434 research outputs found
Hydrodynamic Simulations of Galaxy Formation. I. Dissipation and the Maximum Mass of Galaxies
We describe an accurate, one-dimensional, spherically symmetric, Lagrangian
hydrodynamics/gravity code, designed to study the effects of radiative cooling
and photo-ionization on the formation of protogalaxies. The code can treat an
arbitrary number of fluid shells (representing baryons) and collisionless
shells (representing cold dark matter). As a test of the code, we reproduce
analytic solutions for the pulsation behavior of a polytrope and for the
self-similar collapse of a spherically symmetric, cosmological perturbation. In
this paper, we concentrate on the effects of radiative cooling, examining the
ability of collapsing perturbations to cool within the age of the universe. In
contrast to some studies based on order-of- magnitude estimates, we find that
cooling arguments alone cannot explain the sharp upper cutoff observed in the
galaxy luminosity function.Comment: 33 pages, uuencoded compressed postscript with figures, Ap.J. (in
press), corrections to axes in Fig
Hydrodynamic simulations of galaxy formation
We have developed an accurate, one-dimensional, spherically symmetric, Lagrangian hydrodynamics/gravity code, designed to study the effects of radiative cooling and photo-ionization on the formation of protogalaxies. We examine the ability of collapsing perturbations to cool within the age of the universe. In contrast to some studies based on order-of-magnitude estimates, we find that cooling arguments alone cannot explain the sharp upper cutoff observed in the galaxy luminosity function. We also look at the effect of a photoionizing background on the formation of low-mass galaxies
The Old Halo metallicity gradient: the trace of a self-enrichment process
Based on a model of globular cluster self-enrichment published in a previous
paper, we present an explanation for the metallicity gradient observed
throughout the galactic Old Halo. Our self-enrichment model is based on the
ability of globular cluster progenitor clouds to retain the ejecta of a first
generation of Type II Supernovae. The key point is that this ability depends on
the pressure exerted on the progenitor cloud by the surrounding protogalactic
medium and therefore on the location of the cloud in the protoGalaxy. Since
there is no significant (if any) metallicity gradient in the whole halo, we
also present a review in favour of a galactic halo partly build via accretions
and mergers of satellite systems. Some of them bear their own globular clusters
and therefore ``contaminate'' the system of globular clusters formed ``in
situ'', namely within the original potential well of the Galaxy. Therefore, the
comparison between our self-enrichment model and the observational data should
be limited to the genuine galactic globular clusters, the so-called Old Halo
group.Comment: 11 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
The self-enrichment of galactic halo globular clusters : a clue to their formation ?
We present a model of globular cluster self-enrichment. In the protogalaxy,
cold and dense clouds embedded in the hot protogalactic medium are assumed to
be the progenitors of galactic halo globular clusters. The massive stars of a
first generation of metal-free stars, born in the central areas of the
proto-globular cluster clouds, explode as Type II supernovae. The associated
blast waves trigger the expansion of a supershell, sweeping all the material of
the cloud, and the heavy elements released by these massive stars enrich the
supershell. A second generation of stars is born in these compressed and
enriched layers of gas. These stars can recollapse and form a globular cluster.
This work aims at revising the most often encountered argument against
self-enrichment, namely the presumed ability of a small number of supernovae to
disrupt a proto-globular cluster cloud. We describe a model of the dynamics of
the supershell and of its progressive chemical enrichment. We show that the
minimal mass of the primordial cluster cloud required to avoid disruption by
several tens of Type II supernovae is compatible with the masses usually
assumed for proto-globular cluster clouds. Furthermore, the corresponding
self-enrichment level is in agreement with halo globular cluster metallicities.Comment: 12 pages, 7 figures. Accepted for publication in Astronomy and
Astrophysic
A new seismic analysis of Alpha Centauri
Models of alpha Cen A & B have been computed using the masses determined by
Pourbaix et al. (2002) and the data derived from the spectroscopic analysis of
Neuforge and Magain (1997). The seismological data obtained by Bouchy and
Carrier (2001, 2002) do help improve our knowledge of the evolutionary status
of the system. All the constraints are satisfied with a model which gives an
age of about 6 Gyr for the binary.Comment: to be published in Astronomy and Astrophysic
Breaking the core-envelope symmetry in p-mode pulsating stars
It has been shown that there is a potential ambiguity in the asteroseismic
determination of the location of internal structures in a pulsating star. We
show how, in the case of high-order non-radial acoustic modes, it is possible
to remove this ambiguity by considering modes of different degree. To support
our conclusions we have investigated the seismic signatures of sharp density
variations in the structure of quasi-homogeneous models.Comment: 3 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Cosmological Formation of Low-Mass Objects
We investigate the early formation of bound objects with masses comparable to
the cosmological Jeans mass (10^5 solar masses). We follow the growth of
isolated spherically symmetric density peaks starting from the linear
perturbative regime. The initial parameters correspond to density peaks of
various widths and heights in a Cold Dark Matter cosmology. We use a
one-dimensional spherical Lagrangian hydrodynamics code to follow the
dynamical, thermal, and non-equilibrium chemical evolution of the gas. The
system includes a collisionless dark matter component and a baryonic component
composed of the nine species H, H^-, H^+, He, He^+, He^{++}, H_2, H_2^+, and
e^-. All relevant chemical reactions between these species and their cooling
mechanisms are included in the calculations. We find that radiative cooling by
H_2 affects the collapse dynamics of the gas only after it has already
virialized and become part of the bound object. Therefore, radiative cooling is
unlikely to have triggered the initial collapse of perturbations at redshifts
z>10. Nevertheless, objects with baryonic masses well below the linear-theory
Jeans mass (<10^3 solar masses) collapse due to shell crossing by the dark
matter. Such objects could be the progenitors of a primordial population of
high-mass stars in the intergalactic medium.Comment: 40 pages, uuencoded compressed Postscript, 14 figures included as
three separate file
ANOMALOUS PERTURBATIVE TRANSPORT IN TOKAMAKS DUE TO DRIFT-WAVE TURBULENCE
A new method for calculating the anomalous transport in tokamak plasmas is presented. The renormalized nonlinear plasma response function is derived using the direct-interaction approximation (DIA). A complete calculation for the case of electrostatic drift-wave turbulence is presented. Explicit expressions for all coefficients of the anomalous transport matrix relating particle and heat fluxes to density and temperature gradients in the plasma are obtained. The anomalous transport matrix calculated using the DIA does not have the Onsager symmetry. As an example of application, the parameters of the Texas Experimental Tokamak (TEXT) [Nucl. Technol. Fusion 1, 479 (1981)] are used to evaluate all transport coefficients numerically, as well as the spectrum modulation. The relation between the theoretical results and the experimental data is discussed. Although this paper focuses on electron transport for simplicity, the method can also be used to calculate anomalous transport due to ion instabilities, such as the ion-temperature-gradient instability
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