Stellar and circumstellar activity of the Be star mu Centauri III. Multiline nonradial pulsation modeling

After the description and time series analysis of the variability of the ci rcumstellar and stellar lines, respectively, in Papers I and II of this ser ies, this paper sets out to model the stellar variability in terms of multi mode nonradial pulsation (nrp), but also adds another 109 echelle spectra t o the database, obtained in 1999. While the near-circumstellar emission has faded further, the six periods and the associated line profile variability (lpv) have remained unchanged. For the modeling, P-1 of the periods P-1-P- 4 close to 0.5 day, and P-5 of the two periods P-5 and P-6 near 0.28 day we re selected, because they have the largest amplitude in their respective gr oups, which are characterized by their own distinct phase-propagation patte rn. Permissable ranges of mass, radius, effective temperature, projected eq uatorial rotation velocity, and inclination angle were derived from calibra tions and observations available in the literature. A total of 648 differen t combinations of these parameters were used to compute a number of trial s eries of line profiles for comparison with the observations. Next to reprod ucing the observed variability, the primary constraint on all models was th at the two finally adopted solutions for P-1 and P-5 had to be based on onl y one common set of values of these quantities. This was, in fact, accompli shed. Townsend's (1997b) code BRUCE was deployed to model the pulsational p erturbations of the rotationally distorted stellar surface. With the help o f KYLIE, from the same author, these perturbations were converted into obse rvable quantities. The local flux and the atmosphere structure were obtaine d from a grid of ATLAS9 models with solar metallicity, while the formation of 5967 spectral lines was calculated with the LTE code of Baschek et al. ( 1996). An initial coarse grid of models using all these ingredients was com puted for all 12 nrp modes with l less than or equal to 3 and m not equal 0 . Comparison with the observed variability of C II 4267, which is the best compromise between contamination by circumstellar emission and significance of the variability, yielded (l = 2, m = +2) and P-1 (and, by implication, P-2-P-4) and (l = 3, m = +3) for P-5 (and P-6) as the best matching nrp mod es. At 9 M circle dot / 3.4 R circle dot and 440 kms(-1), respectively, the mass-to-radius ratio and the equatorial velocity are on the high side, but not in fundamental conflict with established knowledge. The photometric va riations of all six modes combine at most to maximal peak-to-peak amplitude of 0.015 mag, consistent with the non-detection of any of the spectroscopi c periods by photometry. Without inclusion of additional physical processes , present-day linear nrp models are fundamentally unable to explain major r ed-blue asymmetries in the power distribution, which however seem to be lim ited to only some lines and the modes with the highest amplitudes. Neverthe less, the model reproduces very well a wide range of observed details. Most notable among them are: (i) Although all modeling was done on there residu als from the mean profiles only, the mean spectrum predicted by the model c losely fits the observed one. (ii) Dense series of high-quality spectra obt ained as early as 1987 and as recently as 1990, published independently but not included in the modeling efforts of this paper, are matched in great d etail by the multiperiodic nrp model. As in omega CMa, the inferred modes a re retrograde in the corotating frame and in the observer's frame appear pr ograde only because of the rapid rotation. This has implications for models of the ejection of matter during line emis sion outbursts, which in mu Cen are correlated with the beating of modes in the 0.5-d group of periods. The length of the corotating periods as well a s the horizontal-to-vertical velocity amplitude ratios suggest a g-mode cha racter.