The Dog that Didn't Bark: Non-Variable Field Stars in the RR Lyrae Instability Strip

TL;DR

This study discovers non-variable stars within the RR Lyrae instability strip using Gaia DR3 data, revealing that a significant fraction of stars in this region do not pulsate as previously assumed, challenging existing stellar models.

Contribution

The paper reports the first identification of non-variable stars in the RR Lyrae instability strip, highlighting a new stellar population that questions current pulsation theories.

Findings

At least 15% of stars in the instability strip are non-variable.

Non-variable stars are mostly bright, nearby, and on cold Galactic orbits.

These stars have a mean metallicity of about -0.5 dex.

Abstract

RR Lyrae stars (RRLs) are easy to identify thanks to their large photometric variation and short periods. All stars in the RRL instability strip are pulsators is often a hidden assumption in most stellar population studies using RRLs. Non-variable stars in the instability strip have been discovered for Cepheids and $\delta$ Scuti, and in this paper, we report the discovery of non-variable filed stars in the RRL instability strip. Using a high-quality sample selected from Gaia DR3, we find at least 15% of the stars in the empirical instability strip where the variable fraction is > 0.7 have near-zero photometric variations or variations that are significantly smaller than typical RRLs. The non-variable stars are mostly bright and close by, on cold orbits in the Galactic plane. Metallicity from Gaia BP/RP spectra suggests the non-variable stars have an average metallicity is ~ -0.5 dex, with a peak at 0. The discovery of these non-variable stars in the RRL instability strip challenges our understanding of stellar physics and further investigation is needed to understand the origin of these stars.