Astron. Nachr. / AN 325, No. 6–8, 483–489 (2004) / DOI 10.1002/asna.200410266

Stellar activity and the long-term use of robotic telescopes

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

� Dept. of Physics and Astronomy, Catania University, Via S. Sofia 78, 95123 Catania, Italy� National Institute for Astrophysics, INAF HQ-Rome, Viale del Parco Mellini 84, 00136 Roma, Italy� INAF, Catania Astrophysical Observatory, Via S. Sofia 78, 95123 Catania, Italy

Received 6 August 2004; accepted 10 September 2004; published online 31 October 2004

Abstract. A number of automated and robotic telescopes are nowadays devoted to the systematic monitoring of magnet-ically active stars and binary systems at several astronomical institutions, all over the world, and their number is steadilyincreasing. Standard equipments include wide– and narrow–band photometers and, more recently, spectroscopic capabilities.The long-term time series that those telescopes are providing turn out to be of paramount importance in order to significantlyprogress in our understanding of solar-like stellar activity of magnetic origin, that seemingly affect most of late-type dwarfsand subgiants. Our principal aim is to illustrate which key parameters, that can be derived from such long-term time series,determine the appearance and evolution of stellar activity phenomena in different astrophysical environments other than so-lar, and their role in determining the physical characteristics of starspots, their surface distribution, filling factor, migration inlatitude and longitude, and evolution in time. By using spots as tracers of stellar rotation, reliable data on stellar differentialrotation, the prime motor of magnetic activity, can be derived. Moreover, the activity cycle is the additional fundamentalparameter that can be provided by long-term time series. In order to properly address the study of stellar activity, an interna-tionally coordinated network of 1-2 m class robotic telescopes dedicated to multi-wavelength systematic observations shouldbe established.

Key words: binaries: close – stars: activity – stars: late-type – stars: rotation – stars: spots – stars: variables

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1. Introduction

The systematic use of Automated Photometric Telescopes(”APT”s) or even fully Robotic Telescopes (”Robo-tel”) inthe past dozen years has allowed us to collect scientificallyprecious long–term photometric time series that are essen-tial for the purpose of studying the time behaviour of late-type magnetically-active stars and for modelling their spottedphotospheres. From these data it has now become possible toderive detailed surface maps, as well as the physical charac-teristics of starspots, their area and distribution, the rotationrates, differential rotation regimes, and the evolution in timeof activity features. This implies that a direct comparison be-tween theoretical predictions and observational results is nowfeasible. In particular, the key parameters presiding over theseveral facets of stellar activity due to the operation of an� � � dynamo, that provides intensified magnetic fields thatare the prime motors of stellar activity, can be compared withphysical parameters derived from observations. In this con-text, some of the achieved highlights will be illustrated.

Correspondence to: mro@ct.astro.it

2. Key parameters of stellar activity

A qualitative description of the operation of an oscillating hy-dromagnetic � � � dynamo in the frozen-field approxima-tion, i.e., when the Reynolds number (the ratio between theconvective and diffusive terms in the induction equation) ismuch larger than unity is schematically shown in Fig. 1. Asimple inspection of this Figure indicates which are the dy-namical agents of the dynamo and its key parameters, namely,rotation, differential rotation, and convection regimes – morespecifically, the rotation period (� ) or the rotation frequency�, the Æ���, the Rossby number or the inverse of the convec-tive turn–over time (� ) given by the ratio between the mix-ing length (�) and the mean convective velocity (��), and theactivity cycle period (����). All of these parameters, can beobtained, directly or indirectly, from the analysis and mod-elling of long-term photometric time series. In addition, theseobservations have shown previously unknown peculiar be-haviours, such as antisolar differential rotation, preferentiallyactive longitudes and the so-called ”flip-flop” phenomenon,consisting in two preferred longitudes, about 180 deg apart,where spot activity alternatively concentrates. As usual, novel

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Fig. 1. An operation scheme of the �� � dynamo process [adaptedfrom Paterno (1998) and Rodono (2000)]. Rotation and convection,giving rise to differential rotation, lead to the azimuthal twisting offeeble poloidal field lines and produce an intensified toroidal field.Both the toroidal and poloidal fields would progressively vanish bydiffusion if differential rotation and the �-effect, respectively, wouldnot regenerate them.

