19 research outputs found
BBN and the Primordial Abundances
The relic abundances of the light elements synthesized during the first few
minutes of the evolution of the Universe provide unique probes of cosmology and
the building blocks for stellar and galactic chemical evolution, while also
enabling constraints on the baryon (nucleon) density and on models of particle
physics beyond the standard model. Recent WMAP analyses of the CBR temperature
fluctuation spectrum, combined with other, relevant, observational data, has
yielded very tight constraints on the baryon density, permitting a detailed,
quantitative confrontation of the predictions of Big Bang Nucleosynthesis with
the post-BBN abundances inferred from observational data. The current status of
this comparison is presented, with an emphasis on the challenges to astronomy,
astrophysics, particle physics, and cosmology it identifies.Comment: To appear in the Proceedings of the ESO/Arcetri Workshop on "Chemical
Abundances and Mixing in Stars in the Milky Way and its Satellites", eds., L.
Pasquini and S. Randich (Springer-Verlag Series, "ESO Astrophysics Symposia"
Seismic Test of Solar Models, Solar Neutrinos and Implications for Metal-Rich Accretion
The Sun is believed to have been the recipient of a substantial amount of
metal-rich material over the course of its evolution, particularly in the early
stages of the Solar System. With a long diffusion timescale, the majority of
this accreted matter should still exist in the solar convection zone, enhancing
its observed surface abundance, and implying a lower-abundance core. While
helioseismology rules out solar models with near-zero metallicity cores, some
solar models with enhanced metallicity in the convection zone might be viable,
as small perturbations to the standard model. Because of the reduced interior
opacity and core temperature, the neutrino flux predicted for such models is
lower than that predicted by the standard solar model. This paper examines how
compatible inhomogeneous solar models of this kind are with the observed low
and intermediate degree p-mode oscillation data, and with the solar neutrino
data from the SNO Collaboration. We set an upper limit on how much metal-rich
accretion took place during the early evolution of the Sun at about 2 Earth
masses of iron (or about 40 Earth masses of meteoric material).Comment: Revision: 23 pages, 8 figures, 3 tables, to be published in ApJ, Sept
2002. Used newer neutrino cross sections, modified text accordingly, other
minor revisions as wel
Sensitive Observations of Radio Recombination Lines in Orion and W51: The Data and Detection of Systematic Recombination Line Blueshifts Proportional to Impact Broadening
Sensitive spectral observations made in two frequency bands near 6.0 and 17.6
GHz are described for Orion and W51. Using frequency switching we were able to
achieve a dynamic range in excess of 10,000 without fitting sinusoidal or
polynomial baselines. This enabled us to detect lines as weak as T\Delta n$ as
high as 25 have been detected in Orion. In the Orion data, where the lines are
stronger, we have also detected a systematic shift in the line center
frequencies proportional to linewidth that cannot be explained by normal
optical depth effects.Comment: 22 pages, 13 figures. Accepted for publication in Astrophysics and
Space Scienc
Multiwavelength photometry in the Globular Cluster M2
We present a multiwavelength photometric analysis of the globular cluster M2.
The data-set has been obtained by combining high-resolution (HST/WFPC2 and ACS)
and wide-field (GALEX) space observations and ground based (MEGACAM-CFHT,
EMMI-NTT) images. The photometric sample covers the entire cluster extension
from the very central regions up to the tidal radius and beyond. It allows an
accurate determination of the cluster center of gravity and other structural
parameters derived from the star count density profile. Moreover we study the
BSS population and its radial distribution. A total of 123 BSS has been
selected, and their radial distribution has been found to be bimodal (highly
peaked in the center, decreasing at intermediate radii and rising outward), as
already found in a number of other clusters. The radial position of the minimum
of the BSS distribution is consistent with the radius of avoidance caused by
the dynamical friction of massive objects over the cluster age. We also
searched for gradients in the red giant branch (RGB) and the asymptotic giant
branch (AGB) populations. At the level we found an overabundance of
AGB stars within the core radius and confirmed the result of Sohn et al.(1996)
that the central region of M2 is bluer than the outer part. We show that the
latter is due to a deficit of very luminous RGB stars in the central region.Comment: 35 pages, 18 figures; Accepted for publication by ApJ
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
The blue straggler star population in NGC 6229
We have used a combination of high-resolution Hubble Space Telescope Wide Field Planetary Camera 2 and wide-field ground-based observations in ultraviolet and optical bands to study the blue straggler star (BSS) population of the outer halo globular cluster NGC 6229 over its entire radial extent. A total of 64 bright BSS (with m 255 64 21.30, corresponding to m 555 64 20.75) has been identified. The BSS-projected radial distribution is found to be bimodal, with a high central peak, a well-defined minimum at intermediate radii (r 3c 40arcsec) and an upturn in the outskirts. From detailed star counts even in the very inner region, we compute the centre of gravity of the cluster and the most accurate and extended radial density profile ever published for this system. The profile is reasonably well reproduced by a standard King model with an extended core (r c 43 9.5arcsec) and a modest value of the concentration parameter (c 43 1.49). However, a deviation from the model is noted in the most external region of the cluster (at r > 250arcsec from the centre). This feature needs to be further investigated in order to assess the possible presence of a tidal tail in this cluster