Subionization and decelerated-flow in the vicinity of a B-shell star

Extract

Three types of spectra have been extensively observed among mainsequence B stars. B-normal is the classical absorption spectrum which defines a normal main-sequence B star. Be is a B-normal spectrum except : (i) some absorption lines, notably the first hydrogen Balmer series members, are replaced by emission lines; (ii) some lines from some singlyionized metals, not normally present in B stars, sometimes appear, either in emission or absorption. Be-shell is a Be spectrum with narrow and deep absorption cores in the Balmer and singly-ionized metal lines. A fourth type, B-shell, has been identified as a B-normal, absorption, spectrum except for the presence of FeII lines, and narrow, deep absorption cores in these and the hydrogen Balmer lines. Once thought to each represent a different kind of star, these spectra are now realized to simply represent different temporal phases, which one and the same star can traverse, apparently in no (as yet) fixed order. Some of the brightest stars --- eg γ Cas, 59 Cyg, Pleione --- have been observed in all of the 3 prominant phases; some stars, in only some of them; 70 % of the B stars have been observed only in the B-normal phase.