397 research outputs found
VINCI / VLTI observations of Main Sequence stars
Main Sequence (MS) stars are by far the most numerous class in the Universe.
They are often somewhat neglected as they are relatively quiet objects (but
exceptions exist), though they bear testimony of the past and future of our
Sun. An important characteristic of the MS stars, particularly the solar-type
ones, is that they host the large majority of the known extrasolar planets.
Moreover, at the bottom of the MS, the red M dwarfs pave the way to
understanding the physics of brown dwarfs and giant planets. We have measured
very precise angular diameters from recent VINCI/VLTI interferometric
observations of a number of MS stars in the K band, with spectral types between
A1V and M5.5V. They already cover a wide range of effective temperatures and
radii. Combined with precise Hipparcos parallaxes, photometry, spectroscopy as
well as the asteroseismic information available for some of these stars, the
angular diameters put strong constraints on the detailed models of these stars,
and therefore on the physical processes at play.Comment: 5 pages, 3 figures. To appear in the Proceedings of IAU Symposium
219, "Stars as Suns", Editors A. Benz & A. Dupree, Astronomical Society of
the Pacifi
Abundances in Stars from the Red Giant Branch Tip to the Near Main Sequence in M71: II. Iron Abundance
We present [Ffe/H] abundance results that involve a sample of stars with a
wide range in luminosity from luminous giants to stars near the turnoff in a
globular cluster. Our sample of 25 stars in M71 includes 10 giant stars more
luminous than the RHB, 3 horizontal branch stars, 9 giant stars less luminous
than the RHB, and 3 stars near the turnoff. We analyzed both Fe I and Fe II
lines in high dispersion spectra observed with HIRES at the W. M. Keck
Observatory. We find that the [Fe/H] abundances from both Fe I and Fe II lines
agree with each other and with earlier determinations. Also the [Fe/H] obtained
from Fe I and Fe II lines is constant within the rather small uncertainties for
this group of stars over the full range in Teff and luminosity, suggesting that
NLTE effects are negligible in our iron abundance determination. In this
globular cluster, there is no difference among the mean [Fe/H] of giant stars
located at or above the RHB, RHB stars, giant stars located below the RHB and
stars near the turnoff.Comment: Minor changes to conform to version accepted for publication, with
several new figures (Paper 2 of a pair
Stellar Iron Abundances: non-LTE Effects
We report new statistical equilibrium calculations for Fe I and Fe II in the
atmosphere of Late-Type stars. We used atomic models for Fe I and Fe II having
respectively 256 and 190 levels, as well as 2117 and 3443 radiative
transitions. Photoionization cross-sections are from the Iron Project. These
atomic models were used to investigate non-LTE effects in iron abundances of
Late-Type stars with different atmospheric parameters.
We found that most Fe I lines in metal-poor stars are formed in conditions
far from LTE. We derived metallicity corrections of about 0.3 dex with respect
to LTE values, for the case of stars with [Fe/H] ~ -3.0. Fe II is found not to
be affected by significant non-LTE effects. The main non-LTE effect invoked in
the case of Fe I is overionization by ultraviolet radiation, thus classical
ionization equilibrium is far to be satisfied. An important consequence is that
surface gravities derived by LTE analysis are in error and should be corrected
before final abundances corrections.
This apparently solves the observed discrepancy between spectroscopic surface
gravities derived by LTE analyses and those derived from Hipparcos parallaxes.
