Spectroscopy of the hot pulsating star β Cephei. Velocities and EWs from C, N, O and Si lines

Abstract

Frequencies in oscillating β Cephei stars are usually inferred by means of radial velocities measured from the Si iii triplet λλ 4552–4574 Å. These lines, relatively insensitive to the variation of T eff through a pulsation cycle, show small equivalent width variations. In this study, we aimed to verify if the behavior of radial velocities and equivalent widths measured from other ions are compatible with the one observed from Si iii lines and than to verify the possible vertical stratification along the stellar atmosphere. For this reason we selected from our spectra a number of, unblended and well isolated, C ii, N ii and O ii lines besides the famous Si iii triplet. All those lines cover the range in optical depth between −2.1 and −0.5. Unfortunately, we did not find any differences in the radial velocities behavior line-by-line and then we derived the frequency of the principal radial mode combining all the velocities derived from each spectral line separately. The inferred frequency was f 1 = 5.249677 ± 0.000007 c d −1. Another important task we would like to accomplish with this paper is to make available to the community our large sample of spectroscopic data, that is 932 velocities and equivalent widths measured from our sample of C, N, O and Si lines. All the spectra were acquired at the 1-m class telescope of the stellar station of the INAF – Osservatorio Astrofisico di Catania, in the period starting from July, the 27th 2005 to November, the 1st 2006.