Astron. Nachr./AN 322 (2001) 5/6, 333–342

The Catania Automatic Photoelectric Telescope on Mt. Etna: asystematic study of magnetically active stars

M. RODONO , G. CUTISPOTO , A.F. LANZA and S. MESSINA

Catania Astrophysical Observatory, V. S. Sofia 78, I-95123 CataniaDepartment of Physics and Astronomy, University of Catania, V. S. Sofia 78, I-95123 Catania

Received 2001 November 22; accepted 2001 November 26

Abstract. A photometric monitoring of about 50 magnetically active stars, that are spread almost all over the H-R diagram,was initiated at the mountain station of Catania Observatory on Mt. Etna (1750-m a.s.l.) in 1992 with an 80-cm robotictelescope (APT-80) built by AutoScope Co. (USA). This systematic survey is now approaching its 10th year anniversary. Formost of the stars, quite well defined solar-like spot maps have been derived from UBV data obtained in different epochs.These data have allowed us to investigate some relevant characteristics of spot activity and variability on stars, and to obtainclear evidence of long-term activity cycles, in the range from a few to about 10 years, on some of the observed targets.Starspot maps are constructed by using advanced tools, such as massive parallel computing and are based on MaximumEntropy and Tikhonov regularization criteria. Selected results are here presented.Our systematic observation program is still underway and a second APT80/2, equipped with a CCD camera, will pair theAPT80/1 on the same site. Its operation is foreseen for mid 2002.

Key words: stars: magnetic fields – stars: activity – telescopes

1. The APT-80/1

The APT80/1 telescope is installed at the M. G. Fracastoromountain station of Catania Astrophysical Observatory onMt. Etna (1750-m a.s.l) (Fig. 1). It was built by AutoScopeCo. (Mesa, AZ, USA) and it is the first Automatic Photoelec-tric Telescope (APT) installed outside the USA. It is a 0.8-mCassegrain telescope (model 800A) with an f/8 equivalent fo-cal ratio and an aluminium horseshoe mount. The telescope’soptical configuration is detailed in Table 1.

The telescope is equipped with a four-port instrument se-lector (IS-400) with attached the following instruments: i) anRP-500 computer controlled photometer with an uncooledHamamatzu R1414 SbCs photomultiplier and Johnson’s stan-dard UBV filters; ii) a fast CCD camera (Model LYNXX PC192x165 Pxl) for setting purpose.

The APT80/1 is a completely automated PC-controlledtelescope, that is capable of performing robotic photoelec-tric observations according to a target schedule loaded into aPC. A specifically designed computer code, Automatic Tele-scope Instruction Set (ATIS), is used to load the target list,

The APT80/1 characteristics, photometric data archiveand highlight of the applications can be also retrievedfrom the Catania Astrophysical Observatory web site athttp://web.ct.astro.it/sln/apt80.html

Table 1. The APT-80/1 optical configuration

Optical configuration CassegrainMain mirror (Pyrex) diameter 0.8 m

thickness 0.05 mcurvature paraboloidfocal length 1.6 mfocal ratio f/2

Secondary mirror diameter 0.14 mCassegrain focus equivalent focal length 6.4 m

equivalent focal ratio f/8

the observation sequence and parameters. As a rule, in ad-dition to the target stars, a bright navigation star, which isthe first target the APT-80/1 hunts, a non-variable comparisonstar, with respect to which the target is observed differentially,and two or three check stars, to check the non variability ofthe comparison star, are also observed, according to a prese-lected sequence. This observation sequence constitutes a so-called group. A schedule priority is given to the groups bestplaced in the visible sky, so that the telescope efficiency andmeasurement precision are optimized. Alternatively, a fixedgroup sequence can be loaded, in order to observe a groupat a particular time or several times with assigned repetitiontimes.

© WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2001 0044-6337/01/5-612-0333 $ 17.50+.50/0

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Fig. 1. The APT80/1 telescope on Mt. Etna.

