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A&A411,517–531(2003) Astronomy DOI:10.1051/0004-6361:20031414 & (cid:13)c ESO2003 Astrophysics The weak-line T Tauri star V410Tau I. A multi-wavelength study of variability B.Stelzer1,M.Fernańdez2,V.M.Costa3,4,J.F.Gameiro3,5,K.Grankin6,A.Henden7,E.Guenther8,S.Mohanty9, E.Flaccomio1,V.Burwitz10,R.Jayawardhana11,P.Predehl10,andR.H.Durisen12 1 OsservatorioAstronomicodiPalermo,PiazzadelParlamento1,90134Palermo,Italy 2 InstitutodeAstrof´ısicadeAndaluc´ıa,CSIC,CaminoBajodeHue´tor24,18008Granada,Spain 3 CentreforAstrophysics,UniversityofPorto,RuadasEstrelas,4150Porto,Portugal 4 DepartamentodeMatema´tica,InstitutoSuperiordeEngenhariadoPorto,4150Porto,Portugal 5 DepartamentodeMatema´ticaAplicada,FaculdadedeC´ıencasdaUniversidadedoPorto,4169Porto,Portugal 6 UlugBegAstronomicalInstitute,Astronomicheskaya33,700052Tashkent,Uzbekistan 7 USRA/USNOFlagsta(cid:11)Station,POBox1149,Flagsta(cid:11),AZ86002-1149,USA 8 Thu¨ringerLandessternwarte,Karl-Schwarzschild-Observatorium,Sternwarte5,07778Tautenburg,Germany 9 Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138,USA 10 Max-PlanckInstitutfu¨rextraterrestrischePhysik,Postfach1312,85741Garching,Germany 11 UniversityofMichigan,953DennisonBuilding,AnnArbor,MI48109,USA 12 IndianaUniversity,727E.3rdStreet,Bloomington,IN47405-7105,USA Received16June2003/Accepted4September2003 Abstract.WepresenttheresultsofanintensivecoordinatedmonitoringcampaignintheopticalandX-raywavelengthrangesof thelow-mass,pre-mainsequencestarV410TaucarriedoutinNovember2001.Theaimofthisprojectwastostudytherelation betweenvariousindicatorsformagneticactivitythat probedi(cid:11)erentemittingregionsandwouldallowustoobtainclueson theinterplayof thedi(cid:11)erent atmospheric layers: optical photometric star spot (rotation) cycle, chromospheric Hαemission, andcoronalX-rays.Ouropticalphotometricmonitoringhasallowedustomeasurethetimeoftheminimumofthelightcurve withhighprecision.JoiningtheresultwithpreviousdataweprovideanewestimateforthedominantperiodicityofV410Tau (1.871970(cid:6)0.000010d).Thisupdatedvalueremovessystematico(cid:11)setsofthetimeofminimumobservedindatatakenoverthe lastdecade.Therecurrenceoftheminimumintheopticallightcurveoversuchalongtimescaleemphasizestheextraordinary stabilityof the largestspot. Thisisconfirmed byradial velocitymeasurements: data from1993 and 2001 fitalmost exactly onto each other when folded with the new period. The combination of the new data from November 2001 with published measurementstakenduringthelastdecadeallowsustoexaminelong-termchangesinthemeanlightlevelofthephotometry ofV410Tau.Avariationonthetimescaleof5.4yrissuggested.AssumingthatthisbehavioristrulycyclicV410Tauisthefirst pre-mainsequencestaronwhichanactivitycycleisdetected.TwoX-raypointingswerecarriedoutwiththeChandrasatellite simultaneouslywiththeopticalobservations, andcenterednearthemaximumandminimumlevelsoftheopticallightcurve. A relation of their di(cid:11)erent count levels to the rotation period of the dominating spot is not confirmed by a third Chandra observationcarriedoutsomemonthslater,duringanotherminimumofthe1.87dcycle.Similarlywefindnoindicationsfor acorrelationoftheHαemissionwiththespots’rotationalphase.Thelackofdetectedrotationalmodulationintwoimportant activitydiagnosticsseemstoargueagainstadirectassociationofchromosphericandcoronalemissionwiththespotdistribution. Keywords.stars:individual:V410Tau–stars:late-type,coronae,activity–X-rays:stars 1. Introduction a circumstellar disk is responsible for ultraviolet and infrared excess emission and for a moderate to strong emission line V410Tau is an analog forthe youngSun on the pre-mainse- spectrum superimposed on the photospheric spectrum, are quence (PMS). Due to the lack of strong emission lines and calledclassicalTTauristars(cTTS).Indeed,V410Taushows infraredexcessitcanbeclassified asa weak-lineT Tauristar asmallinfraredexcesswhich,however,isattributedtooneor (wTTS).ThistermdefinesPMSstarswithoutobvioussignsfor twoclosecompanionsatsub-arcsecondseparation(Ghezetal. disk accretion, while young stars, in which accretion from 1993;Ghezetal.1997). Sendoffprintrequeststo:B.Stelzer, WTTS representan evolutionarystage wherethe disk has e-mail:[email protected] already dissipated, and therefore their variability is not Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 2. REPORT TYPE 3. DATES COVERED 2003 N/A - 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER The Weak-Line T Tauri Star V410 Tau: 1. A Multi-Wavelength Study