Bisectors of the cross-correlation function applied to stellar spectra

Discriminating stellar activity, oscillations and planets

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile e-mail: tdall@eso.org
² Centro de Astronomia e Astrofísica da Universidade de Lisboa, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
³ Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁴ Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark
⁵ School of Physics, University of Sydney, Sydney, NSW 2006, Australia

Received: 14 February 2006
Accepted: 10 April 2006

Abstract

Context.Bisectors of strong, single spectral lines, usually the  6252 line, have traditionally been used to examine the velocity fields in stellar atmospheres. This requires high S/N often achieved by summing many individual spectra.

Aims.We investigate whether bisectors derived from cross-correlation functions (CCF) of single-exposure spectra can be used to provide information on stellar atmospheres, and whether they can be used to discriminate between radial velocity changes caused by planets, magnetic activity and oscillations.

Methods.Using a sample of bright stars observed with the HARPS spectrograph, we examine the shapes of the bisectors of individual strong spectral lines in summed spectra, comparing with similar studies in the literature. Moreover, we examine four different quantitative CCF bisector measures for correlations with radial velocity and stellar parameters.

Results.We show that CCF bisector measures can be used for quantitative analysis, employing both the absolute values and the variations. From absolute values, and absolute magnitude can be approximated, and from the correlations with radial velocity one can distinguish between magnetic activity, oscillations and orbiting planets as the probable cause of radial velocity variations. We confirm that different isolated spectral lines show different bisector shapes, even between lines of the same element, calling for caution in trying to derive global stellar properties from the bisector of a CCF. For the active star HR 1362 we suggest from the bisector shape an extra photospheric heating caused by the chromosphere of several hundred degrees. We confirm the fill-in of spectral lines of the Sun taken on the daylight sky caused by Rayleigh-Brillouin and aerosol scattering, and we show for the first time that the fill-in has an asymmetric component.