1.2 What is a Be star?

Be stars were discovered by Father Angelo Secchi (1867) who observed emission lines (leading to their designation “e”) in Cassiopeia - the archetypal Be star. Classical Be stars are defined to be non-supergiant B-type stars whose spectra are, or have at some time, shown one or more Balmer lines in emission (Collins, 1987). They occupy a region on or near the main sequence of stars in the Hertzsprung-Russell diagram implying that they are still burning hydrogen at their core, prior to the hydrogen shell burning stage and the transition to the red giant phase of stellar evolution. These stars are rapid rotators and are variable in both brightness and spectra. Their spectra usually show broad HeI absorption, and emission at either visual and or ultra-violet (UV) wavelengths. Projected stellar rotational velocities, v_rot sin(i), (where i is the angle of inclination of the star’s polar axis, with respect to our line of sight) can be determined from the broadened profiles of such stellar lines (Schmidt-Kaler, 1982; Slettebak, 1982; Slettebak et al., 1975).

The Be phenomenon is also present in some O and A stars which are known as Oe and Ae stars. Oe stars are the hotter extension of Be stars; they are typically main sequence stars, are rapid rotators (broad lines), and show H emission that is often double-peaked, and their mass loss is believed to produce a disc and/or shell of material around the star. Herbig Ae stars are pre-main sequence stars whose emission lines are generated from the remnants of the proto-stellar environments and not from circumstellar matter generated after the loss of the proto-stellar disc as is the case with Be stars.

Be star photospheric velocities are limited to the critical values given by the Roche model of the outer layers. It can be calculated using:

(1.1)

where G is the universal gravitational constant, M_* is the stellar mass and R_* is the polar stellar radius, and ranges from v_crit ~ 390kms-1 for a B9 star to v_crit ~ 540kms-1 for a B0 star. The factor of 2/3 comes from the fact that, at critical rotation, the equatorial radius is 3/2 times bigger than the polar one (Porter, 1996). Classical Be stars are the fastest rotating bodies, in relation to their angular break up velocities observed in the Universe. To quantify this the following calculation compares the rotational velocities (v) of a Be star and a milli-second pulsar with their associated critical break-up velocities (v_crit), the velocity at which the centrifugal force balances gravitational force. Milli-second pulsars have the highest physical rotations rates observed in the Universe. From Equation (1.1):

	Milli-second Pulsar		Be star
	vcrit =		vcrit =
	= 1.1 × 1010 cms-1		= 4.18 × 107 cms-1

The rotational velocity is v = ,

v = 6.2 × 109 cms-1

v = 0.7 × 4.18 × 107 cms-1

The fractional relationship between the critical and rotational velocities allows direct comparison of the milli-second pulsar and the Be star.

= 0.56

= 0.7

(1.2)

where Be data are extracted from Schmidt-Kaler (1982) and milli-second pulsar data are extracted from Shu (1982). Be stars are observed to be rotating close to their break up speeds (Slettebak, 1982, see also Equation (1.2)) with a modal value of ~ 0.7v_crit (Porter, 1996). As such they are essential to our knowledge of stellar astrophysics, particularly the internal dynamics of stars.

If Be stars’ rapid rotation is their sole cause of ejection of circumstellar material they would have to be rotating at their critical velocities. Since this appears not to be the case it is clear that whilst rotation plays a part in disc creation it is not the sole cause.