20 research outputs found
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
Element Diffusion in the Solar Interior
We study the diffusion of helium and other heavy elements in the solar
interior by solving exactly the set of flow equations developed by Burgers for
a multi-component fluid, including the residual heat-flow terms. No
approximation is made concerning the relative concentrations and no restriction
is placed on the number of elements considered. We give improved diffusion
velocities for hydrogen, helium, oxygen and iron, in the analytic form derived
previously by Bahcall and Loeb. These expressions for the diffusion velocities
are simple to program in stellar evolution codes and are expected to be
accurate to . Our complete treatment of element diffusion can be
directly incorporated in a standard stellar evolution code by means of an
exportable subroutine, but, for convenience, we also give simple analytical
fits to our numerical results.Comment: TeX document, 25 pages, for hardcopy with figures contact
[email protected]. Institute for Advanced Study number AST 93/1
Modules for Experiments in Stellar Astrophysics (MESA): Convective Boundaries, Element Diffusion, and Massive Star Explosions
We update the capabilities of the software instrument Modules for Experiments
in Stellar Astrophysics (MESA) and enhance its ease of use and availability.
Our new approach to locating convective boundaries is consistent with the
physics of convection, and yields reliable values of the convective core mass
during both hydrogen and helium burning phases. Stars with
become white dwarfs and cool to the point where the electrons are degenerate
and the ions are strongly coupled, a realm now available to study with MESA due
to improved treatments of element diffusion, latent heat release, and blending
of equations of state. Studies of the final fates of massive stars are extended
in MESA by our addition of an approximate Riemann solver that captures shocks
and conserves energy to high accuracy during dynamic epochs. We also introduce
a 1D capability for modeling the effects of Rayleigh-Taylor instabilities that,
in combination with the coupling to a public version of the STELLA radiation
transfer instrument, creates new avenues for exploring Type II supernovae
properties. These capabilities are exhibited with exploratory models of
pair-instability supernova, pulsational pair-instability supernova, and the
formation of stellar mass black holes. The applicability of MESA is now widened
by the capability of importing multi-dimensional hydrodynamic models into MESA.
We close by introducing software modules for handling floating point exceptions
and stellar model optimization, and four new software tools -- MESAWeb,
MESA-Docker, pyMESA, and mesastar.org -- to enhance MESA's education and
research impact.Comment: 64 pages, 61 figures; Accepted to AAS Journal
An Improved Method of Photometric Mode Identification: Applications to Slowly Pulsating B, beta Cephei, delta Scuti and gamma Doradus Stars
peer reviewedWe present an improved version of the method of photometric mode identification based upon the inclusion of non-adiabatic eigenfunctions determined in the stellar atmosphere, according to the formalism recently proposed by Dupret et al. (2002). We apply our method to beta Cephei, Slowly Pulsating B, delta Scuti and gamma Doradus stars. Besides identifying the degree l of the pulsating stars, our method is also a tool for improving the knowledge of stellar interiors and atmospheres, by imposing constraints on the metallicity for beta Cephei and SPBs, the characteristics of the superficial convection zone for delta Scuti and gamma Doradus stars and the limb-darkening law
Understanding Dwarf Galaxies in order to Understand Dark Matter
Much progress has been made in recent years by the galaxy simulation
community in making realistic galaxies, mostly by more accurately capturing the
effects of baryons on the structural evolution of dark matter halos at high
resolutions. This progress has altered theoretical expectations for galaxy
evolution within a Cold Dark Matter (CDM) model, reconciling many earlier
discrepancies between theory and observations. Despite this reconciliation, CDM
may not be an accurate model for our Universe. Much more work must be done to
understand the predictions for galaxy formation within alternative dark matter
models.Comment: Refereed contribution to the Proceedings of the Simons Symposium on
Illuminating Dark Matter, to be published by Springe
Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
peer reviewedWe update the capabilities of the open-knowledge software instrument Modules for Experiments in Stellar Astrophysics (MESA). RSP is a new functionality in MESAstar that models the non-linear radial stellar pulsations that characterize RR Lyrae, Cepheids, and other classes of variable stars. We significantly enhance numerical energy conservation capabilities, including during mass changes. For example, this enables calculations through the He flash that conserve energy to better than 0.001 %. To improve the modeling of rotating stars in MESA, we introduce a new approach to modifying the pressure and temperature equations of stellar structure, and a formulation of the projection effects of gravity darkening. A new scheme for tracking convective boundaries yields reliable values of the convective-core mass, and allows the natural emergence of adiabatic semiconvection regions during both core hydrogen- and helium-burning phases. We quantify the parallel performance of MESA on current generation multicore architectures and demonstrate improvements in the computational efficiency of radiative levitation. We report updates to the equation of state and nuclear reaction physics modules. We briefly discuss the current treatment of fallback in core-collapse supernova models and the thermodynamic evolution of supernova explosions. We close by discussing the new MESA Testhub software infrastructure to enhance source-code development
Element Diffusion in the Solar Interior
We study the diffusion of helium and other heavy elements in the solar interior, using the flow equations developed by Burgers (1969) for a multi-component fluid. The set of equations is solved exactly, including the residual heat flow terms. No approximation is made about the relative concentrations and no restriction is placed on the number of elements considered. For helium diffusion, we compare our results with those obtained by Bahcall and Loeb (1990) using a simplified treatment with heat flows neglected. We find that the inclusion of the residual heat flow terms leads to a significant increase in the helium diffusion velocity. However, we also find that the temperature and charge-dependence of the Coulomb logarithm has the opposite effect, leading to a decrease in the helium diffusion velocity. By coincidence, the two effects partially cancel each other out throughout most of the solar interior. Our complete treatment of element diffusion could be directly incorporated in a standard stellar evolution code, but, for convenience, we also give simple analytical fits of our numerical results. Burgers, J.M. 1969, Flow equations for composite gases(Academic Press, New York). Bahcall, J.N., and Loeb, A. 1990, Ap.J.,360, 267
Testing the forward approach in modelling β Cephei pulsators: setting the stage
peer reviewedThe information on stellar parameters and on the stellar interior we can get by studying pulsating stars depends crucially on the available observational constraints: both seismic constraints (precision and number of detected modes, identification, nature of the modes) and ``classical'' observations (photospheric abundances, effective temperature, luminosity, surface gravity). We consider the case of β Cephei pulsators and, with the aim of estimating quantitatively how the available observational constraints determine the type and precision of our inferences, we set the stage for Hare&Hound exercises. In this contribution we present preliminary results for one simple case, where we assume as ``observed'' frequencies a subset of frequencies of a model and then evaluate a seismic merit function on a dense and extensive grid of models of B-type stars. We also compare the behaviour of χ^2 surfaces obtained with and without mode identification
Improving stellar evolution models with atomic diffusion from asteroseismology of intermediate-mass stars
One of the biggest enigmas of stellar evolution concerns the transport of chemical elements. As this process is already ill-constrained during the core-hydrogen burning phase, the uncertainties that are introduced propagate into models of more evolved stars. The current state-of-the-art models are not able to predict the observed oscillation frequencies in intermediate-mass stars with their uncertainties. In this talk, we discuss the effect of atomic diffusion (including radiative levitation) on the frequencies of gravity modes and its implications for forward seismic modelling of intermediate-mass stars. Our results show significant differences in frequencies when including atomic diffusion, resulting in different derived masses, ages and metallicities. We will present our findings of our improved forward modelling scheme applied to a slowly rotating pulsator