Abundance Anomalies In Tidal Disruption Events

TL;DR

This paper explores how stellar evolution causes abundance anomalies in tidal disruption events, particularly in helium, nitrogen, and carbon, which can explain observed spectral diversity and nitrogen-rich quasars.

Contribution

It identifies specific abundance anomalies in TDE debris due to stellar evolution, linking them to observed spectral features and rare nitrogen-rich quasars.

Findings

Helium is enhanced by up to 25% in TDE debris.

Nitrogen to carbon ratio increases by factors of 3-10 in certain TDEs.

High N/C ratios explain nitrogen-rich quasars and UV spectra of ASASSN-14li.

Abstract

The ~10% of tidal disruption events (TDEs) due to stars more massive than the Sun should show abundance anomalies due to stellar evolution in helium, carbon and nitrogen, but not oxygen. Helium is always enhanced, but only by up to ~25% on average because it becomes inaccessible once it is sequestered in the high density core as the star leaves the main sequence. However, portions of the debris associated with the disrupted core of a main sequence star can be enhanced in helium by factors of 2-3 for debris at a common orbital period. These helium abundance variations may be a contributor to the observed diversity of hydrogen and helium line strengths in TDEs. A still more striking anomaly is the rapid enhancement of nitrogen and the depletion of carbon due to the CNO cycle -- stars more massive than the Sun quickly show an increase in their average N/C ratio by factors of 3-10. Because low mass stars evolve slowly and high mass stars are rare, TDEs showing high N/C will almost all be due to 1-2Msun stars disrupted on the main sequence. Like helium, portions of the debris will show still larger changes in C and N, and the anomalies decline as the star leaves the main sequence. The enhanced [N/C] abundance ratio of these TDEs provides the first natural explanation for the rare, nitrogen rich quasars and also explains the strong nitrogen emission seen in ultraviolet spectra of ASASSN-14li.