Making Bright Giants Invisible At The Galactic Centre

TL;DR

This paper proposes that the apparent lack of bright giants near the Galactic Center is due to their atmospheres being stripped by dense stellar disc regions, causing them to become faint or invisible.

Contribution

It introduces a stellar evolution model showing how envelope stripping shortens the giant phase, explaining the observed core-like distribution of bright stars at the GC.

Findings

Red giants are rapidly stripped of their atmospheres in dense disc regions.

Envelope stripping causes giants to quickly become white dwarfs, explaining their invisibility.

The non-linearity factor depends on local physical conditions and clump properties.

Abstract

Current observations of the Galactic Center (GC) seem to display a core-like distribution of bright stars from $\sim 5"$ inwards. On the other hand, we observe young, massive stars at the GC, with roughly 20-50\% of them in a disc, mostly in the region where the bright giants appear to be lacking. In a previous publication we put the idea forward that the missing stars are deeply connected to the presence of this disc. The progenitor of the stellar disc is very likely to have been a gaseous disc that at some point fragmented and triggered star formation. This caused the appearance of overdensity regions in the disc that had high enough densities to ensure stripping large giants of their atmospheres and thus rendering them very faint. In this paper we use a stellar evolution code to derive the properties that a red giant would display in a colour-magnitude diagram, as well as a non-linearity factor required for a correct estimate of the mass loss. We find that in a very short timescale, the red giants (RGs) leave their standard evolutionary track. The non-linearity factor has values that not only depend on the properties of the clumps, but also on the physical conditions the giant stars, as we predicted analytically. According to our results, envelope stripping works, moving stars on a short timescale from the giant branch to the white dwarf stage, thus rendering them invisible to observations.