3,623 research outputs found
The Chemical Evolution of the Galaxy: the two-infall model
In this paper we present a new chemical evolution model for the Galaxy which
assumes two main infall episodes for the formation of halo-thick disk and thin
disk, respectively. We do not try to take into account explicitly the evolution
of the halo but we implicitly assume that the timescale for the formation of
the halo was of the same order as the timescale for the formation of the thick
disk. The formation of the thin-disk is much longer than that of the thick
disk, implying that the infalling gas forming the thin-disk comes not only from
the thick disk but mainly from the intergalactic medium. The timescale for the
formation of the thin-disk is assumed to be a function of the galactocentric
distance, leading to an inside-out picture for the Galaxy building. The model
takes into account the most up to date nucleosynthesis prescriptions and adopts
a threshold in the star formation process which naturally produces a hiatus in
the star formation rate at the end of the thick disk phase, as suggested by
recent observations. The model results are compared with an extended set of
observational constraints. Among these constraints, the tightest one is the
metallicity distribution of the G-dwarf stars for which new data are now
available. Our model fits very well these new data. We show that in order to
reproduce most of these constraints a timescale Gyr for the
(halo)-thick-disk and of 8 Gyr for the thin-disk formation in the solar
vicinity are required. We predict that the radial abundance gradients in the
inner regions of the disk () are steeper than in the outer
regions, a result confirmed by recent abundance determinations, and that the
inner ones steepen in time during the Galactic lifetime.Comment: 48 pages, 20 Postscript figures, AASTex v.4.0, to be published in
Astrophysical Journa
Hurdles for Recent Measures in Eternal Inflation
In recent literature on eternal inflation, a number of measures have been
introduced which attempt to assign probabilities to different pocket universes
by counting the number of each type of pocket according to a specific
procedure. We give an overview of the existing measures, pointing out some
interesting connections and generic predictions. For example, pairs of vacua
that undergo fast transitions between themselves will be strongly favored. The
resultant implications for making predictions in a generic potential landscape
are discussed. We also raise a number of issues concerning the types of
transitions that observers in eternal inflation are able to experience.Comment: 15 PRD-style pages, 5 figures, expanded discussion of measures in
Sec. II, added reference
The Chemical Compositions of the SRd Variable Stars-- II. WY Andromedae, VW Eridani, and UW Librae
Chemical compositions are derived from high-resolution spectra for three
stars classed as SRd variables in the General Catalogue of Variable Stars.
These stars are shown to be metal-poor supergiants: WY And with [Fe/H] = -1.0,
VW Eri with [Fe/H] = -1.8, and UW Lib with [Fe/H] = -1.2. Their compositions
are identical to within the measurement errors with the compositions of
subdwarfs, subgiants, and less evolved giants of the same FeH. The stars are at
the tip of the first giant branch or in the early stages of evolution along the
asymptotic giant branch (AGB). There is no convincing evidence that these SRd
variables are experiencing thermal pulsing and the third dredge-up on the AGB.
The SRds appear to be the cool limit of the sequence of RV Tauri variables.Comment: 14 pages, 1 figure, 4 table
Mixing along the Red Giant Branch in Metal-poor Field Stars
We have determined Li, C, N, O, Na, and Fe abundances, and 12C/13C isotopic
ratios for a sample of 62 field metal-poor stars (plus 43 taken from the
literature). This large sample was used to show that small mass lower-RGB stars
(i.e., fainter than the RGB bump) have abundances of light elements in
agreement with theoretical predictions from classical evolutionary models. A
second, distinct mixing episode occurs just after the RGB bump, reaching
regions of incomplete CNO burning. No O-Na anticorrelation, as observed in
globular cluster stars, is found in field stars. This means that the mixing
episode is not deep enough to reach regions where ON-burning occurs.Comment: 6 pages, 3 encapsulated figures, LateX, uses crckapb.sty; invited
talk, in "The Chemical Evolution of the Milky Way: Stars vs Clusters, Vulcano
(Italy), 20-24 September 1999, F. Matteucci and F. Giovannelli eds, Kluwer,
in pres
Someone\u27s Eyes
Photograph of Miss Luella E. Davis surrounded by bordered framehttps://scholarsjunction.msstate.edu/cht-sheet-music/7624/thumbnail.jp
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
Observational evidence for a different IMF in the early Galaxy
The unexpected high incidence of carbon-enhanced, s-process enriched
unevolved stars amongst extremely metal-poor stars in the halo provides a
significant constraint on the Initial Mass Function (IMF) in the early Galaxy.
We argue that these objects are evidence for the past existence of a large
population of intermediate-mass stars, and conclude that the IMF in the early
Galaxy was different from the present, and shifted toward higher masses.Comment: 14 pages, 1 color figure, accepted for publication on Ap
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