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| Paper: |
Luminous cool stars in the upper HR diagram or 'stars near the edge of existence' |
| Volume: |
9, Cool Stars, Stellar Systems, and the Sun: Sixth Cambridge Workshop |
| Page: |
387 |
| Authors: |
Humphreys, Roberta M. |
| Abstract: |
The present study examines the properties of the most luminous known cool supergiants, the role they may play in defining a stability limit in the upper H-R diagram, and the recent efforts at understanding the physical origin of their instability. As the most massive stars (above 50 solar masses) evolve away from the main sequence they develop instabilities in their atmospheres accompanied by enhanced and high mass loss. Many of the intermediate-type hypergiants show evidence of shell ejections. Consideration is given to the enigmatic behaviour of Variable A in M33 and the new variable recently reported in M31 by Rich et al. (1989). The following are proposed as physical causes of instability: radiation pressure, i.e., the modified Eddington limit, which dominates the hot stars, and the turbulent pressure gradient in the atmospheres of the cool hypergiants which sets an upper boundary to their luminosities independent of the effects of radiation pressure in the hot stars. |
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