14,655 research outputs found
Really Cool Stars and the Star Formation History at the Galactic Center
We present R=550 to 1200 near infrared H and K spectra for a magnitude
limited sample of 79 asymptotic giant branch and cool supergiant stars in the
central ~ 5 pc (diameter) of the Galaxy. We use a set of similar spectra
obtained for solar neighborhood stars with known Teff and Mbol that is in the
same range as the Galactic center (GC) sample to derive Teff and Mbol for the
GC sample. We then construct the Hertzsprung--Russell (HRD) diagram for the GC
sample. Using an automated maximum likelihood routine, we derive a coarse star
formation history of the GC. We find (1) roughly 75% of the stars formed in the
central few pc are older than 5 Gyr; (2) the star formation rate (SFR) is
variable over time, with a roughly 4 times higher star formation rate in the
last 100 Myr compared to the average SFR; (3) our model can only match
dynamical limits on the total mass of stars formed by limiting the IMF to
masses above 0.7 M. This could be a signature of mass segregation or of
the bias toward massive star formation from the unique star formation
conditions in the GC; (4) blue supergiants account for 12 % of the total sample
observed, and the ratio of red to blue supergiants is roughly 1.5; (5) models
with isochrones with [Fe/H] = 0.0 over all ages fit the stars in our HRD better
than models with lower [Fe/H] in the oldest age bins, consistent with the
finding of Ramirez et al. (2000) that stars with ages between 10 Myr and 1 Gyr
have solar [Fe/H].Comment: ApJ, accepted. Latex, 65 pages including 19 figure
Quasi-chemical approximation for polyatomic mixtures
The statistical thermodynamics of binary mixtures of polyatomic species was
developed on a generalization in the spirit of the lattice-gas model and the
quasi-chemical approximation (QCA). The new theoretical framework is obtained
by combining: (i) the exact analytical expression for the partition function of
non-interacting mixtures of linear -mers and -mers (species occupying
sites and sites, respectively) adsorbed in one dimension, and its extension
to higher dimensions; and (ii) a generalization of the classical QCA for
multicomponent adsorbates and multisite-occupancy adsorption. The process is
analyzed through the partial adsorption isotherms corresponding to both species
of the mixture. Comparisons with analytical data from Bragg-Williams
approximation (BWA) and Monte Carlo simulations are performed in order to test
the validity of the theoretical model. Even though a good fitting is obtained
from BWA, it is found that QCA provides a more accurate description of the
phenomenon of adsorption of interacting polyatomic mixtures.Comment: 27 pages, 8 figure
Comment on "Feynman Effective Classical Potential in the Schrodinger Formulation"
We comment on the paper "Feynman Effective Classical Potential in the
Schrodinger Formulation"[Phys. Rev. Lett. 81, 3303 (1998)]. We show that the
results in this paper about the time evolution of a wave packet in a double
well potential can be properly explained by resorting to a variational
principle for the effective action. A way to improve on these results is also
discussed.Comment: 1 page, 2eps figures, Revte
Elemental Abundances of Solar Sibling Candidates
Dynamical information along with survey data on metallicity and in some cases
age have been used recently by some authors to search for candidates of stars
that were born in the cluster where the Sun formed. We have acquired high
resolution, high signal-to-noise ratio spectra for 30 of these objects to
determine, using detailed elemental abundance analysis, if they could be true
solar siblings. Only two of the candidates are found to have solar chemical
composition. Updated modeling of the stars' past orbits in a realistic Galactic
potential reveals that one of them, HD162826, satisfies both chemical and
dynamical conditions for being a sibling of the Sun. Measurements of
rare-element abundances for this star further confirm its solar composition,
with the only possible exception of Sm. Analysis of long-term high-precision
radial velocity data rules out the presence of hot Jupiters and confirms that
this star is not in a binary system. We find that chemical tagging does not
necessarily benefit from studying as many elements as possible, but instead
from identifying and carefully measuring the abundances of those elements which
show large star-to-star scatter at a given metallicity. Future searches
employing data products from ongoing massive astrometric and spectroscopic
surveys can be optimized by acknowledging this fact.Comment: ApJ, in press. Tables 2 and 4 are available in full in the "Other
formats: source" downloa
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