of 5b. GRANT NUMBER Variability 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION Library U.S. Naval Observatory 3450 Massachusetts Avenue, N.W. REPORT NUMBER Washington, D.C. 20392-5420 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release, distribution unlimited 13. SUPPLEMENTARY NOTES 14. ABSTRACT 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF ABSTRACT OF PAGES RESPONSIBLE PERSON a. REPORT b. ABSTRACT c. THIS PAGE UU 15 unclassified unclassified unclassified Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 518 B.Stelzeretal.:Amulti-wavelengthstudyofV410Tau.I. related to accretion processes but believed to be a manifesta- Hatzes 1995). Petrov et al. (1994) suggest a chromospheric tion of magnetic activity, similar to that observed in the Sun, origin for the narrow emission peak that is often superim- butenhancedbyseveralordersofmagnitudes.Magneticactiv- posed on a wide platform (see also Hatzes 1995; Fernańdez ity seemstoexplainalltheopticalvariabilityphenomenadis- &Miranda1998),whiletheplatformorflatextendedemission coveredonV410Tau,comprisingalargerangeoftimescales: wingsmightarisefromthecircumstellargasenvironment. i) variability of the amplitude of the optical light curve on Thedi(cid:11)erentlinefluxesbetweenobservationstakenatdif- timescalesofyears;ii)changesintheHαprofileandintensity ferent epochs may be due to flaring. Flares are magnetic re- recordedwithinmonths;iii)repeatedmodulationoftheoptical connectionevents,andcanbeasshortas(cid:24)1h,suchthattheir brightnesswithaperiodof1.87d;andiv)suddenbrightenings evolution is di(cid:14)cult to trace in non-continuous observations. and/oremissionlineenhancementsevolvingontimescalesof For this reason the number of flares reported for V410Tau is hours. quitelimited.Mostoftheseeventswereobservedspectroscop- Items i) and iii) point directly at the presence of star ically (e.g. Welty & Ramsey 1995),and some flares were de- spots. The periodic1.87d variability of the optical lightcurve tected also in photometric monitoring programs (Rydgren & of V410Tau was first reported by Rydgren & Vrba (1983), Vrba1983;Vrbaetal.1988). and confirmed by subsequent observations (Vrba et al. 1988; In addition to these well-established activity phenomena Bouvier&Bertout1989).Itisattributedtostellarspotswhich observed in the optical, magnetic processes further up in the produce a periodic rotation pattern. From these studies it is atmosphere,i.e. the corona, are expected to give rise to radio shown that about 15−45% of the stellar surface of V410Tau and X-ray emission. Indeed, the radio emission of V410Tau should be covered by spots in order to explain the large am- is highly variable both in intensity and spectral index (Cohen plitudes observed in the optical bands. The models indicated etal.1982;Biegingetal.1984),leadingtotheconclusionthat temperatures for the spot(s) that are 500K to 1200K cooler it must arise from a non-thermalprocess, probably related to than the photosphere,similar to the solar case (Wallace et al. magnetic activity (see Bieging & Cohen 1989 and references 1996). Independent evidence confirming the spot hypothesis therein).Bieging&Cohen(1989)monitoredtheradiofluxden- was provided by high-resolution spectroscopic observations: sity at monthly intervals over one year. They failed to detect large ((cid:21)10%) fractions of the stellar surface at about 1000K strong radio flares, but found evidence for a modulation with belowthephotospherictemperaturecanconsiderablyalterthe a period that is half of the period reported from optical ob- profileofphotosphericlines(Vogt&Penrod1983;Vogtetal. servations.X-raydataofV410Tauhasbeenpresentedbye.g. 1987). The first Doppler images of V410Tau were published Strom & Strom (1994), Neuhaüser et al. (1995), and Stelzer byJoncouretal.(1994)andStrassmeieretal.(1994),andwere & Neuhaüser (2001). Costa et al. (2000) has extensively dis- confirmed by Hatzes (1995) and Rice & Strassmeier (1996). cussedarchivedInternationalUltra-violetExplorer(IUE)and Allofthemshowalargecoolspotthatextendsoveroneofthe ROSATobservationsofthisstar.Despiteitsobviousvariability stellarpolesandsmallercoolfeatureslocatedatlowlatitudes. intheX-rayregimenodirectsignsforrotationalmodulationor Longtermvariabilityofthephotometricamplitudecouldbe X-rayflareshavebeenobservedfromV410Tausofar. related to changes in the number, size and location of spots. The detailed relations among the various atmospheric Longterm periodic changes in photometric lightcurves of a