447 research outputs found
An Update on the 0Z Project
We give an update on our 0Z Survey to find more extremely metal poor (EMP)
stars with [Fe/H] < -3 dex through mining the database of the Hamburg/ESO
Survey. We present the most extreme such stars we have found from ~1550
moderate resolution follow up spectra. One of these, HE1424-0241, has highly
anomalous abundance ratios not seen in any previously known halo giant, with
very deficient Si, moderately deficient Ca and Ti, highly enhanced Mn and Co,
and low C, all with respect to Fe. We suggest a SNII where the nucleosynthetic
yield for explosive alpha-burning nuclei was very low compared to that for the
hydrostatic alpha-burning element Mg, which is normal in this star relative to
Fe. A second, less extreme, outlier star with high [Sc/Fe] has also been found.
We examine the extremely metal-poor tail of the HES metallicity distribution
function (MDF). We suggest on the basis of comparison of our high resolution
detailed abundance analyses with [Fe/H](HES) for stars in our sample that the
MDF inferred from follow up spectra of the HES sample of candidate EMP stars is
heavily contaminated for [Fe/H](HES) < -3 dex; many of the supposed EMP stars
below that metallicity are of substantially higher Fe-metallicity, including
most of the very C-rich stars, or are spurious objects.Comment: to appear in conference proceedings "First Stars III", ed. B. O'Shea,
A. Heger & T.Abel, 4 pages, 2 figure
C, N and O abundances in red clump stars of the Milky Way
The Hipparcos orbiting observatory has revealed a large number of
helium-core-burning "clump" stars in the Galactic field. These low-mass stars
exhibit signatures of extra-mixing processes that require modeling beyond the
first dredge-up of standard models. The 12C/13C ratio is the most robust
diagnostic of deep mixing, because it is insensitive to the adopted stellar
parameters. In this work we present 12C/13C determinations in a sample of 34
Galactic clump stars as well as abundances of nitrogen, carbon and oxygen.
Abundances of carbon were studied using the C2 Swan (0,1) band head at 5635.5
A. The wavelength interval 7980-8130 A with strong CN features was analysed in
order to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen
abundances were determined from the [O I] line at 6300 A. Compared with the Sun
and dwarf stars of the Galactic disk, mean abundances in the investigated clump
stars suggest that carbon is depleted by about 0.2 dex, nitrogen is enhanced by
0.2 dex and oxygen is close to abundances in dwarfs. Comparisons to
evolutionary models show that the stars fall into two groups: the one is of
first ascent giants with carbon isotope ratios altered according to the first
dredge-up prediction, and the other one is of helium-core-burning stars with
carbon isotope ratios altered by extra mixing. The stars investigated fall to
these groups in approximately equal numbers.Comment: 8 pages 6 figures Accepted for publication in MNRA
The s-Process in Rotating Asymptotic Giant Branch Stars
(abridged) We model the nucleosynthesis during the thermal pulse phase of a
rotating, solar metallicity AGB star of 3M_sun. Rotationally induced mixing
during the thermal pulses produces a layer (~2E-5M_sun) on top of the CO-core
where large amounts of protons and C12 co-exist. We follow the abundance
evolution in this layer, in particular that of the neutron source C13 and of
the neutron poison N14. In our AGB model mixing persists during the entire
interpulse phase due to the steep angular velocity gradient at the
core-envelope interface. We follow the neutron production during the interpulse
phase, and find a resulting maximum neutron exposure of tau_max =0.04 mbarn^-1,
which is too small to produce any significant s-process. In parametric models,
we then investigate the combined effects of diffusive overshooting from the
convective envelope and rotationally induced mixing. Models with overshoot and
weaker interpulse mixing - as perhaps expected from more slowly rotating stars
- yield larger neutron exposures. We conclude that the incorporation of
rotationally induce mixing processes has important consequences for the
production of heavy elements in AGB stars. Through a distribution of initial
rotation rates it may lead to a natural spread in the neutron exposures
obtained in AGB stars of a given mass - as appears to be required by
observations. Our results suggest that both processes, diffusive overshoot and
rotational mixing, may be required to obtain a consistent description of the
s-process in AGB stars which fulfils all observational constraints. Finally, we
find that mixing due to rotation within our current framework does increase the
production of N15 in the partial mixing zone, however still falling short of
what seems required by observations.Comment: 50 pages, 13 figures, ApJ in press, tentatively scheduled for v593 n2
August 20, 200
Oxygen and Magnesium Abundance in the Ultra-Metal-Poor Giants CS22949-037 and CS29498-043: Challenges in Models of Atmospheres
We report the results of a non-LTE Fe, O and Mg abundance analysis of the
carbon-nitrogen-rich ultra-metal-poor giants CS29498--043 and CS22949--037. The
abundance of oxygen has been derived from measurements of the oxygen triplet at
7771-5 A in high resolution spectra obtained with KeckI/HIRES and the forbidden
line [OI] 6300 A detected in the TNG/SARG spectra of CS29498-043. Detailed
non-LTE analysis of Fe lines has provided reliable stellar parameters which,
however, do not resolve the oxygen abundance conflict as derived from the OI
7771-5 A triplet and the [OI] 6300 A forbidden lines. We obtained the following
oxygen abundance: for CS22949-037 [O/Fe] =3.13, 1.95; and for CS29498-043;
[O/Fe]=3.02, 2.49, based on the OI 7771-5 A triplet and the [OI] 6300 A
forbidden line, respectively. A similar conflict appears to exist between the
forbidden resonance line MgI 4571 A and several subordinate lines, such as MgI
5172 and 5183 A. Our analysis demonstrates the failure of standard
plane--parallel atmosphere models to describe the physical conditions in the
line-forming regions of these ultra-metal-poor giants.Comment: 16 pages, 15 figures, accepted by Astronomy and Astrophysic
Abundances of metal-weak thick-disc candidates
High resolution spectra of 5 candidate metal-weak thick-disc stars suggested
by Beers & Sommer-Larsen (1995) are analyzed to determine their chemical
abundances. The low abundance of all the objects has been confirmed with
metallicity reaching [Fe/H]=-2.9. However, for three objects, the astrometric
data from the Hipparcos catalogue suggests they are true halo members. The
remaining two, for which proper-motion data are not available, may have
disc-like kinematics. It is therefore clear that it is useful to address
properties of putative metal-weak thick-disc stars only if they possess full
kinematic data. For CS 22894-19 the abundance pattern similar to those of
typical halo stars is found, suggesting that chemical composition is not a
useful discriminant between thick-disc and halo stars. CS 29529-12 is found to
be C enhanced with [C/Fe]=+1.0; other chemical peculiarities involve the s
process elements: [Sr/Fe]=-0.65 and [Ba/Fe]=+0.62, leading to a high [Ba/Sr]
considerably larger than what is found in more metal-rich carbon-rich stars,
but similar to LP 706-7 and LP 625-44 discussed by Norris et al (1997a).
