320 research outputs found
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
On the Origins of the High-Latitude H-alpha Background
The diffuse high-latitude H-alpha background is widely believed to be
predominantly the result of in-situ recombination of ionized hydrogen in the
warm interstellar medium of the Galaxy. Instead, we show that both a
substantial fraction of the diffuse high-latitude H-alpha intensity in regions
dominated by Galactic cirrus dust and much of the variance in the high-latitude
H-alpha background are the result of scattering by interstellar dust of H-alpha
photons originating elsewhere in the Galaxy. We provide an empirical relation,
which relates the expected scattered H-alpha intensity to the IRAS 100um
diffuse background intensity, applicable to about 81% of the entire sky. The
assumption commonly made in reductions of CMB observations, namely that the
observed all-sky map of diffuse H-alpha light is a suitable template for
Galactic free-free foreground emission, is found to be in need of
reexamination.Comment: 26 pages, 5 figures, Accepted for publication in Ap
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&
PHOTONS/AERONET sunphotometer network overview. Description – Activities - Results
Fourteenth International Symposium on Atmospheric and Ocean Optics/Atmospheric Physics celebrado del 24 al 30 de junio de 2007 en Buryatia, Russia
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
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