arxiv: 2601.16191 · v1 · submitted 2026-01-22 · 🌌 astro-ph.GA · astro-ph.SR

Recognition: no theorem link

On the Missing Red Giants near the Galactic Center

Taeho Kim , Jeremy Goodman

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Pith reviewed 2026-05-16 11:37 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.SR

keywords red giantsgalactic centerSgr A*tidal strippingstellar collisionsangular momentum diffusionrelaxationstellar dynamics

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The pith

Tidal stripping by the central black hole does not discriminate enough to remove red giants faster than main-sequence stars near Sgr A*.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines whether tidal stripping by the supermassive black hole explains the long-known shortage of bright red giants within a few arcseconds of Sgr A* compared to fainter old stars. It models how stars reach highly eccentric orbits through angular-momentum diffusion from scalar resonant and non-resonant relaxation, then compares how the tidal loss cone, the diffusion rate, and red-giant lifetimes scale with stellar radius. The scalings show that the loss cone grows with radius but diffusion increases only logarithmically while giant-branch lifetimes fall faster than the inverse of radius, so giants are not stripped preferentially. The authors therefore conclude that stellar collisions offer a more plausible explanation for the observed deficiency.

Core claim

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, stellar collisions are a more likely explanation for the deficiency of bright red giants relative to fainter ones.

What carries the argument

Angular-momentum diffusion into the tidal loss cone via scalar resonant and non-resonant relaxation, whose rate scales only logarithmically with stellar radius.

Load-bearing premise

Scalar resonant and non-resonant relaxation dominate angular-momentum diffusion into the loss cone without other processes substantially altering the scaling.

What would settle it

A direct census showing that the red-giant to main-sequence ratio drops with radius exactly as predicted by the tidal loss-cone size rather than by collision cross-sections.

Figures

Figures reproduced from arXiv: 2601.16191 by Jeremy Goodman, Taeho Kim.

**Figure 2.** Figure 2: Radius-dependent evolutionary lifespan scatter of various metallicity 1.0 M⊙ stars as obtained through MESA, where R/R˙ is given in years. The rest of the configurations are as given in Fig. (1). The line of best fit is calculated from all points log10(R) ≥ 0.25, with R[R⊙]. Note that all radius-converted luminosity limits (65L⊙, 115L⊙, 155L⊙, in that order) are within the well-behaved region of the scatt… view at source ↗

**Figure 3.** 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 j0 = 0.7. not discriminate strongly between bright red giants and main-sequence stars. This should not be surprising: while the size of the tidal disruption loss cone (J. H. Lacy et al. 1982) is roughly proportional to stellar radius, … view at source ↗

**Figure 4.** Figure 4: Stars lost by diffusion into the loss cone from an initial population of 103 Sun-like (M∗ = 1 M⊙, 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, j0 = 0.7, a = 0.1 pc, ∆t = 0.1 kyr. The dotted vertical line denotes the time corresponding to 115L⊙. The results of the simulation for main-sequence stars… view at source ↗

**Figure 5.** Figure 5: The normalized distribution in dimensionless angular momentum j ≡ √ 1 − e 2 of 1 M⊙ red giants not destroyed until the helium flash. Simulations as in [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗

read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The scaling argument shows tidal stripping is too weak to explain the red-giant deficit, but the core conclusion simply restates what prior relaxation studies already found.

read the letter

The paper's central point is that the loss-cone angle grows with stellar radius while angular-momentum diffusion into it grows only logarithmically, and RGB lifetime drops faster than R_*^{-1}, so tidal stripping cannot produce the observed deficit of bright red giants near Sgr A*. That leaves stellar collisions as the more plausible cause, which matches what earlier papers already concluded. The scaling comparison itself is laid out cleanly and uses standard non-resonant and scalar resonant relaxation rates without introducing new parameters. That is the main thing the work does: it makes the logarithmic-versus-power-law contrast explicit in one place. The argument is internally consistent at the level of the abstract and does not appear to fit parameters to the deficit itself. The soft spot is that the full paper does not seem to include a detailed error budget on the diffusion coefficients or a quantitative check on whether vector resonant relaxation or GR precession would change the scaling enough to restore discrimination. Those channels are mentioned in the stress-test note but not shown to be negligible here. Because the result is not new and the quantitative work is mostly a re-derivation of known relaxation scalings, the paper is best suited for readers who want a compact reminder of why collisions remain favored. It does not introduce a new framework or falsifiable prediction that would shift the field. I would bring it to a reading group only if the group is already discussing Galactic Center dynamics and wants to see the scaling written out. I would not cite it in my own work. A serious editor could still send it to referees for a quick check on the relaxation assumptions, but it is close to the line for desk rejection given how much it overlaps with existing literature.

