Tidal Disruptions of Main Sequence Stars of Varying Mass and Age: Inferences from the Composition of the Fallback Material

TL;DR

This study models the changing composition of fallback material from tidally disrupted main sequence stars of various masses and ages, revealing how these changes can inform us about the disrupted star's properties and improve event characterization.

Contribution

It introduces a framework to predict the time evolution of fallback material composition based on stellar mass and age, aiding in identifying the disrupted star in tidal disruption events.

Findings

Nitrogen enhancements and carbon/oxygen depletions develop over stellar lifetimes.

Compositional features become prominent after the fallback rate peak.

These features can help distinguish the disrupted star's properties.

Abstract

We use a simple framework to calculate the time evolution of the composition of the fallback material onto a supermassive black hole arising from the tidal disruption of main sequence stars. We study stars with masses between 0.8 and 3.0 $M_\odot$, at evolutionary stages from zero-age main sequence to terminal-age main sequence, built using the Modules for Experiments in Stellar Astrophysics code. We show that most stars develop enhancements in nitrogen ($^{14}$N) and depletions in carbon ($^{12}$C) and oxygen ($^{16}$O) over their lifetimes, and that these features are more pronounced for higher mass stars. We find that, in an accretion-powered tidal disruption flare, these features become prominent only after the time of peak of the fallback rate and appear at earlier times for stars of increasing mass. We postulate that no severe compositional changes resulting from the fallback material should be expected near peak for a wide range of stellar masses and, as such, are unable to explain the extreme helium-to-hydrogen line ratios observed in some TDEs. On the other hand, the resulting compositional changes could help explain the presence of nitrogen-rich features, which are currently only detected after peak. When combined with the shape of the light curve, the time evolution of the composition of the fallback material provides a clear method to help constrain the nature of the disrupted star. This will enable a better characterization of the event by helping break the degeneracy between the mass of the star and the mass of the black hole when fitting tidal disruption light curves.