Time-resolved high-resolution spectroscopy of the bright pulsating subdwarf B star Balloon 090100001^*

¹ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain e-mail: jht@not.iac.es
² Dr. Remeis Sternwarte Bamberg, Universität Erlangen-Nürnberg, Germany
³ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Belgium
⁴ Department of Physics and Astronomy, Århus University, Denmark

Received: 6 August 2008
Accepted: 23 October 2008

Abstract

We have obtained, for the first time, a time-resolved high-resolution spectroscopic data set of the bright sdBV Balloon 090100001. In the autumn of 2006 we gathered over 1600 time-resolved high-resolution spectra with the aim to identify the pulsation modes in Balloon 090100001 by studying the line-profile variations. In this paper we present the results obtained from 842 spectra secured in August 2006, aiming to identify the dominant pulsation mode in this star. 
We phase fold the spectra onto the known dominant period of 2.80746 mHz, to study the abundances and line-broadening as a function of pulsation phase. We find that the abundances do not stand out against those of other subdwarf B stars. Consequently, there is no way to demonstrate that the richness of the frequency spectrum and/or the high pulsation amplitudes are related to abundance effects. The abundances do not change with pulsational phase. 
The metal lines in Balloon 090100001 are much broader than for non-pulsating subdwarfs, and during the cycle of the main pulsation the line broadening varies with ~2.5 km s^-1 amplitude. Hence, we find clear evidence of pulsational broadening of the lines. 
We combine the information content of 56 narrow absorption lines through a cross-correlation technique, to create cross-correlation functions that for each individual pulsation phase approximate the average line-profile shape. The resulting profiles have a sufficient signal-to-noise ratio for a mode-identification analysis. From the cross-correlation analysis we find that the pulsation amplitude of the main mode decreased from 19 km s^-1 in 2004 to 14.5 km s^-1 in 2006. 
We present, for an sdB star, the first application of pulsation-mode identification based on line-profile variations of lines of heavy elements. To fit the line profiles we consider all modes with degree and associated azimuthal order m, and we use a model of the pulsational surface-velocity field. The models with do not fit the profiles well, and consequently we can rule out quadrupole modes as the origin of the main pulsation mode in Balloon 090100001. Best fits are obtained from the model of a radial pulsation () and the model of a dipole pulsation ().