115 research outputs found
SPITZER survey of dust grain processing in stable discs around binary post-AGB stars
Aims: We investigate the mineralogy and dust processing in the circumbinary
discs of binary post-AGB stars using high-resolution TIMMI2 and SPITZER
infrared spectra. Methods: We perform a full spectral fitting to the infrared
spectra using the most recent opacities of amorphous and crystalline dust
species. This allows for the identification of the carriers of the different
emission bands. Our fits also constrain the physical properties of different
dust species and grain sizes responsible for the observed emission features.
Results: In all stars the dust is oxygen-rich: amorphous and crystalline
silicate dust species prevail and no features of a carbon-rich component can be
found, the exception being EPLyr, where a mixed chemistry of both oxygen- and
carbon-rich species is found. Our full spectral fitting indicates a high degree
of dust grain processing. The mineralogy of our sample stars shows that the
dust is constituted of irregularly shaped and relatively large grains, with
typical grain sizes larger than 2 micron. The spectra of nearly all stars show
a high degree of crystallinity, where magnesium-rich end members of olivine and
pyroxene silicates dominate. Other dust features of e.g. silica or alumina are
not present at detectable levels. Temperature estimates from our fitting
routine show that a significant fraction of grains must be cool, significantly
cooler than the glass temperature. This shows that radial mixing is very
efficient is these discs and/or indicates different thermal conditions at grain
formation. Our results show that strong grain processing is not limited to
young stellar objects and that the physical processes occurring in the discs
are very similar to those in protoplanetary discs.Comment: 22pages, 50 figures (in appendix), accepted for A&
Resolving the compact dusty discs around binary post-AGB stars using N-band interferometry
We present the first mid-IR long baseline interferometric observations of the
circumstellar matter around binary post-AGB stars. Two objects, SX Cen and HD
52961, were observed using the VLTI/MIDI instrument during Science
Demonstration Time. Both objects are known binaries for which a stable
circumbinary disc is proposed to explain the SED characteristics. This is
corroborated by our N-band spectrum showing a crystallinity fraction of more
than 50 % for both objects, pointing to a stable environment where dust
processing can occur. Surprisingly, the dust surrounding SX Cen is not resolved
in the interferometric observations providing an upper limit of 11 mas (or 18
AU at the distance of this object) on the diameter of the dust emission. This
confirms the very compact nature of its circumstellar environment. The dust
emission around HD 52961 originates from a very small but resolved region,
estimated to be ~ 35 mas at 8 micron and ~ 55 mas at 13 micron. These results
confirm the disc interpretation of the SED of both stars. In HD 52961, the dust
is not homogeneous in its chemical composition: the crystallinity is clearly
concentrated in the hotter inner region. Whether this is a result of the
formation process of the disc, or due to annealing during the long storage time
in the disc is not clear.Comment: 12 pages, 10 figures, accepted for publication in A &
Dusty Blue Supergiants: News from High-Angular Resolution Observations
An overview is presented of the recent advances in understanding the B[e]
phenomenon among blue supergiant stars in light of high-angular resolution
observations and with an emphasis on the results obtained by means of long
baseline optical stellar interferometry. The focus of the review is on the
circumstellar material and evolutionary phase of B[e] supergiants, but recent
results on dust production in regular blue supergiants are also highlighted.Comment: 8 pages, 2 figures. Published in "Advances in Astronomy" by Hindawi
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Keplerian discs around post-AGB stars: a common phenomenon?
Aims: We aim at showing that the broad-band SED characteristics of our sample
of post-AGB stars are best interpreted, assuming the circumstellar dust is
stored in Keplerian rotating passive discs.
Methods: We present a homogeneous and systematic study of the Spectral Energy
Distributions (SEDs) of a sample of 51 post-AGB objects. The selection criteria
to define the whole sample were tuned to cover the broad-band characteristics
of known binary post-AGB stars. The whole sample includes 20 dusty RV Tauri
stars from the General Catalogue of Variable Stars (GCVS). We supplemented our
own Geneva optical photometry with literature data to cover a broad range of
fluxes from the UV to the far-IR.
Results: All the SEDs display very similar characteristics: a large IR excess
with a dust excess starting near the sublimation temperature, irrespective of
the effective temperature of the central star. Moreover, when available, the
long wavelength fluxes show a black-body slope indicative of the presence of a
component of large mm sized grains.
Conclusions: We argue that in all systems, gravitationally bound dusty discs
are present. The discs must be puffed-up to cover a large opening angle for the
central star and we argue that the discs have some similarity with the passive
discs detected around young stellar objects. We interpret the presence of a
disc to be a signature for binarity of the central object, but this will need
confirmation by long-term monitoring of the radial velocities. We argue that
dusty RV Tauri stars are those binaries which happen to be in the Population II
instability strip.Comment: 29 pages, 5 figures, accepted for publication 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
Post-AGB stars with hot circumstellar dust: binarity of the low-amplitude pulsators
While the first binary post-AGB stars were serendipitously discovered, the
distinct characteristics of their Spectral Energy Distribution (SED) allowed us
to launch a more systematic search for binaries. We selected post-AGB objects
which show a broad dust excess often starting already at H or K, pointing to
the presence of a gravitationally bound dusty disc in the system. We started a
very extensive multi-wavelength study of those systems and here we report on
our radial velocity and photometric monitoring results for six stars of early F
type, which are pulsators of small amplitude. To determine the radial velocity
of low signal-to-noise time-series, we constructed dedicated auto-correlation
masks. The radial velocity variations were subjected to detailed analysis to
differentiate between pulsational variability and variability due to orbital
motion. Finally orbital minimalisation was performed to constrain the orbital
elements. All of the six objects are binaries, with orbital periods ranging
from 120 to 1800 days. Five systems have non-circular orbits. The mass
functions range from 0.004 to 0.57 solar mass and the companions are likely
unevolved objects of (very) low initial mass. We argue that these binaries must
have been subject to severe binary interaction when the primary was a cool
supergiant. Although the origin of the circumstellar disc is not well
understood, the disc is generally believed to be formed during this strong
interaction phase. The eccentric orbits of these highly evolved objects remain
poorly understood. With the measured orbits and mass functions we conclude that
the circumbinary discs seem to have a major impact on the evolution of a
significant fraction of binary systems.Comment: 13 pages, 15 figures, accepted for Astronomy and Astrophysic
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