On the Missing Red Giants near the Galactic Center

TL;DR

This study tackles the observed paucity of bright red giants near the Galactic Center by examining two competing depletion channels: tidal stripping via diffusion into the black hole's loss cone and destructive stellar collisions. It couples NR and RR diffusion analyses, solved with a backward Fokker-Planck approach and complemented by Monte Carlo diffusion that evolves RGB radii, and then adds collision processes with orbit-averaged rates. The key finding is that tidal diffusion alone yields mean exit times longer than RGB lifetimes and depends only logarithmically on the loss-cone size, making it unlikely to discriminate between RGB and main-sequence stars; in contrast, collisions, whose cross sections scale strongly with stellar radius, emerge as a more plausible driver of the bright RGB depletion, particularly for larger radii and certain orbital ranges. The study also predicts a tangential anisotropy in late-type stars near the center as a potential observational signature, with future work planned to incorporate a realistic mass spectrum and detailed hydrodynamic collision modeling. This work thus emphasizes collisions as a primary factor in the RGB depletion near Sgr A*, refining our understanding of stellar dynamics in the Galactic Center.

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.

Figures (5)

• Figure 1: Evolution in luminosity (upper panel) and radius (lower panel) of solar-mass stars with various metallicities as computed with MESA version 2024-03-1. Dashed lines denote the luminosity and radius corresponding to the $K_{s}\approx15$ (65$L_{\astrosun}$, 115$L_{\astrosun}$, 155$L_{\astrosun}$) for a solar-type red giant. The insets describe the last Gyr of luminosity/radius evolution for each metallicity for ease of comparison.
• Figure 2: Radius-dependent evolutionary lifespan scatter of various metallicity 1.0 $M_{\astrosun}$ stars as obtained through MESA, where $R/\dot{R}$ is given in years. The rest of the configurations are as given in Fig. (\ref{['fig1']}). The line of best fit is calculated from all points $\log_{10}(R)\geq 0.25$, with $R[R_{\astrosun{}}]$. Note that all radius-converted luminosity limits ($65 L_{\astrosun}$, 115$L_{\astrosun}$, 155$L_{\astrosun}$, in that order) are within the well-behaved region of the scatter.
• Figure 3: Illustration of the dependence of mean exit time on stellar radius and on the power-law index of the density profile of field stars, given a test star with solar mass, where $j_{0}=0.7$.
• Figure 4: Stars lost by diffusion into the loss cone from an initial population of $10^{3}$ Sun-like ($M_*=1\,M_\odot$, $Z=0.02$) test stars, simulated by Monte-Carlo methods over 2 Gyr on the main sequence (blue) and the sub-giant and red-giant branches (red). In all cases, $j_0=0.7$, $a=0.1$ pc, $\Delta t = 0.1$ kyr. The dotted vertical line denotes the time corresponding to $115L_{\astrosun}$.
• Figure 5: The normalized distribution in dimensionless angular momentum $j\equiv\sqrt{1-e^2}$ of $1\,M_\odot$ red giants not destroyed until the helium flash. Simulations as in Table \ref{['tab:collisions_clean']} with $\gamma=1.75$, but for semimajor axes $a=0.1\,\mathrm{pc}$ (red) and $0.36\,\mathrm{pc}$ (blue).