Fig. 2. The rotationally modulated V-band fluxes from EK Dra andK1 Cet (Messina & Guinan 2003).

observations allow us to answer some old questions, but in-evitably rise new questions to be answered.

3. Automated and robotic telescopes at work

Limiting ourselves to APT’s and Robo-tel’s devoted to stel-lar activity monitoring programs, as the ”core” or one of the”ancillary” scientific goals, we should like to quote:

– the APT and Robo-tel complex at Fairborn Observatoryin Patagonia (AZ, USA), were about a dozen telescopeswith apertures from 25-cm to 80-cm are currently oper-ated, e.g., the ”Phoenix APT’s”, the ”Four College APT”,and the twin Vienna Obs. ”Wolfgang & Amadeus”

– the Catania Observatory (Italy) 80-cm APT-1, operatedsince 1992 on Mt. Etna, and a second APT of 80-cm, al-ready built and to be soon installed;

– the refurbished and automated 60-cm telescope ofKonkoly Observatory (Hungary);

– the 75-cm SAAO ”Alan Cousin” APT in Southerland(SA), mostly devoted to pulsating stars;

– the 60-cm REM (Rapid Eye Mount) Robo-tel, a collabo-rative project of Brera-Merate, Catania, Roma and Tri-este INAF Observatories, recently installed at La Silla(Chile) and primarily devoted to GRB optical and near-IR follow-up, stellar activity monitoring being one of the”additional” scientific goals;

– the 200-cm Liverpool (UK) Robo-tel, recently installed atLa Palma (Canary Islands, Spain), mainly devoted to thesearch and study of outer planetary systems;

– the Stromgren 50-cm APT of Copenhagen Observatory(Denmark) at La Silla (Chile);

– the Nassau Station 90-cm Robo-tel of the Case WesternReserve University (Ohio, USA);

– the RoboScope 41-cm of Indiana University at Morgan-Monroe (IN, USA);

– the Tennessee State Univ (”TSU”) automated spectro-scopic telescope of 210-cm.

4. What have we learned from long-term timeseries provided by APT’s and Robo-tel’s?

The long-term photometric monitoring of active stars byAPT’s or Robo-tel’s provides a powerful tool to derive rel-evant parameters of the stars themselves and of their surfaceactivity. Some highlights are illustrated in the following para-graphs.

4.1. Rotational modulation and spot modelling

The periodic or semi-periodic modulations of the observedoptical flux from late-type stars are the signature of the pres-ence of transient surface features on their photospheres, suchas cool spots and hot faculae. These features, coupled withthe stellar rotation, give rise to rotationally modulated fluxand, because of their non-permanent character, the resultinglight variation (cf. Fig. 2) generally shows a variable ampli-tude and shape, that sometimes almost vanishes. This lastevent, however, may result from the disappearence of discretefeatures previously on the photosphere, as well as from theirmore or less uniform re-distribution in longitude, includingthe extreme case of evenly distributed features along a con-stant latitude belt or, if saturation occurs, the entire stellar sur-face. In order to understand whether the observed null vari-ability is related to a maximum or null activity phase, it is im-portant to adopt a long-term observation strategy that allowsus to systematically monitor the mean brightness behaviourof the target object.

The straightforward observation of the variable flux mod-ulated by the changing fraction of spots on the stellar hemi-sphere facing the observer, as the star rotates, allows us a di-rect measurement of one of the fundamental stellar parame-ters, i.e. the rotation period or frequency.

A number of computer codes have been developed to in-terpret a single or a sequence of light curves in terms of dis-crete surface features. The most recent and fast of such codes

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M. Rodono et al.: Stellar activity and the long-term use of robotic telescopes 485

Fig. 3. (online colour at www.an-journal.org) The light curve andspot maps of AR Lac 1999 (Rodono et al. 2004, work in progress).

(MARA, Magnetically Active Rotator Analysis) was devel-oped in parallel mode by Becciani et al. (2004) and has re-cently become available on internet at the Italian Astrocomp-Grid Portal (http://www.astrocomp.it). The MARA code doesnot assume any a priori assumption on the spot size and dis-tribution and it allows us to analyze also eclipsing binary lightcurves by taking into account the typical photometric distor-tion due to proximity effects that characterize close binaries.As an example, the AR Lac 1999 case is presented in Fig. 3.