Starting from the so called home position, i.e., the telescopeat the mechanical reference point, the APT80/1 is capableof centering the first star of the first group to be observedwithin 1-2 minutes of time. The other stars belonging to thesame group are centered within 10 seconds. The first starof a following group is centered within 1 minute. Thanksto such pointing performances, the typical duty cycle of theAPT80/1, as the fraction between actual measurement timeand total observing time, is or better, while for con-ventional photometric telescope (such as the ESO50 at LaSilla, Chile) it is usually less than .The typical precision of UBV measurements are of the orderof 0.015, 0.010, 0.007 magnitudes, respectively. On a moon-less clear nights, the limiting V magnitude is about 13 mag.The observations are automatically started and ended at twi-light and a Weather Station by means of a dedicated software(Meteorologic Data Collector) can interrupt the observations,if necessary, and close the dome at any time. For each starwe can select the integration time in each filter, separately forthe object and the sky, the number of measurements and theoff-set position where sky measurements have to be done.During the years 1992-2000 the APT-80/1 achieved

hours of observations, measurements.The distribution of the observing nights and observed groupsper year is shown in Fig. 2.

Fig. 2. Statistics of the observing nights and observed groups duringthe period 1992-2000.

0.0 0.5 1.0 1.5

10.0

5.0

0.0

V(a): Hipparcos MSand error range

V(b): theoretical MS

III: Hipparcos giant sequence

Fig. 3. The selected known or suspected chromospherically activestars on the H-R diagram.

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Table 3. The APT-80/2 optical configuration

Optical configuration Ritchey-ChretienMain mirror diameter 0.8 m

thickness 0.11 mcurvature hyperboloidfocal length 2.4 mfocal ratio f/3

Secondary mirror diameter 0.26 mCassegrain focus equivalent focal length 6.4 m

equivalent focal ratio f/8

Fig. 4. The APT80/2 telescope under construction.

¿From August 1992, when it became fully operative, theAPT-80/1 is being devoted to the systematic photometricmonitoring of a selected sample of known or suspected chro-mospherically active stars (see Table 2 and Fig. 3).

2. The APT-80/2

The APT80/2 is currently under construction by CostruzioniOttico-Meccaniche Marcon (Italy) and will pair the APT80/1on the same site (Fig. 4). Its operation is forseen for mid 2002.It is a 0.8-m Ritchey-Chretien telescope with an f/8 focal ratioand an equatorial fork mount. The APT80/2 optical configu-ration is detailed in Table 3. The telescope will be completelyautomated and PC-controlled. Equipped with a CCD camerait will be devoted primarily to photometry, while its spectro-scopic use is under consideration.

3. Highlights

3.1. Collaborative monitoring campaigns

The APTs can play a fundamental role in collaborative cam-paigns organized to study time variable phenomena on mag-netically active stars and in variable objects in general. Wefocus here on selected results obtained for active stars dur-ing some of recently organized collaborations. The most in-teresting investigations were based on simultaneous or nearsimultaneous multi-band observations that covered not onlythe optical band but also the radio, UV, EUV and X-ray do-mains. The study of the quiescent emission, i.e., outside-of-flare events, provided us with information on the atmosphericfilling factor of the active regions by measuring the ampli-tude of the rotational modulation of the broad-band or linefluxes. The phase correlation between the flux modulation inthe different spectral domains was used to infer the three-dimensional structure of the active regions in rapidly rotatingstars (cf., e.g., Neff et al. 1996). However, quiescent phaseslasting for one entire rotation are not the rule, but rather theexception in very active stars. Actually, during most of thecampaigns large flares were detected, sometimes in the formof a continuous sequence of flares lasting a few days. Theselong-lasting and energetic events provided constraints on theenergy content of the atmospheric magnetic fields and themechanisms for impulsive energy release and plasma heat-ing. The energy released in the optical bands in the case ofthe most energetic events amounted up to erg andthe total energy budget was certainly larger, even by one or-der of magnitude, because a substantial part of the energy wasreleased as kinetic energy or accelerated particles (cf., e.g.,Foing et al. 1994; van den Oord et al. 1996). Close binariesshowed, on average, more powerful events, possibly origi-nated from the release of magnetic energy stored in structuresinterconnecting the two components.

