OPTICAL AND ULTRAVIOLET ANALYSES OF ZZ CETI STARS AND STUDY OF THE ATMOSPHERIC CONVECTIVE EFFICIENCY IN DA WHITE-DWARFS

New high signal-to-noise optical spectrophotometry is presented for 22 ZZ Ceti stars. The atmospheric parameters (T-eff, log g) and the mass are derived for each object using new model atmospheres and synthetic spectra calculated within the mixing-length theory, as well as recent published mass-radius relationships. Various parameterizations of the convective efficiency are explored. The mass distribution obtained from the optical solutions indicate that the so-called ML2 parameterization of the mixing-length theory yields a mean mass of 0.58 M., in excellent agreement with that of hotter DA stars (0.59 M.) whose atmospheres are completely radiative. ML1 and ML3 models, on the other hand, yield mean masses which are, respectively, too high (0.70 M.) and too low (0.51 M.). With ML2 models, ZZ Ceti stars are found within a narrow instability strip located in the range 13,650 greater than or equal to T-eff greater than or equal to 11,960 K. A similar analysis of IUE and HST spectroscopic observations is presented as well. It is first shown that a unique solution for T-eff and log g cannot be achieved on the basis of ultraviolet spectroscopy alone, and that one of these parameters needs to be constrained independently. When log g values from the optical analysis are adopted, the analysis of the ultraviolet data requires a parameterization less efficient than ML2. Models calculated with ML2/alpha = 0.6 are shown to provide an excellent internal consistency between ultraviolet and optical temperatures. The corresponding instability strip becomes cooler and narrower (12,460 greater than or equal to T-eff greater than or equal to 11,160 K) than that inferred from ML2 models. Furthermore, the atmospheric parameters obtained with these models are consistent with the observed photometry, the trigonometric parallax measurements, and the gravitational redshift masses. However, the mean mass of the sample increases to a value similar to 0.06 M. larger than that of hotter DA stars. An explanation for this discrepancy is offered in the light of recent nonadiabatic calculations. The overall consistency of our analysis for DA stars outside the ZZ Ceti instability strip is discussed as well.