石田 俊人

A recently found double-mode Cepheid with the shortest known period in our Galaxy and abnormal period ratio, V371 Per, is investigated by linear nonadiabatic pulsation models. V371 Per is likely to be crossing the instability strip for the first time, because the mass derived from pulsation models is larger than the evolution mass for the second or higher crossing objects. This result seems to support the conclusion obtained from spectroscopic observation. We also found that models with observed period and period ratio of V371 Per need to have mass and T-eff in a narrow range which shifts as heavy element abundance Z changes. We have checked the agreement between T-eff ranges estimated observationally and derived from pulsation models using observational Z. We found that those ranges overlap marginally. We need more spectroscopic estimations of T-eff and [Fe/H], and more photometric monitoring to estimate the evolutional period change for confirmation of our result.

Procedures for calculating linear nonadiabatic pulsations are rediscussed. Pulsation properties of yellow bright stars are calculated by using Zalewski's procedures. The usefulness of this procedure is indicated. The fundamental periods obtained in the present work are very close to previously published results.Stellar pulsationYellow bright starsNonadiabatic oscillation

We have derived the modal coupling coefficients of a model cepheid from the result of hydrodynamic calculations. It is found that the character of the model is marginal to change from a single-periodic behavior to a double-periodic one.Stellar pulsationDouble-mode cepheidsHydrodynamic model

We have constructed a helium-enriched model cepheid to compare the amplitude-amplitude relationship of radial modes. The helium-enriched model shows still single-period characteristic similar to the normal model.Stellar PulsationDouble-mode CepheidsHydrodynamic model