46 research outputs found
On metal-deficient barium stars and their link with yellow symbiotic stars
This paper addresses the question of why metal-deficient barium stars are not
yellow symbiotic stars (YSyS). Samples of (suspected) metal-deficient barium
(mdBa) stars and YSyS have been collected from the literature, and their
properties reviewed. It appears in particular that the barium nature of the
suspected mdBa stars needs to be ascertained by detailed abundance analyses.
Abundances are therefore derived for two of them, HD 139409 and HD 148897,
which reveal that HD 148897 should not be considered a barium star. HD 139409
is a mild barium star, with overabundances observed only for elements belonging
to the first s-process peak (Y and Zr). The evidence for binarity among mdBa
stars is then reviewed, using three different methods: (i) radial-velocity
variations (from CORAVEL observations), (ii) Hipparcos astrometric data, and
(iii) a method based on the comparison between the Hipparcos and Tycho-2 proper
motions. A first-time orbit is obtained for HIP 55852, whereas evidence for the
(so far unknown) binary nature of HIP 34795, HIP 76605, HIP 97874 and HIP
107478 is presented. Two stars with no evidence for binarity whatsoever (HIP
58596 and BD +3 2688) are candidates low-metallicity thermally-pulsing
asymptotic giant branch stars, as inferred from their large luminosities. The
reason why mdBa stars are not YSyS is suggested to lie in their different
orbital period distributions: mdBa stars have on average longer orbital periods
than YSyS, and hence their companion accretes matter at a lower rate, for a
given mass loss rate of the giant star. The definite validation of this
explanation should nevertheless await the determination of the orbital periods
for the many mdBa stars still lacking periods, in order to make the comparison
more significant.Comment: Astronomy & Astrophysics, in press; 16 pages, 14 figures; also
available at http://www.astro.ulb.ac.be/Html/ps.html#PR
A search for diffuse bands in the circumstellar envelopes of post-AGB stars
In this work we present the results of a systematic search for diffuse bands
(DBs, hereafter) in the circumstellar envelopes of a carefully selected sample
of post-AGB stars. We concentrated on the analysis of 9 of the DBs most
commonly found in the interstellar medium. The strength of these features is
determined using high resolution optical spectroscopy and the results obtained
are compared with literature data on field stars affected only by interstellar
reddening. Based on the weak features observed in the subsample of post-AGB
stars dominated by circumstellar reddening we conclude that the carrier(s) of
these DBs must not be present in the circumstellar environment of these
sources, or at least not under the excitation conditions in which DBs are
formed. The conclusion is applicable to all the post-AGB stars studied,
irrespective of the dominant chemistry or the spectral type of the star
considered. A detailed radial velocity analysis of the features observed in
individual sources confirms this result, as the Doppler shifts measured are
found to be consistent with an interstellar origin.Comment: Accepted for A&
Convective-reactive proton-C12 combustion in Sakurai's object (V4334 Sagittarii) and implications for the evolution and yields from the first generations of stars
Depending on mass and metallicity as well as evolutionary phase, stars
occasionally experience convective-reactive nucleosynthesis episodes. We
specifically investigate the situation when nucleosynthetically unprocessed,
H-rich material is convectively mixed with a He-burning zone, for example in
convectively unstable shell on top of electron-degenerate cores in AGB stars,
young white dwarfs or X-ray bursting neutron stars. Such episodes are
frequently encountered in stellar evolution models of stars of extremely low or
zero metal content [...] We focus on the convective-reactive episode in the
very-late thermal pulse star Sakurai's object (V4334 Sagittarii). Asplund etal.
(1999) determined the abundances of 28 elements, many of which are highly
non-solar, ranging from H, He and Li all the way to Ba and La, plus the C
isotopic ratio. Our simulations show that the mixing evolution according to
standard, one-dimensional stellar evolution models implies neutron densities in
the He that are too low to obtain a significant neutron capture nucleosynthesis
on the heavy elements. We have carried out 3D hydrodynamic He-shell flash
convection [...] we assume that the ingestion process of H into the He-shell
convection zone leads only after some delay time to a sufficient entropy
barrier that splits the convection zone [...] we obtain significantly higher
neutron densities (~few 10^15 1/cm^3) and reproduce the key observed abundance
trends found in Sakurai's object. These include an overproduction of Rb, Sr and
Y by about 2 orders of magnitude higher than the overproduction of Ba and La.