3.2. Stellar Flares

The UBV monitoring of active stars is a powerful tool to de-tect flares and to study their statistical distribution in activestars. The optical flux can be correlated, on average, withthe flux in other spectral domains using simple scaling laws,which allows us to estimate the total energy budget of a flare.Sometimes strong flares are observed which put severe con-straints on the atmospheric magnetic field intensities (cf., e.g.,Pagano et al. 1997). Long-term monitoring is extremely use-ful to characterize the contribution of flares to the stellar over-all energy budget and to detect flare preferential longitudes,as in the active dwarf EV Lac (Leto et al. 1997). Of course,the APTs can do a precious job in these investigations, whenused to monitor a given objects in a systematic way (see, e.g.,Fig. 5). More information on the properties of stellar flaresand the underlying physical process in the framework of thesolar-stellar connection can be found in Haisch, Strong &Rodono (1991).

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Table 2. APT-80/1 target stars

HD 8357 AR Psc HD 114519 RS CVn HD 209813 HK LacHD 8358 BI Cet HD 116204 BM CVn HD 210334 AR LacHD 9902 BG Psc HD 116544 IN Vir HD 216489 IM PegHD 12545 XX Tri HD 117555 FK Com HD 218738 KZ AndHD 17433 VY Ari HD 119213 CQ UMa HD 219113 SZ PscHD 20629 XX Ari HD 134319 IU Dra HD 224085 II PegHD 21242 UX Ari HD 136901 UV CrB HD 234601HD 21845 V577 Per HD 143271 HD 234677 BY DraHD 22403 V837 Tau HD 143313 MS Ser HD 250810 CQ AurHD 22468 V711 Tau HD 144110 EV Dra HD 283750 V833 TauHD 26337 EI Eri HD 150708 WW Dra HD 337518 V511 LyrHD 32008 63 Eri HD 160538 DR Dra HD 341475 MM HerHD 37394 HD 166181 V815 Her SAO 91772 LN PegHD 37824 V1149 Ori HD 167605 SAO 130113 BY CetHD 52452 HD 170527 BD+16 516 V471 TauHD 65626 AE Lyn HD 171488 V889 Her BD+16 4908HD 86590 DH Leo HD 175742 V775 Her BD+20 2465 AD LeoHD 106225 HU Vir HD 179094 V1762 Cyg BD+43 4305 EV LacHD 106677 DK Dra HD 184398 V1817 Cyg BD+48 3686 V383 LacHD 107146 HD 199178 V1794 Cyg BD+49 2392HD 108102 IL Com HD 200391 ER Vul BD+61 1211 DM UMaHD 112313 IN Com

Fig. 5. Flares detected by the APT80/1 in the U-band during the1999 coordinated observing campaign on EV Lac.

3.3. Light curve analysis and mapping techniques

The systematic use of APTs allows us to achieve a long-termsystematic UBV monitoring of active stars, either single orin close binaries. The amount of physical information in abroad-band optical light curve is not comparable with thatin high-resolution spectra, but continuous monitoring with astandardized and stable equipment provides us with a uniqueopportunity to acquire homogeneous data sets and to studythe evolution of the surface features in a representative sam-ple of active stars. Specifically, we can identify and map sur-face inhomogeneities and to study their typical behaviour andevolutionary timescales by means of specific techniques, suchas the pooled variance diagrams for long-term sequences oflight curves (cf., e.g., Rodono et al. 2000). The presence ofactive longitudes and their time evolution can be derived from

the same kind of data and the results compare well with thosederived from Doppler imaging maps, when simultaneouslyavailable (cf. Korhonen et al. 2001) . Surface features canalso be used as rotation tracers to detect stellar differentialrotation and its time variations, as well as the long-term mi-gration of the activity belts along activity cycles (cf., e.g.,Rodono, Lanza & Catalano 1995).

