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Register a new userTesting the stability of supersonic ionized Bondi accretion flows with radiation hydrodynamics
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We investigate the general stability of 1D spherically symmetric ionized Bondi accretion onto a massive object in the specific context of accretion onto a young stellar object. We first derive a new analytic expression for a steady state two temperature solution that predicts the existence of compact and hypercompact HII regions. We then show that this solution is only marginally stable if ionization is treated self-consistently. This leads to a recurring collapse of the HII region over time. We derive a semi-analytic model to explain this instability, and test it using spatially converged 1D radiation hydrodynamical simulations. We discuss the implications of the 1D instability on 3D radiation hydrodynamics simulations of supersonic accreting flows.
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Binary stars often move through an ambient medium from which they accrete material and angular momentum, as in triple-star systems, star-forming clouds, young globular clusters and in the centres of galaxies. A binary form of Bondi-Hoyle-Lyttleton accretion results whereby the accretion rate depends on the binary properties: the stellar masses and separation, and the relative wind speed. We present the results of simulations performed with the hydrodynamic code GANDALF, to determine the mass accretion rates over a range of binary separations, inclinations and mass ratios. When the binary separation is short, the binary system accretes like a single star, while accretion onto stars in wide binaries is barely affected by their companion. We investigate intermediate-separation systems in some detail, finding that as the binary separation is increased, accretion rates smoothly decrease from the rate equal to that of a single star to the rate expected from two isolated stars. The form of this decrease depends on the relative centre-of-mass velocity of the binary and the gas, with faster-moving binaries showing a shallower decrease. Accretion rates vary little with orbital inclination, except when the orbit is side-on and the stars pass through each others wakes. The specific angular momentum accretion rate also depends on the inclination but is never sufficient to prevent the binary orbit from contracting. Our results may be applied to accretion onto protostars, pollution of stars in globular and nuclear clusters, and wind mass-transfer in multiple stellar systems.
Hermann Bondis 1952 paper On spherically symmetrical accretion is recognized as one of the foundations of accretion theory. Although Bondi later remarked that it was not much more than an examination exercise, his mathematical analysis of spherical accretion on to a point mass has found broad use across fields of astrophysics that were embryonic or non-existent at the time of the papers publication. In this non-technical review, I describe the motivations for Bondis work, and briefly discuss some of the applications of Bondi accretion in high energy astrophysics, galaxy formation, and star formation.
We present a method for analyzing the interaction between radiation and matter in regions of intense, relativistic shear that can arise in many astrophysical situations. We show that there is a simple velocity profile that should be manifested in regions of large shear that have lost memory of their boundary conditions, and we use this self-similar velocity profile to construct the surface of last scattering, or $tau simeq 1$ surface, as viewed from any comoving point within the flow. We demonstrate that a simple treatment of scattering from this $tau simeq 1$ surface exactly conserves photon number, and derive the rate at which the radiation field is heated due to the shear present in the flow. The components of the comoving radiation energy-momentum tensor are calculated, and we show that they have relatively simple, approximate forms that interpolate between the viscous (small shear) and streaming (large shear) limits. We put our expression for the energy-momentum tensor in a covariant form that does not depend on the explicit velocity profile within the fluid and, therefore, represents a natural means for analyzing general, radiation-dominated, relativistic shear flows.
Gravitational collapse of molecular cloud or cloud core/clump may lead to the formation of geometrically flattened, rotating accretion flow surrounding the new born star or star cluster. Gravitational instability may occur in such accretion flow when the gas to stellar mass ratio is high (e.g. over $sim$10%). This paper takes the OB cluster-forming region G10.6-0.4 as an example. We introduce the enclosed gas mass around its central ultra compact (UC) Htextsc{ii} region, addresses the gravitational stability of the accreting gas, and outline the observed potential signatures of gravitational instability. The position-velocity (PV) diagrams of various molecular gas tracers on G10.6-0.4 consistently show asymmetry in the spatial and the velocity domain. We deduce the morphology of the dense gas accretion flow by modeling velocity distribution of the azimuthally asymmetric gas structures, and by directly de-projecting the PV diagrams. We found that within the 0.3 pc radius, an infall velocity of 1-2 km,s$^{-1}$ may be required to explain the observed PV diagrams. In addition, the velocity distribution traced in the PV diagrams can be interpreted by spiral arm-like structures, which may be connected with exterior infalling gas filaments. The morphology of dense gas structures we propose appears very similar to the spatially resolved gas structures around the OB cluster-forming region G33.92+0.11 with similar gas mass and size, which however is likely to be approximately in a face-on projection. The dense gas accretion flow around G10.6-0.4 appears to be Toomre unstable, which is consistent with the existence of large-scale spiral arm-like structures, and the formation of localize gas condensations.
The fully analytical solution for isothermal Bondi accretion on a black hole (MBH) at the center of two-component Jaffe (1983) galaxy models is presented. In a previous work we provided the analytical expressions for the critical accretion parameter and the radial profile of the Mach number in the case of accretion on a MBH at the center of a spherically symmetric one-component Jaffe galaxy model. Here we apply this solution to galaxy models where both the stellar and total mass density distributions are described by the Jaffe profile, with different scale-lengths and masses, and to which a central MBH is added. For such galaxy models all the relevant stellar dynamical properties can also be derived analytically (Ciotti & Ziaee Lorzad 2018). In these new models the hydrodynamical and stellar dynamical properties are linked by imposing that the gas temperature is proportional to the virial temperature of the galaxy stellar component. The formulae that are provided allow to evaluate all flow properties, and are then useful for estimates of the scale-radius and the mass flow rate when modeling accretion on massive black holes at the center of galaxies. As an application, we quantify the departure from the true mass accretion rate of estimates obtained using the gas properties at various distances from the MBH, under the hypothesis of classical Bondi accretion.
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