Can Star-Disk Collisions Explain the Missing Red Giants Problem in the Galactic Center?

TL;DR

This paper investigates whether collisions between stars and a fragmenting accretion disk can explain the observed scarcity of red giants in the Galactic Center by modeling star-disk interactions through hydrodynamic simulations.

Contribution

It introduces a detailed hydrodynamic simulation approach to assess how star-disk collisions can strip red giant envelopes, offering a potential explanation for their missing population.

Findings

Multiple collisions can strip significant stellar envelopes.

Repeated impacts increase mass loss efficiency.

Stripped stars may have decayed orbits and higher rotation.

Abstract

Observations have revealed a relative paucity of red giant (RG) stars within the central 0.5pc in the Galactic Center (GC). Motivated by this finding we investigate the hypothesis that collisions of stars with a fragmenting accretion disk are responsible for the observed dearth of evolved stars. We use 3-dimensional hydrodynamic simulations to model a star with radius $10 R_{\odot}$ and mass $1 M_{\odot}$, representative of the missing population of RGs, colliding with high density clumps. We find that multiple collisions with clumps of column density $\gtrsim10^{8}\, {\rm g\,cm^{-2}}$ can strip a substantial fraction of the star's envelope and in principle render it invisible to observations. Simulations confirm that repeated impacts are particularly efficient in driving mass loss as partially stripped RGs expand and have increased cross sections for subsequent collisions. Because the envelope is unbound on account of the kinetic energy of the star, any significant amount of stripping of the RG population in the GC should be mirrored by a systematic decay of their orbits and possibly by their enhanced rotational velocity. To be viable, this scenario requires that the total mass of the fragmenting disk has been several orders of magnitude higher than that of the early type stars which now form the stellar disk in the GC.