Early Spectroscopy of the 2010 Outburst of U Scorpii

We present early spectroscopy of the recurrent nova U Sco during an outburst in 2010. We successfully obtained time-series spectra during the period of t_d = 0.37-0.44 d, where t_d is the time that elapsed since the discovery of the present outburst. This is the first time-resolved spectroscopy on the first night of U Sco outbursts. At t_d ∼ 0.4 d, the Hα line consists of a blue-shifted (-5000 km s^-1) narrow absorption component and a wide, triple-peak emission one: blue (∼ -3000 km s^-1), central (∼0 km s^-1), and red (∼+3000 km s^-1) peaks. The blue and red peaks developed more rapidly than the central one for 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_d = 1.4 d, the Hα line has a nearly flat-topped profile with weak blue and red peaks at ∼±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_d = 9 d.