regions involved in stellar magnetic activity (photosphere, numberofsolar-analogs(Messina&Guinan2002;Berdyugina chromosphereand corona) remain unexplained and call for a et al. 2002) and more evolved RSCVn binaries (e.g. Henry systematic investigation.Towards this end we organizedcon- et al. 1995; Rodono´ et al. 2000; Olah et al. 2000) have temporaneous optical and X-ray observations of V410Tau. been interpreted as magnetic activity cycles. Among younger Dedicated photometric as well as intermediate- and high- PMS stars a lack of dedicated monitoring programs has long resolution spectroscopic observations in the optical were impededanysystematicinvestigationofthisissue.Firstresults plannedforatimeintervalof(cid:24)11d,simultaneoustothreeex- onlongtermphotometryincludingPMSstarswerereportedby posures with the Chandra X-ray satellite. This enabled us to Strassmeier et al. (1997) based on observations at automated study activity on V410Tau that evolves on short timescales, photoelectric telescopes. Thanks to the e(cid:11)orts of Vrba et al. i.e. rotationale(cid:11)ects and flares. In addition we examinedhis- (1988), Herbst (1989),Petrov et al. (1994),Strassmeier et al. toricalphotometricdataofV410Tautoexamineitslong-term (1997),andGrankin(1999)V410Tauhasbyfarthelargestdata variability. base of photometry among the PMS stars. Although smooth We will present our results in a series of two papers. In changes in the amplitude of the optical light curve have been the presentpaperwe focuson the examinationof the relation reported,nocyclicorpredictablebehaviorhasbeenfoundyet. betweenemissioninvariousenergybandsandtheopticalrota- Theoriginofvariationsinemissionlines[itemsii)andiv) tioncycle,andthelong-termphotometry,whileinasubsequent of the abovelist] is sometimes less obvious.Hα observations paper(Fernańdezetal.,inprep.)wewillconcentrateonthenu- ofV410Taurevealslowchangesbothintheemissionlinepro- merousflaresfoundduringourcampaign.Thepresentpaperis file and its intensity. There are two kinds of changes: smooth structuredasfollows.InSect.2wepresentthelayoutofourob- variations of the line profile and intensity within weeks (e.g. servations.Wediscusstheevolutionofthephotometric1.87d Petrov et al. 1994), that sometimes correlate with the photo- cycle over the years in Sect. 3. This section includes a new metric 1.87d period (Fernańdez & Miranda 1998) and other, determination of this period and the ephemeris for the mini- stronger, variations that are noticed when comparing data mum brightness,and the tentative detection of an activity cy- taken months apart from each other (previous references; cle.Therelationbetweenspotsandvariousactivitydiagnostics B.Stelzeretal.:Amulti-wavelengthstudyofV410Tau.I. 519 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SierraNevada LLLLLLLLiiiiiiiicccccccckkkkkkkk The photometric observations carried out at the Observatory r. ct of Sierra Nevada (Granada, Spain) were performed with a pe SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiieeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeerrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrraaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeevvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa S Stro¨mgren photometer attached to the 90cm telescope. The pt. photometer is equipped with identical six-channel uvby spec- O CCCCCCCCCCCaaaaaaaaaaalllllllllllaaaaaaaaaaarrrrrrrrrrr AAAAAAAAAAAllllllllllltttttttttttooooooooooo trographphotometersfor simultaneousmeasurementsin uvby or the narrow and wide Hβ channels (Nielsen 1983). We carried out uvby measurements. Integration times were 60s SSSiiieeerrrrrraaa NNNeeevvvaaadddaaa forV410Tauandforitstwocomparisonstars, HD27159and oto. HD283561, as well as for the background sky. Di(cid:11)erential h MMMMMMtttttt......MMMMMMaaaaaaiiiiiiddddddaaaaaannnnnnaaaaaakkkkkk P photometrywas carriedoutduringall the photometricnights, pt. except during the last one, in which the absolute calibration O FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff wasdone. s The error bars of the di(cid:11)erential photometry, as esti- ray CChhaannddrraa mated from the di(cid:11)erence between the two comparisonstars, - X are 0.005mag in the u band and less than 0.001mag for 0 1 2 3 4 5 6 7 8 9 10 JD - 2452230 the vby bands. Nevertheless, we note that both comparison starsareabouttwomagnitudesbrighterthanV410Tau,there- Fig.1. Observing Journal for our monitoring of V410Tau in fore,slightly largererrorsare expectedfor the targetstar. For Nov.2001.Note,thatourcampaignincludesathirdChandraobser- the absolute photometry, the residuals of the standard stars vationwhichwasperformedinMarch2002andisnotshowninthis diagram. (di(cid:11)erence between computed and published values) are less than 0.026mag for the V band and less than 0.008, 0.010 and0.017magforthe(b−y),m1andc1indexes,respectively. 