Entropic-acoustic instability of shocked Bondi accretion I. What does perturbed Bondi accretion sound like ?

In the radial flow of gas into a black hole (i.e. Bondi accretion), the infall of any entropy or vorticity perturbation produces acoustic waves propagating outward. The dependence of this acoustic flux on the shape of the perturbation is investigated in detail. This is the key process in the mechanism of the entropic-acoustic instability proposed by Foglizzo & Tagger (2000) to explain the instability of Bondi-Hoyle-Lyttleton accretion. These acoustic waves create new entropy and vorticity perturbations when they reach the shock, thus closing the entropic-acoustic cycle. With an adiabatic index 1<gamma<=5/3, the linearized equations describing the perturbations of the Bondi flow are studied analytically and solved numerically. The fundamental frequency of this problem is the cut-off frequency of acoustic refraction, below which ingoing acoustic waves are refracted out. This cut-off is significantly smaller than the Keplerian frequency at the sonic radius and depends on the latitudinal number l of the perturbations. When advected adiabatically inward, entropy and vorticity perturbations trigger acoustic waves propagating outward, with an efficiency which is highest for non radial perturbations l=1. The outgoing acoustic flux produced by the advection of vorticity perturbations is always moderate and peaks at rather low frequency. By contrast, the acoustic flux produced by an entropy wave is highest close to the refraction cut-off. It can be very large if gamma is close to 5/3. These results suggest that the shocked Bondi flow with gamma=5/3 is strongly unstable with respect to the entropic-acoustic mechanism.Comment: 14 pages, 11 figures, accepted for publication in A&

Numerical Models of Binary Neutron Star System Mergers. I.: Numerical Methods and Equilibrium Data for Newtonian Models

The numerical modeling of binary neutron star mergers has become a subject of much interest in recent years. While a full and accurate model of this phenomenon would require the evolution of the equations of relativistic hydrodynamics along with the Einstein field equations, a qualitative study of the early stages on inspiral can be accomplished by either Newtonian or post-Newtonian models, which are more tractable. In this paper we offer a comparison of results from both rotating and non-rotating (inertial) frame Newtonian calculations. We find that the rotating frame calculations offer significantly improved accuracy as compared with the inertial frame models. Furthermore, we show that inertial frame models exhibit significant and erroneous angular momentum loss during the simulations that leads to an unphysical inspiral of the two neutron stars. We also examine the dependence of the models on initial conditions by considering initial configurations that consist of spherical neutron stars as well as stars that are in equilibrium and which are tidally distorted. We compare our models those of Rasio & Shapiro (1992,1994a) and New & Tohline (1997). Finally, we investigate the use of the isolated star approximation for the construction of initial data.Comment: 32 pages, 19 gif figures, manuscript with postscript figures available at http://www.astro.sunysb.edu/dswesty/docs/nspap1.p

Black Hole - Neutron Star Mergers as Central Engines of Gamma-Ray Bursts

Hydrodynamic simulations of the merger of stellar mass black hole - neutron star binaries (BH/NS) are compared with mergers of binary neutron stars (NS/NS). The simulations are Newtonian, but take into account the emission and backreaction of gravitational waves. The use of a physical nuclear equation of state allows us to include the effects of neutrino emission. For low neutron star to black hole mass ratios the neutron star transfers mass to the black hole during a few cycles of orbital decay and subsequent widening before finally being disrupted, whereas for ratios near unity the neutron star is already distroyed during its first approach. A gas mass between about 0.3 and about 0.7 solar masses is left in an accretion torus around the black hole and radiates neutrinos at a luminosity of several 10^{53} erg/s during an estimated accretion time scale of about 0.1 s. The emitted neutrinos and antineutrinos annihilate into electron-positron pairs with efficiencies of 1-3% percent and rates of up to 2*10^{52} erg/s, thus depositing an energy of up to 10^{51} erg above the poles of the black hole in a region which contains less than 10^{-5} solar masses of baryonic matter. This could allow for relativistic expansion with Lorentz factors around 100 and is sufficient to explain apparent burst luminosities of up to several 10^{53} erg/s for burst durations of approximately 0.1-1 s, if the gamma emission is collimated in two moderately focussed jets in a fraction of about 1/100-1/10 of the sky.Comment: 8 pages, LaTex, 4 postscript figures, 2 tables. ApJ Letters, accepted; revised and shortened version, Fig. 2 change

A Solution to the Protostellar Accretion Problem

Accretion rates of order 10^-8 M_\odot/yr are observed in young protostars of approximately a solar mass with evidence of circumstellar disks. The accretion rate is significantly lower for protostars of smaller mass, approximately proportional to the second power of the stellar mass, \dot{M}_accr\propto M^2. The traditional view is that the observed accretion is the consequence of the angular momentum transport in isolated protostellar disks, controlled by disk turbulence or self--gravity. However, these processes are not well understood and the observed protostellar accretion, a fundamental aspect of star formation, remains an unsolved problem. In this letter we propose the protostellar accretion rate is controlled by accretion from the large scale gas distribution in the parent cloud, not by the isolated disk evolution. Describing this process as Bondi--Hoyle accretion, we obtain accretion rates comparable to the observed ones. We also reproduce the observed dependence of the accretion rate on the protostellar mass. These results are based on realistic values of the ambient gas density and velocity, as inferred from numerical simulations of star formation in self--gravitating turbulent clouds.Comment: 4 pages, 2 figures, ApJ Letters, in pres

Bondi-Hoyle-Lyttleton Accretion onto a Protoplanetary Disk

Young stellar systems orbiting in the potential of their birth cluster can accrete from the dense molecular interstellar medium during the period between the star's birth and the dispersal of the cluster's gas. Over this time, which may span several Myr, the amount of material accreted can rival the amount in the initial protoplanetary disk; the potential importance of this `tail-end' accretion for planet formation was recently highlighted by Throop & Bally (2008). While accretion onto a point mass is successfully modeled by the classical Bondi-Hoyle-Lyttleton solutions, the more complicated case of accretion onto a star-disk system defies analytic solution. In this paper we investigate via direct hydrodynamic simulations the accretion of dense interstellar material onto a star with an associated gaseous protoplanetary disk. We discuss the changes to the structure of the accretion flow caused by the disk, and vice versa. We find that immersion in a dense accretion flow can redistribute disk material such that outer disk migrates inwards, increasing the inner disk surface density and reducing the outer radius. The accretion flow also triggers the development of spiral density features, and changes to the disk inclination. The mean accretion rate onto the star remains roughly the same with and without the presence of a disk. We discuss the potential impact of this process on planet formation, including the possibility of triggered gravitational instability; inclination differences between the disk and the star; and the appearance of spiral structure in a gravitationally stable system.Comment: Accepted to ApJ. Version 2 replaces a mislabeled figure. Animations of the simulations and a version of the paper with slightly less-compressed images can be found at http://origins.colorado.edu/~moeckel/BHLpape