deaths are also prime contributors to the chemical enrichment of galaxies as a function of age.
The most-massive stars are also notoriously unstable objects. Their high temperatures combined with significant outflows are a natural source of strong emission lines, making them among the most prominent sources of Galactic emission-lines. We typically classify these stars by their emission-line properties in the optical and IR, but this is a dangerous undertaking given the fact that the overwhelming majority of their energy output is in the unobservable UV range - as illustrated in a quasi-famous illustration by Peter Conti (Figure 7.23). Furthermore, in an excellent review of the IR spectra of massive stars,
Morris et al. (1996) show that these stars can even change their classification from one class to another and back again on timescales of just a few years!
In the subsections below, we consider in turn the emission-line properties of Be and B[e] stars, Of stars, luminous blue variables, and Wolf-Rayet stars.
Be stars are the most numerous class of massive stars exhibiting significant line emission. As their name implies, they are basically "normal" B stars with a strong emission line - typically hydrogen lines. Ha is the primary means for identifying these stars, though BrY is the only such diagnostic for deeply embedded stars or those located at large distances in the disk of the Milky Way.
The obvious question one would next ask is "What makes some B stars have emission lines?" The most-prominent theory for the last 50 years in Be stars is that, like some young stellar objects (YSOs), Be stars have a circumstellar disk of material. The relatively hard radiation field from the B-star photosphere impinges on the cooler disk material and thus radiatively pumps the observed line emission - primarily from neutral hydrogen. The disk-like geometry produces a classical double-peaked emission-line profile like the one shown in Figure 7.24.
However, unlike YSOs, Be-star disks are very likely "excretion disks" as opposed to accretion disks - that is, they result from outflows rather than inflows. It is now fairly well established that for Be stars there is a clear correlation between stellar surface rotation (v sin i) and Ha FWHM (e.g. Hanuschik 1988). This is taken to indicate that the disks are rotationally supported, probably by coupling of the stellar photosphere (possibly magnetically?) to the disk itself. This basic concept is illustrated in Figure 7.25.
Tycner et al. (2006) have recently obtained interferometry in the Ha line for two very-nearby Be stars, in which they resolve the emission-line region. They find that the disk, rather than having a standard "thin-disk" profile (roughly equivalent to a square step function in latitude centered on the stellar equator as shown in Figure 7.25), actually has a broad Gaussian-like profile - more closely resembling an equatorial enhancement to a quasi-spherical outflow than a classical disk.
B[e] stars, are both similar to and different from Be stars, as their name hints. They are fundamentally B stars with emission lines. However, for B[e] stars many "metal" lines are also present, together with, as the brackets indicate, forbidden lines. Be stars, on the other hand, are dominated by hydrogen Balmer transitions and little or no metal forbidden-line transitions. Figure 7.26 shows a spectrum of the B[e] star CI Cam. Note the large number of metal transitions - Jaschek and Andillat (2000) report 450 identified line transitions in this single star spectrum! Optical emission lines of the B[e] star MWC 349 have "flat-topped" profiles typical of an optically thick, spherically symmetric geometry. Furthermore, for many B[e] stars there are IR excesses consistent with warm dust in their circumstellar disks. This combination of features, as well as the typical lack of clearly identifiable normal "photospheric" absorption lines, seems to indicate that the B[e] stars are highly evolved stars, each of which is deeply enshrouded in its own circumstellar envelope.
Luminous blue variables (LBVs) are the most-luminous known individual stars. With luminosities several million times that of the Sun and apparent masses approaching (or exceeding) the theoretical limits of star formation, they are not surprisingly highly unstable as well. The most-famous LBVs include n Car and P Cygni. P Cygni is well known as the prototypical star for the line profile that bears its name. A typical "P
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