996 research outputs found
Oxygen Abundances in Nearby FGK Stars and the Galactic Chemical Evolution of the Local Disk and Halo
Atmospheric parameters and oxygen abundances of 825 nearby FGK stars are
derived using high-quality spectra and a non-LTE analysis of the 777 nm O I
triplet lines. We assign a kinematic probability for the stars to be thin-disk
(P1), thick-disk (P2), and halo (P3) members. We confirm previous findings of
enhanced [O/Fe] in thick-disk (P2>0.5) relative to thin-disk (P1>0.5) stars
with [Fe/H]<-0.2, as well as a "knee" that connects the mean [O/Fe]-[Fe/H]
trend of thick-disk stars with that of thin-disk members at [Fe/H]>-0.2.
Nevertheless, we find that the kinematic membership criterion fails at
separating perfectly the stars in the [O/Fe]-[Fe/H] plane, even when a very
restrictive kinematic separation is employed. Stars with "intermediate"
kinematics (P1<0.7, P2<0.7) do not all populate the region of the [O/Fe]-[Fe/H]
plane intermediate between the mean thin-disk and thick-disk trends, but their
distribution is not necessarily bimodal. Halo stars (P3>0.5) show a large
star-to-star scatter in [O/Fe]-[Fe/H], but most of it is due to stars with
Galactocentric rotational velocity V-200 km/s
follow an [O/Fe]-[Fe/H] relation with almost no star-to-star scatter. Early
mergers with satellite galaxies explain most of our observations, but the
significant fraction of disk stars with "ambiguous" kinematics and abundances
suggests that scattering by molecular clouds and radial migration have both
played an important role in determining the kinematic and chemical properties
of solar neighborhood stars.Comment: ApJ, in press. Complete tables 2-6 are available in the source
(Download: Other formats -> Source
Chemical Abundances from the Continuum
The calculation of solar absolute fluxes in the near-UV is revisited,
discussing in some detail recent updates in theoretical calculations of
bound-free opacity from metals. Modest changes in the abundances of elements
such as Mg and the iron-peak elements have a significant impact on the
atmospheric structure, and therefore self-consistent calculations are
necessary. With small adjustments to the solar photospheric composition, we are
able to reproduce fairly well the observed solar fluxes between 200 and 270 nm,
and between 300 and 420 nm, but find too much absorption in the 270-290 nm
window. A comparison between our reference 1D model and a 3D time-dependent
hydrodynamical simulation indicates that the continuum flux is only weakly
sensitive to 3D effects, with corrections reaching <10% in the near-UV, and <2%
in the optical.Comment: 10 pages, 5 figures, to appear in the proceedings of the conference A
Stellar Journey, a symposium in celebration of Bengt Gustafsson's 65th
birthday, June 23-27, 2008, Uppsal
Validation of stellar population and kinematical analysis of galaxies
3D spectroscopy produces hundreds of spectra from which maps of the
characteristics of stellar populations (age-metallicity) and internal
kinematics of galaxies can be derived. We carried on simulations to assess the
reliability of inversion methods and to define the requirements for future
observations. We quantify the biases and show that to minimize the errors on
the kinematics, age and metallicity (in a given observing time) the size of the
spatial elements and the spectral dispersion should be chosen to obtain an
instrumental velocity dispersion comparable to the physical dispersion.Comment: 5 pages, 3 figures, extended version of a poster proceeding to appear
in "Science Perspectives for 3D Spectroscopy", eds. M. Kissler-Patig, M. M.
Roth and J. R. Walsh, ESO Astrophysics Symposia. (The two last pages with
figures are not in the conference proceedings.
Lithium abundances in nearby FGK dwarf and subgiant stars: internal destruction, Galactic chemical evolution, and exoplanets
We derive atmospheric parameters and lithium abundances for 671 stars and
include our measurements in a literature compilation of 1381 dwarf and subgiant
stars. First, a "lithium desert" in the effective temperature (Teff) versus
lithium abundance (A_Li) plane is observed such that no stars with Teff~6075 K
and A_Li~1.8 are found. We speculate that most of the stars on the low A_Li
side of the desert have experienced a short-lived period of severe surface
lithium destruction as main-sequence or subgiant stars. Next, we search for
differences in the lithium content of thin-disk and thick-disk stars, but we
find that internal processes have erased from the stellar photospheres their
possibly different histories of lithium enrichment. Nevertheless, we note that
the maximum lithium abundance of thick-disk stars is nearly constant from
[Fe/H]=-1.0 to -0.1, at a value that is similar to that measured in very
metal-poor halo stars (A_Li~2.2). Finally, differences in the lithium abundance
distribution of known planet-host stars relative to otherwise ordinary stars
appear when restricting the samples to narrow ranges of Teff or mass, but they
are fully explained by age and metallicity biases. We confirm the lack of a
connection between low lithium abundance and planets. However, we find that no
low A_Li planet-hosts are found in the desert Teff window. Provided that subtle
sample biases are not responsible for this observation, this suggests that the
presence of gas giant planets inhibit the mechanism responsible for the lithium
desert.Comment: ApJ, in press. Complete Tables 1 and 3 are available upon reques
Accounting for Convective Blue-Shifts in the Determination of Absolute Stellar Radial Velocities
For late-type non-active stars, gravitational redshifts and convective
blueshifts are the main source of biases in the determination of radial
velocities. If ignored, these effects can introduce systematic errors of the
order of ~ 0.5 km/s. We demonstrate that three-dimensional hydrodynamical
simulations of solar surface convection can be used to predict the convective
blue-shifts of weak spectral lines in solar-like stars to ~ 0.070 km/s. Using
accurate trigonometric parallaxes and stellar evolution models, the
gravitational redshifts can be constrained with a similar uncertainty, leading
to absolute radial velocities accurate to better than ~ 0.1 km/s.Comment: To appear in the proceedings of the Joint Discussion 10, IAU General
Assembly, Rio de Janeiro, August 10-11, 200
Granulation across the HR diagram
We have obtained ultra-high quality spectra (R=180,000; S/N>300) with
unprecedented wavelength coverage (4400 to 7400 A) for a number of stars
covering most of the HR diagram in order to test the predictions of models of
stellar surface convection. Line bisectors and core wavelength shifts are both
measured and modeled, allowing us to validate and/or reveal the limitations of
state-of-the-art hydrodynamic model atmospheres of different stellar
parameters. We show the status of our project and preliminary results.Comment: 4 pages, 3 figures; proceedings article for Joint Discussion 10 at
the IAU General Assembly, Rio de Janeiro, Brazil, August 200
The initial-final mass relationship of white dwarfs in common proper motion pairs
A promising approach to decrease the uncertainties in the initial-final mass relationship, which is still poorly constrained, is to study white dwarfs for which external constraints are available, for instance, white dwarfs in common proper motion pairs (CPMPs). Important information of the white dwarf can be inferred from the study of the companion, since they were born at the same time and with the same initial chemical composition. In this contribution, we report new results obtained from spectroscopic observations of both members of several CPMPs composed of a F, G or K type star and a DA white dwarf
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