In order to perform these investigations we need to ex-tract information on the position and area of the surface fea-tures in single stars, as well as in the components of closebinaries. The study of eclipsing binaries is particularly inter-esting because one can use eclipses to increase the resolutionof photometrically-based mapping techniques. The method isknow as eclipse mapping and uses the scanning of the com-ponent, being occulted by the disk of the component in fronton the disk, to map its brightness inhomogeneities. The geo-metrical resolution depends on the photometric accuracy, theintrinsic light fluctuations (e.g., flares or pulsations), as wellas on the duration and depth of the eclipse. Typical resolu-tions are in the case of RS CVn binaries observedwith our APTs. It is important to stress that the constructionof a map using broad-band photometric data is not a well-posed problem, even in the cases when eclipses provide uswith the inclination of the stellar rotation axis and the relativeluminosities of the binary components. In order to obtain aunique and stable solution it is necessary to add some kindof a priori information to the data. Usually this is done byassuming a simple geometrical shape for the brightness in-homogeneities, such as rectangular or polar cap spots of uni-form effective temperature. This approach is well suited forsingle or non-eclipsing binaries, but it fails to reproduce thedetails of the light curves during eclipses. In order to fullyexplore the effective increase of resolution during eclipses, itis necessary to use more sophisticated a priori assumptions

M. Rodono et al.: The Catania Automatic Photoelectric Telescope on Mt. Etna 337

which can warrant the uniqueness and stability of the solu-tion, while allowing a continuously varying brightness distri-bution on the surface of the star. The Maximum Entropy tech-nique (e.g., Cameron & Hilditch 1997; Lanza et al. 1998a) –hereinafter MEM – and the Tikhonov regularization (Lanza etal. 1998a) – hereinafter TIK – proved to be especially suitedfor light curve inversions and allowed us to fully exploit theincreased of resolution during eclipses.

An example of the application of those methods is givenin Fig. 6 and Fig. 7 where the light curve fits and the mapsof the active RS CVn-type binary AR Lacertae in the secondhalf of 2000 are shown. The application of both regularizationcriteria to the same data set is particularly interesting becauseit allows us to distinguish between the map’s features actu-ally required by the data and the artifacts introduced by thea priori assumptions implied by each regularization. In thecase of photometric data, the intrinsic information content ofthe data is usually lower than in the case of a sequence ofhigh resolution spectra, thus giving more weight to the sta-tistical assumptions introduced by the regularization method.Therefore, the simultaneous use of independent regulariza-tion methods is highly advisable in the present case.

At a first glance, the MEM and the TIK maps do not ap-pear very similar, but a more detailed analysis shows that thelongitude location of the relative maxima of the brightnessdistribution is similar in both cases. Their total area is sys-tematically larger in the TIK maps, because they provide thesmoothest maps compatible with the data, while the MEMmaps tend to display smaller and contrasted features. How-ever, the seasonal variations are very similar, unless the in-trinsic data content is insufficient to derive the total spottedarea (cf. Lanza et al. 1998a).

3.4. Active longitudes and activity cycles

The systematic application of the approach discussed in theprevious Section to our photometric data sequences has beenpursued in a number of papers dedicated to the analysis ofthe long-term photometry of the prototype active binary RSCVn (Rodono et al. 1995), AR Lac (Lanza et al. 1998a), IIPeg (Rodono et al. 2000), SZ Psc (Lanza et al. 2001) andRT Lac (Lanza et al. 2002). Moreover, a number of singlestars have been investigated (cf., e.g., Messina et al. 1999;Messina, Guinan & Lanza 1999). Other RS CVn binaries willsoon be added to this list, among which UX Ari, RT And and

And, complementing the Catania data with those obtainedat other Observatories.

