Applegate Mechanism Research Articles

Detection of a period decrease in NN Ser with ULTRACAM: evidence for strong magnetic braking or an unseen companion

We present results of high time resolution photometry of the eclipsing pre-cataclysmic variable NN Ser. NN Ser is a white dwarf/M dwarf binary with a very low-mass secondary star (∼0.2 M⊙). We observed 13 primary eclipses of NN Ser using the high-speed CCD camera ULTRACAM and derived times of mid-eclipse, from fitting of light-curve models, with uncertainties as low as 0.06 s. The data show that the period of the binary is decreasing, with an average rate of , which has increased to a rate of over the last 2 yr. These rates of period change appear difficult to reconcile with any models of orbital period change. If the observed period change reflects an angular momentum loss, the average loss rate is consistent with the loss rates (via magnetic stellar wind braking) used in standard models of close binary evolution, which were derived from observations of much more massive cool stars. Observations of low-mass stars such as NN Ser's secondary predict rates of ∼100 times lower than we observe. The alternatives are either magnetic activity-driven changes in the quadrupole moment of the secondary star (Applegate's mechanism) or a light traveltime effect caused by the presence of a third body in a long (∼ decades) orbit around the binary. We show that Applegate's mechanism fails by an order of magnitude on energetic grounds, but that the presence of a third body with mass 0.0043 < M3 < 0.18 M⊙ and orbital period 30 < P3 < 285 yr could account for the observed changes in the timings of NN Ser's mid-eclipses. We conclude that we have either observed a genuine angular momentum loss for NN Ser, in which case our observations pose serious difficulties for the theory of close binary evolution, or we have detected a previously unseen low-mass companion to the binary.

Period and light variations for the cool, overcontact binary BX Pegasi

New charge-coupled device photometric observations of the W UMa-type binary BX Pegasi (BX Peg) were collected on four nights from 1999 October to 2000 September. The light curve was covered completely in each season. Seven new times of minimum light were determined. It was found that the orbital period of the system has varied recently in a sinusoidal way, superimposed on a downward parabolic variation. The long-term period decrease rate is deduced as dP/dt = -8.62 or 9.59 × 10 -8 d yr -1 , which can be interpreted as either mass transfer from the more massive cool star to the less massive hot component, or as the combination of mass transfer and angular momentum loss due to a magnetic stellar wind. The period and amplitude of the sinusoidal period variation were calculated to be about 35.3 yr and 0.015 d, respectively. The light curves of BX Peg are asymmetric and show year-to-year light variability. A spot model has been applied to analyse these light curves. After using the light curves of 1999 as reference ones, we solve those of 2000 by adjusting only the spot parameters. One cool-spot model on the cool secondary satisfies the observed light curves of both 1999 and 2000 quite well and shows a good representation of the BX Peg system for both the photospheric and spot descriptions. The brightness variations of BX Peg are not coincident with the period variations and so do not conform to a prediction of the Applegate mechanism. We think the most likely cause of the cyclical variation is the light-time effect due to a third body, although no third light was detected in the light-curve analysis. If it exists, the hypothetical object could be a very red main-sequence star or a white dwarf. We have solved anew the historical published light curve for only the spot parameters and these closely resemble our spot parameters. We speculate that this result is associated with the small coronal saturation of the cool star of the system.

The Chromospherically Active Contact Binary CE Leonis

We present complete VRI light curves of the contact binary CE Leo and provide detailed photometric solutions, as well as an analysis of its period variation. A total of 73 times of minima over 50 years, including our observations, were used for the period study. The complex period variation can be sorted into a linear period improvement, a period of P = 0.30342771 days, a secular period increase at the rate of /P = +3.05 × 10-7 days yr-1, and a 22.6 ± 0.5 yr periodic component. The ∼22.6 yr periodic variation in the O-C residuals most likely arises from the light-travel time effect from a low-mass (m3 ∼ 0.3M⊙ sin i) dM tertiary companion moving in an eccentric (e' = 0.61 ± 0.04) orbit. However, it is also possible that this periodic variation arises from the effects of a magnetic activity cycle, known as the Applegate mechanism. The light curves show asymmetries in the two different maxima. The 1998 light curves show that primary maximunm was 0.042, 0.038, and 0.038 mag brighter than secondary maximum in V, R, and I, respectively. The durations of totality for three epochs were 18.3 minutes in 1989, 19.7 minutes in 1998, and 18.8 minutes in 2001. The relative depth of primary minimum was found to vary between 1.20 and 1.32 mag. We have analyzed the light curves from the three epochs using the 1993 version of the Wilson-Devinney differential corrections computer code to find a unique solution for CE Leo. The corresponding spectral type of the secondary star from the colors and effective temperature is ∼dK2. The asymmetric light curves can be explained by the effects of starspots. We find that the light curves are best fitted by employing a cool spot on the cooler, larger component of the system. The spot-effect parameter SE = 0.05 found for CE Leo is relatively large for W UMa binaries.

Orbital Motion and Magnetic Activity in Close Binaries and Planetary Systems

The connection between orbital period variation and magnetic activity cyclic behaviour in close binaries with late-type components is addressed by discussing recent observational studies of Algols, RS CVn's, W UMa's and CVs. A theoretical model based on the Applegate's mechanism seems capable of explaining the observed orbital period modulation in terms of cyclic changes of a gravitational quadrupole moment induced by a magnetic activity cycle affecting one of the binary components. In such a case, the study of orbital period modulations offers a promising tool to investigate hydromagnetic dynamos operating in the interior of active stars, in particular, to address the fundamental question of the interaction between rotation and magnetic fields in nonlinear dynamo regimes. Moreover, interesting applications to planetary systems with a magnetically active central star are discussed.

Doin’ the twist: secular changes in the surface differential rotation on AB Doradus

We present measurements of the rotation rates of individual starspots on the rapidly rotating young K0 dwarf AB Doradus, at six epochs between 1988 December and 1996 December. The equatorial rotation period of the star decreased from 0.5137 to 0.5129 days between 1988 December and 1992 January. It then increased steadily, attaining a value of 0.5133 days by 1996 December. The latitude dependence of the rotation rate mirrored the changes in the equatorial rotation rate. The beat period between the equatorial and polar rotation periods dropped from 140 days to 70 days initially, then rose steadily. The most rigid rotation, in 1988 December, occurred when the starspot coverage was at a maximum. The time-dependent part of the differential rotation is found to have Delta Omega / Omega ~ 0.004, which should alter the oblateness of the star enough to explain the period changes observed in several close binaries via the mechanism of Applegate (1992).

An active binary XY UMa revisited

New extensive multicolour photoelectric photometry, performed since 1994, is presented. 17 more- or-less complete light curves were obtained and analyzed. The Wilson-Devinney code applied to the BVRI light curves without the maculation eect together with published spectroscopic mass ratio and semi-major axis yielded new absolute parameters of the eclipsing pair: m1 =1 :10 M, m2 =0 :66 M, R1 =1 :16 R, R2 =0 :63 R, a =3 :107 R and the orbital inclination i =8 0:9. The observed orbital period changes are conclusively explained by the mutual action of the third body in the system (P3 = 30 years) and the maculation eects. Simultaneous analysis of the period changes and the visual brightness excludes the possibility of their explanation by Applegate's mechanism. The dierences in the maxima heights caused by the maculation exhibit variations with the period of 70910 days. The distance to the system d =8 65 pc determined from the absolute dimensions and luminosities of the components is larger than the Hipparcos astrometric value d =6 6 6p c.