Quite generally, the resulting spots or actually spotted ar-eas may cover up to 30-40% of the star surface and they arefrom a few hundreds to one thousand degree cooler that thesurrounding photosphere. Their typical lifetime is from tensof days to years. Spots often cluster around preferred longi-tudes and, especially in close binaries, they appear placed atthe sub-stellar point and 180-degree apart. Moreover, a newphenomenon has emerged (Collier-Cameron & Donati 2002,Korhonen et al. 2002, Lanza et al. 2002): the most active ofthe two preferred longitudes alternate in time, producing asort of ”flip-flop” behaviour, as clearly shown in Fig. 4. Anew theoretical challenge just being addressed (Kitchatinov& Rudiger, 2004).

4.2. Long-term activity cycles

One of the parameters that is required for a correct modellingof the observed light curve of an active star is its unspottedmagnitude. This requires the availability of a long-term mon-itoring from which the time behaviour of the given star canbe derived. The brightest magnitude ever observed is gener-ally assumed as the best possible approximation to the truevalue of the unspotted level. Actually, a uniform distribution

Fig. 4. The spot maps of RT Lac from 1965 to 2000 showing prefer-entially active longitudes and the flip-flop phenomenon (Lanza et al.2002).

of residual spots or a permanent polar cap can not be ex-cluded, a priori, even at the brightest observed magnitude.Otherwise, only the asymmetric component of the spot dis-tribution can be derived.

Long-term monitorings provide another essential param-eter, namely the length of the activity cycle (����), i.e., theperiod of the dynamo operating in a given star. Photomet-ric time series covering tens of years since the late ’60s,and more recently boosted by robotic telescopes, are nowavailable (e.g., Rodono et al. 1995, 2002; Strassmeier et al.1997; Lanza et al. 1998, 2002; Cutispoto 1998; Cutispoto etal. 2001, 2003; Olah et al. 2000; Olah & Strassmeier 2002;Messina et al. 2001; Messina & Guinan 2002). These data,together with systematic monitoring of the Ca II K line flux(S index) started by Olin Wilson also in the late ’60 (e.g.,Wilson 1978, Baliunas et al. 1995), constitute a precious database for activity cycle studies. Whenever a sufficiently ex-tended monitoring time is available, a solar-like more or lessdefined activity cycle period, of the order of a few years totens of years (Fig. 5), is found on most of the observed stars.The first evidence that this might be true also at coronal levelhas been recently found (Favata et al. 2004). This is not sur-prising because, as demonstrated by Messina et al. (2002),photospheric and coronal activity diagnostics correlate fairlywell each other.

By applying the Scargle’s (1982) periodogram analysisor the false-alarm-probability method by Horne & Baliunas(1986) to long-term photometric time series, almost invari-ably multiple periods are found. As shown in Fig. 6 fromMessina & Guinan (2002), the dependence of the ratio be-

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Fig. 5. A few examples of short- and long-term activity cycles: the dashed lines are best fits to the seasonal mean V magnitudes (CataniaGroup on Active Stars & Systems (GASS) 2004, work in progress).

Fig. 6. The cycle frequency over the rotation frequency versus the in-verse Rossby number. The youngest objects (dots) have the smallestratios and the smallest values of (����). Inactive, active, and super-active objects show different slopes. Symbols joined by a verticalline indicate multi-frequency cycles on the same star (from Messina& Guinan 2002)

tween the activity cycle frequency and the rotation frequencyon the inverse Rossby number, and consequently on the con-vective turnover time, indicates that the stellar structure andits rotation regime are the key ingredients for stellar activityto develop.

4.3. Rotation period variation and differential rotation

In a differential rotation regime, if the mean latitude of spotformation migrates towards the equator along the activity cy-cle, as on the Sun, the photometric periods determined byusing spots as rotation tracers are subject to change. The firstindication of such a behaviour was found on the prototypeactive system RS CVn (Rodono 1965): the photometric pe-riod of the so-called wave-like distortion, that characterizesthe outside-of-eclipse light curve of RS CVn, was slightlyshorter than the orbital period and it was variable. This wassuggestive of a differentially rotating spotted star. A funda-mental contribution on this subject was given by Henry et al.(1995) and, more recently, by Collier-Cameron et al. (2002).

