Pulsating hot O subdwarfs in Omega Cen: mapping a unique instability strip on the Extreme Horizontal Branch

TL;DR

This study surveys Extreme Horizontal Branch stars in omega Cen, discovering rapid pulsators and mapping an empirical instability strip that reveals two distinct temperature regions where pulsations occur, supporting the kappa-mechanism as the driving force.

Contribution

It provides the first detailed empirical mapping of the omega Cen instability strip, identifying two separate pulsation regions and linking observations to theoretical models.

Findings

Five rapid multi-periodic variables discovered between 85-125 s.

An additional star shows possible binary-induced luminosity variation.

The instability strip is bifurcated, with pulsations occurring below 36,000 K and above 50,000 K.

Abstract

We present an extensive survey for rapid pulsators among Extreme Horizontal Branch (EHB) stars in omega Cen. The observations performed consist of nearly 100 hours of time-series photometry, as well as low-resolution spectroscopy. We obtained photometry for some 300 EHB stars. Based on the spectroscopy, we derive reliable values of log g, Teff and log(N(He)/N(H)) for 38 targets, as well as estimates of the effective temperature for another nine targets. The survey uncovered a total of five rapid variables with multi-periodic oscillations between 85 and 125 s. Spectroscopically, they form a homogeneous group of hydrogen-rich subdwarf O stars clustered between 48,000 and 54,000 K. For each of the variables we are able to measure between two and three significant pulsations believed to constitute independent harmonic oscillations. In addition to the rapid variables, we found an EHB star with an apparently periodic luminosity variation of ~2700 s, which we tentatively suggest may be caused by ellipsoidal variations in a close binary. Using the overlapping photometry and spectroscopy sample we are able to map an empirical omega Cen instability strip. This can be directly compared to the pulsation driving predicted from the Montreal "second-generation" models. We find that the region where p-mode excitation occurs is bifurcated, and the well-known instability strip between 29,000-36,000 K where the rapid subdwarf B pulsators are found is complemented by a second one above 50,000 K. While significant challenges remain at the quantitative level, we believe that the same kappa-mechanism that drives the pulsations in hot B subdwarfs is also responsible for the excitation of the rapid oscillations observed in the omega Cen variables.