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Hydrogen emissions of RR Lyrae variables are the imprints of shock wavestraveling through their atmospheres. We develop a pattern recognitionalgorithm, which is then applied to single-epoch spectra of SDSS and LAMOST.These two spectroscopic surveys covered $\sim$ 10,000 photometrically confirmedRR Lyrae stars. We discovered in total 127 RR Lyrae stars with blueshiftedBalmer emission feature, including 103 fundamental mode (RRab), 20first-overtone (RRc), 3 double-mode (RRd), and 1 Blazhko type (temporaryclassification for RR Lyrae stars with strong Blazhko modulation in Catalinasky survey that cannot be characterized) RR Lyrae variable. This forms thelargest database to date of the properties of hydrogen emission in RR Lyraevariables. Based on ZTF DR5, we carried out a detailed light-curve analysis forthe Blazhko type RR Lyrae star with hydrogen emission of long-term modulations.We characterize the Blazhko type RR Lyrae star as an RRab and point out apossible Blazhko period. Finally, we set up simulations on mock spectra to testthe performance of our algorithm and on the real observational strategy toinvestigate the occurrence of the "first apparition".

Blueshifted hydrogen emission and shock wave of RR Lyrae variables in SDSS and LAMOST