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

Abstract

Context. 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. Aims. 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 new indices characterizing the properties of CEs in Be stars and properties relating Be and Bn stars. Methods. 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 Hα line. Results. Correlations of the aspect and intensity of the sBD and the emission in the Hα line with the stellar parameters and the Vsin i are presented. Some Bn stars exhibit the sBD in absorption, which may indicate the presence of rather dense CEs. Six Bn stars show emission in the Hα line, so they are reclassified as Be stars. The sBD in emission appears in Be stars with Vsin i 250 km s −1 , and in absorption in both Be and Bn stars with Vsin i 50 km s −1 . Low-mass Be and Bn stars share the same region in the Hertzsprung-Russell diagram. The distributions of rotational to critical velocity ratios of Be and Bn stars corresponding to the current stellar evolutionary stage are similar, while distributions inferred for the zero-age main sequence have different skewness. Conclusions. We found emission in the Hα line and signs of a CE in some Bn stars, which motivated us to think that Bn and Be stars probably belong to the same population. It should be noted that some of the most massive Bn stars could display the Be phenomenon at any time. The similarities found among Be and Bn stars deserve to be more deeply pursued.

Hubble spectroscopy of LB-1: comparison with B+black-hole and Be+stripped-star models

“…While the Be nature of the broad lined star has been attributed to the presence of a disk, as implied by the characteristic Balmer emission lines and IR excess, the SED displays no evidence for a second BD or emission in the Mg ii 2800 Å doublet, as is often observed in classical Be stars (Cochetti et al 2020;Slettebak 1994). Liu et al (2020) have discussed the Balmer and Paschen emission line spectrum at length, and detect emission wings to a velocity of ±250 km s −1 , that for a Keplerian disk measure the projected velocity of the inner edge of the disk.…”

MEGARA-GTC stellar spectral library – II. MEGASTAR first release

“…The comparison of the Hα line profiles in our HR-R spectrum to those reported by Shultz & Wade (2017) (for the 2015 period) and Martins et al (2010) (for the period 2007-2009) suggests that HD000108 might be moving towards a high emission state, since it is much stronger than in both observations. Finally, HD058343 is a runaway Be star (Tetzlaff, Neuhäuser, & Hohle 2011;Hoogerwerf, de Bruijne, & de Zeeuw 2001) for which the SIMBAD database provides a spectral classification of B2 Vne, although in the literature we can find spectral types that vary between B2 and B4 (e.g., INDO-US library, Cochetti et al 2020;Ahmed & Sigut 2017;Silaj et al 2010). Stellar parameters and Hα line profile variability has been investigated by Arcos et al (2018) and Ahmed & Sigut (2017) within the BeSOS and MiMeS surveys, respectively.…”

Ultraviolet Spectropolarimetry with Polstar: on the origin of rapidly rotating B stars

“…rapid rotation and span a wide range of spectral sub-types usually B7 to A2), but have never been observed to have a disk. The 'n' in 'Bn' indicates nebulous absorption lines (usually metals), which are widened due to high v sin i (Cochetti et al 2020). Despite the name, the origin of these lines is now understood as being purely photospheric.…”