Are the stars of a new class of variability detected in NGC~3766 fast rotating SPB stars?

TL;DR

This study investigates whether the unexpected variability of stars in NGC~3766 can be explained by fast rotating SPB stars, considering effects like gravity darkening and mode excitation.

Contribution

It demonstrates that gravity darkening and Coriolis effects in fast rotating SPB stars can produce variability periods and luminosity shifts matching the observed anomalies.

Findings

Prograde sectoral modes are excited at the red edge of the SPB instability strip.

These modes have periods of 0.2--0.5 days influenced by the Coriolis effect.

Gravity darkening can make low-mass SPB stars appear fainter and between known instability strips.

Abstract

A recent photometric survey in the NGC~3766 cluster led to the detection of stars presenting an unexpected variability. They lie in a region of the Hertzsprung-Russell (HR) diagram where no pulsation are theoretically expected, in between the $\delta$ Scuti and slowly pulsating B (SPB) star instability domains. Their variability periods, between $\sim$0.1--0.7~d, are outside the expected domains of these well-known pulsators. The NCG~3766 cluster is known to host fast rotating stars. Rotation can significantly affect the pulsation properties of stars and alter their apparent luminosity through gravity darkening. Therefore we inspect if the new variable stars could correspond to fast rotating SPB stars. We carry out instability and visibility analysis of SPB pulsation modes within the frame of the traditional approximation. The effects of gravity darkening on typical SPB models are next studied. We find that at the red border of the SPB instability strip, prograde sectoral (PS) modes are preferentially excited, with periods shifted in the 0.2--0.5~d range due to the Coriolis effect. These modes are best seen when the star is seen equator-on. For such inclinations, low-mass SPB models can appear fainter due to gravity darkening and as if they were located between the $\delta$~Scuti and SPB instability strips.