Be and Bn stars: Balmer discontinuity and stellar-class relationship

ZOREC, J.; AIDELMAN, Y; GRANADA, A.; CIDALE, L. S.; A. TORRES; COCHETTI, Y. R.; ARIAS, M. L.; FREMAT, Y

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2019

0004-6361

A significant number of Be stars show a second Balmer Discontinuity (sBD) attributed to an extended circumstellar envelope (CE). The fast rotational velocity of Be stars undoubtedly plays a significant role in the formation of the CE. However, Bn stars, which are also B-type rapidly rotating stars, do not all present clear evidence of being surrounded by circumstellar material.We aim to characterize the populations of Be and Bn stars and discuss the appearance of the sBD as a function of the stellar parameters. We expect to find: a) new indices characterizing the properties of CEs in Be stars; b) properties relating Be and Bn stars.We obtained low- and high-resolution spectra of a sample of Be and Bn stars, derived stellar parameters, characterized the sBD and measured the emission in the H$alpha$ line.Correlations of the sBD and the emission in the H$alpha$ line with the stellar parameters and the $V!sin i$ are presented. Some Bn stars exhibit the sBD in absorption, which can reveal the presence of rather dense CEs. Six Bn show emission in H$alpha$ line profile, so they are considered Be stars. The sBD in emission appears in Be stars with $V!sin ilesssim250$ km,s$^{-1}$, while in absorption in both Be and Bn stars with mbox{$V!sin igtrsim250$ km,s$^{-1}$}. Low-mass Be and Bn stars share the same region in the HR diagram. The distributions of rotational to critical velocity ratios of Be and Bn stars corresponding to their present day stellar evolutionary stage are similar, while while distributions inferred for the ZAMS have different skewness.We found emission in the H$alpha$ line and signs of a CE in some Bn stars which motivate us to think that Bn and Be stars probably belong to the same population. It should be noted that some Bn stars among the more massive ones might at any time display the Be phenomenon. The similarities found among Be and Bn stars deserve to be more deeply pursued.