# Just how hot are the $\omega$ Centauri extreme horizontal branch pulsators?

## TL;DR

This study uses UV spectra to re-evaluate the effective temperatures of pulsating stars in $ ext{omega}$ Cen, finding they are hotter than previously thought, which aligns them with the theoretical instability strip.

## Contribution

The paper demonstrates that UV spectral analysis reveals higher temperatures for $ ext{omega}$ Cen pulsators, resolving previous discrepancies with theoretical models.

## Key findings

- UV spectra indicate temperatures around 60,000 K, higher than prior estimates.
- Pulsation periods confirmed through FUV lightcurves.
- Stars now fall within the predicted instability strip.

## Abstract

Past studies based on optical spectroscopy suggest that the five $\omega$ Cen pulsators form a rather homogeneous group of hydrogen-rich subdwarf O stars with effective temperatures of around 50 000 K. This places the stars below the red edge of the theoretical instability strip in the log $g$ $-$ Teff diagram, where no pulsation modes are predicted to be excited. Our goal is to determine whether this temperature discrepancy is real, or whether the stars' effective temperatures were simply underestimated. We present a spectral analysis of two rapidly pulsating extreme horizontal branch (EHB) stars found in $\omega$ Cen. We obtained Hubble Space Telescope/COS UV spectra of two $\omega$ Cen pulsators, V1 and V5, and used the ionisation equilibrium of UV metallic lines to better constrain their effective temperatures. As a by-product we also obtained FUV lightcurves of the two pulsators. Using the relative strength of the N IV and N V lines as a temperature indicator yields Teff values close to 60 000 K, significantly hotter than the temperatures previously derived. From the FUV light curves we were able to confirm the main pulsation periods known from optical data. With the UV spectra indicating higher effective temperatures than previously assumed, the sdO stars would now be found within the predicted instability strip. Such higher temperatures also provide consistent spectroscopic masses for both the cool and hot EHB stars of our previously studied sample.