Robert M. Hueckstaedt

Hydrodynamic Simulations of He Shell Flash Convection

We present the first hydrodynamic, multidimensional simulations of He shell flash convection. We investigate the properties of shell convection immediately before the He luminosity peak during the 15th thermal pulse of a stellar evolution track with initially 2 solar masses and metallicity Z = 0.01. This choice is a representative example of a low-mass asymptotic giant branch thermal pulse. We construct the initial vertical stratification with a set of polytropes to resemble the stellar evolution structure. Convection is driven by a constant volume heating in a thin layer at the bottom of the unstable layer. We calculate a grid of two-dimensional simulations with different resolutions and heating rates, plus one low-resolution three-dimensional run. The flow field is dominated by large convective cells that are centered in the lower half of the convection zone. It generates a rich spectrum of gravity waves in the stable layers both above and beneath the convective shell. The magnitude of the convective velocities from our one-dimensional mixing-length theory model and the rms-averaged vertical velocities from the hydrodynamic model are consistent within a factor of a few. However, the velocity profile in the hydrodynamic simulation is more asymmetric and decays exponentially inside the convection zone. Both g-modes and convective motions cross the formal convective boundaries, which leads to mixing across the boundaries. Our resolution study shows consistent flow structures among the higher resolution runs, and we see indications for convergence of the vertical velocity profile inside the convection zone for the highest resolution simulations. Many of the convective properties, in particular the exponential decay of the velocities, depend only weakly on the heating rate. However, the amplitudes of the gravity waves increase with both the heating rate and the resolution.

RADIATIVE AND KINETIC FEEDBACK BY LOW-MASS PRIMORDIAL STARS

Ionizing UV radiation and supernova (SN) flows amidst clustered minihalos at high redshift regulated the rise of the first stellar populations in the universe. Previous studies have addressed the effects of very massive primordial stars on the collapse of nearby halos into new stars, but the absence of the odd-even nucleosynthetic signature of pair-instability SNe in ancient metal-poor stars suggests that Population III stars may have been less than 100 M ☉. We extend our earlier survey of local UV feedback on star formation to 25-80 M ☉ stars and include kinetic feedback by SNe for 25-40 M ☉ stars. We find radiative feedback to be relatively uniform over this mass range, primarily because the larger fluxes of more massive stars are offset by their shorter lifetimes. Our models demonstrate that prior to the rise of global UV backgrounds, Lyman-Werner (LW) photons from nearby stars cannot prevent halos from forming new stars. These calculations also reveal that violent dynamical instabilities can erupt in the UV radiation front enveloping a primordial halo, but that they ultimately have no effect on the formation of a star. Finally, our simulations suggest that relic H II regions surrounding partially evaporated halos may expel LW backgrounds at lower redshifts, allowing stars to form that were previously suppressed. We provide fits to radiative and kinetic feedback on star formation for use in both semianalytic models and numerical simulations.

Self‐gravity driven instabilities of interfaces in the interstellar medium

In order to understand star formation it is important to understand the dynamics of atomic and molecular clouds in the interstellar medium (ISM). Nonlinear hydrodynamic flows are a key component to the ISM. One route by which nonlinear flows arise is the onset and evolution of interfacial instabilities. Interfacial instabilities act to modify the interface between gas components at different densities and temperatures. Such an interface may be subject to a host of instabilities, including the Rayleigh-Taylor, Kelvin-Helmholtz, and Richtmyer-Meshkov instabilities. Recently, a new density interface instability was identified. This self-gravity interfacial instability (SGI) causes any displacement of the interface to gr ow on roughly a free-fall time scale, even when the perturbation wavelength is much less than the Jeans length. In previous work, we used numerical simulations to confirm the expectations of linear theory and examine the nonlinear evolution of the SGI. We now continue our study by generalizing our initial conditions to allow the acceleration due to self-gravity to be non-zero across the interface. We also consider the behaviour of the SGI for perturbation wavelengths near the Jeans wavelength. We conclude that the action of self-gravity across a density interface may play a significant role in the ISM either by fueling the growth of new instabilities or modifying the evolution of existing instabilities.

Local Radiative Feedback: The Rise of Early Stellar Populations

How the first stars regulated the formation of later generations by their intense UV flux is key to the assembly of primeval galaxies, the rise of the first stellar populations, and the onset of cosmological reionization. It is commonly held that photoevaporation of cosmological halos by nearby Pop III stars quenched new star formation. We present a survey of halo photoevaporation by high‐mass and low‐mass primordial stars with simulations that self‐consistently solve hydrodynamics, radiative transfer, and primordial gas chemistry. We find that ionizing and LW radiative feedback from a nearby star is much less destructive to star formation than is generally believed, and that it can even accelerate the collapse of baryons into new stars in some cases.

Self‐Gravity Driven Instabilities at Accelerated Interfaces

Abstract: Nonlinear hydrodynamic flows are ubiquitous in the interstellar medium (ISM). Such flows play an important role in shaping atomic and molecular clouds and determining the initial conditions for star formation. One mechanism by which nonlinear flows arise is the onset and growth of interfacial instabilities. Any interface of discontinuous density is subject to a host of instabilities, including Rayleigh‐Taylor, Kelvin‐Helmholtz, and Richtmyer‐Meshkov. As part of an ongoing study of structure formation in the ISM, Hunter, Whitaker, and Lovelace discovered an additional density interface instability. This instability is driven by self‐gravity and termed the self‐gravity interfacial instability (SGI). The SGI causes any displacement of the interface to grow on roughly a free‐fall time scale, even when the perturbation wavelength is much less than the Jeans length. Numerical simulations have confirmed the expectations of linear theory, including the near scale invariance of the growth rate. Here, we build upon previous work by considering an initial condition in which the acceleration due to self‐gravity is non‐zero at the interface.