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M. Rodono et al.: Stellar activity and the long-term use of robotic telescopes 487

Fig. 7. Variation of the spot area mean latitude (upper two panels),of the photometric period and of the spot distribution asymmetriccomponent for three consecutive activity cycles on II Peg from spotmodelling by adopting the maximum entropy (ME) and the Tikonov(Tik) regularization criteria. Note the flip-flop phenomenon clearlyshown by the ME data (Rodono et al. 2000).

7.20

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Fig. 8. The activity cycle of DX Leo showing changes of the rotationperiod with different slopes in the successive cycles suggesting ananti-solar differential rotation regime (Messina & Guinan 2003).

Nowadays several systems have been extensively stud-ied (see Strassmeier, 2004 for a recent update). The caseof II Peg and DX Leo are illustrated in Fig. 7 (Rodono etal., 2000) and Fig. 8 (Messina & Guinan, 2003). These twoFigures clearly illustrate that solar-like and antisolar-like dif-ferential rotation patterns are encountered: a new challengefor the dynamo theory. Contradicting results are sometimesobtained (e.g., Weber & Strassmeier, 2001) The rotation pe-riod variation (�� ) correlates quite well with the rotation pe-riod (� ) according to a power law with exponent� ��� and,consequently, the cycle frequency (����) strongly correlateswith the differential rotation amplitude(����), as apparentin Fig. 9.

Fig. 9. The correlation between the cycle frequency (����)and thedifferential rotation (����) from Messina & Guinan (2003). Dif-ferent symbols denote different ages: 0.13 Gyr (dots), 0.30 Gyr(stars).

4.4. Correlations among activity indicators and globalstellar parameters

As anticipated in section 5.1, the key parameters of stellar ac-tivity are the differential rotation and convection regimes, thelatter implying the star internal structure. Hence, in a givenstar if the activity indicators at different atmospheric levelscorrelate each other, we may expect that they also correlatewith global stellar parameters, such as the Rossby number.Actually, Messina et al. (2001, 2002) showed that the follow-ing correlations exist among photospheric, chromospheric,transition region and coronal activity diagnotics, such as themaximum observed amplitude of the rotational modulationdue to photospheric spots (����), the luminosity of Mg IIand C IV emission lines, formed in the chromosphere andtransition region, respectively, and in X-ray:

�������� � ����

���� � ����� � � .

The most recent and illustrative example of the correla-tion between photospheric and overlying chromospheric ac-tivity features is given in Fig. 10 kindly provided before pub-blication by Lanzafame et al. (2004).

The dependence on global stellar parameters, e.g.,the differential rotation, is given by the equation:

���� � � ��� �

where the coefficient � � ������ ����� is equal for thethree age branches where the data concentrate (from inactiveto very active stars) as shown in Fig. 9, while the coefficientchanges from one age branch to the other (Messina & Guinan2003). The same authors have shown that the � � ����

amplitude correlates very well with the inverse of Rossbynumber �� (Fig. 11).

5. Newly emerged challenging questions

In addition to antisolar differential rotation and the ”flip-flop” phenomenon already introduced in the previous sec-tion, the long-term monitorings of active stars have shownanother interesting phenomenon, i.e., from close binaries

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Fig. 10. The correlation between photospheric spots and chromo-spheric plages on II Peg from contemporary V-band photometry and�� equivament widths (Lanzafame et al., 2004).

eclipse timings, it appears that the orbital period undergoescyclic changes that in some cases are correlated with the ac-tivity cycle (e.g., Rodono et al. 1995, 2000). This new find-ing has been interpreted in terms of gravitational quadrupolechanges of the active star figure of equilibrium due to a cycle-dependent magnetic reaction, as illustrated in more details inthe following paragraph.

5.1. Cyclic variation of the orbital and rotation period inclose binaries, and activity cycles

The long-term collection of eclipse times has revelaled thatthe orbital period (� ) of several types of close binary systemswith deeply convective late-type secondaries, i.e., potentiallycapable of exciting a hydromagnetic dynamo, are cyclicallymodulated with period ����, namely:

– RS CVn’s and Algol’s binaries with a component laterthan G0, with ���� � 30-100 yr and �� / � � ����;

– W UMa, with ���� � 10-100 yr and �� / � � ����;– Cataclysmic variables (CV) with ���� � 5-30 yr and�� / � � ��

��.

