Metallic line profiles in cepheid variables
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A grid of synthetic line profiles incorporating a new treatment of macroturbulence is calculated for a Cepheid atmosphere in radial motion. Projection factors are determined to be up to 10 per cent higher than previous calculations. A relationship is found between the asymmetry of the line profiles and γ, the ratio of pulsational velocity to line half-width at half-height. A programme of high-resolution spectroscopic observations of Cepheids has been carried out at Mt John University Observatory for several years. Radial velocities and asymmetries have been measured for selected metallic lines and the asymmetries compared with those of the synthetic profiles. The line profiles from the observations show a larger asymmetry than the synthetic profiles at phases of inward pulsational velocity. The asymmetry at phases of outward velocity is smaller and in agreement with the synthetic profiles for some Cepheids. However, for others the asymmetry at phases of outward pulsation is in the direction expected for inward motion. A hydrodynamic model of an 11-day Cepheid has been calculated. A set of flux-constant, line-blanketed model atmospheres have been converged based on the density structure predicted by the hydrodynamic model. A method for calculating the source function and mean intensity in a moving atmosphere has been developed and incorporated into the ATLAS code. Synthetic line profiles have been calculated from the set of dynamic model atmospheres. A function giving the contribution of different atmospheric layers to the absorption of a spectral line has been derived. Flux contribution contour diagrams based on this function have been introduced and used to study the effects that velocity fields in a stellar atmosphere have on spectral line formation. In the presence of a constant velocity, the trailing wing of a spectral line is no longer formed at deep atmospheric layers close to the continuum. The presence of a velocity gradient can have a large effect on the region of line formation. Multiple centres of absorption, separated in wavelength and in physical depth, can occur which will not usually be resolved by observation. The use of spectral lines of different strength and excitation potential to probe depth-dependent phenomena in pulsating stars, will give incorrect results if based on static models of line formation depths. Projected radial velocities of spectral lines, synthesised from the set of dynamic model atmospheres, represent the motion of a mass zone in the hydrodynamic model with an rms error of ~ 1 km s-⁻₹ if bump phases are excluded. A mapping of the radial velocities to the motion of the photospheric radius has an rms error of ~5 km s-⁻₹. This is due to a changing phase lag between the photospheric radius and the line formation region. In particular, the radial velocities significantly overestimate the photospheric velocity for ~0.1 cycle near radial velocity maximum. The inverse line asymmetry observed in some Cepheids during phases of expansion may be due to velocity gradients in their atmospheres or to radial macroturbulence. The enhanced asymmetry in Cepheids at contraction phases is most likely due to an increase in anisotropic macroturbulence.