The Keplerian Three-body Encounter. I. Insights on the Origin of the S-stars and the G-objects in the Galactic Center

Abstract

Recent spectroscopic analysis has set an upper limit to the age of the S-stars, the ∼30 B-type stars in highly eccentric orbits around the supermassive black hole (SMBH) in the Galactic center. The inferred age (<15 Myr) is in tension with the binary break-up scenario proposed to explain their origin. However, the new estimate is compatible with the age of the disk of O-type stars that lies at a farther distance from the SMBH. Here we investigate a new formation scenario, assuming that both S-stars and the O-type stars were born in the same disk around SgrA*. We simulate encounters between binaries of the stellar disk and stellar black holes from a dark cusp around SgrA*. We find that B-type binaries can be easily broken up by the encounters and their binary components are kicked into highly eccentric orbits around the SMBH. In contrast, O-type binaries are less frequently disrupted and their members remain in low eccentricity orbits. This mechanism can reproduce 12 S-stars just by assuming that the binaries initially lie within the stellar disk as observed nowadays. To reproduce all the S-stars, the original disk must have been extended down to 0.006 pc. However in this case many B-and O-type stars remain in low eccentricity orbits below 0.03 pc, in contrast with the observations. Therefore, some other mechanism is necessary to disrupt the disk below 0.03 pc. This scenario can also explain the high eccentricity of the G-objects, if they have a stellar origin.

The Hills Mechanism and the Galactic Center S-stars

“…We focus on smaller semimajor axes to avoid contamination from stars that may have been kicked out of the disk by other mechanisms (e.g. by vector resonant relaxation Szölgyén & Kocsis 2018, or by scattering between stellar mass black holes and disk binaries Trani et al 2019). We implicitly assume these stars all belong to a single, few Myr-old population, although some admixture of older stars is possible considering (i) only 8 of the 19 stars have spectroscopic age measurements (ii) spectroscopy of recently-discovered, faint S-stars indicate ages closer to ∼ 100 Myr (Peißker et al 2020).…”

The Keplerian Three-body Encounter. II. Comparisons with Isolated Encounters and Impact on Gravitational Wave Merger Timescales

“…In order to set up the 4-body simulations in which a binary and a single object undergo a 3-body encounter while orbiting an SMBH, we follow the same method described in Trani et al (2019). The orbits of the binary and the single around the SMBH are set up to be almost intersecting except for a small impact parameter, randomly sampled between 0 and twice the binary semimajor axis a bin .…”

“…We term the three-body encounters in which all encountering bodies are in orbit around the SMBH as Keplerian three-body encounters. Trani et al (2019) investigated Keplerian three-body encounters as a mechanism to produce S-stars and G-objects in the Galactic center via ionizing encounters between young stellar binaries and stellar mass black holes. Here we investigate the fundamental difference between isolated and Keplerian three-body encounters, by comparing our numerical experiments to the statistical escape theory of three-body breakups in isolation (Valtonen & Karttunen 2005).…”

K2-264: a transiting multiplanet system in the Praesepe open cluster

“…We use the probabilistic mass-radius relation of Wolfgang et al (2016) to estimate the masses of the planets given their measured radii, yielding m b = 7.7±2.3 and m c = 9.5±2.7 M ⊕ for planets b and c, respectively. We use the TSUNAMI code (Trani et al 2016(Trani et al , 2018 to simulate the orbital evolution of 500 realizations. Consistent with the planets' orbital inclinations from the measured transit geometry, we set their mutual inclination to zero and sampled the eccentricity of the outer planet between 0 and 0.6.…”