Spectroscopic analysis of the candidate β Cephei star σ Cas: Atmospheric characterization and line-profile variability

Abstract

This study is an exploratory search to ascertain for the first time the detailed properties of σ Cas as a candidate pulsator of β Cephei type in view of future observing campaigns. This search is devoted to a precise characterization of the parameters of the star for verifying its location with respect to the β Cephei instability region and to a preliminary analysis of a possible spectral line-profile variability.

The characterization of the parameters of the star was based both on spectroscopic observations carried out by us on October 2007 and literature data used to build its spectral energy distribution. The suspected pulsational variability of the star have been studied by analyzing the line-profile variability of the two spectral lines HeI λ 5875.6 Å and SiIIIλ4552.6 Å. The search for periodicity was performed by adopting the pixel-by-pixel method.

From the analysis of the composite spectrum of σ Cas we derived the effective temperature and surface gravity. From literature values of distance, visual magnitude, bolometric correction and extinction we determined the luminosity and radius. By means of evolutionary tracks and isochrones we derived also mass and age. The values obtained pose σ Cas with a good certainty inside the β Cephei instability region. The line-profile variability analysis indicate a clear excess of oscillation power in the range 2-8 cycles d^-1 , with two prominent peaks at 3.00 and 5.32cyclesd^-1 , in the expected typical range of frequency of β Cephei pulsations. In this framework, it emerged that, besides the use of the canonical SiIIIλ4552.6 Å line for β Cephei pulsation studies, the HeIλ5875.6 Å line might be an additional suitable candidate for period and mode analysis in β Cephei stars.