A search for non-pulsating, chemically normal stars in the $\delta$ Scuti instability strip using Kepler data

TL;DR

This study searches for chemically normal stars within the $ ext{delta}$ Scuti instability strip that do not pulsate, using Kepler data, and investigates whether binarity or other factors explain their non-pulsation.

Contribution

It identifies chemically normal non-pulsating stars in the $ ext{delta}$ Scuti strip and explores potential explanations like binarity or unknown mechanisms suppressing pulsations.

Findings

Most non-pulsators are near the strip edges, possibly outside the strip within uncertainties.

Unresolved binaries can mimic non-pulsation signatures, explaining some cases.

One chemically normal star in the strip challenges current pulsation theories.

Abstract

We identify stars in the $\delta$ Sct instability strip that do not pulsate in p modes at the 50-$\mu$mag limit, using Kepler data. Spectral classification and abundance analyses from high-resolution spectroscopy allow us to identify chemically peculiar stars, in which the absence of pulsations is not surprising. The remaining stars are chemically normal, yet they are not $\delta$ Sct stars. Their lack of observed p modes cannot be explained through any known mechanism. However, they are mostly distributed around the edges of the $\delta$ Sct instability strip, which allows for the possibility that they actually lie outside the strip once the uncertainties are taken into account. We investigated the possibility that the non-pulsators inside the instability strip could be unresolved binary systems, having components that both lie outside the instability strip. If misinterpreted as single stars, we found that such binaries could generate temperature discrepancies of $\sim$300 K -- larger than the spectroscopic uncertainties, and fully consistent with the observations. After these considerations, there remains one chemically normal non-pulsator that lies in the middle of the instability strip. This star is a challenge to pulsation theory. However, its existence as the only known star of its kind indicates that such stars are rare. We conclude that the $\delta$ Sct instability strip is pure, unless pulsation is shut down by diffusion or another mechanism, which could be interaction with a binary companion.