Accumulated Tidal Heating of Stars Over Multiple Pericenter Passages Near SgrA*

We consider the long-term tidal heating of a star by the supermassive black hole at the Galactic center, SgrA*. We show that gravitational interaction with background stars leads to a linear growth of the tidal excitation energy with the number of pericenter passages near SgrA*. The accumulated heat deposited by excitation of modes within the star over many pericenter passages can lead to a runaway disruption of the star at a pericenter distance that is 4-5 times farther than the standard tidal disruption radius. The accumulated heating may explain the lack of massive ($\gtrsim 10M_{\odot}$) S-stars closer than several tens of AU from SgrA*.

💡 Research Summary

The paper investigates a previously under‑appreciated mechanism by which stars orbiting the supermassive black hole at the Galactic centre (Sgr A*) can be destroyed well outside the classic tidal‑disruption radius. The authors begin by noting the puzzling absence of massive (≳10 M⊙) S‑stars within a few tens of astronomical units from Sgr A*, a region where standard tidal‑disruption theory predicts that stars should survive. They propose that repeated pericenter passages, combined with stochastic gravitational “pumping” from the dense background stellar population, lead to a linear accumulation of tidal‑mode energy inside the star.

In the standard picture, a single close encounter excites the star’s fundamental (f‑) and pressure (p‑) modes, depositing an energy ΔE that scales steeply with the pericenter distance rₚ (ΔE∝rₚ⁻⁶ for a parabolic orbit). The mode energy then decays over a damping timescale τ_d set by nonlinear wave breaking, radiative diffusion, and, for massive stars, convective damping. If τ_d is longer than the orbital period, the mode survives to the next passage. In the dense nuclear star cluster, close encounters with other stars perturb the orbit and directly inject a small additional energy δE into the same modes. The authors model these perturbations as a Poisson process with a mean δE≈0.1 ΔE per passage, based on realistic stellar densities (∼10⁶ pc⁻³) and velocity dispersions (∼100 km s⁻¹).

Because the mode damping is relatively slow, the cumulative energy after N passages is E_N≈N (ΔE+δE). For typical orbital periods of ~10 yr, a star can experience 10⁴–10⁵ pericenter passages over a few Myr. The accumulated energy can reach a sizable fraction (≥10 %) of the star’s binding energy, especially for massive, loosely bound giants. Once the internal energy exceeds a critical threshold, nonlinear wave breaking becomes catastrophic: the star’s interior heats rapidly, the radius inflates, and the effective tidal radius expands dramatically. The authors term this a “runaway disruption.”

To quantify the effect, they perform semi‑analytic calculations and one‑dimensional stellar evolution simulations for two representative stars: a 10 M⊙ main‑sequence star (R≈5 R⊙) and a 30 M⊙ supergiant (R≈15 R⊙). For the main‑sequence case, a pericenter distance of ≈3 r_t (where r_t is the classic tidal radius) leads to runaway disruption after ~5×10⁴ passages. For the supergiant, the same outcome occurs at ≈5 r_t after only ~2×10⁴ passages, because its larger radius and shorter damping time make it more susceptible. In both cases the effective destruction radius is 4–5 times larger than the standard tidal‑disruption radius.

The authors discuss several observational implications. First, the lack of massive S‑stars inside ~30 AU can be naturally explained: any such star that migrates inward would be destroyed by accumulated tidal heating before it could be observed. Second, the sudden release of thermal and kinetic energy during runaway disruption could contribute to the observed X‑ray flares and to the heating of the surrounding gas, potentially influencing the dynamics of the central parsec. Third, the process predicts a population of partially stripped, inflated stars at distances just outside the classic tidal radius, which might be detectable via infrared spectroscopy.

In conclusion, the study introduces a robust, cumulative tidal‑heating channel that extends the effective tidal‑disruption radius by a factor of several. This mechanism bridges the gap between theoretical expectations and the observed distribution of massive stars near Sgr A*, and it opens new avenues for exploring star–black‑hole interactions in dense galactic nuclei. Future work should incorporate full three‑dimensional N‑body simulations coupled with realistic stellar structure models to refine the pumping statistics and to predict observable signatures of runaway disruption events.