Such a peculiar nucleosynthesis signature is impossible to obtain with the
mixing predictions in our one-dimensional stellar evolution models. [...] We
determine how our results depend on uncertainties of nuclear reaction rates,
for example for the C13(\alpha, n)O16 reaction.Comment: ApJ in press, this revision contains several changes that improve
clarity of presentation reflecting the suggestions made by the referee; this
version represents no change in substance compared to version 1; some
technical material has been moved to an appendix; an additional appendix
deals in more detail with the combustion time scales; this version is
practically identical to the ApJ versio
Abundance Analysis of HE2148-1247, A Star With Extremely Enhanced Neutron Capture Elements
Abundances for 27 elements in the very metal poor dwarf star HE2148-1247 are
presented, including many of the neutron capture elements. We establish that
HE2148-1247 is a very highly s-process enhanced star with anomalously high Eu
as well, Eu/H about half Solar, demonstrating the large addition of heavy
nuclei at [Fe/H] = -2.3 dex. Ba and La are enhanced by a somewhat larger factor
and reach the solar abundance, while Pb significantly exceeds it. Ba/Eu is ten
times the solar r-process ratio but much less than that of the s-process,
indicating a substantial r-process addition as well. C and N are also very
highly enhanced. We have found that HE2148-1247 is a radial velocity variable.
The C, N and the s-process element enhancements thus presumably were produced
through mass transfer from a former AGB binary companion. The large enhancement
of heavy r-nuclides also requires an additional source as this is far above any
inventory in the ISM at such low [Fe/H]. We further hypothesize that accretion
onto the white dwarf from the envelope of the star caused accretion induced
collapse of the white dwarf, forming a neutron star, which then produced heavy
r-nuclides and again contaminated its companion. (abridged)Comment: Accepted by the Astrophysical Journal. Companion paper by Qian and
Wasserburg follow
Echelle long-slit optical spectroscopy of evolved stars
We present echelle long-slit optical spectra of a sample of objects evolving
off the AGB, most of them in the pre-planetary nebula (pPN) phase, obtained
with the ESI and MIKE spectrographs at Keck-II and Magellan-I, respectively.
The total wavelength range covered with ESI (MIKE) is ~3900 to 10900 A (~3600
to 7200A). In this paper, we focus our analysis mainly on the Halpha profiles.
Prominent Halpha emission is detected in half of the objects, most of which
show broad Halpha wings (up to ~4000 km/s). In the majority of the
Halpha-emission sources, fast, post-AGB winds are revealed by P-Cygni profiles.
In ~37% of the objects Halpha is observed in absorption. In almost all cases,
the absorption profile is partially filled with emission, leading to complex,
structured profiles that are interpreted as an indication of incipient post-AGB
mass-loss. All sources in which Halpha is seen mainly in absorption have F-G
type central stars, whereas sources with intense Halpha emission span a larger
range of spectral types from O to G. Shocks may be an important excitation
agent of the close stellar surroundings for objects with late type central
stars. Sources with pure emission or P Cygni Halpha profiles have larger J-K
color excess than objects with Halpha mainly in absorption, which suggests the
presence of warm dust near the star in the former. The two classes of profile
sources also segregate in the IRAS color-color diagram in a way that intense
Halpha-emitters have dust grains with a larger range of temperatures.
(abridged)Comment: 68 pages, 14 figures, accepted for publication in ApJS (abstract
abridged
Three-Dimensional Spectral Classification of Low-Metallicity Stars Using Artificial Neural Networks
We explore the application of artificial neural networks (ANNs) for the
estimation of atmospheric parameters (Teff, logg, and [Fe/H]) for Galactic F-
and G-type stars. The ANNs are fed with medium-resolution (~ 1-2 A) non
flux-calibrated spectroscopic observations. From a sample of 279 stars with
previous high-resolution determinations of metallicity, and a set of (external)
estimates of temperature and surface gravity, our ANNs are able to predict Teff
with an accuracy of ~ 135-150 K over the range 4250 <= Teff <= 6500 K, logg
with an accuracy of ~ 0.25-0.30 dex over the range 1.0 <= logg <= 5.0 dex, and
[Fe/H] with an accuracy ~ 0.15-0.20 dex over the range -4.0 <= [Fe/H] <= +0.3.
Such accuracies are competitive with the results obtained by fine analysis of
high-resolution spectra. It is noteworthy that the ANNs are able to obtain
these results without consideration of photometric information for these stars.
