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

TL;DR

This study analyzes the pulsation modes of the star PG1605+072, revealing amplitude and frequency variations, and provides evidence that it is a slow rotator, challenging previous assumptions about its rotation speed.

Contribution

The paper presents detailed frequency analysis of PG1605+072, showing amplitude modulation and multiplet structures, suggesting it is a slow rotator, which contradicts earlier claims.

Findings

Identified 85 significant pulsation frequencies including sums and harmonics.

Amplitude of the main mode varies with a ~630-day period, indicating long-term beating.

Evidence for frequency multiplets supports slow rotation hypothesis.

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.