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Magnetic activity and dynamics of close binaries and planetary systems

Rodonò and Lanza (), () addressed the apparent connection between orbital period and magnetic activity cyclic behaviour in close binaries with late-type components by considering recent observational studies of Algols, RS CVn's, W UMa and CVs. Specifically, RS CVn systems offer the best opportunity to study the connection between magnetic activity and orbital dynamics since they are not significantly affected by mass exchange or mass loss and a light-time effect due to a third body can be excluded in most of the cases. In Rodonò and Lanza () a detailed comparison of the most recent data with the proposed theoretical models is presented. It is particularly interesting the possibility that the orbital period cycle has a length twice that of the starspot cycle, as it is suggested by the observations of the prototype RS CVn, AR Lac and RT Lac (see, e.g., Fig. 1.6).

Figure 1.6: Upper panel: The $(O-C)$ of the primary eclipses of RS CVn versus time. The reference ephemeris is: $ Min\; I = 2410102.4280 + 4.797817 \times E$, where $E$ is the number of orbital cycles elapsed from the initial epoch. The dashed line is a third-order polynomial best fit to the data. Middle panel: The residual $(O-C)_{I}$ versus time obtained by subtracting the third-order polynomial fit to the $(O-C)$ variation in the upper panel. Lower panel: The total spotted area on the K2IV active component of RS CVn as derived by Rodonò et al. (1995).
\begin{figure}\centerline{\psfig{file=stars/an742f1.ps,width=8cm}}\end{figure}
Figure 1.7: The expected relative orbital period variation of a star-planet system versus the orbital period of the planet according to an extrapolation of the model by Lanza & Rodonò (1999). The central star is assumed to be a late-type dwarf of mass $M=0.7  M_{\odot }$ and radius $R=0.86  R_{\odot }$. The plots are labelled by the rotation period $P_{rot}$ of the central star: $P_{rot} = 3$ days (solid line), $P_{rot} = 10$ days (dashed line) and $P_{r
ot} = 30$ days (dotted line).
\begin{figure}\centerline{\psfig{file=stars/opv_planet.ps,width=8cm}}\end{figure}
A theoretical model based on an improved version of the Applegate's mechanism seems capable of explaining the observed period variations in terms of changes of the gravitational quadrupole moment induced by the presence of a magnetic activity cycle in the binary secondary component. If so, the study of orbital period modulations can offer a promising tool to investigate hydromagnetic processes in the interior of active stars, in particular, the interaction between rotation and magnetic fiels in non-linear dynamo regimes, leading to new insights into the long-term evolution of angular momentum and magnetic breaking in cool stars. An extrapolation of the results obtained for close binary systems to the case of the recently discovered extrasolar planetary systems suggests that the orbital period modulation may play a role also in those systems. It is important to notice that, thanks to the small ratio between the planet and the star radii, the long-term timing of planetary transits may allow us to measure relative orbital period variations as small as $\Delta P/P \sim 10^{-10} -
10^{-9}$ (cf. Fig. 1.7).

Frasca and Lanza () studied orbital period changes in the spectroscopic close binary HR 1099, one of the brightest members of the RS CVn class of magnetically active binary systems. Intermediate-resolution optical spectroscopy and IUE archive spectra were used to build radial-velocity curves yielding epochs of superior conjunction with an accuracy of 0.01 days (see Fig. 1.8).

Figure 1.8: Radial-velocity curves (circles for Fekel (1983) data, asterisks for IUE, squares for Frasca & Lanza () data) and best-fit solution for HR 1099. The solid lines are the radial-velocity curves computed according to Fekel's ephemeris, the dashed lines are the actual radial-velocity curves, fitted to the observational data by varying the phase of superior conjunction $\phi_{0}$. This sequence of radial velocity curves make evident the orbital period change of HR 1099 versus time.
1cm
\psfig{file=stars/1007_f2a.ps,width=8cm}
\psfig{file=stars/1007_f2b.ps,width=8cm}
The complete database ranged from 1976 to 2002 and allowed them to better assess the amplitude of the orbital period variation and its timescale. The system radial velocity was constant within $\pm 3$ km s$^{-1}$ during the 1976-2002 time interval, indicating that an apparent period variation due to a light-time effect can be ruled out. On the basis of such results, the mechanisms proposed to explain the period change are briefly discussed giving further support to the possible connection between the orbital period modulation and the change of the gravitational quadrupole moment of the K1 subgiant component, in the framework of the model by Lanza et al. (1998).


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Next: Magnetic structures in the Up: Magnetic activity and variability Previous: Structure and modelling of   Contents   Index
Innocenza Busa' 2005-11-14