the orbital phase of superior conjunction reveal
the secondary component to be the origin of the fast (max. v~560^{-1}\sun_{max} \ge 17,0002 -
5\times10^{-5}\sun^{-1}\sun$, about 5% of the
luminosity of the entire system. (abridged)Comment: Accepted for publication in Ap
The evolved circumbinary disk of AC Her: a radiative transfer, interferometric and mineralogical study
We aim to constrain the structure of the circumstellar material around the
post-AGB binary and RV Tauri pulsator AC Her. We want to constrain the spatial
distribution of the amorphous as well as of the crystalline dust. We present
very high-quality mid-IR interferometric data that were obtained with
MIDI/VLTI. We analyse the MIDI data and the full SED, using the MCMax radiative
transfer code, to find a good structure model of AC Her's circumbinary disk. We
include a grain size distribution and midplane settling of dust
self-consistently. The spatial distribution of crystalline forsterite in the
disk is investigated with the mid-IR features, the 69~m band and the
11.3~m signatures in the interferometric data. All the data are well
fitted. The inclination and position angle of the disk are well determined at
i=50+-8 and PA=305+-10. We firmly establish that the inner disk radius is about
an order of magnitude larger than the dust sublimation radius. Significant
grain growth has occurred, with mm-sized grains being settled to the midplane
of the disk. A large dust mass is needed to fit the sub-mm fluxes. By assuming
{\alpha}=0.01, a good fit is obtained with a small grain size power law index
of 3.25, combined with a small gas/dust ratio <10. The resulting gas mass is
compatible with recent estimates employing direct gas diagnostics. The spatial
distribution of the forsterite is different from the amorphous dust, as more
warm forsterite is needed in the surface layers of the inner disk. The disk in
AC Her is very evolved, with its small gas/dust ratio and large inner hole.
Mid-IR interferometry offers unique constraints, complementary to mid-IR
features, for studying the mineralogy in disks. A better uv coverage is needed
to constrain in detail the distribution of the crystalline forsterite in AC
Her, but we find strong similarities with the protoplanetary disk HD100546.Comment: update with final version published in A&
The optically bright post-AGB population of the LMC
The detected variety in chemistry and circumstellar shell morphology of the
limited sample of Galactic post-AGB stars is so large that there is no
consensus yet on how the different objects are linked by evolutionary channels.
The evaluation is complicated by the fact that their distances and hence
luminosities remain largely unknown. Via cross-correlation of the Spitzer SAGE
catalogue with optical catalogues we selected a sample of LMC post-AGB
candidates based on their [8]-[24] colour index and estimated luminosity. We
determined the fundamental properties of the central stars of 105 of these
objects using low-resolution, optical spectra that we obtained at Siding Spring
Observatory and SAAO, and constructed a catalogue of 70 high probability and
1337 candidate post-AGB stars that is available at the CDS. The sample forms an
ideal testbed for stellar evolution theory predictions of the final phase of
low- and intermediate-mass stars, because the distance and hence luminosity and
also the current and initial mass of these objects is well constrained. About
half of the objects in our sample of post-AGB candidates show a spectral energy
distribution (SED) that is indicative of a disc rather than an expanding and
cooling AGB remnant. Like in the Galaxy, the disc sources are likely associated
with binary evolution. Important side products of this research are catalogues
of candidate young stellar objects, candidate supergiants with circumstellar
dust, and discarded objects for which a spectrum was obtained. These too are
available at the CDS
Time resolved spectroscopy of BD+46 442: gas streams and jet creation in a newly discovered evolved binary with a disk
Previous studies have shown that many post-AGB stars with dusty disks are
associated with single-lined binary stars. To verify the binarity hypothesis on
a larger sample, we started a high-resolution spectral monitoring of about 40
field giants, whose binarity was suspected based on either a light curve, an
infrared excess, or a peculiar chemical composition. Here we report on the
discovery of the periodic RV variations in BD+46 442, a high-latitude F giant
with a disk. We interpret the variations due to the motion around a faint
companion, and deduce the following orbital parameters: Porb = 140.77 d, e =
0.083, asini=0.31 AU. We find it to be a moderately metal-poor star
([M/H]=-0.7) without a strong depletion pattern in the photospheric abundances.
Interestingly, many lines show periodic changes with the orbital phase: Halpha
switches between a double-peak emission and a PCyg-like profiles, while strong
metal lines appear split during the maximum redshift. Similar effects are
likely visible in the spectra of other post-AGB binaries, but their regularity
is not always realized due to sporadic observations. We propose that these
features result from an ongoing mass transfer from the evolved giant to the
companion. In particular, the blue-shifted absorption in Halpha, which occurs
only at superior conjunction, may result from a jet originating in the
accretion disk around the companion and seen in absorption towards the luminous
primary.Comment: 16 pages, accepted in A&
An interferometric study of the post-AGB binary 89 Herculis. II Radiative transfer models of the circumbinary disk
The presence of disks and outflows is widespread among post-AGB binaries. In
the first paper of this series, a surprisingly large fraction of optical light
was found to be resolved in the 89 Her post-AGB system. The data showed this
flux to arise from close to the central binary. Scattering off the inner rim of
the circumbinary disk, or in a dusty outflow were suggested as two possible
origins. With detailed dust radiative transfer models of the disk we aim to
discriminate between these two configurations. By including Herschel/SPIRE
photometry, we extend the SED such that it now fully covers UV to sub-mm
wavelengths. The MCMax radiative transfer code is used to create a large grid
of disk models. Our models include a self-consistent treatment of dust settling
as well as of scattering. A Si-rich composition with two additional opacity
sources, metallic Fe or amorphous C, are tested. The SED is fit together with
mid-IR (MIDI) visibilities as well as the optical and near-IR visibilities of
Paper I, to constrain the structure of the disk and in particular of its inner
rim. The near-IR visibility data require a smooth inner rim, here obtained with
a two-power-law parameterization of the radial surface density distribution. A
model can be found that fits all the IR photometric and interferometric data
well, with either of the two continuum opacity sources. Our best-fit passive
models are characterized by a significant amount of mm-sized grains, which are
settled to the midplane of the disk. Not a single disk model fits our data at
optical wavelengths though, the reason being the opposing constraints imposed
by the optical and near-IR interferometric data. A geometry in which a passive,
dusty, and puffed-up circumbinary disk is present, can reproduce all the IR but
not the optical observations of 89 Her. Another dusty, outflow or halo,
component therefore needs to be added to the system.Comment: 15 pages, in pres
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