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

摘要

This study is an exploratory search to ascertain for the first time the detailed properties of sigma Cas as a candidate pulsator of beta 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 beta 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 lambda 5875.6 angstrom and SiIII lambda 4552.6 angstrom. The search for periodicity was performed by adopting the pixel-by-pixel method. From the analysis of the composite spectrum of sigma 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 sigma Cas with a good certainty inside the beta 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 5.32 cycles d(-1), in the expected typical range of frequency of beta Cephei pulsations. in this framework, it emerged that, besides the use of the canonical SiIII lambda 4552.6 angstrom line for beta Cephei pulsation studies, the HeI lambda 5875.6 angstrom line might be an additional suitable candidate for period and mode analysis in beta Cephei stars.