2,636 research outputs found
Homogeneous abundance analysis of dwarf, subgiant and giant FGK stars with and without giant planets
We have analyzed high-resolution and high signal-to-noise ratio optical
spectra of nearby FGK stars with and without detected giant planets in order to
homogeneously measure their photospheric parameters, mass, age, and the
abundances of volatile (C, N, and O) and refractory (Na, Mg, Si, Ca, Ti, V, Mn,
Fe, Ni, Cu, and Ba) elements. Our sample contains 309 stars from the solar
neighborhood (up to the distance of 100 pc), out of which 140 are dwarfs, 29
are subgiants, and 140 are giants. The photospheric parameters are derived from
the equivalent widths of Fe I and Fe II lines. Masses and ages come from the
interpolation in evolutionary tracks and isochrones on the HR diagram. The
abundance determination is based on the equivalent widths of selected atomic
lines of the refractory elements and on the spectral synthesis of C_2, CN, C I,
O I, and Na I features. We apply a set of statistical methods to analyze the
abundances derived for the three subsamples. Our results show that: i) giant
stars systematically exhibit underabundance in [C/Fe] and overabundance in
[N/Fe] and [Na/Fe] in comparison with dwarfs, a result that is normally
attributed to evolution-induced mixing processes in the envelope of evolved
stars; ii) for solar analogs only, the abundance trends with the condensation
temperature of the elements are correlated with age and anticorrelated with the
surface gravity, which is in agreement with recent studies; iii) as in the case
of [Fe/H], dwarf stars with giant planets are systematically enriched in [X/H]
for all the analyzed elements, except for O and Ba (the former due to
limitations of statistics), confirming previous findings in the literature that
not only iron has an important relation with the planetary formation; and iv)
giant planet hosts are also significantly overabundant for the same metallicity
when the elements from Mg to Cu are combined together.Comment: 20 pages, 16 figures, 8 table
Chemical Evolution of the Galaxy Based on the Oscillatory Star Formation History
We model the star formation history (SFH) and the chemical evolution of the
Galactic disk by combining an infall model and a limit-cycle model of the
interstellar medium (ISM). Recent observations have shown that the SFH of the
Galactic disk violently variates or oscillates. We model the oscillatory SFH
based on the limit-cycle behavior of the fractional masses of three components
of the ISM. The observed period of the oscillation ( Gyr) is reproduced
within the natural parameter range. This means that we can interpret the
oscillatory SFH as the limit-cycle behavior of the ISM. We then test the
chemical evolution of stars and gas in the framework of the limit-cycle model,
since the oscillatory behavior of the SFH may cause an oscillatory evolution of
the metallicity. We find however that the oscillatory behavior of metallicity
is not prominent because the metallicity reflects the past integrated SFH. This
indicates that the metallicity cannot be used to distinguish an oscillatory SFH
from one without oscillations.Comment: 21 pages LaTeX, to appear in Ap
The metallicity distribution of G dwarfs in the solar neighbourhood
We derive a new metallicity distribution of G dwarfs in the solar
neighbourhood, using uvby photometry and up-to-date parallaxes. Our
distribution comprises 287 G dwarfs within 25 pc from the Sun, and differs
considerably from the classic solar neighbourhood distribution of Pagel &
Patchett and Pagel by having a prominent single peak around [Fe/H] = -0.20 dex.
The raw data are corrected for observational errors and cosmic scatter assuming
a deviation of 0.1. In order to obtain the true abundance distribution, we use
the correction factors given by Sommer-Larsen, which take into account the
stellar scale heights. The distribution confirms the G dwarf problem, that is,
the paucity of metal-poor stars relative to the predictions of the simple model
of chemical evolution. Another feature of this distribution, which was already
apparent in previous ones, is the small number of metal-rich stars again in
comparison with the simple model. Our results indicate that it is very
difficult to fit the simple model to this distribution, even with the
definition of an `effective yield'. A comparison with several models from the
literature is made. We find that models with infall are the most appropriate to
explain the new metallicity distribution. We also show that the metallicity
distribution is compatible with a major era of star formation occurring 5 to 8
Gyr ago, similar to results found by several authors.Comment: Tex, uses mn.tex v1.6, 13 pages, 8 figures available upon request,
accepted for publication in Monthly Notices of Roy. Astr. So
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