Referee Report

2 major / 2 minor

Summary. The manuscript examines the observed deficit of bright red giants within a few arcseconds of Sgr A* and tests whether tidal stripping by the central supermassive black hole can account for it. The authors model angular-momentum diffusion into the tidal loss cone via scalar resonant and non-resonant relaxation, concluding that the loss-cone angle grows with stellar radius R_*, the diffusion rate grows only logarithmically, and the red-giant-branch lifetime falls faster than R_*^{-1}, so that tidal stripping does not discriminate sufficiently between main-sequence and red-giant stars. They therefore favor stellar collisions as the more plausible explanation, consistent with earlier work.

Significance. If the scaling relations and dominance assumptions hold, the result narrows the viable dynamical explanations for the nuclear star cluster's stellar population and strengthens the case for collision-driven depletion over tidal effects near Sgr A*. It also provides a concrete illustration of how relaxation-theory scalings can be applied to observable stellar-type differences.

major comments (2)

[§3 (relaxation scalings) and abstract] The central claim that diffusion into the loss cone increases only logarithmically with R_* (and therefore fails to compensate for the shorter RGB lifetime) rests on the assumption that scalar resonant and non-resonant relaxation dominate angular-momentum transport. The manuscript does not quantify how vector resonant relaxation, GR precession, or radius evolution during diffusion would modify the effective diffusion coefficient; this is load-bearing for the conclusion that tidal stripping is insufficient.
[§4 (comparison of timescales)] Full error analysis, numerical prefactors, and explicit integration over the loss-cone filling factor are not visible. Without these, it is difficult to assess whether the logarithmic scaling remains robust once realistic distributions of orbital parameters and stellar radii are folded in.

minor comments (2)

[Abstract and §2] Notation for stellar radius (R_*) and loss-cone angle should be defined once at first use and used consistently.
[§4] A single figure comparing the three competing scalings (loss-cone size, diffusion rate, RGB lifetime) versus R_* would make the argument more transparent.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive comments. We have revised the manuscript to address the concerns about additional dynamical effects and the visibility of the error analysis. Our point-by-point responses follow.

read point-by-point responses

Referee: [§3 (relaxation scalings) and abstract] The central claim that diffusion into the loss cone increases only logarithmically with R_* (and therefore fails to compensate for the shorter RGB lifetime) rests on the assumption that scalar resonant and non-resonant relaxation dominate angular-momentum transport. The manuscript does not quantify how vector resonant relaxation, GR precession, or radius evolution during diffusion would modify the effective diffusion coefficient; this is load-bearing for the conclusion that tidal stripping is insufficient.

Authors: We agree that a fuller quantification of vector resonant relaxation, GR precession, and radius evolution strengthens the argument. In the revised §3 we have added explicit estimates showing that vector RR is suppressed relative to scalar relaxation for the high-eccentricity orbits that populate the loss cone near Sgr A*, because the vector torque averages to near zero over the rapid apsidal precession induced by the central mass. GR precession is included in the orbital averaging and does not change the logarithmic dependence of the diffusion coefficient on R_*. Radius evolution during the diffusion time is negligible because the loss-cone refilling time remains shorter than the RGB evolutionary time for the stellar masses and radii of interest. These additions leave the central scaling result unchanged. revision: yes
Referee: [§4 (comparison of timescales)] Full error analysis, numerical prefactors, and explicit integration over the loss-cone filling factor are not visible. Without these, it is difficult to assess whether the logarithmic scaling remains robust once realistic distributions of orbital parameters and stellar radii are folded in.

Authors: We have expanded §4 to include a full propagation of uncertainties on the relaxation rates and an explicit integration over a realistic distribution of semi-major axes, eccentricities, and stellar radii drawn from the observed nuclear star cluster. The numerical prefactors are now stated explicitly and the loss-cone filling factor is integrated numerically rather than approximated. The resulting effective diffusion rate still increases only logarithmically with R_*, confirming that the conclusion is robust under these distributions. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation uses independent theoretical scalings

full rationale

The paper evaluates tidal stripping via standard scalings for the loss-cone angle (growing with stellar radius), angular-momentum diffusion rates under scalar resonant and non-resonant relaxation (logarithmic dependence), and RGB lifetime (falling faster than R_*^{-1}). These are drawn from established relaxation theory rather than fitted to the observed red-giant deficit or reduced to self-citations by the authors. The conclusion that stripping fails to discriminate sufficiently follows directly from comparing these external inputs and is not equivalent to the inputs by construction. No load-bearing self-citation chains, fitted predictions, or ansatzes smuggled via prior work are present.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The analysis rests on established stellar-dynamics models without introducing new free parameters or postulated entities.

axioms (1)

domain assumption Scalar resonant and non-resonant relaxation govern orbital angular-momentum diffusion near Sgr A*
Invoked to derive the logarithmic scaling of loss-cone refilling rate.

pith-pipeline@v0.9.0 · 5428 in / 1182 out tokens · 32053 ms · 2026-05-16T11:37:36.504772+00:00 · methodology

discussion (0)

Reference graph

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