PG 1657+416: a new fast pulsating sdB star

Photometric observations and theoretical model comparison

¹ Instituto de Astrofísica de Canarias, 38200 La Laguna, Spain e-mail: ror@iac.es
² Dpto. de Astrofísica, Universidad de La Laguna, 38200 Tenerife, Spain
³ Isaac Newton Group, Aptdo. correos 321, 38700 Santa Cruz de la Palma, Spain
⁴ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Oslo, Norway
⁵ Department of Physics and Astronomy, University of Delaware, DE 19716, USA
⁶ Dpto. Física Aplicada, Universidade Vigo, 36310 Vigo, Spain

Received: 1 March 2006
Accepted: 15 August 2006

Abstract

Context.Since the discovery in 1997 of stellar oscillations in some B-type hot subdwarfs (sdBs), much effort is being made both observationally and theoretically to characterize this new group of pulsating objects.

Aims.To increase the number of members of such pulsators and to perform a theoretical analysis of new pulsating stars.

Methods.The Nordic Optical Telescope was used to measure the brightness as function of time for a list of selected sdBs. Pulsator candidates were selected by deriving the effective temperatures and gravities of sdB stars from various spectroscopic surveys, and picking targets in the instability region. Pulsational properties were investigated by Fourier analysis. For the theoretical analysis, the predicted frequencies of sdB structural models in the region of interest were computed.

Results.From our list of candidates we detected one, PG 1657+416, as a new oscillating star. Follow-up photometry revealed at least four frequencies in the range 6.9-7.9 mHz, whose amplitudes vary from one data set to another. The theoretical comparison indicates that the observed periods are well reproduced by structural models whose physical parameters are in good agreement with the effective temperature and gravity estimates from a spectral energy distribution fit, which also shows that the sdB has a G5 main sequence companion with K.