H beta in Be stars

Forums Spectroscopy strange Hb in Be (48 Per)? H beta in Be stars

Robin Leadbeater

Hi Kevin,

I don’t know 48 Per specifically but I think what you see is typical in some Be stars. The Be star spectrum is a combination of the normal main sequence B star (dominated by broad winged Balmer lines with He also)  plus a Balmer line emission component from the disc, H alpha being the strongest but also sometimes in H beta if the emission is strong. If the ssytem is seen pole on as would seem to be the case here, then there is no doppler component from the rotation of the disc so the emission is a single narrow line which is narrower than the photospheric absorption line (because of the lower gas density in the disc). When the two are superimposed, the H alpha emission can dominate, hiding the whole absorption line including the wings, but where the emission is less as here in H beta we still see the photospheric absorption line with a small emission in the central core. (The same effect is also sometimes seen at H alpha in cases where the emission is particularly weak.)