Hipparcos data have been used to calculate the space velocities of 25 candidate
metal-weak thick-disc stars, thus allowing us to identify 3 bona fide members,
which support the existence of a metal-poor tail of the thick-disc, at variance
with a claim to the contrary by Ryan & Lambert (1995).Comment: to be published in MNRA
RR-Lyrae-type pulsations from a 0.26-solar-mass star in a binary system
RR Lyrae pulsating stars have been extensively used as tracers of old stellar
populations for the purpose of determining the ages of galaxies, and as tools
to measure distances to nearby galaxies. There was accordingly considerable
interest when the RR Lyr star OGLE-BLG-RRLYR-02792 was found to be a member in
an eclipsing binary system4, as the mass of the pulsator (hitherto constrained
only by models) could be unambiguously determined. Here we report that
RRLYR-02792 has a mass of 0.26 M_sun and therefore cannot be a classical RR
Lyrae star. Through models we find that its properties are best explained by
the evolution of a close binary system that started with 1.4 M_sun and 0.8
M_sun stars orbiting each other with an initial period of 2.9 days. Mass
exchange over 5.4 Gyr produced the observed system, which is now in a very
short-lived phase where the physical properties of the pulsator happen to place
it in the same instability strip of the H-R diagram occupied by RR Lyrae stars.
We estimate that samples of RR Lyr stars may contain a 0.2 percent
contamination with systems similar to this one, implying that distances
measured with RR Lyrae stars should not be significantly affected by these
binary interlopers
Near-Infrared Properties of Metal-poor Globular Clusters in the Galactic Bulge Direction
Aims. J, H, and K' images obtained from the near-infrared imager CFHTIR on
the Canada-France-Hawaii Telescope are used to derive the morphological
parameters of the red giant branch (RGB) in the near-infrared color-magnitude
diagrams for 12 metal-poor globular clusters in the Galactic bulge direction.
Using the compiled data set of the RGB parameters for the observed 12 clusters,
in addition to the previously studied 5 clusters, we discuss the properties of
the RGB morphology for the clusters and compare them with the calibration
relations for the metal-rich bulge clusters and the metal-poor halo clusters.
Methods. The photometric RGB shape indices such as colors at fixed magnitudes
of MK = MH = (-5.5, -5, -4, and -3), magnitudes at fixed colors of (J - K)o =
(J - H)o = 0.7, and the RGB slope are measured from the fiducial normal points
defined in the near- infrared color-magnitude diagrams for each cluster. The
magnitudes of RGB bump and tip are also estimated from the differential and
cumulative luminosity functions of the selected RGB stars. The derived RGB
parameters have been used to examine the overall behaviors of the RGB
morphology as a function of cluster metallicity. Results. The correlations
between the near-infrared photometric RGB shape indices and the cluster
metallicity for the programme clusters compare favorably with the previous
observational calibration relations for metal-rich clusters in the Galactic
bulge and the metal-poor halo clusters. The observed near-infrared magnitudes
of the RGB bump and tip for the investigated clusters are also in accordance
with the previous calibration relations for the Galactic bulge clusters.Comment: 12 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
The Milky Way Bulge: Observed properties and a comparison to external galaxies
The Milky Way bulge offers a unique opportunity to investigate in detail the
role that different processes such as dynamical instabilities, hierarchical
merging, and dissipational collapse may have played in the history of the
Galaxy formation and evolution based on its resolved stellar population
properties. Large observation programmes and surveys of the bulge are providing
for the first time a look into the global view of the Milky Way bulge that can
be compared with the bulges of other galaxies, and be used as a template for
detailed comparison with models. The Milky Way has been shown to have a
box/peanut (B/P) bulge and recent evidence seems to suggest the presence of an
additional spheroidal component. In this review we summarise the global
chemical abundances, kinematics and structural properties that allow us to
disentangle these multiple components and provide constraints to understand
their origin. The investigation of both detailed and global properties of the
bulge now provide us with the opportunity to characterise the bulge as observed
in models, and to place the mixed component bulge scenario in the general
context of external galaxies. When writing this review, we considered the
perspectives of researchers working with the Milky Way and researchers working
with external galaxies. It is an attempt to approach both communities for a
fruitful exchange of ideas.Comment: Review article to appear in "Galactic Bulges", Editors: Laurikainen
E., Peletier R., Gadotti D., Springer Publishing. 36 pages, 10 figure
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