7 research outputs found
Near-Infrared Spectroscopy of Carbon-Enhanced Metal-Poor Stars. I. A SOAR/OSIRIS Pilot Study
We report on an abundance analysis for a pilot study of seven Carbon-Enhanced
Metal-Poor (CEMP) stars, based on medium-resolution optical and near-infrared
spectroscopy. The optical spectra are used to estimate [Fe/H], [C/Fe], [N/Fe],
and [Ba/Fe] for our program stars. The near-infrared spectra, obtained during a
limited early science run with the new SOAR 4.1m telescope and the Ohio State
Infrared Imager and Spectrograph (OSIRIS), are used to obtain estimates of
[O/Fe] and 12C/13C. The chemical abundances of CEMP stars are of importance for
understanding the origin of CNO in the early Galaxy, as well as for placing
constraints on the operation of the astrophysical s-process in very
low-metallicity Asymptotic Giant Branch (AGB) stars.
This pilot study includes a few stars with previously measured [Fe/H],
[C/Fe], [N/Fe],[O/Fe], 12C/13C, and [Ba/Fe], based on high-resolution optical
spectra obtained with large-aperture telescopes. Our analysis demonstrates that
we are able to achieve reasonably accurate determinations of these quantities
for CEMP stars from moderate-resolution optical and near-infrared spectra. This
opens the pathway for the study of significantly larger samples of CEMP stars
in the near future. Furthermore, the ability to measure [Ba/Fe] for (at least
the cooler) CEMP stars should enable one to separate stars that are likely to
be associated with s-process enhancements (the CEMP-s stars) from those that do
not exhibit neutron-capture enhancements (the CEMP-no stars).Comment: 27 pages, including 5 tables, 6 figures, accepted for publication in
The Astronomical Journa
HD 209621: Abundances of neutron-capture elements
High resolution spectra obtained from the Subaru Telescope High Dispersion
Spectrograph have been used to update the stellar atmospheric parameters and
metallicity of the star HD 209621. We have derived a metallicity of [Fe/H] =
-1.93 for this star, and have found a large enhancement of carbon and of heavy
elements, with respect to iron. Updates on the elemental abundances of four
s-process elements (Y, Ce, Pr, Nd) along with the first estimates of abundances
for a number of other heavy elements (Sr, Zr, Ba, La, Sm, Eu, Er, Pb) are
reported. The stellar atmospheric parameters, the effective temperature, Teff,
and the surface gravity, log g (4500 K, 2.0), are determined from LTE analysis
using model atmospheres. Estimated [Ba/Eu] = +0.35, places the star in the
group of CEMP-(r+s) stars; however, the s-elements abundance pattern seen in HD
209621 is characteristic of CH stars; notably, the 2nd-peak s-process elements
are more enhanced than the first peak s-process elements. HD 209621 is also
found to show a large enhancement of the 3rd-peak s-process element lead (Pb)
with [Pb/Fe] = +1.88. The relative contributions of the two neutron-capture
processes, r- and s- to the observed abundances are examined using a parametric
model based analysis, that hints that the neutron-capture elements in HD 209621
primarily originate in s-process.Comment: 16 pages,8 figures. Accepted for publication in MNRA
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
Detailed Analysis of Nearby Bulgelike Dwarf Stars III. Alpha and Heavy-element abundances
The present sample of nearby bulgelike dwarf stars has kinematics and
metallicities characteristic of a probable inner disk or bulge origin. Ages
derived by using isochrones give 10-11 Gyr for these stars and metallicities
are in the range -0.80< [Fe/H]< +0.40. We calculate stellar parameters from
spectroscopic data, and chemical abundances of Mg, Si, Ca, Ti, La, Ba, Y, Zr
and Eu are derived by using spectrum synthesis.
We found that [alpha-elements/Fe] show different patterns depending on the
element. Si, Ca and Ti-to-iron ratios decline smoothly for increasing
metallicities, and follow essentially the disk pattern. O and Mg, products of
massive supernovae, and also the r-process element Eu, are overabundant
relative to disk stars, showing a steeper decline for metallicities [Fe/H] >
-0.3 dex. [s-elements/Fe] roughly track the solar values with no apparent trend
with metallicity for [Fe/H] < 0, showing subsolar values for the metal rich
stars. Both kinematical and chemical properties of the bulgelike stars indicate
a distinct identity of this population when compared to disk stars.Comment: 21 pages, 9 figures, to appear in Ap
CS 30322-023: an ultra metal-poor TP-AGB star?
