H$\alpha$ long term monitoring of the Be star $\beta$ Cep Aa

TL;DR

This study long-term monitored the Hα emission of the Be star component in the β Cep binary, revealing its variable emission profile and estimating its rotational velocity and disk size through spectral analysis.

Contribution

It developed a method to disentangle the secondary's Hα profile in a binary system, providing new insights into the Be star's behavior and properties.

Findings

Secondary exhibits typical Be star Hα emission with strong variability.

Estimated the secondary's projected rotational velocity and disk size.

Reconstructed the secondary's Hα profile over a long time period.

Abstract

Recent papers published in the last years contributed to resolve the enigma on the hypothetical Be nature of the hot pulsating star $\beta$ Cep. This star shows variable emission in the H$\alpha$ line, typical for Be stars, but its projected rotational velocity is very much lower than the critical limit, contrary to what is expected for a typical Be star. The emission has been attributed to the secondary component of the $\beta$ Cep spectroscopic binary system. In this paper, using both ours and archived spectra, we attempted to recover the H$\alpha$ profile of the secondary component and to analyze its behavior with time for a long period. To accomplish this task, we first derived the atmospheric parameters of the primary: T$_{\rm eff}$ = 24000 $\pm$ 250 K and $\log g$ = 3.91 $\pm$ 0.10, then we used these values to compute its synthetic H$\alpha$ profile and finally we reconstructed the secondary's profile disentangling the observed one. The secondary's H$\alpha$ profile shows the typical two peaks emission of a Be star with a strong variability. We analyzed also the behavior versus time of some line width parameters: equivalent width, V/R, FWHM, peaks separation and radial velocity of the central depression. Projected rotational velocity ($v \sin i$) of the secondary and the dimension of the equatorial surrounding disk have been estimated, too.