2.1.2. Mt.Maidanak areexaminedinSect.4.WedevoteSect.5tothedescriptionof the X-ray spectrum of V410Tau. The results are discussed in Diaphragm photometry was performed at the Mt. Maidanak Sect.6.Section7presentsabriefsummary. Observatory,Uzbekistan.Allobservationswereobtainedwith the 48cm telescope equipped with a one-channel pulse- countingphotometer.WecarriedoutJohnsonUBVRmeasure- ments with a 2800 diaphragm. A detailed description of the 2. Observationsanddatareduction equipmentandmeasuringtechniqueswasgivenbyShevchenko (1980).Theexposuretimewastypically40−120s,depending Simultaneous optical and X-ray observations were planned on the band and sky-backgroundbrightness. As a rule, seven for V410Tau from Nov. 15 to 26, 2001 (UT). Optical pho- standard stars from the list of Landolt (1992) were observed tometry and spectroscopy were carried out from several ob- on each night to determine the atmospheric extinction coe(cid:14)- servatories (see below) while three X-ray observations were cients. The instrumental photometric system was reduced to scheduled with Chandra using the Advanced CCD Imaging thestandardsystembythemethodofNikonov(1976).Thestar SpectrometerforSpectroscopy(ACIS-S).Ourobservingstrat- BD+28 643 was used as a secondarystandard. The rms error egywastomaximizetheprobabilityofdetectinglargeampli- ofasinglemeasurementonamoonlessnightwas0.032,0.007, tudevariabilityintheX-rayobservations.IftheX-rayemission 0.005,and0.005maginU,B,V,andR,respectively. comes fromspotted regionsthe maximumof the X-ray emis- sionshouldoccurattheminimumoftheopticallightcurve,i.e. 2.1.3. USNOFlagstaff atphaseswhenthespotisonthevisiblehemisphere,andvice versa.Therefore,theChandraobservationswerescheduledfor ObservationsfromtheUSNO,Flagsta(cid:11)Station(NOFS,USA) the time near minimum and maximum of the known rotation used the NOFS 1.0 m f/7.3 R/C telescope along with a cycle,respectively. SITe/Tektronix2048(cid:2)2048CCD.AmaskaroundtheCCDlim- Nevertheless, the third Chandra observation was de- itsitsusableareato1815(cid:2)1815pixels,orabout20(cid:2)200.Bypo- layed by several months because at the anticipated time in sitioningV410Tauinonecornerofthearraythelargestnum- November2001 the satellite had to be shut down due to high ber of candidate comparison stars could be included. Several solar activity. Therefore,optical data is available only for the nightspriortothe campaign,plusthe photometricnightsdur- first two Chandra observations. The complete observing log ingthecampaign,wereusedtocalibrateallpotentialcompar- for the campaign in Nov. 2001 is given in Fig. 1. In the fol- isonstarsinthefield.UBVR I filterswereused,alongwitha c c lowing the individualobservations and the data reduction are largenumberofLandoltstandardstars(seeLandolt1983and described. Landolt 1992), to calibrate the comparison stars. Five of the 520 B.Stelzeretal.:Amulti-wavelengthstudyofV410Tau.I. potentialcomparisonstarswerefoundtobeconstantduringthe Table1.ObservinglogforthethreeChandraACIS-Sobservationsof surveyinterval,andbrightenoughtogivereasonablesignal-to- V410Tau.Rotationalphases arecomputed usingthenewephemeris noise (S/N) for di(cid:11)erentialphotometry.Di(cid:11)erential photome- giveninTable2.Valuesrefertothe0.2−8keVenergyband. try at UBVR I was performed extensively on the five nights c c Nov.16throughNov.20UT,plusafewdatapointsonnights Seq.# StartDate Exp.Time φ Rate rot priortoandsubsequenttothisperiod. [d/m/yh:m] [s] [10−3cps] 200130 16/11/0120:11 14925.3 0.91−1.00 296(cid:6)4 2.2.Opticalspectroscopy 200190 19/11/0123:37 11007.6 0.58−0.65 361(cid:6)6 2.2.1. Intermediate-resolutionspectroscopy 200191 07/03/0206:16 17734.2 0.89−0.99 324(cid:6)4 Intermediate resolution spectroscopy was carried out at the 1.5mtelescopeontheObservatoryofSierraNevada(Granada, spectra). Wavelength calibration was done using spectra of a Spain) using the spectrograph ALBIREO (see Sanchez et al. Th-Ar lamp. Finally, for each order we normalized the ex- 2000)inlong-slitmode.ThedetectorwasaEEV8821/TCCD tracted spectra by means of a polynomial fit to the observed of 1152(cid:2)770 pixels with a 22.5µm pixel size. Observations