A sequence of V -band light curves of the active close binary RT Lacertae (G5+G9 IV), extending from 1965 to 2000, is presented and analysed to derive the spot distribution and evolution on the component stars. In our modelling approach, the Roche geometry and Kurucz's atmospheric models were adopted. The resulting maps of the spot surface distribution were regularized by means of the Maximum Entropy and Tikhonov criteria to take full advantage of the increased geometrical resolution during eclipses. By comparing the maps obtained with these two criteria, it was possible to discriminate between surface features actually required by the data and artifacts introduced by the regularization process. Satisfactory ts were obtained assuming spots on both components and the unspotted V -band luminosity ratio: LG5=LG9 IV =0 :65 0:05. The more massive G5 primary appears to be the most active star in the system and its spotted areas are mainly responsible for the light curve distortions. The yearly spot distributions on both components indicate that their spot patterns consist of two components, one uniformly and the other non-uniformly distributed in longitude, the latter suggesting the presence of preferential longitudes. In particular, spots are concentrated around the substellar points and their antipodes on both stars. The eclipse scanning reveals spots with diameters of40, or possibly smaller, on the hemisphere of the primary star being occulted. The primary shows clear evidence for a short-term activity cycle with a period of8: 5y r and a possible long-term cycle with a period of approximately 35 yr. The variation of the spot migration rate may be related with surface dierential rotation, with a lower limit of = 3:2 10 3 . The G9 IV secondary does not show evidence for an activity cycle, its spot coverage appearing rather constant at15 20% of its surface. The relative amplitude of its surface dierential rotation, as indicated by the variation of the spot migration rate, is = 2:7 10 3 . The variation of the orbital period shows a correlation with the activity level of the primary component. Specically, the decreases of the orbital period appear to be associated with minimum spottedness and sizeable changes of the surface spot distribution that may be related to increases of the rotation rate of the spot pattern. Conversely, an episode of increase of the orbital period was related to an increase of the spotted area on the primary star. Such results support the recently proposed models that connect the perturbations of the orbital dynamics with the variation of the gure of equilibrium of the active components, due to the operation of non-linear hydromagnetic dynamos in their extended convective envelopes.

/pdf/long-term-starspot-evolution-activity-cycle-and-orbital-rpxueg514x.pdf

Long-term starspot evolution, activity cycle and orbital period variation of RT Lacertae

A sequence of V -band light curves of the active close binary SZ Piscium (F8 V-IV+ K1 IV), extending from 1957 to 1998, is presented and analysed to derive the spot distribution and evolution on the component stars. In our modelling approach the Roche geometry and Kurucz's atmospheric models were adopted. The resulting maps of the spot surface distribution were regularized by means of the Maximum Entropy and Tikhonov criteria to take full advantage of the increased geometrical resolution during eclipses. By comparing the maps obtained with these two criteria, it was possible to discriminate between surface features actually required by the data and artifacts introduced by the regularization process. Satisfactory ts were obtained assuming spots on both components and the unspotted V -band luminosity ratio: L1=L2 =0 :35 0:035. The derived yearly spot distributions indicate that the spot patterns consist of two components, one uniformly and the other non-uniformly distributed in longitude, the latter suggesting the presence of preferential longitudes. Starspots at latitudes higher than75 were not required to reproduce the photometric modulation. For the spot pattern on the less luminous F8 V primary component there are only some hints of the possible presence of spots around the substellar point facing the K1 IV secondary component. On the more luminous and larger secondary component there is clear evidence for the presence of three active longitudes. A stable active longitude around the substellar point facing the primary star appears to be quite compact with an extension of40 on several maps. The other two active longitudes display irregular changes of the spot area and location. The variation of the total spotted area on the secondary component shows sizeable fluctuations and a possible short-term cycle of about 13 yr, which is more clearly apparent in the variation of the spotted area unevenly distributed in longitude. A longer-term cycle of about 30 yr with a markedly non-sinusoidal variation might also be present. The spatial association between photospheric spots and chromospheric plages (as detected in the UV lines) appears signicant for the active region around the substellar point in the K1 IV component. A possible connection between magnetic activity and orbital period variation is suggested by comparing the variation of the total spotted area with the O C diagram of the primary eclipse times. The time span covered by the available data is, however, not long enough for us to draw any denite conclusions.

/pdf/long-term-starspot-evolution-activity-cycle-and-orbital-1za48s4szx.pdf

Long-term starspot evolution, activity cycle and orbital period variation of SZ Piscium

We present surface spot maps of the K2V primary star in the pre-cataclysmic variable binary system, V471 Tau. The spot maps show the presence of large high latitude spots located at the sub-white dwarf longitude region. By tracking the relative movement of spot groups over the course of four nights (eight rotation cycles), we measure the surface differential rotation rate of the system.Our results reveal that the star is rotating rigidly with a surface shear rate, dOmega = 1.6 +/- 6 mrad/d. The single active star AB Dor has a similar spectral type, rotation period, and activity level as the K star in V471 Tau but displays much stronger surface shear (46 < dOmega < 58 mrad/d). Our results suggest that tidal locking may inhibit differential rotation; this reduced shear, however, does not affect the overall magnetic activity levels in active K dwarfs.