The most interesting results we obtained concern the du-ration of the cyclic behaviour of the total spot area in activeclose binaries and the identification of active longitudes. Inthe K0 IV component of AR Lac the analysis of about 25 yof optical photometry provided evidence of a large and per-sistent active region located around the substellar point, i.e.,on the stellar surface along the line joining the centers of thetwo components. Two other active longitudes showing a lesspersistent spot activity were found at from the mostactive one (Lanza et al. 1998a). A similar behaviour seemsto be present in SZ Psc (Lanza et al. 2001). It is interesting

to compare such behaviour with what observed on HK Lac,where two active longitudes, separated by , appear tohave been present for more than 30 y (Olah et al. 1997). Onthe contrary, the K2 IV active component of RS CVn showsevidence of a migration of the most active longitude with re-spect to the orbital time reference frame. Recently, Berdyug-ina & Tuominen (1998) suggested that in EI Eri and II Peg(and possibly in Gem and HR 7275) the most active longi-tude may switch by in a few months in a way rem-iniscent of the so-called flip-flop phenomenon in the singleFK Com star (Jetsu, Pelt & Tuominen 1993). A recent anal-ysis by Rodono et al. (2000) confirmed the occurrence of theflip-flop on the active component of II Peg and the durationof the switching cycle was estimated. More precisely, threeactive longitudes were identified on the star: one active lon-gitude was continuously present and characterized by a spotcycle of y and the other two alternatively active with acycle of y (see Fig. 8 and Fig. 9).

It is important to note that when only sparse data areavailable for a given star, it is very difficult to discriminatebetween a regular migration of the active longitudes and aswitching of the activity among different fixed longitudes.Therefore, a great improvement in our understanding of ac-tive longitudes is expected in the forthcoming years as moreextended and nearly continuous data sets will be made avail-able through the extensive use of APTs.

On the theoretical side, mean field dynamo models forsingle stars by assuming a dynamo action extended overthe entire convective envelope and allowing for a non-axisymmetric field distribution have been explored by Mosset al. (1991, 1995). When the differential rotation is not large( ), non-axisymmetric field modes with an az-imuthal wavenumber can be excited at dynamo num-bers lower than the modes with . The extension ofsuch an approach to binaries is still very preliminary, butMoss & Tuominen (1997) suggested a preference for themean field to concentrate around the line joining the centersof the two components. However, other relevant aspects ofthe starspot longitude distributions are not explained by thesemodels, in particular the possible migration of the active lon-gitudes versus time and the flip-flop phenomenon (see, how-ever, Berdyugina & Tuominen 2000).

The correlation of the optical modulation with the modu-lation of the flux in the chromospheric lines and in the EUVlines allows us to study the three dimensional structure ofthe stellar active regions. The cool spots in the photosphereare usually associated with enhanced emission in the chromo-spheric and transition region layers, although they may not beexactly co-spatial. In AR Lac there is clear evidence for X-rayemission arising from coronal structures overlying the photo-spheric spots located around the substellar points (Lanza etal. 1998a). Similar evidence was found for SZ Psc from theanalysis of the rotational modulation and eclipses in the op-tical continuum and the EUV lines (Lanza et al. 2001). Theanalysis of the H line excess emission is a powerful tool tostudy stellar chromosphere. The H excess due to chromo-spheric faculae usually appears to be anti-correlated with theV-band flux modulation due to photospheric spots, indicat-

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Fig. 6. The maximum entropy fit of the AR Lac light curve obtained in the second half of 2000 by the Catania and Phoenix APTs. Themaps of the surface components at the indicated phases are plotted in the lower panels.

Fig. 7. The Tikhonov fit of the AR Lac light curve obtained in the second half of 2000 by the Catania and Phoenix APTs. The maps of thesurface components at the indicated phases are plotted in the lower panels.

M. Rodono et al.: The Catania Automatic Photoelectric Telescope on Mt. Etna 339

Fig. 8. V-band photometry of II Peg in the 1974-1998 interval. Open diamonds: photometric data collected from the literature; filled circles:APT80/1 photometry.