continuum. were done on two spectral ranges: 4000−5160 Å (blue) and 5645−6790 Å (red). The full width half maximum 2.3.X-rays (FWHM) of the lines of the calibration lamps were 1.7 Å and 1.5 Å for the blue and red ranges, respectively. Several TheChandraobservationswere performedwith V410Tau on spectrophotometricstandard stars were observed on the three theback-illuminatedACIS-S3CCD. Inordertoavoidpile-up photometric nights, with the aim of correcting the slope of (thedetectionofmorethanonephotonasasingleeventwhich thecontinuumbutnofluxcalibration.Asetoftemplates,with leads to distortion of the spectral shape and underestimate of spectraltypesrangingfromK2toK5werealsoobserved. thecountrate)thesourcewasplaced40 o(cid:11)theaimpoint.This Thedatareductionandanalysiswasdonewiththelongslit way, for the expected X-ray brightness of V410Tau in the package of IRAF (Image Reduction and Analysis Facility1). Chandraenergyband,pile-upshouldbebelow10%.However, Thelongslittaskswererequiredduetothestrongdistortionof weperformedadditionalchecksonthecountrateandthespec- theskylinesalongthespatialdirection.Individualspectrahave trumtoverifythatpile-upisnegligible(seeSect.5).Ourdata aS/Nofabout30. analysis is based on the events level1 data provided by the pipeline processing at the Chandra X-ray Center (CXC), and wascarriedoutusingtheCIAOsoftwarepackage2version2.3. 2.2.2. High-resolutionspectroscopy In the process of converting the level1 events file to a High-resolution spectroscopic observations were performed level2 events file for each of the observations we performed using the Fiber Optics Cassegrain Echelle Spectrograph thefollowingsteps:Wefilteredtheeventsfileforeventgrades (FOCES) and a 2048(cid:2)2048 pixel SITe CCD attached to the (retainingthestandardASCAgrades0,2,3,4,and6),andap- 2.2mtelescopeattheCalarAltoObservatory(Almeria,Spain). pliedthestandardgoodtimeinterval(GTI)file.Eventsflagged The spectra include some seventy orders, covering a range as cosmic ray afterglow were retained after inspection of the from 4200 Å to 7000 Å, with a nominal resolving power of images revealed that a substantial number of source photons λ/(cid:1)λ (cid:25) 30000. Technical details regarding FOCES can be erroneously carry this flag. We also checked the astrometry foundinPfei(cid:11)eretal.(1998). for any known systematic aspect o(cid:11)sets using CIAO soft- Additionally, high-resolution spectra were taken at the ware,andperformedthenecessarycorrectionsbyupdatingthe University of California’s Lick Observatory on Mt. Hamilton FITSheaders. using the Hamilton echelle spectrograph on the 3m-Coude To findthe preciseX-raypositionofV410Tau weranthe telescope. The instrument yielded 107 spectral orders span- wavdetectsourcedetectionroutine(Freemanetal. 2002).For ning a wavelength rangeof (cid:24)3500−10000Å. A thinned Ford the source detection we used a binned image of the ACIS-S3 2048 (cid:2) 2048 pixel CCD with 15 µm pixel size was used. chipwith(cid:24)300(cid:2)300 pixels.Thesignificancethresholdwasset Using an aperture plate above the slit gave a slit width to 10−6, and wavelet scales between 1 and 8 were applied. of 640 µm, corresponding to 1.200 projected on the sky, AfterV410Tauwaslocatedinthiswayweextractedthecounts and (cid:24)2pixels on the CCD. This gave a 2-pixel spectral reso- from a circular regionof 400 radius aroundthe position found lutionof(cid:24)0.1ÅFWHM(i.e.,2-pixelR(cid:25)60000). bywavdetect.Thisareaincludes(cid:25)97%ofthe sourcephotons All high-resolution spectra have been reduced and ex- (for a monochromatic1.49keV source).The backgroundwas tracted using the standard IRAF reduction procedures (bias extracted from an annulus in a source-free region surround- subtraction, flat-field division and optimal extraction of the ingV410Tau.TheobservinglogforthethreeChandraexpo- sures and the average ACIS-S count rates of V410Tau in the 1 IRAF is distributed by the National Optical Astronomy 0.2−8keVenergyrangearegiveninTable1. Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agree- 2 CIAOismadeavailablebytheCXCandcanbedownloadedfrom mentwiththeNationalScienceFoundation. htp://cxc.harvard.edu/ciao/download-ciao-reg.html B.Stelzeretal.:Amulti-wavelengthstudyofV410Tau.I. 521 Table2.EphemerisofV410Tau:valuebyP94andourupdate. However,suchalargedisplacementisclearlyinconsistentwith the relatively unchangedshape of the photometriclightcurve. Reference T [JD] P [d] Furthermore,Dopplermapsobtainedintheyears1990to1994 min rot seemnottoshowasystematicmovementofthemainspot. P94 2446659.4389 1.872095(cid:6)0.000022 here 2452234.285971 1.871970(cid:6)0.000010 3.2.Anupdateoftheperiodandephemeris In this section we examine the second possibility for the 3. Variabilityintheoptical observed systematic shift of the minimum in the optical lightcurve, namely the need for a refinement of the value for The rotational period of V410Tau was first determined by therotationalperiod.Inthefollowing,wheneverwerefertothe Rydgren&Vrba(1983),whofoundavalueof1.92dfrompho- “rotationalperiod” or “rotation cycle” the reader should keep tometricdataobtainedovera6-dayinterval.Vrbaetal.(1988) inmindthatthisistheperiodrepresentativeforthemainspot combined data taken during five observing seasons between thatdominatestheobservedlightcurve. 1981and1987,andimprovedthemeasurementoftheperiod. OnlyoneyearlaterHerbst(1989)notedalineartrendpresent For our update of the period we made use of historical intheO–C(observedminuscomputed)diagramforminimum data from the T Tauri photometry data base3. We split the lightofV410TaubasedontheephemerisbyVrbaetal.(1988) available V band photometry on a yearly basis, and derived indicating the need for a further revision. The most recent themostsignificantperiodfor eachofthe observingintervals and most widely used ephemeris for the photometric mini- using the string length method (Dworetsky 1983). Each sea- mumofV410TauistheonepresentedbyPetrovetal.(1994): sonal lightcurve was folded with its period to determine the JD(min) = JD2446659.4389+1.872095((cid:6)0.000022)E.This time of minimum T0 by fitting a polynomial to the folded resultisbasedondatafrom1986−1992. lightcurve. We used the value for the period given by P94 to We haveused this periodand ephemeristo phase-foldthe compute the number of cycles elapsed between each of the photometryfromNovember2001,andfoundthattheminimum seasonal points T0 and the time of minimum observed by us iso(cid:11)setfromphaseφ = 0by(cid:1)φ (cid:24) 0.14(Stelzeretal.2002). in November 2001, using this last observation as a reference A shiftof the opticalminimumwith respect to the ephemeris point.ThenwecomputedtheO–Cdiagramfortheminima T0 given by P94 had already been identified by Grankin (1999) as a function of cycle number. The residuals in this diagram inhisinvestigationofseasonallightcurvesacquiredwithinthe canbeminimizedbymodifyingtheperiod.ButtheO–Cresid- last decade. Our measurementcontinues the monotonic trend uals show systematic non-linear trends for data acquired be- observedsince1990.Thismigrationoftheminimumcouldei- fore 1990. The large scatter for these earlier years may indi- ther indicate a change in the latitude of the spots or the need catethatthespotsmigratedinanirregularwayonthesurface foranewestimateoftheperiod.Thestructureofthelightcurve ofV410Tau.Foryearslaterthan1990weobserveamonotonic of V410Tau undergoes changes over the years (Fig. 3 and trend in the O–C residuals. Therefore we used only observa- Sect.3.3)suggestingthatindeedthedistributionofsurfacefea- tionsobtainedin 1991and later for the finaldeterminationof turesisvariablein time.However,spotmodelsbasedonlyon theperiod.Thiswayabestfitperiodof1.871970(10)disfound photometricdatacannotprovideuniqueinformationaboutthe (seeTable2forthenewephemerisofV410Tau),slightlylower shapeandlatitudeofthespots. thantheearlierdeterminationbyP94whichwasbasedondata from1986to1992.TheresidualsintheO–Cplotforthenew bestfitperiodareshowninFig.2.Inthefollowingweusethe 3.1.Migratingspotsanddifferentialrotation ephemerisandperiodnewlyderivedhere. As outlined above, the systematic phase shift of the minimum observed over the last (cid:24)10yrs could be due to a lat- 3.3.Changesinthespots itudinal migration of the spots on the di(cid:11)erentially rotating surface. The di(cid:11)erential rotation of V410Tau is known from Provided the newly derived ephemeris represents the correct Doppler Imaging, where the di(cid:11)erential rotation parameter period for the surface structure on V410Tau, the minima of κ = (cid:1)Ω/Ω (Ω is the rotation rate at the equator) for an phasefoldedlightcurvesforeachobservingseasonshouldnow eq eq assumedsolar-likerotationlawwasfoundtobe(cid:25)0.001(Rice coincide with φ = 0. This is verified in Fig. 3 where the & Strassmeier 1996).Thisvalue is muchsmaller than the so- phase plots of the yearly averages of the V band lightcurve lar di(cid:11)erential rotation which is (cid:25)0.2 from equator to pole of V410Tau since 1981 are shown. Most of the photometry (Snodgrass1983),butnotuntypicalforafastrotatingstar(see fromearlier yearsis available onlyin graphicalform,andthe e.g.CollierCameron2002). time information can not be extracted accurately enough for The phase shift accumulatedoverthe last decade with re- studies of the 1.87d periodicity (original references Romano specttotheephemerisbyP94is(cid:1)φ (cid:25) 0.14(see Stelzeretal. 