Spectroscopic study of Be-shell stars : 4 Her and 88 Her

Abstract

We present an optical spectroscopic study based on 41 spectr a of 4 Her and 32 spectra of 88 Her, obtained over a period of 6 months. We estimate the rotationa l velocity of these stars from He I lines in the blue spectral region (4000–4500 ̊ A). We find that these stars are likely to be rotating at a fract ional critical rotation of ∼ 0.80. We measure the average I p /I c ratio to quantify the strength of the H α line and obtain 1.63 for 4 Her and 2.06 for 88 Her. The radius of the H α emission region is estimated to be R d /R ∗ ∼ 5.0, assuming a Keplerian disk. These stars are thus found to be fa st rotators with a relatively small H α emission region. We detect V/R variation of the H α spectral line during the observed period. We re-estimate th e periods for both stars and obtain a period of ∼ 46 d and its harmonic of 23.095 d for 4 Her, and a period of ∼ 86 d for 88 Her. As these two cases are shell stars with binarie s and have low H α EW with the emission region closer to the central star, the V/R variation and a change in period may be an effect of the binary on the circumstellar disk.