Fig. 9. (a) The II Peg longitudes of maximum spottedness, derived from ME and Tik solutions, versus time. The active longitude A ispermanent, whereas B and C are alternatively active. The vertical dashes mark the beginning of the activity cycles of longitude A. (b) Theangular separation between the permanent active longitude A and the switching longitudes versus time. The continuous line is a sinusoidalfit with a period of 6.8 y, corresponding to the period of the flip-flop switching. (c) The photometric period with its uncertainty, as derivedby periodogram analysis. Continuous lines are linear best fits to the data. (d) The variation of the unevenly distributed spot area A versustime. The continuous lines are periodic fits with P=9.5 y.

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Fig. 10. H emission curve (continuous line) and V-band light curve(dashed line) for UX Ari in 1989, 1994, 1996 and 1997. The H ex-cess of chromospheric faculae appears to be anti-correlated with theV-band flux modulation due to photospheric spots. Circled symbolsrepresent the H equivalent width contaminated by flares.

ing that bright faculae overlie cool spots in stellar active re-gions, as expected on the basis of solar analogy (see Fig. 10)In some cases, chromospheric faculae may be systematicallyshifted in longitude with respect to the photospheric spots(Catalano et al. 1996; Catalano et al. 2000). It must be noted,however, that in very active stars, the faculae extension, muchwider than photospheric spots, may cover almost completelythe star chromosphere and the question of spatial correlationbetween spots and plages can not be addressed.

Magnetically active stars with outer convection zonesgenerally show long-term cyclic modulations of their activ-ity indicators, as in the Sun. The simplest and less indirectproxies of activity cycles are the quasi-periodic modulationof the total starspot area, as derived from spot models, andthe variation of the mean stellar magnitude. It is important tonote that in the case of the Sun the average bolometric lumi-nosity increases when there are more spots, an effect due tothe larger area and brightness of the faculae and photosphericmagnetic elements (Foukal & Lean 1988). In the case of veryactive stars, however, the colour indexes indicate that coolspots usually dominate the photometric modulation (Henryet al. 1995, cf. also Foukal 1993, Solanki 1999).

Our long-term photometric monitoring allowed us to de-tect activity cycles with periods from to y. Longercycles can not be ruled out given the limited time interval cov-ered by the data and are indeed suggested by the long-termtrends of the total spotted area (cf., e.g., Fig. 11). Moreover,in the case of the best studied active stars, only two or, in afew cases, three cycles of the spot area modulation have beenobserved. Therefore, the best approach to study the relation-ship between activity cycles and stellar parameters is a statis-tical one. Such an approach suggests that the hydromagneticdynamos in very active stars may be working in a differentway than in moderately active solar-like stars (cf. Baliunas etal. 1996; Saar & Brandenburg 1998; Olah, Kollath & Strass-meier 2000).

In addition to the long-term variation of the spotted area,Henry et al. (1995) proposed other indicators of activity cy-cles, i.e., the modulation of the mean colour indexes andthe modulation of the orbital period in close binaries. Spruit(1982) and Spruit & Weiss (1986) convincingly ruled out anydetectable change of the color indexes due to the surface re-distribution of the flux blocked by starspots. Moreover, theobserved long-term variation of the index is usuallyvery small and may be affected by factors not directly con-nected with spot activity (cf. Amado & Byrne 1997). There-fore, color index variations should not be regarded as goodproxy indicators of activity cycles.

