Black Hole Survival Guide: Searching for Stars in the Galactic Center That Endure Partial Tidal Disruption

TL;DR

This work investigates whether stars that survive partial tidal disruption by the Galactic Center's supermassive black hole can exist as observable remnants and potentially explain some G objects. It combines high-resolution 3D FLASH hydrodynamics with 1D MESA stellar evolution to model the immediate post-disruption phase and the long-term evolution of remnants, spanning from $\lesssim 10^5$ years to $\sim$Gyr timescales. The study finds an initial, dramatic bright phase ( luminosities up to $\sim 500\times$ and envelope inflation by $4$–$6\times$) that fades within $\sim 10^5$ years, followed by a long-lived evolution where remnants are cooler and fainter than their progenitors but brighter than mass-equivalent MS stars; mixing during disruption leads to He and N enrichment and C depletion in the envelopes, with more pronounced effects for higher-$\beta$ events. Detecting these survivors would rely on a combination of kinematic signatures and spectroscopic fingerprints, particularly enhanced He and N abundances, offering a concrete route to identifying a population of TDE remnants in the Milky Way and informing gravity-dominated stellar evolution in galactic nuclei.

Abstract

Once per 10,000-100,000 years, an unlucky star may experience a close encounter with a supermassive black hole (SMBH), partially or fully tearing apart the star in an exceedingly brief, bright interaction called a tidal disruption event (TDE). Remnants of partial TDEs are expected to be plentiful in our Galactic center, where at least six unexplained, diffuse, star-like "G objects" have already been detected which may have formed via interactions between stars and the SMBH. Using numerical simulations, this work aims to identify the characteristics of TDE remnants. We take 3D hydrodynamic FLASH models of partially disrupted stars and map them into the 1D stellar evolution code MESA to examine the properties of these remnants from tens to billions of years after the TDE. The remnants initially exhibit a brief, highly luminous phase, followed by an extended cooling period as they return to stable hydrogen burning. During the initial stage (< 100,000 yr) their luminosities increase by orders of magnitude, making them intriguing candidates to explain a fraction of the mysterious G objects. Notably, mild TDEs are the most common and result in the brightest remnants during this initial phase. However, most remnants exist in a long-lived stage where they are only modestly offset in temperature and luminosity compared to main-sequence stars of equivalent mass. Nonetheless, our results indicate remnants will sustain abnormal, metal-enriched envelopes that may be discernible through spectroscopic analysis. Identifying TDE survivors within the Milky Way could further illuminate some of the most gravitationally intense encounters in the Universe.

Figures (6)

• Figure 1: Slices in 2D of the four TDE remnants studied in this paper, selected from the STARS Library LawSmith2020. The left three columns show remnants that begin as a 1 $M_\odot$, 4.8 Gyr main sequence star before disruption, and the fourth displayed in the rightmost column begins as a 3 $M_\odot$, 0.3 Gyr main sequence star. Each is subjected to varying degrees of tidal disruption. The impact parameter ($\beta$) increases from left to right, indicating how deeply the star plunged past the SMBH's tidal radius. Colorbars indicate temperature in Kelvin (top row) and diffusion time in years (bottom row). The left panels have width 10 $R_{\odot}$, while the rightmost column has width 22 $R_{\odot}$. All snapshots are captured at roughly 80 $t_{\rm dyn}$ after pericenter, corresponding to $\approx 38$ hr after pericenter for the left three columns and $\approx 124$ hr after pericenter for the right column.
• Figure 2: Luminosity, temperature and radius evolution of the three tidally disrupted remnants (red solid) with progenitor mass $M_{i} = 1\, M_{\odot}$. These remnants are compared to mass-equivalent stars (blue dash-dotted) that evolve from the pre-main sequence (PMS) to the main sequence (MS) and up the red giant branch (RGB). The masses of the TDE remnants and their associated mass-equivalent stars are $0.956\, M_{\odot}$, $0.708\, M_{\odot}$, and $0.386\, M_{\odot}$ respectively from left to right. The start of core hydrogen fusion is denoted by star symbols for remnants and mass-equivalent stars. In addition, the evolution of an undisturbed $M = 1\, M_{\odot}$ star (black dotted) is shown from the time of disruption onward in order to represent the comparative evolution of the progenitor had it not undergone the TDE. Immediately after disruption, the remnants appear larger and more luminous than the progenitor, then shrink over the next several million years. After reigniting core hydrogen fusion, they stabilize near the MS locations of their mass-equivalent stars.
• Figure 3: Luminosity evolution for the first few 100 Myr of the TDE remnants. The $1M\beta1$, $1M\beta1.5$, and $1M\beta2$ TDE remnants from Figure \ref{['fig:evol']} are shown with dashed lines and are compared here with their $1\, M_{\odot}$ progenitor (black dash-dotted). Additionally, the $3M\beta4$ remnant is shown (solid) to remain brighter than its mass-equivalent star (black dash-dotted), but dimmer than its progenitor (black dotted), throughout the PMS + MS. On the right, a histogram of the number of G-type objects ($N_G$) per luminosity bin is shown (data from Ghez2004Ghez2005Hornstein2007Phifer2013Sitarski2016PhDTWitzel2014).
• Figure 4: Hertzsprung-Russell diagram of simulated stars on the hydrogen-burning main sequence. Data from Gaia DR3Gaia2016Gaia2021 of the closest 50,000 stars is plotted in the background as a logarithmic histogram where grayscale shading indicates number density, to show the natural spread of Milky Way stars. The filled circles are main sequence single stars simulated in MESA, with the ZAMS masses listed in the upper right legend. The open diamonds represent the $1M\beta1$, $1M\beta1.5$, and $1M\beta2$ TDE remnants, and the open square is the $3M\beta4$ TDE remnant; all are shown at snapshots when they have returned to stable hydrogen burning, and the final masses of each remnant are noted in the bottom legend. All remnants appear substantially cooler and dimmer than their respective progenitors, while appearing subtly hotter and brighter than their mass-equivalent stars (or extremely hotter and brighter, in the case of the $3M\beta4$ remnant).
• Figure 5: Surface mass fractions of selected elements $X_{i}$ in our remnants, scaled to solar metallicity abundances $X_{i,\odot}$. We show results for the elements $X_{i}$ listed in the legend. Lines end just before core He ignition. The $1M\beta1$ remnant retains near-solar abundances until a dredge-up phase when it becomes a red giant. For the disrupted stars with $\beta > 1$, by the main sequence the envelopes are enriched in nitrogen (orange dashes) and depleted in carbon (red dots), both indicators of CNO fusion processes in the core. The presence of core-like CNO ratios on the stars' surfaces is a result of convective mixing that occurs during the first $10^5$ yr after disruption. Enhancement of helium is also observed in each of these models. These differences may manifest as spectroscopic signatures in the remnants of partial TDEs.