arXiv 1002.12202010-02-05astro-ph.GA

银河系中心的质量分离

Mass Segregation in the Galactic Centre

Clovis Hopman, Ann-Marie Madigan

备注：: Proceedings article to be published in "The Galactic Center: A Window on the Nuclear Environment of Disk Galaxies", ed. Mark Morris, Daniel Q. Wang and Feng Yuan

摘要：

围绕大质量黑洞（MBH）运行的恒星之间的两体能量交换导致形成幂律密度分布n（r）~r^（-a），该分布向MBH发散。对于单个质量的星族，a=7/4，星流远小于N（<r）/t_r（每个弛豫时间内的星数）。对于中等质量比的多质量系统或有许多重恒星的系统，这种“零流量”解仍然存在，并且达到3/2<a<2的斜率，对于质量更大的恒星，斜率更陡。然而，如果重恒星是罕见的和大质量的，零流极限打破，并获得更陡峭的分布。我们用Fokker-Planck计算讨论了驱动质量分离的物理过程，并指出在0.2- 0.3t_r范围内达到稳定状态。由于银河系中心的弛豫时间为t_r ~2-3 * 10^（10）yr，尖点的形成时间应小于哈勃时间。GC中没有可见的老恒星尖端对这些模型提出了挑战，这表明除了两体弛豫之外的过程也发挥了作用。我们讨论的天体物理过程中的GC，至关重要的恒星尖点的细节。

英文摘要：

Two-body energy exchange between stars orbiting massive black holes (MBHs) leads to the formation of a power-law density distribution n(r)~r^(-a) that diverges towards the MBH. For a single mass population, a=7/4 and the flow of stars is much less than N(<r)/t_r (enclosed number of stars per relaxation time). This "zero-flow" solution is maintained for a multi-mass system for moderate mass ratios or systems where there are many heavy stars, and slopes of 3/2<a<2 are reached, with steeper slopes for the more massive stars. If the heavy stars are rare and massive however, the zero-flow limit breaks down and much steeper distributions are obtained. We discuss the physics driving mass-segregation with the use of Fokker-Planck calculations, and show that steady state is reached in 0.2-0.3 t_r. Since the relaxation time in the Galactic centre (GC) is t_r ~2-3 * 10^(10) yr, a cusp should form in less than a Hubble time. The absence of a visible cusp of old stars in the GC poses a challenge to these models, suggesting that processes other than two-body relaxation have played a role. We discuss astrophysical processes within the GC that depend crucially on the details of the stellar cusp.

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英文摘要：

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