3.5. Orbital period modulation

Active close binaries belonging to the RS CVn and Algolclasses show cyclic changes of the orbital period with rel-ative amplitudes of . The typi-cal timescales are of the order of y with a medianvalue of y (Hall 1989). The phenomenon is studied bymeans of long-term eclipse timing or radial velocity mon-itoring (cf. Donati 1999; Frasca & Lanza 2000). Explana-tions based on a light-time effect due to the motion arounda third body can account for only a few cases because thechanges usually are not exactly periodic. Apsidal motion canbe excluded because the orbits should have been circularizedby tidal effects. Hall (1989) pointed out that orbital periodchanges of alternate sign are observed only in close bina-ries with rapidly rotating components and deep outer con-vection zones, the basic ingredients for a strong magnetic ac-tivity. Several physical mechanisms have been proposed toexplain the connection between magnetic activity and orbitalperiod modulation, but they have encountered serious diffi-culties in accounting for the time scale of the modulation,which is shorter than the spin-orbit tidal coupling, and theconservation of energy. The only mechanism that is capableof accounting for the short time scales and can satisfy therequired energetic constraints is that proposed by Applegate(1992) and revisited by Lanza, Rodono & Rosner (1998b) andLanza & Rodono (1999). Applegate (1992) suggested that anon-linear hydromagnetic dynamo may modify the distribu-tion of the internal angular momentum of the active compo-nent along the activity cycle. An internal exchange of a fewper cents of the total angular momentum produces a slight

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Fig. 11. (a) The total spotted area (average between MEM andTIK models) of the RT Lac primary component, (b) the longitude ofthe centroid of the spot pattern and (c) the diagram vs.time. The solid line connecting the points in panel b) was drawn onlythe help the eye. The four intervals of approximately constant orbitalperiod described in the text are indicated in panel c) and the timesof orbital period changes are marked by the dotted vertical lines.The long-dashed lines in panel c) are linear best fits to the

points during intervals of approximately constant orbital period(after Lanza et al. 2002).

modification of the oblateness of the star and of its gravita-tional quadrupole moment. Therefore, the outer gravitationalfield of the active component fluctuates with the same periodof the activity cycle and the gravitational acceleration of thecompanion star is perturbed with the same period. Therefore,when the quadrupole moment is maximum, the companion isforced to move closer and faster leading to a minimum of theorbital period; when the quadrupole moment is minimum, theorbital period reaches a maximum. The relative orbital periodchange is:

(1)

where is the gravitational quadrupole moment, the massof the star, and the semi-major axis of the orbit (see Apple-gate 1992 for more details). The models proposed by Lanzaet al. (1998b) and Lanza & Rodono (1999) explored the con-nection between Applegate’s mechanism and the non-linearhydromagnetic dynamo models showing that they can indeedproduce variation of the quadrupole moment of the requiredorder of magnitude. They also suggested that the relationshipbetween the orbital period cycle and the activity cycle may bemore complex than supposed by Applegate (1992) due to theaction of the internal Lorentz force on the star figure of equi-librium. The observations of RS CVn (Rodono et al. 1995),AR Lac (Lanza et al. 1998a), SZ Psc (Lanza et al. 2001) andRT Lac (Lanza et al. 2002) gave support to the improvedmodel. Lanza & Rodono (1999) further investigated the en-

ergetic constraints on the quadrupole moment variations byapplying the model to a sample of 46 close binaries.

Long-term photometric monitoring is of fundamental im-portance to study the orbital period variations of close bi-naries and the correlation of the orbital period changes withthe variations of the starspot area on the active components.Rodono et al. (1995) showed that the orbital period of RSCVn decreases when the total spot area on the active K2 IVcomponent decreases and Lanza et al. (2001) found an in-crease of the orbital period associated with an increase ofthe mean spotted area on the active component of SZ Psc.The recent study by Lanza et al. (2002) shows that the or-bital period decrease of RT Lac is indeed associated with therelative minima of the spotted area and possibly, with the in-crease of the stellar rotation rate, as shown by Fig. 11. Thetotal starspot area on the more massive primary and the lon-gitude of the centroid of the spot distribution are correlatedwith the O–C diagram of the system orbital period. The lon-gitude reference frame is defined in such a way that longitudeincreases in the direction opposite to stellar rotation. Our ob-servations thus suggest that the quadrupole moment is at aminimum and the orbital period is at a maximum when themagnetic energy of the star is at a maximum, as indicated bythe relative maximum of the spot activity. Conversely, whenthe magnetic energy is at a minimum, the quadrupole mo-ment is maximum and the orbital period is minimum. Sucha result can be understood in the framework of the operationof a stellar dynamo which converts alternatively kinetic rota-tional energy into magnetic energy, as discussed in Lanza etal. (1998b) and Lanza & Rodono (1999).

