Amplitude variation and multiplet structures: Is PG1605+072 a slow rotator?

Abstract

The subdwarf B star PG1605+072, with an unusually low log g ~ 5.3, shows a high number of non-radial pulsation modes, making it a promising candidate for asteroseismology. This could allow probing the star's interior to gain important insights in its structure and evolution. Comparison of previous work conducted over the last decade shows clear amplitude variation and hints of frequency variation. We analyse white light photometric data of the Multi-Site Spectroscopic Telescope (MSST) and Whole Earth Telescope (WET) XCov22 campaigns using prewhitening techniques and O-C diagrams. A total of 85 significant frequencies are identified, among them more than 20 frequency sums and harmonics. Moreover, it is shown that the main mode's amplitude varies like a sine with a period of ~630 d, indicative of long-term beating. Strong hints for the existence of frequency multiplets support the hypothesis that PG1605+072 is a slow rotator with V_eq < 0.9 km/s, contrary to previous claims. Existing asteroseismic models mainly suggest that the star possesses a high mass of ~0.7 M_Sun, presuming the main pulsation mode to be radial. This calls for an alternate formation channel.