The multi-site spectroscopic telescope campaign - II. effective temperature and gravity variations in the multi-periodic pulsating subdwarf B star PG 1605+072

Abstract

Context. Stellar oscillations are an important tool to probe the interior of a star. Subdwarf B stars are core helium burning objects, but their formation is poorly understood as neither single star nor binary evolution can fully explain their observed properties. Since 1997, an increasing number of sdB stars has been found to pulsate forming two classes of stars, the V361 Hya and V1093 Her stars. Aims. We focus on the bright V 361 Hya star PG 1605+ 072 to characterise its frequency spectrum. While most previous studies relied on light variations, we have measured radial velocity variations for as many as 20 modes. In this paper, we aim at characterising the modes from atmospheric parameter and radial velocity variations. Methods. Time resolved spectroscopy (approximate to 9000 spectra) has been carried out to detect line profile variations from which variations of the effective temperature and gravity are extracted by means of a quantitative spectral analysis. Results. We measured variations of effective temperatures and gravities for eight modes with semi- amplitudes ranging from Delta T-eff = 880 K to as small as 88 K, and. log g of 0.08 dex to as low as 0.008 dex. Gravity and temperature vary almost in phase, whereas phase lags are found between temperature and radial velocity. Conclusions. This profound analysis of a unique data set serves as a sound basis for the next step towards an identification of pulsation modes. As rotation may play an important role, the modelling of pulsation modes is challenging but feasible.