Van Hoof effect with hydrogen

  

Figure: Differential velocity $\Delta\,V$, radius $\Delta\,R$ and acceleration $\Delta\,a$ between H$\alpha$$\lambda\lambda$6562.817 and FeII$\lambda\lambda$4923.921 lines. a: $\Delta\,V=\dot{R}_{{\rm H}\alpha}-\dot{R}_{{\rm Fe\,II}}$[km.s^-1]. b: $\Delta\,R=\int\Delta\,V{\rm d}\varphi$ [R$_{\odot}$]. c: $\Delta\,a=\frac{{\rm d}}{{\rm d}\varphi}(\Delta\,V)$ [m.s^-2]

Figure gives the H$\alpha$-FeII differential curves. They are similar to those obtained in Paper I except that all amplitudes are quite larger. Indeed $\Delta\,V$ reaches 134km/s instead 40km/s in Paper I, while for $\Delta\,R$ we have respectively 1.43R$_{\odot}$ and 0.28R$_{\odot}$ and for $\Delta\,a$, 155m.s^-2 and 22m.s^-2. It is true that the values of Paper I were given for H$\beta$ which is formed a little bit lower but we can conclude that these new amplitudes appear considerably larger with our new observations. Apart from a double resolving power, their time resolution is 5 times better. Consequently an effective average effect affects our first observations (July 1990) and certainly explains in part this large difference in amplitude. In any event, this demontrates well the importance of obtaining high quality data.