Steeper Stellar Cusps in Galactic Centers from Binary Disruption

Giacomo Fragione, Re'Em Sari

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34 Scopus citations


The relaxed distribution of stars around a massive black hole is known to follow a cusp profile, p(r) μ r-α, with a characteristic slope α = 7/4. This follows from energy conservation and a scattering rate given by two-body encounters. However, we show that the injection of stars close to the black hole, i.e., a source term in the standard cusp picture, modifies this profile. In the steady-state configuration, the cusp develops a central region with a typical slope α= 9/4 in which stars diffuse outward. Binary disruption by the intense tidal field of the massive black hole is among the phenomena that take place in the Galactic Center (GC). In such a disruption, one of the binary members remains bound to the black hole, thus providing a source term of stars close to the black hole. Assuming a binary fraction of 0.1 and an orbital circularization efficiency of 0.35, we show that this source is strong enough to modify the cusp profile within ≈0.07 pc of the GC. If the binary fraction at the influence radius is of order unity and the orbits of all captured stars are efficiently circularized, the steeper cusp extends almost as far as the radius of influence of the black hole.

Original languageAmerican English
Article number51
JournalAstrophysical Journal
Issue number1
StatePublished - 1 Jan 2018

Bibliographical note

Funding Information:
This research was partially supported by an ISF and an iCore grant. We acknowledge Tal Alexander, Bence Kocsis, Scott Tremaine, Eugene Vasiliev, and Ben Bar-Or for useful discussions, and the anonymous referee for constructive comments.


  • Galaxy: center
  • Galaxy: kinematics and dynamics
  • binaries: general
  • stars: kinematics and dynamics


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