Hydrogen lines in LAMOST low-resolution spectra of RR Lyrae stars

The LAMOST pilot survey has produced a data release containing over 600,000 stellar spectra. By cross-checking with a large time series photometric database of RR Lyrae stars in high Galactic latitude regions, we found a total number of 157 RR Lyrae stars that have been observed with LAMOST. In this sample, we successfully captured three RR Lyrae stars in the fast expansion phase, all of them showing hypersonic shock wave features in the Balmer line region. We fit the shape of Hα line region and determine that the emission feature seen within the broadened Hα absorption line suggests hypersonic relative motion in the atmospheres of these three objects. With a further LAMOST survey of millions of stars, we plan to capture a large sample of RR Lyrae stars in their hypersonic expansion phase, and therefore provide a large database for the study of the internal structure and the pulsation mechanism of RR Lyrae stars.