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Astron. Astrophys. 332, 149-154 (1998)

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6. Summary and conclusion

In the present paper we have shown that the dynamo mechanism present in the active star of the UX Arietis binary system presents two alternating "regimes"; the active and the quiescent one, with a total period of 158.7 days. The duration of the active state seems to be 20% longer than that of the quiescent one.

During the active phase the activity does not take place randomly, but shows a periodical trend with two periods of 14.4 and 25.5 days.

Sudden flux decreases at [FORMULA] which always take place at the same orbital phase, are ascribed to the disappearence of the radio source behind the star.

The sign of the circular polarization seems to reverse within the dominant activity cycle of 25 days, returning at its initial value after 56 [FORMULA] 4 days. If this polarization reversal is due to a polarity inversion of the general field of the star, this is strongly reminiscent of what happens on the Sun. It is very well known that the solar activity is cyclical with a period of 11 years, while the general magnetic field of the Sun (and hence the polarity of the preceeding and following spots in the spots groups of each hemisphere) reverse every 11 years, returning at its initial configuration after 22 years, namely after two consecutive cycles of activity.

We would like to point out that these sequences of very active periods followed by quiescent ones, confirm the predictions of theoretical models on the existence of two different convective states with a different angular velocity distribution (Tobias et al 1995). Such a theory has so far been applied only to the Sun, where the activity cycle of 11 years (22 years taking into account the magnetic field polarity) appears to be modulated with a period of about 90 years. According to recent observations of the average sunspot numbers (Mouradian and Soru-Escaut,1995), the long term periodicity modulating the 11 years cycle seems to have increased its period to about 110 years. The statistical relevance of those observations is however limited, since only two such long term periods in the solar activity ihave taken place since 1770! This fact increases the interest of the results presented in this paper, since they may provide a way of constructing more reliable statistics to compare with the predictions of dynamo theories having the same phenomenon at much shorter time scale: 158 days for UX Arietis instead of more than one hundred years as in the case of the Sun.

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© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998