217 research outputs found
HD172481: a super lithium-rich metal-deficient post-AGB binary with a red AGB companion
We present in this paper a study on the peculiar supergiant HD172481. Its
spectral type (F2Ia), high galactic latitude (b=-10.37), circumstellar dust,
high radial velocity and moderate metal deficiency ([Fe/H]=-0.55) confirm the
post-AGB character of this object. A detailed chemical analysis shows slight
but real s-process overabundances, however no CNO-enhancement was detected.
Furthermore, the spectral energy distribution and the TiO bands in the red part
of the spectrum reveal a red luminous companion. The luminosity ratio of the
hot F type component and this cool M type companion L(F)/L(M) is derived for a
reddening of E(B-V)=0.44 (L(F)/L(M)=1.8) and indicates that the companion must
also be strongly evolved and probably evolving along the AGB. Neither our
photometric data-set, nor our radial velocity monitoring show evidence for
orbital variability which may indicate that the period is too large for direct
binary interaction. Most interestingly, a strong lithium resonance line is
detected, which yields an abundance of log(Li)=3.6. Several explanations for
this large lithium content are explored.Comment: 11 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
A Spectroscopic and Photometric Study of the Metal-Poor, Pulsating, Post-AGB Binary HD 46703
The metal-poor post-AGB star HD 46703 is shown to be a single-line
spectroscopic binary with a period of 600 days, a high velocity of -94 km/s,
and an orbital eccentricity of 0.3. Light curve studies show that it also
pulsates with a period of 29 days. High-resolution, high signal-to-noise
spectra were used for a new abundance study. The atmospheric model determined
is T(eff) = 6250 K, log(g) = 1.0, V(t) = 3.0 km/s, and a metal abundance of
[M/H] = -1.5. A low carbon abundance and lack of s-process element enhancement
indicate that the star has not experienced third dredge-up on the AGB. The
sulfur and zinc abundances are high compared with iron, and the chemical
abundances show a clear anti-correlation with condensation temperature. The
abundance depletion pattern is similar to that seen in other post-AGB binaries,
and, like them, is attributed to the chemical fractionation of refractory
elements onto dust stored in a circumbinary disk and the re-accretion of
volatiles in the stellar atmosphere. The infrared excess is small but the
excess energy distribution is very similar to what can expected from a disk. HD
46703 joins the growing list of depleted, post-AGB stars which are likely
surrounded by a dusty and stable circumbinary disk.Comment: Machine readable files not include
Testing eccentricity pumping mechanisms to model eccentric long period sdB binaries with MESA
Hot subdwarf-B stars in long-period binaries are found to be on eccentric
orbits, even though current binary-evolution theory predicts those objects to
be circularised before the onset of Roche-lobe overflow (RLOF). We aim to find
binary-evolution mechanisms that can explain these eccentric long-period
orbits, and reproduce the currently observed period-eccentricity diagram. Three
different processes are considered; tidally-enhanced wind mass-loss,
phase-dependent RLOF on eccentric orbits and the interaction between a
circumbinary disk and the binary. The binary module of the stellar-evolution
code MESA (Modules for Experiments in Stellar Astrophysics) is extended to
include the eccentricity-pumping processes. The effects of different input
parameters on the final period and eccentricity of a binary-evolution model are
tested with MESA. The end products of models with only tidally-enhanced wind
mass-loss can indeed be eccentric, but these models need to lose too much mass,
and invariably end up with a helium white dwarf that is too light to ignite
helium. Within the tested parameter space, no sdBs in eccentric systems are
formed. Phase-dependent RLOF can reintroduce eccentricity during RLOF, and
could help to populate the short-period part of the period-eccentricity
diagram. When phase-dependent RLOF is combined with eccentricity pumping via a
circumbinary disk, the higher eccentricities can be reached as well. A
remaining problem is that these models favour a distribution of higher
eccentricities at lower periods, while the observed systems show the opposite.
The models presented here are potentially capable of explaining the
period-eccentricity distribution of long-period sdB binaries, but further
theoretical work on the physical mechanisms is necessary.Comment: 18 pages, 9 figures, accepted for publication in A&
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