Other
Scientific paper
Jun 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001phdt..........d&link_type=abstract
PhD Thesis Wroclaw University, June 2001
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3
&Beta, Cephei Stars, Pulsation, Mode Identification, Photometric And Spectroscopic Observables
Scientific paper
An essential step for asteroseismology is the mode identification, that is for a given frequency to determine three quantum numbers n,l and m. In my Ph.D. thesis I present how we can use to this aim the observables connected with nonadiabaticity of the pulsation. The photometric and spectroscopic observables of β Cephei stars were studied using the linear nonadiabatic theory of pulsation of Dziembowski & Pamyatnykh (1993) as well as line-blanketed models of the stellar atmospheres of Kurucz (1996). The first part of the thesis contains an analysis of the photometric nonadiabatic observables, i.e. the amplitude ratios and phase differences for brightness variations in two selected filters or colours. The photometric diagrams were constructed for Main Sequence stellar models with masses of 8 - 16 Modot for passbands in Strömgren photometry. These diagrams are useful to determine the harmonic degree and the radial order for l=0. The effect of mass, metallicity and microturbulent velocity on their diagnostic properties were examined. It turned out that neither chemical composition nor microturbulence influence the diagram for passbands u and y. However, metallicity is important in the case of the diagram involving the m1 index. The second part of the thesis deals with the use of spectroscopy in asteroseismology. The new method of mode identification is proposed, especialy for the azimuthal order m. This method consists in constructing diagrams with the amplitude ratios and phase differences, however both derived from the variations of various line profile parameters. The term spectroscopic nonadiabatic observables was adopted for these quantities by analogy to the photometric nonadiabatic observables. For this purpose time series of Si III 455.26 nm line profiles were calculated for a 10 Modot star at selected epochs of its Main Sequence evolution. In most of the spectroscopic diagrams the modes are grouped in separated regions with respect to the azimuthal order m. But it turned out that the diagram for the first and second moment (M1,M2) is the best one to assign this quantum number because it is entirely independent of inclination. However, as far as the equatorial rotation is concerned, this method works for ve>20 km s-1. In the presented method the identification of the harmonic degree is not so unambiguous, but a few of the investigated diagrams are suitable also to this aim, e.g. the diagram for (FWHM,Vhm). Moreover in many diagrams the zonal modes are located in different regions with respect to l. On the basis of almost each diagram, independently of inclination and equatorial rotation, we are able to distinguish the zonal modes from those with m≠0, and we can say whether a mode is prograde (m<0) or retrograde (m>0). The spectroscopic diagrams are complementery to the photometric ones. In the next part of the thesis the influence of nonadiabatic effects on another spectroscopic methods of mode identification is studied, considering the same spectral line. There are i.e. monochromatic diagrams of amplitudes and phases, moment method, and various parameters of the line profile. It was found that the nonadiabatic effects are of the greatest importance in the models of β Cephei stars only in the equivalent width (the zeroth moment of the line profile) as well as in the residual intensity. In the β Cephei stars the pulsation mechanism is strictly connected with the abundance of the Fe-group elements. In the last part of the thesis, the metallicity parameter, [m/H], of these variables was derived from the IUE satellite ultraviolet spectra. The metallicity obtained for the field β Cephei stars are contained in the range of [-0.3, +0.3], and for the cluster stars: [-0.1, 0.1] for NGC 3293, [-0.5, -0.1] for NGC 4755, and [+0.1, +0.2] for NGC 6231. The obtained values of the metallicity are independent of the effective temperature and gravity. They are also not correlated with the rotation velocity and with pulsational parameters such as the dominant period or the radial velocity amplitude. However it turned out that on the whole the multiperiodic β Cep stars have higher values of the metallicity than the monoperiodic ones: [m/H]=+0.02± 0.03 and [m/H]=-0.23±0.06, respectively. This result is in agreement with the linear nonadiabatic theory of pulsation. However the frequency range appeared to be inversely proportional to the [m/H]. This result is surprising and demands farther studies.
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