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

TL;DR

This spectroscopic study of Be-shell stars 4 Her and 88 Her reveals their rapid rotation, small Hα emission regions, and V/R variations likely influenced by binary interactions, providing insights into their circumstellar disks.

Contribution

First detailed optical spectroscopic analysis of 4 Her and 88 Her focusing on rotational velocities, emission region sizes, and V/R variations in relation to binary effects.

Findings

Stars are fast rotators with ~0.80 fractional critical rotation.

Hα emission regions are about 5 times the stellar radius.

Detected V/R variations and periodicities suggest binary influence on disks.

Abstract

We present the optical spectroscopic study based on 41 spectra of 4 Her and 32 spectra of 88 Her, obtained over a period of 6 months. We estimate the rotational velocity of these stars from He{\sc i} lines in the blue spectral region (4000--4500 \AA). We find that these stars are likely to be rotating at a fractional critical rotation of $\sim$ 0.80. We measure the average $I_p/I_c$ ratio to quantify the strength of the H$\alpha$ line and obtain 1.63 for 4 Her and 2.06 for 88 Her. The radius of the H$\alpha$ emission region is estimated to be $R_d/R_*$ $\sim$ 5.0, assuming a Keplerian disk. These stars are thus found to be fast rotators with relatively small H$\alpha$ emission region. We detect V/R variation of H$\alpha$ spectral line during the observed period. We re-estimate the periods for both the stars and obtain two periods of $\sim$ 46 days and its harmonic of 23.095 days for 4 Her and $\sim$ 86 days for 88 Her. As these two are shell stars with binaries and have low H$\alpha$ 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.