3.6. Photometric parameters of active close binaries

Photospheric cool spots produce significant distortions onwide-band light curves of close active binaries and they canaffect the determination of the photometric and system pa-rameters. Therefore, it is necessary to model accurately thespot effects in order to estimate the true luminosity ratio, thefractionary radii and the inclination of the orbital plane in aconsistent way. Rodono, Lanza & Becciani (2001) addressedsuch a problem for the prototype active binary RS Canum Ve-naticorum, for which the availability of long-term and accu-rate series of light curves allowed us to minimize the starspoteffects on the estimation of the quoted parameters. The lightcurve sequence was analysed by using a specific parallel codewe developed for such an application by solving simultane-ously for the system parameters and spot configuration. Theuniqueness and stability problems related to the spot map-ping were solved by means of the regularization approachesdescribed above.

As an application of the derived results, Rodono et al.(2001) studied the positions of the components of RS CVnon the H-R diagram and compared them with computed evo-lutionary tracks. The effective temperature of the cooler com-ponent turned out to be % smaller than expected on thebasis of its radius, thus suggesting some effects of magneticactivity on the internal structure and evolution of the star.

The method developed by Rodono et al. (2001) is alsoof interest to accurately measure the inclination of the or-

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bital planes and to detect precessional motions in young RSCVn binaries, the spin angular momenta of which are not yetaligned with the orbital angular momentum.

3.7. Ground-based support to space mission

Space missions dedicated to long-term observations of stars,e.g., to look for p-mode oscillations, need to be supportedby ground based collaborative campaigns to accurately de-termine the stellar properties and to study stellar variability.This is of fundamental importance when surface activity maysignificantly affect the determination of stellar parameters,e.g., the color indexes or the total energy output. Moreover,magnetic activity may influence in a complex way the excita-tion and the propagation of the p-modes producing frequencyshifts that vary along the activity cycle as observed in the Sun(e.g., Elsworth et al. 1994; Jimenez-Reyes et al. 1998). There-fore, a simultaneous photometric and spectroscopic monitor-ing of the observed stars is strongly recommended (see alsoLanza & Rodono 2002). The space mission COROT, whichwill be launched in 2004, will benefit from such a kind of cov-erage and will also open a new way to look at activity-relatedvariability with the invaluable advantage of a photometric rel-ative accuracy of .

4. Conclusion

We have briefly reviewed the main characteristics of theAPTs implemented by Catania Astrophysical Observatorygiving a short account of the research programs that are beencarried out. The systematic time coverage that APTs allow,also in collaboration with other telescopes, is very valuablefor studying solar-like star variability and other classes ofvariables. The limiting stellar magnitude of the present APTusing photomultipliers is about ; but the newAPT equipped with CCD can allow us to extend the observ-able magnitude range. We have focused our review on theapplications to magnetically active stars, but many other pro-grams are possible including, e.g., the monitoring of Algols,cataclysmic variables, symbiotic stars and long-period vari-ables, as well as extragalactic objects.

Acknowledgements. The acquisition of photometric data over somany years with the Catania APT80/1 has been possible thanks tothe dedicated and highly competent technical assistance of a num-ber of people, notably S. Sardone, P. Bruno and E. Martinetti. Activestar research at Catania Astrophysical Observatory and the Dept. ofPhysics and Astronomy of Catania University is funded by MURST(Ministero dell’Universita e della Ricerca Scientifica e Tecnolog-ica), CNAA (Consorzio Nazionale per l’Astronomia e l’Astrofisica)and the Regione Siciliana, whose financial support is gratefully ac-knowledged.

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