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

Abstract

We identify stars in the δ Sct instability strip that do not pulsate in p modes at the 50-µmag 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 δ 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 δ 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 ∼300 K -larger than the spectroscopic uncertainties, and fully consistent with the observations. After these considerations, there remains one chemically normal nonpulsator 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 δ Sct instability strip is pure, unless pulsation is shut down by diffusion or another mechanism, which could be interaction with a binary companion.

Gaia-derived luminosities ofKeplerA/F stars and the pulsator fraction across the δ Scuti instability strip

“…The simulation seems incompatible with the actual distribution, regardless of which instability strip is used, so we are able to reject the hypothesis that the δ Sct instability strip is actually pure. The conclusion in Murphy et al (2015) that all stars in the instability strip pulsate is now seen to be incorrect, presumably due to the small sample size used there.…”

“…Helium gravitationally settles out of the partial ionization zone where p modes are driven, hence Am stars are expected (and observed) to have a low pulsator fraction (Breger 1970;Kurtz 1989;Smalley et al 2011Smalley et al , 2017. Suggestions that the nonpulsators in the instability strip are actually Ap/Am stars have naturally followed (Murphy 2014;Murphy et al 2015).…”

Near-uniform internal rotation of the main-sequence γ Doradus pulsator KIC 7661054

“…They analysed the spectra and provided atmospheric parameters that identified KIC 7661054 as a slow rotator (v sin i = 16±1 km s −1 ). Using those parameters, Murphy et al (2015) showed KIC 7661054 to lie in the middle of the γ Dor instability strip, and at the red edge of the δ Sct instability strip. The pulsation frequency distribution in Fig.…”

Keplerobservations of A–F pre-main-sequence stars in Upper Scorpius: discovery of six new δ Scuti and one γ Doradus stars

“…Balona & Dziembowski 2011a, and references therein). A recent discussion of this subject is provided by Murphy et al (2015) whose investigation is based on ultra-precise Kepler photometry and high-resolution spectroscopy. Their conclusion is that chemically normal 4 non-variable stars inside the δ Sct instability strip exist but are rare.…”