Just how hot are the ω Centauri extreme horizontal branch pulsators?^⋆

¹ Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: marilyn.latour@fau.de
² ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Département de Physique, Université de Montréal, Succ. Centre-Ville, CP 6128 Montréal, QC H3C 3J7, Canada
⁵ National Optical Astronomy Observatory – AURA, 950 North Cherry Avenue, Tucson, AZ 85719, USA
⁶ Adnet Systems, NASA/GSFC Code 553, Greenbelt, MD 20771, USA

Received: 25 November 2016
Accepted: 20 February 2017

Abstract

Context. Past studies based on optical spectroscopy suggest that the five ω 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−T_eff diagram, where no pulsation modes are predicted to be excited.

Aims. Our goal is to determine whether this temperature discrepancy is real, or whether the stars’ effective temperatures were simply underestimated.

Methods. We present a spectral analysis of two rapidly pulsating extreme horizontal branch (EHB) stars found in ω Cen. We obtained Hubble Space Telescope/COS UV spectra of two ω 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.

Results. Using the relative strength of the N iv and N v lines as a temperature indicator yields T_eff 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.

Conclusions. 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.