Starspot evolution, activity cycle and orbital period variation of the prototype active binary RS Canum Venaticorum.

Details Starspot evolution, activity cycle and orbital period variation of the prototype active binary RS Canum Venaticorum. Starspot evolution, activity cycle and orbital period variation of the prototype active binary RS Canum Venaticorum.

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...301...75r&link_type=abstract

Astronomy and Astrophysics, v.301, p.75

Astronomy and Astrophysics

Astrophysics

64

Stars: Activity, Binaries: Close, Stars: Individual Rs Cvn

Scientific paper

The sequence of RS CVn seasonal light curves obtained at Catania Astrophysical Observatory since 1963, complemented with Keller & Limber (1951), Popper (1961) and Ludington (1978) data, are analysed in the framework of the starspot hypothesis (Eaton & Hall 1979) to deduce the spot evolution in the last 45 years. In the adopted modelling technique the star surface is divided into small elements, the specific intensities of which are iteratively adjusted, by a χ^2^ optimization, to fit the residual flux modulation, after the proximity effects and eclipses have been taken into account. Satisfactory fits are obtained assuming that spots are located only on the K2 subgiant. Maps of the spot distributions are presented, together with an improved value for the luminosity ratio between the two components in the V band ((L_1V_)/(L_2V_)=0.9375+/-0.025). The yearly spot distributions are analysed to derive general activity characteristics. The spot pattern appears to consist of two components, one evenly and the other unevenly distributed in longitude, the latter being suggestive of preferential longitudes. Spots at latitudes higher than 35deg are not required to reproduce the observed light curves within the data accuracy. The total area covered by spots ranges from about 17% to 37% of the active star photosphere and shows a periodic variation with a period of 19.7+/-1.9 years with maxima in 1967 and 1984-1988 and minima in 1973 and 1993. The same periodic variation affects also the area of the evenly distributed spot component. The area of the unevenly distributed component shows the same behaviour up to the 1973 minimum, but the subsequent maximum occurs in 1981, a few years earlier than the maximum of the total spotted area. The longitude of the maximum of the spot distribution migrates in time at a variable speed with respect to a reference frame corotating with the orbital motion, as defined by the times of eclipses. The migration rate remains approximately constant at ~0.1deg/day during the interval 1963-1984 and does not change with the phase of the activity cycle suggested by the cyclic variation of the total spot area. In the years 1988-1993, the migration rate accelerates to ~0.34deg/day while the spot activity decreases toward a cycle minimum. In the latter time interval a solar-like model applies, suggesting a systematic decrease of the latitude of the spot formation vs. cycle phase and a surface differential rotation between 5% and 20% of the solar one. These new results allow us to address also the problem of the orbital period variation. Specifically, the orbital period shows a long-term variation on a timescale of the order of a century, upon which a short-term modulation is superposed. A correlation between the rate of change of the period and the migration rate of the photometric wave is also suggested by the available data. This evidence supports the model proposed by Applegate (1992), but an improvement of the theory is required for a quantitative explanation of the observed phenomenology.

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