Pulsation versus metallicism in Am stars as revealed by LAMOST and WASP

TL;DR

This study analyzes the pulsation behavior of Am stars compared to normal A stars using LAMOST spectral data and WASP light curves, revealing temperature and metallicity effects on pulsations and proposing turbulent pressure as the driving mechanism.

Contribution

It provides new insights into the pulsation characteristics of Am stars, highlighting the role of metallicism and effective temperature, and challenges the traditional $$-mechanism as the primary driver.

Findings

Pulsations in Am stars are confined to 6900-7600 K.

Incidence of pulsations decreases with increasing metallicism.

Turbulent pressure likely drives pulsations in Am stars.

Abstract

We present the results of a study of a large sample of A and Am stars with spectral types from LAMOST and light curves from WASP. We find that, unlike normal A stars, $\delta$ Sct pulsations in Am stars are mostly confined to the effective temperature range 6900 $<$ $T_{\rm eff}$ $<$ 7600 K. We find evidence that the incidence of pulsations in Am stars decreases with increasing metallicism (degree of chemical peculiarity). The maximum amplitude of the pulsations in Am stars does not appear to vary significantly with metallicism. The amplitude distributions of the principal pulsation frequencies for both A and Am stars appear very similar and agree with results obtained from Kepler photometry. We present evidence that suggests turbulent pressure is the main driving mechanism in pulsating Am stars, rather than the $\kappa$-mechanism, which is expected to be suppressed by gravitational settling in these stars.