1975;Ro¨ssiger1981). 2002). This indicates that (cid:1)P/P (cid:25) 6(cid:2)10−5. If we assume a solarrotationlaw,andthatthemajorspotwascenteredonthe 3 TheTTauriphotometrydatabaseiscompiledandmaintainedby poleinitially(θ1 = 90(cid:14)),thischangeinperiodimpliesthatthe W.Herbstandavailableat spot should have moved to a latitude of θ (cid:25) 75(cid:14) by 2001. http://www.astro.wesleyan.edu/(cid:24)bill/ 2 522 B.Stelzeretal.:Amulti-wavelengthstudyofV410Tau.I. Year - 1900 10.4 1981 / (1982) 91 92 93 94 95 96 97 98 99 00 01 0.30 10.6 ] g a m 10.8 [ 0.20 V 11.0 C 10.4 O - 0.10 1983 / 1984 10.6 ] g 0.00 ma 10.8 [ V 11.0 -0.10 -2000 -1500 -1000 -500 0 10.4 N 1984 / 1985 10.6 Fig.2.O–CdiagramfortheseasonallyaveragedtimesofminimumT0 g] in the V band lightcurve of V410Tau versus rotational cycle num- ma 10.8 berN.Opencircles–fortheperiodgivenbyP94,andfilledcircles– V [ afteradaptingtheperiodtominimizetheresiduals.Dataobtainedin 11.0 Nov.2001wereusedasreferencepoint. 10.4 1985 / (1986) The minima of all years included in our determination of 10.6 the period, i.e. 1990 to 2001, are aligned at the same phase, g] a asexpected.Theminimaoftheyears1986and1989doshow m 10.8 [ a shift. This phase migration seems to be irregular and may V 11.0 indeed indicate changes in the spot location or distribution. Between1981and1985thelightcurveofV410Tauwaschar- acterized by a completely di(cid:11)erent, double-peaked structure. 0.0 0.5 1.0 1.5 2.0 DatafromtheseyearshasbeenextensivelydiscussedbyHerbst Phase (1989)whoshowedthatatwo-spotmodelprovidesagoodde- Fig.3. Seasonally averaged V band lightcurves of V410Tau scriptionofthevisiblelightcurve. from 1981 to 2001 phase folded with the new ephemeris given in Changes in the shape of the lightcurve reflect variations Table2(continuedonthenextpage).Dataareextractedfromthedata of the structure of active regions. In order to quantify these basemaintainedbyW.Herbst.Thedatafor2001wasobtainedbyone changes we plot the time-evolution of amplitude, mean, min- ofus(KG)inthemonthspriortoourcoordinatedobservingcampaign. imum,andmaximumoftheV bandlightcurveinFig.4.Over theyearsthelightcurveofV410Tauhassystematicallybecome morestableanditsvariabilitymoreregular. particularthoserelatedtothespotrotationcycle.Tothisendthe FromasimpleharmonicfittothemeanV bandmagnitude Chandraobservationswere purposelyscheduledto coverdif- fordataobtainedafter1990(alsodisplayedinFig.4)atenta- ferentrotationalphasesofthestar.IfX-rayemissionisrelated tivecyclelengthof5.4yrisderived.Inspectionofthephotom- tospottedregionsthemaximumoftheX-rayemissionshould etry in other bands, also available at the T Tauri photometry occur at the minimum of the optical lightcurve, i.e. at times data base, shows the same 5.4yr-patternin the B band with a when the spot is on the visible hemisphere, and vice versa. similaramplitude,supportingtheideathatthisvariationcanbe Therefore, to maximize the amplitude of the expected X-ray attributedtoaspotactivitycycle.ThedataintheRandI band variabilitywe observednearopticalmaximumandminimum. is scarce and inhomogeneous because of the use of di(cid:11)erent In addition our spectroscopic optical monitoring providedin- filters(JohnsonandCousin). formationon the time-evolutionof the Hα line and the radial However,when extrapolatingthe 5.4yr-periodicityto ear- velocity(RV). lieryears(dottedlineinFig.4)thedatafirstrunsoutofphase The V band lightcurve obtained during our monitoring in withthe5.4yrperiodandthenturnsintocompletelyirregular November2001isshowninthe lowestpanelofFig.5 phase- behavior.Theevolutionofboththeshapeofthelightcurveand foldedwiththenewperiodandephemeris.Thedi(cid:11)erentialpho- themeanbrightnessaresuggestiveofatransitiontowardsless tometrycarriedoutin the Stro¨mgrensystem was transformed activebehavior. totheJohnsonsystemwithhelpoftheabsoluteStro¨mgrenpho- tometrydoneduringthelastnight.We pointoutthatthepho- tometricmeasurementsfromthethreeobservingsitescomple- 4. SimultaneousX-rayandopticaldata menteachothertoprovidenearlyfullcoverageoftherotational AmajoraimofthiscampaignwastostudysimultaneousX-ray cycle despite the relatively short duration of the monitoring and