6 Rotational velocities of the gainers

The stars in non-evolved detached close binaries with small eccentricities usually rotate synchronously. However, the situation is usually different in the semi-detached systems because the rotational velocity of the gainer may be significantly higher than the synchronous one (Stothers 1973).

The observed values of the rotational velocities of the gainers $V_{\rm e}$are available only for eight systems of the ensemble analysed here. They are summarized in Table 4 along with the references. This table also contains the ratio of the real equatorial velocity to the synchronous one $V_{\rm e}$/$V_{\rm syn}$. The synchronous value $V_{\rm syn}$ was determined for the parameters of the system given in Table 1. Only in case of IZ Per the value of $V_{\rm e}$/$V_{\rm syn}$ was taken from the solution of the light curve by Wolf & West (1993). Since it is not clear how $V_{\rm e}$ in V 448 Cyg listed by Volkova et al. (1993) was obtained this value should be taken with caution.

Dependence of the degree of asynchronous rotation of the gainer on $R_{\rm g}/a$ for these binaries is plotted in Fig. 20. It can be immediately seen that systems which periods can be considered constant (AQ Cas, V 448 Cyg, IZ Per, V 356 Sgr) or where the change is uncertain (RZ Sct) have gainers rotating significantly faster than the synchronous value (often more than three times). The exception is u Her. The gainers in two systems with variable periods, SX Aur and Z Vul, rotate rather close to the synchronous value. Certainly, the number of the systems is too small to confirm any clear relation of the rotational velocity of the gainer and the observed period changes but it is useful to evaluate the role of rotation of the gainer in the storage of angular momentum of the system (see below).

  

Table 4: Rotational velocities of gainers in the semi-detached ET-systems. $V_{\rm e}$ refers to the observed velocity corrected for the inclination angle while the synchronous value determined for the parameters given in Table 1 is abbreviated as $V_{\rm syn}$. The ratio of the real to the synchronous rotation $V_{\rm e}$/$V_{\rm syn}$ is given, too. The last column gives the source of $V_{\rm e}$

References: 1. Stothers (1973); 2. Etzel & Olson (1993); 3. Van Hamme & Wilson (1990); 4. Wolf and West (1993); 5. Volkova et al. (1993).

  

Figure 20: Dependence of the degree of asynchronous rotation of the gainer on $R_{\rm g}/a$. The symbols are the same as in Fig. 17. See the text for details