274 research outputs found
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
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&
Probing the mass-loss history of the unusual Mira variable R Hydrae through its infrared CO wind
Context. The unusual Mira variable R Hya is well known for its declining period between AD 1770 and 1950, which is possibly attributed to a recent thermal pulse. Aims. The goal of this study is to probe the circumstellar envelope (CSE) around R Hya and to check for a correlation between the derived density structure and the declining period. Methods. We investigate the CSE around R Hya by performing an in-depth analysis of (1.) the photospheric light scattered by three vibration-rotation transitions in the fundamental band of CO at 4.6 m; and (2.) the pure rotational CO J = 1-0 through 6-5 emission lines excited in the CSE. The vibrational-rotational lines trace the inner CSE within 3.5´´, whereas the pure rotational CO lines are sensitive probes of the cooler gas further out in the CSE. Results. The combined analysis bear evidence of a change in mass-loss rate some 220 yr ago (at ~150 or ~1.9 arcsec from the star). While the mass-loss rate before AD 1770 is estimated to be ~ 2 imes 10^ /yr, the present day mass-loss rate is a factor of ~20 lower. The derived mass-loss history nicely agrees with the mass-loss rate estimates by Zijlstra et al. (2002) on the basis of the period decline. Moreover, the recent detection of an AGB-ISM bow shock around R Hya at 100 arcsec to the west by Wareing et al. (2006) shows that the detached shell seen in the 60 m IRAS images can be explained by a slowing-down of the stellar wind by surrounding matter and that no extra mass-loss modulation around 1-2 arcmin needs to be invoked. Conclusions. Our results give empirical evidence to the thermal-pulse model, which is capable of explaining both the period evolution and the mass-loss history of R Hya
Kinematics and H_2 morphology of the multipolar Post-AGB star IRAS 16594-4656
context: The spectrum of IRAS 16594-4656 shows shock excited H_2 emission and
collisionally excited emission lines such as[O I],[C I],and [Fe II]. aim: The
goal is to determine the location of the H_2 and [Fe II] shock emission, to
determine the shock velocities,and constrain the physical properties in the
shock. methods: High resolution spectra of the H_2 1-0 S(1),H_2 2-1 S(1), [Fe
II], and Pa emission lines were obtained with the near infrared
spectrograph Phoenix on Gemini South. results: The position-velocity diagrams
of H_2 1-0 S(1), H_2 2-1 S(1), and [Fe II] are presented. The H_2 and [Fe II]
emission is spatially extended. The collisionally excited [O I] and [C I]
optical emission lines have a similar double peaked profile compared to the
extracted H_2 profile and appear to be produced in the same shock. They all
indicate an expansion velocity of ~8 km/s and the presence of a neutral, very
high density region with about 3 x 10^6 to 5 x10^7 cm. The
[Fe II] emission however is single peaked. It has a gaussian FWHM of 30 km/s
and a total width of 62 km/s at 1% of the peak. The Pa profile is even
wider with a gaussian FWHM of 48 km/s and a total width of 75 km/s at 1% of the
peak. conclusions: The H emission is excited in a slow 5 to 20 km/s shock
into dense material at the edge of the lobes, caused by the interaction of the
AGB ejecta and the post-AGB wind. The 3D representation of the H_2 data shows a
hollow structure with less H_2 emission in the equatorial region. The [Fe II]
emission is not present in the lobes, but originates close to the central star
in fast shocks in the post-AGB wind or in a disk. The Pa emission also
appears to originate close to the star.Comment: 11 pages and 8 figures; A&A in press; the paper includig high
resolution figures can be downloaded from
http://homepage.oma.be/gsteene/publications.htm
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
Spectroscopic binaries among Hipparcos M giants III. The eccentricity-period diagram and mass-transfer signatures
This paper is the third one in a series devoted to studying the properties of
binaries involving M giants. We use a new set of orbits to construct the first
(e-logP) diagram of an extensive sample of M giant binaries, to obtain their
mass-function distribution, and to derive evolutionary constraints for this
class of binaries and related systems. The orbital properties of binaries
involving M giants were analysed and compared with those of related families of
binaries (K giants, post-AGB stars, barium stars, Tc-poor S stars). The orbital
elements of post-AGB stars and M giants are not different, which may very
indicate that, for the considered sample of post-AGB binaries, the post-AGB
star left the AGB at quite an early stage (M4 or so). Neither are the orbital
elements of post-mass-transfer binaries like barium stars very different from
those of M giants, suggesting that the mass transfer did not alter the orbital
elements much, contrary to current belief. Finally, we show that binary systems
with e < 0.4 log P - 1 (with periods expressed in days) are predominantly
post-mass-transfer systems, because (i) the vast majority of barium and S
systems match this condition, and (ii) these systems have companion masses
peaking around 0.6 solar mass, as expected for white dwarfs. The latter
property has been shown to hold as well for open-cluster binaries involving K
giants, for which a lower bound on the companion mass may easily be set.Comment: 14 pages, 12 figures, accepted for publication in A&A, language
editing changes onl
Period and chemical evolution of SC stars
The SC and CS stars are thermal-pulsing AGB stars with C/O ratio close to
unity. Within this small group, the Mira variable BH Cru recently evolved from
spectral type SC (showing ZrO bands) to CS (showing weak C2). Wavelet analysis
shows that the spectral evolution was accompanied by a dramatic period
increase, from 420 to 540 days, indicating an expanding radius. The pulsation
amplitude also increased. Old photographic plates are used to establish that
the period before 1940 was around 490 days. Chemical models indicate that the
spectral changes were caused by a decrease in stellar temperature, related to
the increasing radius. There is no evidence for a change in C/O ratio. The
evolution in BH Cru is unlikely to be related to an on-going thermal pulse.
Periods of the other SC and CS stars, including nine new periods, are
determined. A second SC star, LX Cyg, also shows evidence for a large increase
in period, and one further star shows a period inconsistent with a previous
determination. Mira periods may be intrinsically unstable for C/O ~ 1; possibly
because of a feedback between the molecular opacities, pulsation amplitude, and
period. LRS spectra of 6 SC stars suggest a feature at wavelength > 15 micron,
which resembles one recently attributed to the iron-sulfide troilite. Chemical
models predict a large abundance of FeS in SC stars, in agreement with the
proposed association.Comment: 14 pages, 20 figures. MNRAS, 2004, accepted for publication. Janet
Mattei, one of the authors, died on 22 March, 2004. This paper is dedicated
to her memor
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