With [Fe/H] = -3.5, CS 30322-023 is the most metal-poor star to exhibit a
clear s-process signature and the most metal-poor ``lead star'' known. CS
30322-023 is also remarkable in having the lowest surface gravity (log g <=
-0.3) among the metal-poor stars studied to date. The available evidence
indicates that this star is presently a thermally-pulsing asymptotic giant
branch (TP-AGB) star, with no strong indication of binarity thus far (although
a signal of period 192 d is clearly present in the radial-velocity data, this
is likely due to pulsation of the stellar envelope). We show that low-mass
TP-AGB stars are not expected to be exceedingly rare in a magnitude-limited
sample such as the HK survey, because their high luminosities make it possible
to sample them over a very large volume. The strong N overabundance and the low
12C/13C ratio (4) in this star is typical of the operation of the CN cycle.
Coupled with a Na overabundance and the absence of a strong C overabundance,
this pattern seems to imply that hot-bottom burning operated in this star,
which should then have a mass of at least 2 Msun. However, the luminosity
associated with this mass would put the star at a distance of about 50 kpc, in
the outskirts of the galactic halo. We explore alternative scenarios in which
the observed abundance pattern results from some mixing mechanism yet to be
identified occurring in a single low-metallicity 0.8 Msun AGB star, or from
pollution by matter from an intermediate-mass AGB companion which has undergone
hot-bottom burning. We stress, however, that our abundances may be subject to
uncertainties due to NLTE or 3D granulation effects which were not taken into
consideration.Comment: 17 pages, Astronomy & Astrophysics, in press; also available at
http://www.astro.ulb.ac.be/Html/ps.html#PR
More lead stars
The standard model for the operation of the s-process in asymptotic giant
branch (AGB) stars predicts that low-metallicity ([Fe/H] <= -1) AGB stars
should exhibit large overabundances of Pb and Bi as compared to other
s-elements. Eight CH stars (with [Fe/H] ranging from -1.5 to -2.5) are studied
in the present paper using high-resolution spectra (R = lambda/Delta lambda =
135 000), and five of them appear to be enriched in lead (with [Pb/Ce] ~ 0.7).
Abundances of heavy elements in these `lead stars' (defined as stars enriched
in s-elements and with [Pb/hs] >= 1, hs being any of Ba, La or Ce) are
consistent with the predictions for the s-process operating in low-metallicity
AGB stars as a consequence of the `partial mixing' of protons below the
convective hydrogen envelope. Two among the studied stars (HD 189711 and V Ari)
add to a growing number of low-metallicity stars which do not conform to these
predictions, however. Variations on the canonical proton-mixing scenario for
the operation of the s-process in low-metallicity stars, that could account for
these discrepant stars, are briefly discussed.Comment: 9 pages, 6 figures, accepted for publication in Astronomy &
Astrophysics. Also available at http://www-astro.ulb.ac.be/Html/ps.html#PR
Proper-motion binaries in the Hipparcos catalogue: comparison with radial velocity data
Context. This paper is the last in a series devoted to the analysis of the binary content of the Hipparcos Catalogue. Aims. The comparison of the proper motions constructed from positions spanning a short (Hipparcos) or long time (Tycho-2) makes it possible to uncover binaries with periods of the order of or somewhat larger than the short time span (in this case, the 3 yr duration of the Hipparcos mission), since the unrecognised orbital motion will then add to the proper motion. Methods. A list of candidate proper motion binaries is constructed from a carefully designed χ2 test evaluating the statistical significance of the difference between the Tycho-2 and Hipparcos proper motions for 103 134 stars in common between the two catalogues (excluding components of visual systems). Since similar lists of proper-motion binaries have already been constructed, the present paper focuses on the evaluation of the detection efficiency of proper-motion binaries, using different kinds of control data (mostly radial velocities). The detection rate for entries from the Ninth Catalogue of Spectroscopic Binary Orbits (SB9) is evaluated, as well as for stars like barium stars, which are known to be all binaries, and finally for spectroscopic binaries identified from radial velocity data in the Geneva-Copenhagen survey of F and G dwarfs in the solar neighbourhood. Results. Proper motion binaries are efficiently detected for systems with parallaxes in excess of ∼20 mas, and periods in the range 1000-30 000 d. The shortest periods in this range (1000-2000 d, i.e. once to twice the duration of the Hipparcos mission) may appear only as DMSA/G binaries (accelerated proper motion in the Hipparcos Double and Multiple System Annex). Proper motion binaries detected among SB9 systems having periods shorter than about 400 d hint at triple systems, the proper-motion binary involving a component with a longer orbital period. A list of 19 candidate triple systems is provided. Binaries suspected of having low-mass (brown-dwarf-like) companions are listed as well. Among the 37 barium stars with parallaxes larger than 5 mas, only 7 exhibit no evidence for duplicity whatsoever (be it spectroscopic or astrometric). Finally, the fraction of proper-motion binaries shows no significant variation among the various (regular) spectral classes, when due account is taken for the detection biases. © ESO 2007.SCOPUS: ar.jinfo:eu-repo/semantics/publishe