A table of non-LTE [Fe/H] and log g values for a sample of metal-poor late-type
stars is given.Comment: 22 pages, 9 figures, 1 table, ApJ style, accepte
Abundances in Stars from the Red Giant Branch Tip to Near the Main Sequence Turn Off in M71: III. Abundance Ratios
We present abundance ratios for 23 elements with respect to Fe in a sample of
stars with a wide range in luminosity, from luminous giants to stars near the
turnoff, in the globular cluster M71. The analyzed spectra, obtained with HIRES
at the Keck Observatory, are of high dispersion (R=35,000). We find that the
neutron capture, the iron peak and the alpha-element abundance ratios show no
trend with Teff, and low scatter around the mean between the top of the RGB and
near the main sequence turnoff. The alpha-elements Mg, Ca, Si and Ti are
overabundant relative to Fe. The anti-correlation between O and Na abundances,
observed in other metal poor globular clusters, is detected in our sample and
extends to the main sequence. A statistically significant correlation between
Al and Na abundances is observed among the M71 stars in our sample, extending
to Mv = +1.8, fainter than the luminosity of the RGB bump in M5. Lithium is
varying, as expected, and Zr may be varying from star to star as well. M71
appears to have abundance ratios very similar to M5 whose bright giants were
studied by Ivans et al. (2001), but seems to have a smaller amplitude of
star-to-star variations at a given luminosity, as might be expected from its
higher metallicity. The results of our abundance analysis of 25 stars in M71
provide sufficient evidence of abundance variations at unexpectedly low
luminosities to rule out the mixing scenario. Either alone or, even more
powerfully, combined with other recent studies of C and N abundances in M71
stars, the existence of such abundance variations cannot be reproduced within
the context of our current understanding of stellar evolution.Comment: AJ, in press (June 2002), 18 figure
A toolkit of mechanism and context independent widgets
Most human-computer interfaces are designed to run on a static platform (e.g. a workstation with a monitor) in a static environment (e.g. an office). However, with mobile devices becoming ubiquitous and capable of running applications similar to those found on static devices, it is no longer valid to design static interfaces. This paper describes a user-interface architecture which allows interactors to be flexible about the way they are presented. This flexibility is defined by the different input and output mechanisms used. An interactor may use different mechanisms depending upon their suitability in the current context, user preference and the resources available for presentation using that mechanism
Modeling of Alpha Cen and Procyon using VLTI observations
We present a novel approach to model the nearby stars Alpha Cen A & B and
Procyon A using asteroseismic and interferometric constraints. Using the VINCI
instrument installed at the VLT Interferometer (VLTI), the angular diameters of
the Alpha Centauri system were measured with a relative precision of 0.2% and
0.6%, respectively. From these values, we derive linear radii of R[A] = 1.224
+/- 0.003 R_sun and R[B] = 0.863 +/- 0.005 R_sun. These radii are in excellent
agreement with the models of Thevenin et al. (2002), that use asteroseismic
frequencies as constraints (Bouchy & Carrier 2001; Bouchy & Carrier 2002). With
the same instrument, we also measured the angular diameter of Procyon A. Using
the Hipparcos parallax, we obtain a linear radius of 2.048 +/- 0.025 R_sun. We
use this result together with spectroscopic and photometric constraints to
model this star with the CESAM code. We also computed the adiabatic oscillation
spectrum of our model of Procyon A, giving a mean large frequency separation of
Delta nu_0 = 54.8 uHz, in agreement with the seismic observations by Martic et
al. (2001). Our model favours a mass around 1.4 M_sun for Procyon A.Comment: 8 pages, 5 figures, poster presented at the JD 12 of the IAU General
Assembly, Sydney 200
Oxygen Abundances in Two Metal-Poor Subgiants from the Analysis of the 6300 A Forbidden O I Line
Recent LTE analyses (Israelian et al. 1998 and Bosegaard et al. 1999) of the
OH bands in the optical-ultraviolet spectra of nearby metal-poor subdwarfs
indicate that oxygen abundances are generally higher than those previously
determined. The difference increases with decreasing metallicity and reaches
delta([O/Fe]) ~ +0.6 dex as [Fe/H] approaches -3.0.
Employing high resolution (R = 50000), high S/N (~ 250) echelle spectra of
the two stars found by Israelian et al. (1998) to have the highest
[O/Fe]-ratios, viz, BD +23 3130 and BD +37 1458, we conducted abundance
analyses based on about 60 Fe I and 7-9 Fe II lines. We determined from Kurucz
LTE models the values of the stellar parameters, as well as abundances of Na,
Ni, and the traditional alpha-elements, independent of the calibration of color
vs scales. We determined oxygen abundances from spectral synthesis of
the stronger line (6300 A) of the [O I] doublet.
The syntheses of the [O I] line lead to smaller values of [O/Fe], consistent
with those found earlier among halo field and globular cluster giants. We
obtain [O/Fe] = +0.35 +/- 0.2 for BD +23 3130 and +0.50 +/- 0.2 for BD +37
1458. In the former, the [O I] line is very weak (~ 1 mA), so that the quoted
[O/Fe] value may in reality be an upper limit.
Therefore in these two stars a discrepancy exists between the [O/Fe]- ratios
derived from [O I] and the OH feature, and the origin of this difference
remains unclear. Until the matter is clarified, we suggest it is premature to
conclude that the ab initio oxygen abundances of old, metal-poor stars need to
be revised drastically upward.Comment: 38 pages, 5 tables, 14 figures To appear in July 1999 AJ Updated
April 16, 1999. Fixed typo
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