The role of the outer boundary condition in accretion disk models: theory and application

Feng Yuan(1), Qiuhe Peng(2), Ju-fu Lu(3), Jianmin Wang(4)

(1) Department of Astronomy, Nanjing University, Nanjing 210093,, and Beijing Astrophysics Center, Beijing 100080, China, Email:
(2) Department of Astronomy, Nanjing University, Nanjing 210093, China
(3) Center for Astrophysics, University of Science & Technology of China, Hefei, 230026, China,, and National Astronomical Observatories, Chinese Academy of Sciences
(4) Laboratory of Cosmic Ray and High Energy Astrophysics,, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China

Paper: ApJ, June 2000, in press

EPrint Server: astro-ph/0002068


In a previous paper (Yuan 1999, hereafter Paper I), we find that the outer boundary conditions(OBCs) of an optically thin accretion flow play an important role in determining the structure of the flow. Here in this paper, we further investigate the influence of OBC on the dynamics and radiation of the accretion flow on a more detailed level. Bremsstrahlung and synchrotron radiations amplified by Comptonization are taken into account and two-temperature plasma assumption is adopted. The three OBCs we adopted are the temperatures of the electrons and ions and the specific angular momentum of the accretion flow at a certain outer boundary. We investigate the individual role of each of the three OBCs on the dynamical structure and the emergent spectrum. We find that when the general parameters such as the mass accretion rate \dotM and the viscous parameter alpha are fixed, the peak flux at various bands such as radio, IR and X-ray, can differ by as large as several orders of magnitude under different OBCs in our example. Our results indicate that OBC is both dynamically and radiatively important therefore should be regarded as a new ``parameter'' in accretion disk models. As an illustrative example, we further apply the above results to the compact radio source Sgr A^* located at the center of our Galaxy. Advection-dominated accretion flow (ADAF) model has been turned out to be a great success to explain its luminosity and spectrum. However, there exists a discrepancy between the mass accretion rate favored by ADAF models in the literature and that favored by the three dimensional hydrodynamical simulation, with the former being 10-20 times smaller than the latter. By seriously considering the outer boundary condition of the accretion flow, we find that due to the low specific angular momentum of the accretion gas, the accretion in Sgr A^* should belong to a new accretion pattern, which is characterized by possessing a very large sonic radius. This accretion pattern can significantly reduce the discrepancy between the mass accretion rate. We argue that the accretion occurred in some detached binary systems, the core of nearby elliptical galaxies and active galactic nuclei(AGNs) very possibly belongs to this accretion pattern.

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