Is the model of one-armed oscillations able to explain the long-term V/R changes of Be stars?

¹ Astronomical Institute of the Charles University, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
² Mathematical Institute of the University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany e-mail: roman.firt@uni-bayreuth.de
³ Astronomical Institute of the Academy of Sciences, 251 65 Ondřejov, Czech Republic

Received: 19 January 2005
Accepted: 28 September 2005

Abstract

Context.Many scientists studying Be stars currently adopt the model of one-armed oscillations as the correct explanation of the cyclic long-term variations observed for a number of Be stars. We test the ability of this model to be used for the predictions of V/R variations in real observed Be stars.

Aims.The behavior of the one-armed oscillations can be described as a solution of linearized hydrodynamical equations with the presence of “distorted” gravitational potential and a radiation force.

Methods.We developed a new computer program to model one-armed oscillations in Be star disks, which includes both the pressure force and the quadrupole term in the gravitational potential, related to the obliquity of a rapidly rotating star inside the disk. Moreover, we slightly improved the model in an effort to decrease the number of input parameters with the help of NLTE stellar atmosphere models.

Results.We carried out detailed tests of the dependence of  “periods” predicted by the model on all individual input parameters. We arrived at the following results: (1) the model has great potential to explain not only the cause of the cyclic long-term  changes but also some of the observed statistical properties of the phenomenon. (2) The model in its present linear form cannot be considered as proven. Its ability to predict the duration of  cycles for individual well observed Be stars is insufficient. Changing some of the input parameters of the model, which are still loosely constrained by observations and/on current understanding of the disks, like the radial density distribution in the disk, one can easily arrive at any desired cycle length from, say, 1 to 20 years.

Conclusions. Clearly, a much more sophisticated non-linear and self-consistent model of disk structure and its oscillations will be needed before a truly quantitative test of a one-armed model vs. observations will be possible.