Formation rate of LB-1-like systems through dynamical interactions. (30th April 2020)

• Record Type: Journal Article
• Authors: Tanikawa, Ataru; Kinugawa, Tomoya; Kumamoto, Jun; Fujii, Michiko S
• Is Part Of: Publications of the Astronomical Society of Japan. Volume 72:Number 3(2020:Jun.)
• Publication Date: 2020-04-30
• Subjects: binaries: close -- open clusters and associations: general -- stars: black holes -- stars: individual (LB-1); Astronomy -- Periodicals; 520.5
• DOI: 10.1093/pasj/psaa021
• Languages: English
• ISSNs: 0004-6264

Abstract

Abstract: We estimate formation rates of LB-1-like systems through dynamical interactions in the framework of the theory of stellar evolution before the discovery of the LB-1 system. The LB-1 system contains a ∼70 ${M_{\odot}}$ black hole (BH), a so-called pair instability (PI) gap BH, and a B-type star with solar metallicity, and has nearly zero eccentricity. The most efficient formation mechanism is as follows. In an open cluster, a naked helium star (with ∼20 ${M_{\odot}}$ ) collides with a heavy main sequence star (with ∼50 ${M_{\odot}}$ ) which has a B-type companion. The collision results in a binary consisting of the collision product and the B-type star with a high eccentricity. The binary can be circularized through the dynamical tide with radiative damping of the collision product envelope. Finally, the collision product collapses to a PI-gap BH, avoiding pulsational pair instability and pair instability supernovae because its He core is as massive as the pre-colliding naked He star. We find that the number of LB-1-like systems in the Milky Way galaxy is ∼0.01(ρoc /10 4 ${M_{\odot}}$ pc −3 ), where ρoc is the initial mass densities of open clusters. If we take into account LB-1-like systems with O-type companion stars, the number increases to ∼0.03(ρoc /10 4 ${M_{\odot}}$ pc −3 ). This mechanism can form LB-1-like systems at least ten times more efficiently than the other mechanisms: captures of B-type stars by PI-gap BHs, stellar collisions between other types of stars, and stellar mergers in hierarchical triple systems. We conclude that no dynamical mechanism can explain the presence of the LB-1 system.