134 research outputs found
First detection of photospheric depletion in the LMC
Recent photospheric abundance studies of galactic field RV Tauri stars show
that depletion of refractory elements is rather common in these evolved
objects. The process that creates this chemical anomaly is not understood well,
but it probably requires the presence of gravitationally bound dust in a binary
system. We test for the presence of depletion in extra-galactic objects. A
detailed photospheric abundance study on the basis of high-quality UVES spectra
was performed on the RV Tauri star in the LMC: MACHO82.8405.15. Abundances were
derived using a critically compiled line list with accurate log(gf) values and
the latest Kurucz model atmospheres. With [Fe/H]=-2.6 in combination with
[Zn/Fe]=+2.3 and [S/Ti]=+2.5, MACHO82.8405.15 displays a strong depletion
abundance pattern. The effect of the depletion is comparable to the strongest
depletions seen in field Galactic RV Tauri stars. The chemical analysis of
MACHO82.8405.15 proves that the depletion process also occurs in the
extragalactic members of the RV Tauri pulsation class. Our program star is a
member of a larger sample of LMC RV Tauri objects. This sample is unique, since
the distances of the members are well-constrained. Further studies of this
sample are therefore expected to gain deeper insight into the poorly understood
depletion phenomenon and of the evolutionary status of RV Tauri stars in
general.Comment: 4 pages, 5 figures, accepted by A&A Letter
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&
A study of the s-process in the carbon-rich post-AGB stars IRAS06530-0213 and IRAS08143-4406 on the basis of VLT-UVES spectra
In an effort to extend the still limited sample of s-process enriched
post-AGB stars, high-resolution, high signal-to-noise VLT+UVES spectra of the
optical counterparts of the infrared sources IRAS06530-0213 and IRAS08143-4406
were analysed. The objects are moderately metal deficient by [Fe/H]=-0.5 and
-0.4 respectively, carbon-rich and, above all, heavily s-process enhanced with
a [ls/Fe] of 1.8 and 1.5 respectively. Especially the spectrum of
IRAS06530-0213 is dominated by transitions of s-process species, and therefore
resembling the spectrum of IRAS05341+0852, the most s-process enriched object
known so far. The two objects are chemically very similar to the 21micron
objects discussed in Van Winckel & Reyniers (2000). A homogeneous comparison
with the results of these objects reveals that the relation between the third
dredge-up efficiency and the neutron nucleosynthesis efficiency found for the
21micron objects, is further strengthened. On the other hand, a detailed
comparison with the predictions of the latest AGB models indicates that the
observed spread in nucleosynthesis efficiency is certainly intrinsic, and
proves that different C-13 pockets are needed for stars with comparable mass
and metallicity to explain their abundances.Comment: 14 pages, 10 figures, accepted for publication in A&A; Table 4 is
available at ftp://ftp.ster.kuleuven.ac.be/dist/maarten/filescds/ pending
upload to CD
Stellar population synthesis of post-AGB stars: the s-process in MACHO47.2496.8
The low-metallicity RV Tauri star MACHO47.2496.8, recently discovered in the
Large Magellanic Cloud, is highly enriched in carbon and heavy elements
produced by the slow neutron capture process (s-process), and is most probably
a genuine post-C(N-type) asymptotic giant branch (AGB) star. We use the
analysis of the abundances of MACHO47.2496.8 to constrain free parameters in
AGB models. We test which values of the free parameters describing uncertain
physical mechanisms in AGB stars, namely the third dredge-up and the features
of the 13C neutron source, produce models that better match the abundances
observed in MACHO47.2496.8. We carry out stellar population synthesis coupled
with s-process nucleosynthesis using a synthetic stellar evolution code. The
s-process ratios observed in MACHO47.2496.8 can be matched by the same models
that explain the s-process ratios of Galactic AGB and post-AGB stars of
metallicity > Z_sun/10, except for the choice of the effectiveness of 13C as a
neutron source, which has to be lower by roughly a factor of 3 to 6. The less
effective neutron source for lower metallicities is also required when
comparing population synthesis results to observations of Galactic halo
-enhanced stars, such as Pb stars. The 12C/13C ratio in MACHO47.2496.8
cannot be matched simultaneously and requires the occurrence of extra-mixing
processes. The confirmed trend of the decreased efficiency of the 13C neutron
source with metallicity requires an explanation from AGB s-process models. The
present work is to date the first comparison between theoretical models and the
detailed abundances of an extragalactic post-AGB star.Comment: accepted for publication on Astronomy & Astrophysics Letter
V453 Oph: a s-process enriched, but carbon-deficient RV Tauri star of low intrinsic metallicity
This paper reports the detection of a heavy element enriched RV Tauri
variable with an abundance pattern that differs significantly from a standard
s-process enriched object: V453 Oph. Based on optical high-resolution spectra,
we determined that this object of low intrinsic metallicity ([Fe/H] = -2.2) has
a mild, but significant, enrichment ([s/Fe] ~ +0.5) of heavy elements for which
the distribution points to slow neutron capture nucleosynthesis. This result is
strengthened by a comparative analysis to the non-enriched RV Tauri star DS Aqr
([s/Fe] = 0.0). Although V453 Oph is the first RV Tauri star showing a strong
s-process signature, it is NOT accompanied by C enhancement, challenging our
current nucleosynthetic models of post-AGB stars that predict a simultaneous
enrichment in C and s-process elements. The low N abundance excludes CN cycling
as being responsible for the low C abundance. We explore three different
scenarios to explain the heavy element distribution in this evolved object: an
enrichment of the parental cloud, an accretion scenario in which the chemical
patterns were acquired by mass transfer in a binary system and an intrinsic
enrichment by dredge-up.Comment: Accepted for publication in A&
IRAS\,11472-0800: an extremely depleted pulsating binary post-AGB star
We focus here on one particular and poorly studied object, IRAS11472-0800. It
is a highly evolved post-Asymptotic Giant Branch (post-AGB) star of spectral
type F, with a large infrared excess produced by thermal emission of
circumstellar dust. We deploy a multi-wavelength study which includes the
analyses of optical and IR spectra as well as a variability study based on
photometric and spectroscopic time-series. The spectral energy distribution
(SED) properties as well as the highly processed silicate N-band emission show
that the dust in IRAS11472-0800 is likely trapped in a stable disc. The
energetics of the SED and the colour variability show that our viewing angle is
close to edge-on and that the optical flux is dominated by scattered light.
With photospheric abundances of [Fe/H] = -2.7 and [Sc/H]=-4.2, we discovered
that IRAS11472-0800 is one of the most chemically-depleted objects known to
date. Moreover, IRAS11472-0800 is a pulsating star with a period of 31.16 days
and a peak-to-peak amplitude of 0.6 mag in V. The radial velocity variability
is strongly influenced by the pulsations, but the significant cycle-to-cycle
variability is systematic on a longer time scale, which we interpret as
evidence for binary motion. We conclude that IRAS11472-0800 is a pulsating
binary star surrounded by a circumbinary disc. The line-of-sight towards the
object lies close the the orbital plane making that the optical light is
dominated by scattered light. IRAS11472-0800 is one of the most
chemically-depleted objects known so far and links the dusty RV\,Tauri stars to
the non-pulsating class of strongly depleted objects.Comment: 12 pages, 14 figures Accepted for publication in A&A Main Journa
Detailed abundance study of four s-process enriched post-AGB stars in the Large Magellanic Cloud
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