7 research outputs found
Gas Accretion and Star Formation Rates
Cosmological numerical simulations of galaxy evolution show that accretion of
metal-poor gas from the cosmic web drives the star formation in galaxy disks.
Unfortunately, the observational support for this theoretical prediction is
still indirect, and modeling and analysis are required to identify hints as
actual signs of star-formation feeding from metal-poor gas accretion. Thus, a
meticulous interpretation of the observations is crucial, and this
observational review begins with a simple theoretical description of the
physical process and the key ingredients it involves, including the properties
of the accreted gas and of the star-formation that it induces. A number of
observations pointing out the connection between metal-poor gas accretion and
star-formation are analyzed, specifically, the short gas consumption time-scale
compared to the age of the stellar populations, the fundamental metallicity
relationship, the relationship between disk morphology and gas metallicity, the
existence of metallicity drops in starbursts of star-forming galaxies, the
so-called G dwarf problem, the existence of a minimum metallicity for the
star-forming gas in the local universe, the origin of the alpha-enhanced gas
forming stars in the local universe, the metallicity of the quiescent BCDs, and
the direct measurements of gas accretion onto galaxies. A final section
discusses intrinsic difficulties to obtain direct observational evidence, and
points out alternative observational pathways to further consolidate the
current ideas.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by
Springe
Oxygen abundance in local disk and bulge: chemical evolution with a strictly universal IMF
The empirical differential oxygen abundance distribution (EDOD) is deduced
from subsamples related to two different samples involving solar neighbourhood
(SN) thick disk, thin disk, halo, and bulge stars. The EDOD of the SN thick +
thin disk is determined by weighting the mass, for assumed SN thick to thin
disk mass ratio within the range, 0.1-0.9. Inhomogeneous models of chemical
evolution for the SN thick disk, the SN thin disk, the SN thick + thin disk,
the SN halo, and the bulge, are computed assuming the instantaneous recycling
approximation. The EDOD data are fitted, to an acceptable extent, by their TDOD
counterparts provided (i) still undetected, low-oxygen abundance thin disk
stars exist, and (ii) a single oxygen overabundant star is removed from a thin
disk subsample. In any case, the (assumed power-law) stellar initial mass
function (IMF) is universal but gas can be inhibited from, or enhanced in,
forming stars at different rates with respect to a selected reference case.
Models involving a strictly universal IMF (i.e. gas neither inhibited from, nor
enhanced in, forming stars with respect to a selected reference case) can also
reproduce the data. The existence of a strictly universal IMF makes similar
chemical enrichment within active (i.e. undergoing star formation) regions
placed in different environments, but increasing probability of a region being
active passing from SN halo to SN thick + thin disk, SN thin disk, SN thick
disk, and bulge. On the basis of the results, it is realized that the chemical
evolution of the SN thick + thin disk as a whole cannot be excluded.Comment: 26 pages, 10 tables, and 5 figures; tables out of page are splitted
in two parts in Appendix B; sects.4 and 5 rewritten for better understanding
of the results; further references added. Accepted for publication in
Astrophysics & Space Scienc
The chemical evolution of a Milky Way-like galaxy: the importance of a cosmologically motivated infall law
We aim at finding a cosmologically motivated infall law to understand if the
LambdaCDM cosmology can reproduce the main chemical characteristics of a Milky
Way-like spiral galaxy. In this work we test several different gas infall laws,
starting from that suggested in the two-infall model for the chemical evolution
of the Milky Way by Chiappini et al., but focusing on laws derived from
cosmological simulations which follows a concordance LambdaCDM cosmology. By
means of a detailed chemical evolution model for the solar vicinity, we study
the effects of the different gas infall laws on the abundance patterns and the
G-dwarf metallicity distribution. The cosmological gas infall law predicts two
main gas accretion episodes. By means of this cosmologically motivated infall
law, we study the star formation rate, the SNIa and SNII rate, the total amount
of gas and stars in the solar neighbourhood and the behaviour of several
chemical abundances. We find that the results of the two-infall model are fully
compatible with the evolution of the Milky Way with cosmological accretion
laws. A gas assembly history derived from a DM halo, compatible with the
formation of a late-type galaxy from the morphological point of view, can
produce chemical properties in agreement with the available observations.Comment: This paper has 26 pages, 19 figures and 5 table
Thermonuclear destruction of lithium beryllium and boron inside a 1 solar mass star
The goal of this paper is to show the interest of combining various text analysis techniques (shallow parsing, semantic analysis, etc.) and some information access techniques (indexing, classification, clustering, mapping)) to developp an information analysis system to be used and customized by non-specialists of documentary languages. The paper shows how these techniques can be integrated to for a process chain including : XML reformating, information extraction, clustering, mapping
Comparison of capsule deformations induced by radiation asymmetries in spherical and cylindrical hohlraums lighted by the laser MégaJoule
The nominal configuration of inertial confinement fusion
with the Laser MégaJoule (LMJ) uses a cylindrical hohlraum with two
polar holes for the entrance of 60 laser quads. But the LMJ facility makes
it possible to use a spherical hohlraum with more than two holes. We have
studied two alternate configurations, in which 56 and 44 laser quads enter a
spherical hohlraum through four and six holes respectively, with the same
wall surface as in the nominal cylindrical hohlraum. We have estimated the
intrinsic and random radiation asymmetries on the nominal capsule A1040 in
these two configurations
Primordial nucleosynthesis and nuclear reaction rates uncertainties.
The present analysis is devoted to interdiscourse functions in argumentative activities in psychological consulting on radio. The interdiscourse participates in persuasive premises by introducing sentences in relation to doxa and the topic, and the audience or the clients'reactions. it also occurs in interpretative activities according to the psychological consulting's pattern and contributes to creating vulgates in relation to stabilized theories