The Be stars's garden

by Coralie NEINER

B-type stars :

B-type stars are hot and blue stars, with a high proportion of radiation in the ultraviolet domain. Their temperature varies from about 10000 K at the subtype B9 to nearly 30000 K at B0. Their mass ranges from about 3 to 20 M_⊕ and their luminosity from 100 to 50000 L_⊕.

Moreover, B-type stars are often found together with O stars in OB associations since, being massive, they are short-lived and therefore do not survive long enough to move far from the place where they were formed. Spica and Regulus are two well known examples of B-type stars.

The spectrum of B stars is characterized by the presence of hydrogen and HeI lines in the optical. The strength of hydrogen increases with the subtypes (from B0 to B9) while a maximum strength of the HeI lines is reached around the subtype B2. HeII lines only appear in the B0 stars. Other spectral features may include CaII, CII, CIII, NII, NIII, OII, SiII, SiIII, SiIV, and MgII.

Variability is found in several classes of B stars, for example in beta Cephei, Slowly Pulsating B (SPB) and Be stars.

Be stars :

Be stars are non-supergiant B stars that at least once have displayed Balmer line emission. In the stellar classification, the 'e' in 'Be' stars thus stands for 'emission'. This property applies to about 20% of all B-type stars in our galaxy, but is especially observed at the B1-B2 subtypes.

Some late O and early A stars also show such emission and are considered as an extension of the Be stars. The early (i.e. more massive and hotter) Be stars exhibit strong variable winds evidenced by the rapidly variable UV resonance lines of highly ionised species, as well as by spectral and light variations in the optical on timescales from hours to decades.

The visual inspection through a spectroscope of the first discovered Be star gamma Cas was described in 1867 by Secchi. Other bright examples of Be stars are zeta Tau or delta Sco.

The optical spectrum of a Be star :

The Balmer lines (Halpha, Hbeta, etc) are the ones most commonly seen in emission in Be stars. The level of emission decreases along the Balmer series, the H$\alpha$ line having the strongest emission. Moreover, lines from other ions can also be in emission, in particular the HeI lines at e.g. 4921, 5876 and 6678 A, SiII at 6347 A, MgII at 4481 A, CII at 6578 and 6583 A and the numerous FeII lines. The emission level is highly variable from one Be star to the other but also for a given Be star: it can even completely disappear and reappear years later.

The presence of a circumstellar disk, seen under different angles depending on the star, gives rise to different kinds of emission line profiles. When the star is seen pole-on, i.e. from the top, the equatorial disk is seen face-on and the line has a single emission peak. When the star and the disk are seen edge-on, i.e. from the equator, the line has a double-peak emission with a central absorption. The two emission peaks are then called the Violet (V) and Red (R) peaks. Any line of sight with an angle between 0 and 90 deg will give a line profile with a mixture of emission and absorption.

The presence of a circumstellar disk not only affects the lines but also the continuum. If B stars show one Balmer discontinuity at 3700 A, Be stars, however, show two Balmer discontinuities: the one at 3700 A and a second one at 3647 A. This second discontinuity can be seen in emission when the star is in a phase of strong emission or in absorption in the case of Be-shell stars. This additionna scontinuity is due to the presence of the circumstellar enveloppe.