I continue to compare Be type stars that are designated ‘shell stars.’ A shell stars’ circumstellar decretion disk is seen edge-on so that line of sight looks through disk material that variously obscures our view of the photosphere. Spectra of ‘shell stars’ are typically a combination of H-balmer lines which occur in both the photosphere and the decretion disk, either in absorption or emission. Plus, He I absorption lines, which are primarily photospheric in nature, and, metal lines, Fe II, Ca II and Mg II, referred to as ‘shell lines’ that develop in the cooler decretion disk.
I’ve put together a web page displaying spectra of ‘shell stars’ I have recorded in recent months. The two figures at the top of the page are a comparison of 6 stars that show presence of shell lines, Fe II, in roughly descending intensity. 59 Cyg’s H-alpha and H-beta lines are in strong emission, and likely all of its H-Balmer lines would show some degree of emission with a higher resolution spectrograph. Fe II lines are also in emission in 59 Cyg’s spectrum. The next 4 stars also show H-alpha in emission, with shell absorption line intensities tapering off. The last star, 4 Her, is a well studied ‘shell star’ that presently shows little evidence of a decretion disk, lacking both shell lines and Balmer line emission.
What is fascinating is that all six of these stars have been observed to go through one or more periods of intense change over the last 100 years. Presenting, at some point, spectra of ordinary B type stars (like present day 4 Her), Be type spectra with Balmer emission lines (88 Her), and Be spectra containing weak to strong ‘shell lines.’
All of these stars are fast rotators – 200-450 km/s. Depending upon stellar diameter, many of these stars are rotating at 70% to 80% of critical rotation…..bordering on breaking up, though, no evidence has yet been presented that a star has actually spun itself out of existence.
Five of the six stars are 2-4x the size of the sun with 59 Cyg a whopping 7x solar diameter. All six have suspected or confirmed faint binary companions. 59 Cyg’s is a sub-dwarf O type star, the remaining companions are not classified in the papers I have been reading.
Farther on down the web page you can see individual spectra of these and half a dozen addition stars. The web page is still a work in progress, lacking annotations and commentary which I am presently working on. Be type stars are a fascinating study, even at very low spectral resolution. Spectroscopy rocks!