210 research outputs found
Reddenings of FGK supergiants and classical Cepheids from spectroscopic data
Accurate and homogeneous atmospheric parameters (Teff, log (g), Vt, [Fe/H])
are derived for 74 FGK non-variable supergiants from high-resolution, high
signal-to-noise ratio, echelle spectra. Extremely high precision for the
inferred effective temperatures (10-40 K) is achieved by using the line-depth
ratio method. The new data are combined with atmospheric values for 164
classical Cepheids, observed at 675 different pulsation phases, taken from our
previously published studies. The derived values are correlated with unreddened
B-V colours compiled from the literature for the investigated stars in order to
obtain an empirical relationship of the form: (B-V)o = 57.984 - 10.3587(log
Teff)^2 + 1.67572(log Teff)^3 - 3.356(log (g)) + 0.0321(Vt) + 0.2615[Fe/H] +
0.8833((log (g))(log Teff)). The expression is used to estimate colour excesses
E(B-V) for individual supergiants and classical Cepheids, with a precision of
+-0.05 mag. for supergiants and Cepheids with n=1-2 spectra, reaching +-0.025
mag. for Cepheids with n>2 spectra, matching uncertainties for the most
sophisticated photometric techniques. The reddening scale is also a close match
to the system of space reddenings for Cepheids. The application range is for
spectral types F0--K0 and luminosity classes I and II.Comment: accepted for publication (MNRAS
Oxygen, -element and iron abundance distributions in the inner part of the Galactic thin disc. II
We have derived the abundances of 36 chemical elements in one Cepheid star,
ASAS 181024--2049.6, located R kpc from the Galactic center.
This star falls within a region of the inner thin disc poorly sampled in
Cepheids. Our spectral analysis shows that iron, magnesium, silicon, calcium
and titanium LTE abundances in that star support the presence of a plateau-like
abundance distribution in the thin disc within 5 kpc of the Galactic center, as
previously suggested by \cite{Maret15}. If confirmed, the flattening of the
abundance gradient within that region could be the result of a decrease in the
star formation rate due to dynamic effects, possibly from the central Galactic
bar.Comment: 5 pages, 3 figure
Mixing and Accretion in lambda Bootis Stars
Strong evidence for deep mixing has been uncovered for slowly rotating F, and
A stars of the main sequence. As the accretion/diffusion model for the
formation of lboo stars is heavily dependent on mixing in superficial regions,
such deep mixing may have important repercussions on our understanding of these
stars. It is shown that deep mixing at a level similar to that of FmAm stars
increases the amount of matter that needs to be accreted by the stars with
respect with the standard models by some three orders of magnitude. It is also
shown that significantly larger accretion rates have to be maintained, as high
as ~M_\sun yr^{-1}, to prevent meridional circulation from
canceling the effect of accretion. The existence of old (~Gyr) is
not a likely outcome of the present models for accretion/diffusion with or
without deep mixing. It is argued that lboo stars are potentially very good
diagnostics of mixing mechanisms in moderately fast rotators.Comment: To appear in Astrophysical Journal Letters. 4 pages, 2 fgure
New results on the time variation of the radial abundance gradients from planetary nebulae
New results on the time variation of the radial abundance gradients in the
galactic disk are presented on the basis of four different samples of planetary
nebulae. These comprise both smaller, homogeneous sets of data, and larger but
non-homogeneous samples. Four different chemical elements are considered,
namely, O, S, Ar, and Ne. Other objects such as open clusters, cepheids and HII
regions are also taken into account. Our analysis support our earlier
conclusions in the sense that, on the average, the radial abundance gradients
have flattened out during the last 6 to 8 Gyr, with important consequences for
models of the chemical evolution of the Galaxy.Comment: 2 pages, 1 figure, LaTeX, To be published in the Proceedings of the
IAU Symposium 234: Planetary Nebulae in Our Galaxy and Beyond, ed. M. J.
Barlow, R. H. Mende
Using Cepheids to determine the galactic abundance gradient I. The solar neighbourhood
A number of studies of abundance gradients in the galactic disk have been
performed in recent years. The results obtained are rather disparate: from no
detectable gradient to a rather significant slope of about -0.1 dex kpc -1. The
present study concerns the abundance gradient based on the spectroscopic
analysis of a sample of classical Cepheids. These stars enable one to obtain
reliable abundances of a variety of chemical elements. Additionally, they have
well determined distances which allow an accurate determination of abundance
distributions in the galactic disc. Using 236 high resolution spectra of 77
galactic Cepheids, the radial elemental distribution in the galactic disc
between galactocentric distances in the range 6-11 kpc has been investigated.
Gradients for 25 chemical elements (from carbon to gadolinium) are derived...Comment: 28 pages, 14 postscript figures, LaTeX, uses Astronomy and
Astrophysics macro aa.cls, graphicx package, to be published in Astronomy and
Astrophysics (2002) also available at
http://www.iagusp.usp.br/~maciel/index.htm
Quantifying the uncertainties of chemical evolution studies. II. Stellar yields
This is the second paper of a series which aims at quantifying the
uncertainties in chemical evolution model predictions related to the underlying
model assumptions. Specifically, it deals with the uncertainties due to the
choice of the stellar yields. We adopt a widely used model for the chemical
evolution of the Galaxy and test the effects of changing the stellar
nucleosynthesis prescriptions on the predicted evolution of several chemical
species. We find that, except for a handful of elements whose nucleosynthesis
in stars is well understood by now, large uncertainties still affect the model
predictions. This is especially true for the majority of the iron-peak
elements, but also for much more abundant species such as carbon and nitrogen.
The main causes of the mismatch we find among the outputs of different models
assuming different stellar yields and among model predictions and observations
are: (i) the adopted location of the mass cut in models of type II supernova
explosions; (ii) the adopted strength and extent of hot bottom burning in
models of asymptotic giant branch stars; (iii) the neglection of the effects of
rotation on the chemical composition of the stellar surfaces; (iv) the adopted
rates of mass loss and of (v) nuclear reactions, and (vi) the different
treatments of convection. Our results suggest that it is mandatory to include
processes such as hot bottom burning in intermediate-mass stars and rotation in
stars of all masses in accurate studies of stellar evolution and
nucleosynthesis. In spite of their importance, both these processes still have
to be better understood and characterized. As for massive stars, presupernova
models computed with mass loss and rotation are available in the literature,
but they still wait for a self-consistent coupling with the results of
explosive nucleosynthesis computations.Comment: 28 pages, 23 figures. Accepted for publication in A&
Using Cepheids to determine the galactic abundance gradient II. Towards the galactic center
Based on spectra obtained at the Anglo-Australian Observatory, we present a
discussion of the metallicity of the galactic disc derived using Cepheids at
galactocentric distances 4-6 kpc. Our new results together with previous
gradient determination (Paper I) show that the overall abundance distribution
within the galactocentric distances 4-11 kpc cannot by represented by a single
gradient value. The distribution is more likely bimodal: it is flatter in the
solar neighbourhood with a small gradient, and steepens towards the galactic
center. The steepening begins at a distance of about 6.6 kpc.Comment: 8 pages, 6 postscript figures, LaTeX, uses Astronomy and Astrophysics
macro aa.cls, graphicx package, to be published in Astronomy and Astrophysics
(2002) also available at http://www.iagusp.usp.br/~maciel/index.htm
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