A spectroscopic and dynamical study of binary and other Cepheids
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
High resolution observations have been made of a number of southern Cepheids to make an observational and theoretical study of Cepheid variables using radial velocities. The stars studied were part of a long term programme to observe southern variable stars, from which a valuable database of radial velocities gathered over a long period were available. Sixteen échelle spectrograph orders in the wavelength region 5400 - 8600Å were used, which included a number of absorption lines covering a range of species and excitation potentials. The line bisector technique was used to measure stellar and telluric lines and to obtain radial velocities. To improve the precision of the radial velocities we used telluric lines to calibrate the observations to a common reference frame. The radial velocities have a precision of ~300ms⁻¹ allowing the detection of velocity differences of ~1 kms⁻¹ with confidence. The radial velocity data obtained at Mount John University Observatory (MJUO) was combined with data from various sources to determine the orbits of any Cepheids exhibiting orbital motion. The various orbital parameters were determined for a number of systems and where radial velocities for the companions exist, some estimate of the mass was made. The precision of the radial velocities obtained from MJUO also allowed us to search for line level effects for a number of species among the Cepheid spectra. A number of IAU standard stars were observed to calibrate the radial velocities obtained at MJUO to the IAU standard scale. The radial velocities from MJUO were found not to differ significantly from the IAU values. Binary Cepheids are particularly useful in the determination of Cepheid masses, which are still an active topic for astronomical research. The value of the MJUO data was that it provided a consistent set of data against which other sources of data could be compared. For 8 of the Cepheids new or improved orbital solutions were found. They are Y Car, YZ Car, AX Cir, BP Cir, S Mus, V636 Sco, V350 Sgr, W Sgr and T Mon. Of these 8 systems, 3 had radial velocities for their respective companions which allowed the determination of the mass of the Cepheids. Masses were determined for the 9 day Cepheid S Mus (6.0±0.4M⊙), the double mode Cepheid Y Car (4.5±1.8 M⊙) and the 5 day Cepheid V350 Sgr (6.0±0.9 M⊙). New results presented here include the first orbital solution for the binary Cepheid AX Cir, a completely revised orbital solution for the binary Cepheid YZ Car which established its eccentricity, and orbital motion. The binary Cepheid BP Cir however was found to require a new pulsation period of 2.39819d to fit the observed pulsational velocities. Observations of the suspected binary Y Oph show it to be an unusual Cepheid with no evidence of binarity (~0.5 kms⁻¹) in our data. Finally, the 5 day Cepheid W Sgr was observed to have the lowest orbital amplitude measured. We discussed the line level effects found in our observations, where a number of spectral lines were observed to show departures from the Fei velocity curve. Line level effects were observed in H⍺, CaⅡ, BaⅡ, FeⅡ, SiⅠ and SiⅡ. Most of the Cepheids were observed to show the same progression of line level effects, with the best example being the bright Cepheid β Dor. The SiⅡ velocities indicate that these lines have the lowest velocity amplitude and form deeper in the atmosphere than the FeⅠ, where as the CaⅡ and H⍺ lines were all observed to have much greater velocity amplitudes with the peak of these velocity curves occurring later, in pulsational phase. These observations are consistent with the lines forming at different depths as a density wave propagates through the atmosphere. X Sgr is peculiar due to its interesting spectra which at certain phases show line doubling and at most other phases the lines were asymmetric. These observations are interpreted to support the idea that X Sgr has strong shock waves present and that X Sgr has greater atmospheric transparency in the spectral region near 6000Å. We also present the results of dynamical modelling of a pulsating Cepheid. Using a non-linear radiative hydrodynamic code developed by A. Fokin, we have modelled two Cepheids, AX Cir and YZ Car. These models have then been compared with the observations obtained here. After extensive modelling using the latest OP opacities, it has been determined that the Cepheid AX Cir can be modelled using parameters L = 2050 L⊙, M = 4.8 M⊙ and Teff = 5900K. The model reproduced the observed stellar characteristics, such as the photometric amplitude and pulsational period. Comparison between the observations and the model for the selected spectral lines, FeⅠ 5576Å, SiⅡ 6347Å, BaⅡ 5853Å and CaⅡ 8542Å show good agreement with similar amplitudes and velocity curves. With no strong shock waves being produced by the model, the observed line level effects can be explained by a density wave. The 18 day Cepheid YZ Car was chosen to explore the capabilities of the radiative hydrodynamic code for a long period Cepheid. The best model developed that reproduced the observed stellar characteristics had parameters L = 9350 L⊙, M = 7.7 M⊙ and Teff = 5590K. The period was 18.314 days and the bolometric light curve agreed well with the observed visual light curve. Comparison of the theoretical and observed radial velocities showed good agreement.