The Helium Abundance in the Ejecta of U Scorpii

TL;DR

This study analyzes the 2010 outburst of U Scorpii, deriving its helium abundance from optical and near-infrared spectra, and finds it to be lower than previously estimated, suggesting the secondary star is not helium-rich.

Contribution

First spectroscopic measurement of helium abundance in U Sco's ejecta during the 2010 outburst, challenging prior assumptions about the secondary star’s composition.

Findings

Helium abundance in ejecta is $N$(He)/$N$(H) = 0.073±0.031.

Velocities reach up to 14000 km/s in broad components.

Spectral evolution consistent with previous outbursts.

Abstract

U Scorpii is a recurrent nova which has been observed in outburst on 10 occasions, most recently in 2010. We present near-infrared and optical spectroscopy of the 2010 outburst of U Sco. The reddening of U Sco is found to be $E(B-V) = 0.14\pm0.12$, consistent with previous determinations, from simultaneous optical and near-IR observations. The spectra show the evolution of the line widths and profiles to be consistent with previous outbursts. Velocities are found to be up to 14000\,kms$^{-1}$ in broad components and up to 1800\,kms$^{-1}$ in narrow line components, which become visible around day 8 due to changes in the optical depth. From the spectra we derive a helium abundance of $N$(He)/$N$(H)$ = 0.073\pm0.031$ from the most reliable lines available; this is lower than most other estimates and indicates that the secondary is not helium-rich, as previous studies have suggested.