We have also explored the impact of the signal-to-noise ratio (S/N) on the
behavior of ANNs, and conclude that, when analyzed with ANNs trained on spectra
of commensurate S/N, it is possible to extract physical parameter estimates of
similar accuracy with stellar spectra having S/N as low as 13. Taken together,
these results indicate that the ANN approach should be of primary importance
for use in present and future large-scale spectroscopic surveys.Comment: 51 pages, 11 eps figures, uses aastex; to appear in Ap
New Spectroscopic Observations of the Post-AGB Star V354Lac=IRAS22272+5435
The strongest absorption features with the lower-level excitation potentials
eV are found to be split in the high-resolution optical
spectra of the post-AGB star V354 Lac taken in 2007--2008. Main parameters,
T=5650 K, =0.2, =5.0 km/s, and the abundances of 22
chemical elements in the star's atmosphere are found. The overabundance of the
-process chemical elements (Ba, La, Ce, Nd) in the star's atmosphere is
partly due to the splitting of strong lines of the ions of these metals. The
peculiarities of the spectrum in the wavelength interval containing the LiI
6707 \AA{} line can be naturally explained only by taking the
overabundances of the CeII and SmII heavy-metal ions into account. The best
agreement with the synthetic spectrum is achieved assuming (LiI)=2.0,
(CeII)=3.2, and (SmII)=2.7. The velocity field both in the
atmosphere and in the circumstellar envelope of V354 Lac remained stationary
throughout the last 15 years of our observations.Comment: 16 pages, 6 figures, 2 table
Spectroscopic Analysis of Two Carbon Rich Post-AGB Stars
The chemical compositions of the C-rich pAGB stars IRAS 05113+1347 and IRAS
22272+5424 are determined from high-resolution optical spectra using standard
LTE model atmosphere-based techniques. The stars are C, N, and -process
enriched suggesting efficient operation of the third-dredge up in the AGB star
following a first dredge-up that increased the N abundance. Lithium is present
with an abundance requiring Li manufacture. With this pair, abundance analyses
are now available for 11 C-rich pAGBs. A common history is indicated and, in
particular, the -abundances, especially the relative abundances of light to
heavy -process elements, follow recent predictions for the third dredge-up
in AGB stars.Comment: 41 pages (including 10 figs). 2001, ApJ, Accepte
Spectropolarimetry of the Massive Post-Red Supergiants IRC +10420 and HD 179821
We present medium resolution spectropolarimetry and long term
photo-polarimetry of two massive post-red supergiants, IRC +10420 and HD
179821. The data provide new information on their circumstellar material as
well as their evolution. In IRC +10420, the polarization of the Halpha line is
different to that of the continuum, which indicates that the
electron-scattering region is not spherically symmetric. The observed long term
changes in the polarimetry can be associated with an axi-symmetric structure,
along the short axis of the extended reflection nebulosity. Long term
photometry reveals that the star increased in temperature until the
mid-nineties, after which the photospheric flux in the optical levelled off. As
the photometric changes are mostly probed in the red, they do not trace high
stellar temperatures sensitively. And so, it is not obvious whether the star
has halted its increase in temperature or not. For HD 179821 we find no
polarization effects across any absorption or emission lines, but observe very
large polarization changes of order 5% over 15 years. Unexpectedly, during the
same period, the optical photometry displayed modest variability at the 0.2
magnitude level. Several explanations for this puzzling fact are discussed.
Most of which, involving asymmetries in the circumstellar material, seem to
fail as there is no evidence for the presence of hot, dusty material close to
the star. Alternatively, the variations can be explained by the presence of a
non-radially pulsating photosphere. Changes in the photometry hint at an
increase in temperature corresponding to a change through two spectral
subclasses over the past ten years.Comment: 13 pages, 6 figures, MNRAS accepte
Abundance analysis of barium and mild barium stars
High signal to noise, high resolution spectra were obtained for a sample of
normal, mild barium, and barium giants. Atmospheric parameters were determined
from the FeI and FeII lines. Abundances for Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr,
Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, Eu, and Gd, were
determined from equivalent widths and model atmospheres in a differential
analysis, with the red giant Eps Vir as the standard star. The different levels
of s-process overabundances of barium and mild barium stars were earlier
suggested to be related to the stellar metallicity. Contrary to this
suggestion, we found in this work no evidence of barium and mild barium to
having different range in metallicity. However, comparing the ratio of
abundances of heavy to light s-process elements, we found some evidence that
they do not share the same neutron exposure parameter. The exact mechanism
controlling this difference is still not clear. As a by-product of this
analysis we identify two normal red giants misclassified as mild barium stars.
The relevance of this finding is discussed. Concerning the suggested
nucleosynthetic effects possibly related to the s-process, for elements like
Cu, Mn, V and Sc, we found no evidence for an anomalous behavior in any of the
s-process enriched stars analyzed here. However, further work is still needed
since a clear [Cu/Fe] vs. [Ba/H] anticorrelation exists for other s-process
enriched objects.Comment: 34 pages, 18 figures, A&A accepted (revised version