Spectroscopic Comparison of Metal-Rich RRab Stars of the Galactic Field with Their Metal-Poor Counterparts

TL;DR

This study compares atmospheric properties of metal-rich and metal-poor RRab stars, revealing significant differences in their spectral and pulsation characteristics based on extensive spectroscopic data.

Contribution

It provides a new calibration of the metallicity scale and identifies distinct atmospheric behaviors between metal-rich and metal-poor RRab stars.

Findings

Metal-rich and metal-poor RRab stars differ in Halpha emission strength and line-doubling.

Metal-poor RRab stars show Halpha line-doubling during the bump, absent in metal-rich stars.

Distinct atmospheric properties suggest different evolutionary or structural states.

Abstract

We investigate atmospheric properties of 35 stable RRab stars that possess the full ranges of period, light amplitude, and metal abundance found in Galactic RR Lyrae stars. Our results are derived from several thousand echelle spectra obtained over several years with the du Pont telescope of Las Campanas Observatory. Radial velocities of metal lines and the Halpha line were used to construct curves of radial velocity versus pulsation phase. From these we estimated radial velocity amplitudes for metal lines (formed near the photosphere) and Halpha Doppler cores (formed at small optical depths). We also measured Halpha emission fluxes when they appear during primary light rises. Spectra shifted to rest wavelengths, binned into small phase intervals, and coadded were used to perform model atmospheric and abundance analyses. The derived metallicities and those of some previous spectroscopic surveys were combined to produce a new calibration of the Layden abundance scale. We then divided our RRab sample into metal-rich (disk) and metal-poor (halo) groups at [Fe/H]=-1.0. The atmospheres of RRab families, so defined, differ with respect to (a) peak strength of Halpha emission flux, (b) Halpha radial velocity amplitude, (c) dynamical gravity, (d) stellar radius variation, (e) secondary acceleration during the photometric "bump" that precedes minimum light, and (g) duration of Halpha line-doubling. We also detected Halpha line-doubling during the bump in the metal-poor family, but not in the metal-rich one. Though all RRab probably are core helium-burning horizontal branch stars, the metal-rich group appear to be a species sui generis.