On the Missing Red Giants near the Galactic Center

TL;DR

This paper investigates the cause of the observed scarcity of bright red giants near the Galactic Center, concluding that stellar collisions are a more plausible explanation than tidal stripping by the black hole.

Contribution

The study evaluates tidal stripping effects and finds stellar collisions are a more likely cause for red giant deficiency near Sgr A*.

Findings

Tidal stripping does not sufficiently explain red giant scarcity.

Diffusion rates into the loss cone increase only logarithmically with stellar radius.

Stellar collisions are identified as a more probable cause for the deficiency.

Abstract

There is a long-acknowledged deficiency of bright red giants relative to fainter old stars within a few arc seconds of Sgr A*. We explore whether this could be due to tidal stripping by the central black hole. This requires putting the stars onto highly eccentric orbits, for which we evaluate diffusion by both scalar resonant and non-resonant relaxation of the orbital angular momentum. We conclude that tidal stripping does not discriminate sufficiently between main-sequence and red giant stars. While the tidal loss cone increases with stellar radius, the rate of diffusion into the loss cone increases only logarithmically, whereas the lifetime on the red giant branch decreases more rapidly than $R_*^{-1}$. In agreement with previous studies, we find that stellar collisions are a more likely explanation for the deficiency of bright red giants relative to fainter ones.