Applegate & Patterson (1987), Warner (1988) and Hall(1989, 1990) suggested on theoretical or phenomenologicalgrounds a connection between orbital period modulation andmagnetic activity. Actually, time-light or third body effectscould be rejected because observational or dynamical evi-dence is missing. A critical review of these models by Apple-gate (1992) concluded that a hydrodymagnetic dynamo mayproduce cyclic changes of the internal angular velocity ofthe active component and of its gravitational quadrupole mo-ment, to which an instantaneous change of the orbital dynam-ics and of the orbital period follow. The Applegate’s (1992)model predicts 1-3% variation of the differential rotation andthe cyclic variation of the orbital period to follow closely theactivity cycle. The long-term observations of several binariesdisagree with the amount of the predicted differential rotationchanges and, in at least three cases (RS CVn, RT Lac, andAR Lac), the spot cycle lengths are about half of the short-term orbital period modulation. An improvement of the Ap-

Fig. 11. The correlation between the maximum asymmetric spotcomponent (�� �����) and the inverse Rossby number ����

(Messina & Guinan, 2003).

plegate’s model by Lanza & Rodono (2004, and referencestherein) includes the role of the Lorentz force in perturbingthe figure of equilibrium of the active star when a torsionaloscillation wave perturbs the star rotation velocity and thetoroidal magnetic field. This improved model is consistentwith the observational data: the magnetic activity and orbitalcycle periods will have the same value if the ratio betweenthe perturbed azimuthal field (��������) and the unperturbedfield (��), is� �, while the former will be half of the latterif ����������� � �. Other predictions, such as the oppo-site vatiation of the kinetic (rotational velocity) and magneticenergy (total spotted area), are consistent with the observa-tions. A crucial test will be the detection of internal rotationchanges by asteroseismic techniques from space (Lanza &Rodono, 2002).

5.2. Robotic telescopes for spectroscopy: what they canadd to our understanding of stellar activity

As illustrated in the preceding sections, from long-term pho-tometry a number of activity or activity-related parametersand their variations can be derived, more specifically, a) thestar rotation period, b) rather detailed spot maps, c) the meanlongitude of spot complexes and less accurate estimates oftheir latitude, d) preferential longitudes and the ”flip-flop”behaviour, e) the surface differential rotation, f) the lightcurve amplitudes, i.e. the total spottedness versus time, andg) activity cycles. Flare activity can also be recorded. Theadvent of Robo-tel’s for spectroscopy, such as the TSU (Ten-nessee State University) 2.1-m telescope at Fairborn Obser-vatory (Eaton & Williamson, 2004) and the STELLA tele-scopes of the Potsdam Institute for Astronomy and the Ham-burg Observatory being installed in Tenerife (Strassmeier &the STELLA team, 2004), will significantly add to our knowl-edge of stellar activity because spectroscopy can provide ac-curate radial velocity information, such as rotational veloc-ity, surface and possibly radial differential rotation from linebisector analysis, and Doppler Imaging. Ideally, contempo-rary photometry and spectroscopy should always be acquired

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M. Rodono et al.: Stellar activity and the long-term use of robotic telescopes 489

because of their complementary nature. In addition, spec-tropolarimetry is a unique capability for magnetic mappingsand for the measurements of stellar magnetic fields (cf. Saar1991), i.e, the ultimate motors and the key parameter of stel-lar activity (Parker 1987).

6. Conclusions

Most of the relevant parameters that are needed for a signif-icant progress in understanding the physics of stellar struc-ture and magnetic activity can be provided by long-term con-temporary photometry and spectroscopy. Actually, we shouldaim at acquiring the best possible multi–frequency observa-tions to be possibly complemented by contemporary obser-vations at non-optical wavelengths from space-borne instru-ments.

Owing to a) the fundamental astrophysical relevance ofthese observations in assisting theoretical models and reliableinterpretations, b) the unpredictable and wide time-domainspanned by activity phenomena, from seconds to years, andc) the large range of parameters to be covered, a worldwideinternational effort aimed at building a coordinated networkof 1-2 m class Robo-tel’s, one of which to be installed in An-tartica or possibly on the Moon, is surely worth to be consid-ered and pursued.

Acknowledgements. Stellar activity research at Catania Astrophys-ical Observatory and the Department of Physics and Astronomy ofCatania University is supported by the Italian Ministry for Univer-sities and Research, the National Institute for Astrophysics (INAF),and by the Sicilian Regional Government. Their financial contribu-tions are gratefully acknowledged. Finally, we should like to thankalso an anonymous referee for helpful comments.

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