SPB stars in the open SMC cluster NGC 371

TL;DR

This study observed the SMC cluster NGC 371 and discovered numerous SPB stars, demonstrating that pulsation driven by the kappa mechanism occurs even at low metallicity, contrary to prior expectations.

Contribution

It provides the first observational evidence of SPB stars in a low-metallicity environment, challenging previous assumptions about pulsation suppression in such conditions.

Findings

29 short-period B-type variables identified, many likely SPB stars

Pulsation driven by the kappa mechanism occurs at low metallicity

No beta Cep stars detected with amplitudes above 5 mmag

Abstract

Pulsation in beta Cep and SPB stars are driven by the kappa mechanism which depends critically on the metallicity. It has therefore been suggested that beta Cep and SPB stars should be rare in the Magellanic Clouds which have lower metallicities than the solar neighborhood. To test this prediction we have observed the open SMC cluster NGC 371 for 12 nights in order to search for beta Cep and SPB stars. Surprisingly, we find 29 short-period B-type variables in the upper part of the main sequence, many of which are probably SPB stars. This result indicates that pulsation is still driven by the kappa mechanism even in low metallicity environments. All the identified variables have periods longer than the fundamental radial period which means that they cannot be beta Cep stars. Within an amplitude detection limit of 5 mmag no stars in the top of the HR-diagram show variability with periods shorter than the fundamental radial period. So if beta Cep stars are present in the cluster they oscillate with amplitudes below 5 mmag, which is significantly lower than the mean amplitude of beta Cep stars in the Galaxy. We see evidence that multimode pulsation is more common in the upper part of the main sequence than in the lower. We have also identified 5 eclipsing binaries and 3 periodic pulsating Be stars in the cluster field.