Formation of Ultracompact X-ray Binaries in Dense Star Clusters

• Published in: arXiv.org
• Main Authors: Ivanova, N, Rasio, F A, Lombardi, J C, Jr, Dooley, K L, Proulx, Z F
• Format: Paper
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 10.02.2005
• Subjects: Binary stars; Collisions; Companion stars; Deposition; Globular clusters; Helium; Neutron stars; Neutrons; Red giant stars; Star clusters; White dwarf stars; X ray binaries; X ray stars
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.0501617

Bright, ultracompact X-ray binaries observed in dense star clusters, such as Galactic globular clusters, must have formed relatively recently, since their lifetimes as persistent bright sources are short (e.g., ~10^8 yr above 10^36 erg/s for a 1.4 Msun neutron star accreting from a degenerate helium companion with an initial mass of ~0.2 Msun). Therefore, we can use the present conditions in a cluster core to study possible dynamical formation processes for these sources. Here we show that direct physical collisions between neutron stars and red giants can provide a sufficient formation rate to explain the observed numbers of bright sources. These collisions produce tight, eccentric neutron star -- white dwarf binaries that decay to contact by gravitational radiation on timescales ~10^6-10^10 yr, usually shorter and often much shorter than the cluster age.