Possible evidence of hydrogen emission in the first-overtone and multi-mode RR Lyrae variables

TL;DR

This study provides the first spectroscopic evidence of hydrogen emission indicating shock waves in non-fundamental mode RR Lyrae stars, using pattern recognition on large survey data, and explores potential links to the Blazhko effect.

Contribution

It introduces a novel application of pattern recognition to identify hydrogen emission in RR Lyrae stars, revealing shock phenomena in first-overtone and multi-mode types.

Findings

Hydrogen emission detected in 23 RR Lyrae stars, indicating shock waves.

Radial velocities of emission lines linked to shock conditions.

Possible connection between shock waves and the Blazhko effect.

Abstract

The nature of shock waves in non-fundamental mode RR Lyrae stars remains a mystery because of limited spectroscopic observations. We apply a pattern recognition algorithm on spectroscopic data from SDSS and LAMOST and report the first evidence of hydrogen emission in first-overtone and multi-mode RR Lyrae stars showing the "first apparition", which is the most prominent observational characteristic of shock in RR Lyrae variables. We find ten RRc stars in SDSS, ten RRc stars in LAMOST, and three RRd stars in LAMOST that show blueshifted Balmer emissions. The emission features possibly indicate the existence of shock waves. We calculate the radial velocities of the emission lines, which are related to the physical conditions occurring in the radiative zone of shock waves. Using photometric observations from ZTF, we present a detailed light curve analysis for the frequency components in one of our RRd stars with hydrogen emission, RRdl3, for possible modulations. With the enormous volume of upcoming spectral observations of variable stars, our study raises the possibility of connecting the unexplained Blazhko effect to shock waves in non-fundamental mode RR Lyrae stars.