The Orbital Structure and Selection Effects of the Galactic Center S-star Cluster

Abstract

The orbital distribution of the S-star cluster surrounding the supermassive black hole in the center of the Milky Way is analyzed. A tight dependence of the pericenter distance r p on orbital eccentricity e ⋆ is found, log ( r p ) ∼ ( 1 − e ⋆ ) , which cannot be explained simply by a random distribution of semimajor axis and eccentricities. No stars are found in the region with high e ⋆ and large log ( r p ) or in the region with low e ⋆ and small log ( r p ) . Although the sample is still small, the G-clouds show a very similar distribution. The likelihood P ( log ( r p ) , ( 1 − e ⋆ ) ) to determine the orbital parameters of S-stars is determined. P is very small for stars with large e ⋆ and large log ( r p ) . S-stars might exist in this region. To determine their orbital parameters, one however needs observations over a longer time period. On the other hand, if stars would exist in the region of low log ( r p ) and small e ⋆, their orbital parameters should by now have been determined. That this region is unpopulated therefore indicates that no S-stars exist with these orbital characteristics, providing constraints for their formation. We call this region, defined by log ( r p / AU ) < 1.57 + 2.6 ( 1 − e ⋆ ) , the zone of avoidance. Finally, it is shown that the observed frequency of eccentricities and pericenter distances is consistent with a random sampling of log ( r p ) and e ⋆ if one takes into account the fact that no stars exist in the zone of avoidance and that orbital parameters cannot yet be determined for stars with large r p and large e ⋆.

The Population of Massive Stars in Active Galactic Nuclei Disks

“…The observed populations of young stars near the Galactic center where (mostly B-type) S stars with semimajor axes a å ∼ 5 × 10 −3 -0.04 pc (Genzel et al 2003;Ghez et al 2003;Gillessen et al 2017) are surrounded by a population of clockwise-rotating disk (mostly O/WR type) stars with a å ∼ 0.04-0.5 pc, (Levin & Beloborodov 2003;Lu et al 2006;Yelda et al 2014;von Fellenberg et al 2022). A zone of avoidance highlights the absence of stars with pericenter distance and eccentricity smaller than those of the S stars (Burkert et al 2024). This observed feature is more consistent with the possibility that these young stars formed coevally in SEPAD-type disk models, with  M d increasing with R (X.…”