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

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

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 $\sim 15\%$. 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 $M<8\,{\rm M_\odot}$ 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

Ensemble Asteroseismology of the Young Open Cluster NGC 2244

Our goal is to perform in-depth ensemble asteroseismology of the young open cluster NGC2244 with the 2-wheel Kepler mission. While the nominal Kepler mission already implied a revolution in stellar physics for solar-type stars and red giants, it was not possible to perform asteroseismic studies of massive OB stars because such targets were carefully avoided in the FoV in order not to disturb the exoplanet hunting. Now is an excellent time to fill this hole in mission capacity and to focus on the metal factories of the Universe, for which stellar evolution theory is least adequate. Our white paper aims to remedy major shortcomings in the theory of stellar structure and evolution of the most massive stars by focusing on a large ensemble of stars in a carefully selected young open cluster. Cluster asteroseismology of very young stars such as those of NGC2244 has the major advantage that all cluster stars have similar age, distance and initial chemical composition, implying drastic restrictions for the stellar modeling compared to asteroseismology of single isolated stars with very different ages and metallicities. Our study requires long-term photometric measurements of stars with visual magnitude ranging from 6.5 to 15 in a large FoV with a precision better than 30 ppm for the brightest cluster members (magnitude below 9) up to 500 ppm for the fainter ones, which is well achievable with 2-Wheel Kepler, in combination with high-precision high-resolution spectroscopy and spectro-polarimetry of the brightest pulsating cluster members. These ground-based spectroscopic data will be assembled with the HERMES and CORALIE spectrographs (twin 1.2m Mercator and Euler telescopes, La Palma, Canary Islands and La Silla, Chile), as well as with the spectro-polarimetric NARVAL instrument (2m BLT at the Pic du Midi, French Pyrenees), to which we have guaranteed access.Comment: 10 pages, 3 figures, white paper submitted in response to the NASA call for community input for science investigations the Kepler 2-Wheel spacecraf

The Impact of White Dwarf Luminosity Profiles on Oscillation Frequencies

peer reviewedKIC 08626021 is a pulsating DB white dwarf (WD) of considerable recent interest, and the first of its class to be extensively monitored by Kepler for its pulsation properties. Fitting the observed oscillation frequencies of KIC 08626021 to a model can yield insights into its otherwise-hidden internal structure. Template-based WD models choose a luminosity profile where the luminosity is proportional to the enclosed mass , Lr ∝ Mr, independent of the effective temperature T eff. Evolutionary models of young WDs with T eff 25,000 K suggest that neutrino emission gives rise to luminosity profiles with L r M r. We explore this contrast by comparing the oscillation frequencies between two nearly identical WD models: one with an enforced luminosity profile, and the other with a luminosity profile determined by the star's previous evolution history. We find that the low-order g-mode frequencies differ by up to ≃70 μHz over the range of Kepler observations for KIC 08626021. This suggests that by neglecting the proper thermal structure of the star (e.g., accounting for the effect of plasmon neutrino losses), the model frequencies calculated by using an profile may have uncorrected, effectively random errors at the level of tens of μHz. A mean frequency difference of 30 μHz, based on linearly extrapolating published results, suggests a template model uncertainty in the fit precision of ≃12% in WD mass, ≃9% in the radius, and ≃3% in the central oxygen mass fraction. © 2018. The American Astronomical Society

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

On the Impact of 22Ne on the Pulsation Periods of Carbon-Oxygen White Dwarfs with Helium-dominated Atmospheres

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