Spot patterns and differential rotation in the eclipsing pre-CV binary,V471 Tau

V471 Tauri, an eclipsing system consisting of a hot DA white dwarf (WD) and a dK2 companion in a 12.5-hour orbit, is the prototype of the pre-cataclysmic binaries. The late-type component is magnetically active, due to its being constrained to rotate synchronously with the short orbital period. During a program of UV spectroscopy of V471 Tau, carried out with the Goddard High Resolution Spectrograph (GHRS) onboard the Hubble Space Telescope, we serendipitously detected two episodes in which transient absorptions in the Si III 1206 A resonance line appeared suddenly, on a timescale of <2 min. The observations were taken in a narrow spectral region around Ly-alpha, and were all obtained near the two quadratures of the binary orbit, i.e., at maximum projected separation (~3.3 Rsun) of the WD and K star. 
We suggest that these transient features arise when coronal mass ejections (CME's) from the K2 dwarf pass across the line of sight to the WD. Estimates of the velocities, densities, and masses of the events in V471 Tau are generally consistent with the properties of solar CME's. Given our detection of 2 events during 6.8 hr of GHRS observing, along with a consideration of the restricted range of latitudes and longitudes on the K star's surface that can give rise to trajectories passing in front of the WD as seen from Earth, we estimate that the active V471 Tau dK star emits some 100-500 CME's per day, as compared to 1-3 per day for the Sun. The K dwarf's mass-loss rate associated with CME's is at least (5-25) x 10^{-14} Msun/yr, but it may well be orders of magnitude higher if most of the silicon is in ionization states other than Si III.

Detection of Coronal Mass Ejections in V471 Tauri with the Hubble Space Telescope

V471 Tauri, a white dwarf--red dwarf eclipsing binary in the Hyades, is well known for stimulating development of common envelope theory, whereby novae and other cataclysmic variables form from much wider binaries by catastrophic orbit shrinkage. Our evaluation of a recent imaging search that reported negative results for a much postulated third body shows that the object could have escaped detection or may have actually been seen. The balance of evidence continues to favor a brown dwarf companion about 12 AU from the eclipsing binary. A recently developed algorithm finds unified solutions from three datatypes. New radial velocities (RVs) of the red dwarf and BV RCIC light curves are solved simultaneously along with white dwarf and red dwarf RVs from the literature, uvby data, the MOST mission light curve, and 40 years of eclipse timings. Precision-based weighting is the key to proper information balance among the various datasets. Timewise variation of modeled starspots allows unified solution of multiple data eras. Light curve amplitudes strongly suggest decreasing spottedness from 1976 to about 1980, followed by approximately constant spot coverage from 1981 to 2005. An explanation is proposed for lack of noticeable variation in 1981 light curves, in terms of competition between spot and tidal variations. Photometric spectroscopic distance is estimated. The red dwarf mass comes out larger than normal for a K2V star, and even larger than adopted in several structure and evolution papers. An identified cause for this result is that much improved red dwarf RVs curves now exist.

The V471 Tauri System: A Multi-datatype Probe

The RS CVn-type eclipsing binary AR Lac has been observed in two colours,B andV, in 1978 and 1979. Several times of minima have been obtained during the observations and the new light elements have been calculated. The orbital period of the system was found to be decreasing with an amount of 14.6 s century−1. The light curves of the system obtained inB andV show a significant wave-like distortion outside eclipses. The amplitudes of the wave-like distortion inB andV are about 0
 .
 09 and 0
 .
 10 in 1978, and, 0
 .
 12 and 0
 .
 13 in 1979, respectively. The minima of the wave-like distortion have shifted with an amount of 0
 .
 40 in one year, thus indicating the migration period of the wave-like distortion is about 2.5 yr. A noticeable variation of light during the totality of primary minimum has also been observed. This variation can be attributed to the distortion wave which has been moving towards decreasing phases.

Two-colour photoelectric investigation of AR Lacertae

The eclipsing variable RT Lac, which is classified as an RS CVn-type binary by Hall, was observed in two colours,B andV, during the summer seasons of 1978 and 1979. The observations made during two successive seasons indicate that, outside eclipses, the system has brightened by about 0.12 and 0.15 magnitudes on average inB andV, respectively. Moreover, RT Lac was even fainter when observed by Milone in 1965. While the total brightness of the system remained unchanged at mid-secondary, it increased at mid-primary and outside eclipses. This may be interpreted as indicating that the cooler component is responsible for the peculiar light variations.

Light curve variations of RT lacertae

In this paper we present the observations of the peculiar eclipsing binary RT Lac, which were obtained in 1980 and 1981. Examining the light curves obtained in successive four years we estimate the period of the migrating light curve distortion to be about four years. The orbital period of the system was also found to be decreasing with an amount 34s century−1. The brightnesses both at primary and secondary are changing with the time, which is clearly seen from the last light curves. The variations occurred at primary is three times greater than secondary eclipse. Therefore, we suggest that the star, in front at primary eclipse, is more responsible for the distorted light curves and it should be the earlier spectral type and also the smaller one.

Extensive photometric study of RT lacertae

The observations of the peculiar eclipsing binary RT Lac were continued during the observing seasons of 1982 and 1983. The salient properties of the light curves were presented. The shallowest primary eclipse was observed in 1983. When the light curves, obtained in six years sucessively, are interpreted together it is seen that the variations of the brightness at mid-primary and second maximum are too similar to each other. The colour variations at mid-primary also follow those variations in the brightness. These circumstances indicate that the starspot hypothesis is insufficient alone to explain all the phenomena observed. The earlier spectral type G9 companion may be intrinsically variable and the unusual distortion on the light curves following the secondary eclipse may arise from the gas-stream from larger K star to the G9 star.

Photoelectric photometry of the peculiar system RT Lacertae

Two-colour light curves of Z Her were obtained observing the system from 1978 to 1981. The wave-like distortion at outside eclipses were derived. The amplitudes of the wave increase and its minimum migrates towards the decreasing orbital phase with a period of about 1.4 yr. An attempt was made to represent the wave-like distortions with a truncated Fourier series. The representation was found to be unsatisfactory since the distribution of the present observations is not regular but forms separate groups along the time axis. Therefore, further observations to cover the phases evenly in between the minima are needed.

Photoelectric photometry of Z herculis

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