| |
 |
| Paper: |
Polaris: What Does a Little Pulsation Do to an Atmosphere? |
| Volume: |
154, Cool Stars, Stellar Systems and the Sun: Tenth Cambridge Workshop |
| Page: |
745 |
| Authors: |
Evans, Nancy Remage; Sasselov, Dimitar; Short, C. Ian |
| Abstract: |
Polaris is exhibiting unique behavior for a classical Cepheid in that its pulsation amplitude has decreased over the past 50 years. We can use this change in amplitude and also the comparison between Polaris (low amplitude pulsation) and both full amplitude Cepheids and nonvariable supergiants to see how pulsation affects atmospheric structure and heating. IUE high resolution spectra show no change in the Mg 2 lines between 1978 and 1993 when the amplitude dropped from 2.8 to 1.6 km s^{-1}. We have modeled the Polaris emergent spectrum with self-consistent hydrodynamic pulsation at both epochs, and found the same result. The energy distribution from 1700 AA through V, B, R(KC) and I(KC) is similar to a nonvariable supergiant rather than delta Cep, a full amplitude Cepheid, in that it has nonradiative flux at 1800 AA. The recent Hipparcos parallax for Polaris shows that it is pulsating in the first overtone. Because Polaris shares with other overtone pulsators (so-called s-Cepheids) a period change which is larger than would be expected for its luminosity we propose the following interpretation of period changes. For overtone pulsators the observed period changes reflect the complexities of the envelope acoustic cavity, which overtone frequencies are particularly sensitive to. Mode interaction is strong; acoustic properties change fast. Hence the period variations are no longer a smooth linear function of evolutionary change in temperature and radius. |
|
|
|
|
|
