Early Spectroscopy of the 2010 Outburst of U Scorpii

TL;DR

This paper presents the first time-resolved early spectroscopy of the 2010 outburst of U Sco, revealing rapid changes in spectral line profiles indicative of evolving aspherical wind structures during the initial stages.

Contribution

It provides the first detailed time-series spectra of U Sco during the first night of outburst, highlighting the evolution of wind geometry from aspherical to spherical.

Findings

Rapid development of blue and red peaks in Hα line indicating aspherical wind growth

Transition from aspherical wind to nearly spherical shell within 9 days

First spectroscopic observation of U Sco's early outburst phase

Abstract

We present early spectroscopy of the recurrent nova U~Sco during the outburst in 2010. We successfully obtained time-series spectra at $t_{\rm d}=$0.37--0.44~d, where $t_{\rm d}$ denotes the time from the discovery of the present outburst. This is the first time-resolved spectroscopy on the first night of U Sco outbursts. At $t_{\rm d}\sim 0.4$~d the H$\alpha$ line consists of a blue-shifted ($-5000$ km s$^{-1}$) narrow absorption component and a wide emission component having triple peaks, a blue ($\sim -3000$ km s$^{-1}$), a central ($\sim 0$ km s$^{-1}$) and a red ($\sim +3000$ km s$^{-1}$) ones. The blue and red peaks developed more rapidly than the central one during the first night. This rapid variation would be caused by the growth of aspherical wind produced during the earliest stage of the outburst. At $t_{\rm d}=1.4$~d the H$\alpha$ line has a nearly flat-topped profile with weak blue and red peaks at $\sim \pm 3000$ km s$^{-1}$. This profile can be attributed to a nearly spherical shell, while the asphericity growing on the first night still remains. The wind asphericity is less significant after $t_{\rm d}=9$ d.