4. The FWHM variation
Figs. 7-11 show the observed FWHM of Fe II 4923.921 Å at the Blazhko phases , , , and . To first approximation, the FWHM curves behave similarly, decreasing and increasing together with the pulsation phase and having peaks at the same phases. As previously explained by Fokin et al. (1999), the main peak (phase ) occurs during the line doubling phenomenon, which is due to a shock passage through the photosphere, the second occurs just after maximum luminosity () and is explained by rotation and pulsation effects, and the last peak (), which occurs during secondary acceleration, is due to the propagation of an infalling shock called the "secondary shock"(Chadid & Gillet 1998; Fokin et al. 1999). Thus, the variation of the FWHM of a photospheric absorption line, such as the unblended Fe II 4923.921 Å line, is mainly related to temperature, velocity and turbulence changes occurring in atmospheric layers located just above the photosphere. As shown by Fokin et al. (1996), the variation of the velocity field in pulsating stars, especially when shock waves are propagating in the atmosphere, induces appreciable changes to the FWHM.
A closer inspection of the curves shows that the FWHM curves vary from one night to another, i.e. at least during three consecutive pulsation cycles. For instance, the FWHM shift at is near 6 km/s at and and near 10 km/s at . It clearly appears that the amplitude and the width of the three FWHM peaks are strongly variable from one pulsation period to the next.
© European Southern Observatory (ESO) 2000
Online publication: July 13, 2000