On the use of the Fourier Transform to determine the projected rotational
velocity of line-profile variable B stars

C. Aerts, S. Simon-Diaz, P. G. Groot, P. Degroote

Institute of Astronomy, KULeuven, Belgium

Dept. of Astrophysics, IMAPP, Radboud University Nijmegen, the Netherlands

Instituto de Astrofisica de Canarias, 38200, La Laguna, Tenerife, Spain

Departamento de Astrofisica, Universidad de La Laguna, 38205, La Laguna,
Tenerife, Spain

The Fourier Transform method is a popular tool to derive the rotational velocities of stars from their spectral line profiles. However, its domain of validity does not include line-profile variables with
time-dependent profiles. We investigate the performance of the method for
such cases, by interpreting the line-profile variations of spotted B stars, and
of pulsating B stars, as if their spectral lines were caused by uniform surface
rotation along with macroturbulence. We perform time-series analysis and
harmonic least-squares fitting of various line diagnostics and of the outcome
of several implementations of the Fourier Transform method. We find that
the projected rotational velocities derived from the Fourier Transform vary
appreciably during the pulsation cycle whenever the pulsational and rotational
velocity fields are of similar magnitude. The macroturbulent velocities
derived while ignoring the pulsations can vary with tens of km/s
during the pulsation cycle. The temporal behaviour of the deduced rotational
and macroturbulent velocities are in antiphase with each other. The
rotational velocity is in phase with the second moment of the line profiles.
The application of the Fourier method to stars with considerable pulsational
line broadening may lead to an appreciable spread in the values of the
rotation velocity, and, by implication, of the deduced value of the
macroturbulence. These two quantities should therefore not be derived from
single snapshot spectra if the aim is to use them as a solid diagnostic for
the evaluation of stellar evolution models of slow to moderate rotators.

Reference: Accepted for publication in A&A
Status: Manuscript has been accepted

Weblink: http://adsabs.harvard.edu/abs/2014arXiv1407.6611A


Email: conny.aerts@ster.kuleuven.be