optical observations in order to look for correlations, in (11d;seeFig.1).PhotometricdataintheUBRIbandswasalso B.Stelzeretal.:Amulti-wavelengthstudyofV410Tau.I. 523 10.4 10.4 1986 / 1987 1994 / (1995) 10.6 10.6 ] ] g g a a m 10.8 m 10.8 [ [ V V 11.0 11.0 10.4 10.4 1987 / 1988 1995 / (1996) 10.6 10.6 ] ] g g a a m 10.8 m 10.8 [ [ V V 11.0 11.0 10.4 10.4 1988 / 1989 1996 / (1997) 10.6 10.6 ] ] g g a a m 10.8 m 10.8 [ [ V V 11.0 11.0 10.4 10.4 1989 / 1990 1997 / (1998) 10.6 10.6 ] ] g g a a m 10.8 m 10.8 [ [ V V 11.0 11.0 10.4 10.4 1990 / 1991 1998 / (1999) 10.6 10.6 ] ] g g a a m 10.8 m 10.8 [ [ V V 11.0 11.0 10.4 10.4 1991 / (1992) 1999 / (2000) 10.6 10.6 ] ] g g a a m 10.8 m 10.8 [ [ V V 11.0 11.0 10.4 10.4 1992 / (1993) 2000 / (2001) 10.6 10.6 ] ] g g a a m 10.8 m 10.8 [ [ V V 11.0 11.0 10.4 10.4 1993 / (1994) 2001 / (2002) 10.6 10.6 ] ] g g a a m 10.8 m 10.8 [ [ V V 11.0 11.0 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 Phase Phase Fig.3.continued. 524 B.Stelzeretal.:Amulti-wavelengthstudyofV410Tau.I. Year - 1900 78 79 80 81 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 0.7 0.6 0.5 mpl 0.4 Va 0.3 0.2 0.1 10.8 mean 10.9 V 11.0 10.4 10.6 10.8 min V , max V 11.0 11.2 11.4 0 2000 4000 6000 8000 10000 HJD - 2443000 Fig.4.Longtermbehaviorofamplitude,mean,andmaximumandminimumoftheV bandlightcurveofV410Taufrom1978to2001.Data pointsrepresentseasonalaverages.Thenumbersonthetopy-axisstandfortheyearinwhichtheobservingseasonstarted. obtained, but does not provide new information on the phas- fromMt.Maidanak,hintingatapossiblerelationbetweenthe ingofthe1.87dcycle.Wewillusetheselatterlightcurvesfor opticalandX-rayemissionsites. the discussionofflaresin anaccompanyingpaper(Fernańdez A third Chandra pointing was obtained in March 2002 etal.,inprep.).SomeoftheseflarescanbeseenintheV band near optical minimum, i.e. coinciding in phase with the first lightcurvedisplayedinFig.5.However,ouraiminthissection November observation. As this observation is not simultane- istoexamineactivityparametersforapossiblerelationtothe ous with the optical lightcurvewe do not display it in Fig. 5. 1.87dcycle,whereshort-termrandomprocessessuchasflares The March data does not show significant time-variability, arenotofinterest. and its count rate is somewhat higher than the lowest (quies- cent)levelmeasuredintheNovemberobservationtakenatthe samerotationalphase.Thus,whilethetwoNovemberpointings 4.1.X-rays seemtosuggestthattheX-rayemissiondependsonrotational phase,thisconclusionisnotsupportedbytheobservationfrom The meanACIS countrateslisted in Table 1 indicate that the March. X-rayemissionofV410Tauwasvariable.Tochecktherelation with the optical photometry we generated X-ray lightcurves, and phase-folded them with the 1.871970d period derived 4.2.Hα in Sect. 3.2. The result for the two observations obtained in November2001 is shown in Fig. 5. A rapid rise of the X-ray The Hα line is in the spectralrange of both the intermediate- countratetookplacenearopticalminimumintheobservation andthehigh-resolutionspectraobtainedwithinthiscampaign. fromNovember,probablyindicatingthe onsetofa flare. This Hα is always weak in emission. We display the equivalent event was not accompanied by simultaneous observations in widthWHα measuredatdi(cid:11)erentphasesofthephotometricro- theoptical,butjustafewhourslatertwoflareswereobserved tation cycle in the second panel of Fig. 5. On Nov. 20 both B.Stelzeretal.:Amulti-wavelengthstudyofV410Tau.I. 525 Fig.5.Multi-wavelengthphaseplotforthe1.87dcycleofV410Tau.Fromtoptobottom:Radialvelocity,equivalentwidthofHα,X-raycount rate, and V band photometry. Except for some data points in the radial velocity curve all measurements are obtained quasi-simultaneously inNovember2001.ThelargeflarevisibleintheV bandlightcurvewasalsoobservedspectroscopically,buttheequivalentwidthsofHαare outsidetherangedisplayedinthisfigure(seeFernańdezetal.,inprep.foradetaileddiscussion).ThefastincreaseoftheX-raycountratenear φ=0(markedwithanarrow)probablyindicatesthebeginningofaflare(seeSect.5). intermediate- and high-resolution spectra were obtained, and Duetopoorweatherconditions,thephasecoverageofthe the measuredequivalentwidthsare in goodagreementwithin Hα data is limited. While the equivalent width seems to un- theuncertainties.Theerrorbarsforthehigh-resolutionspectra dergo (short-term) changes within each night, no clear trend are based on the assumptionof an accuracyin the continuum relatedtotherotationalphasecanbefound. normalizationof10%.Thetypicalerrorbarforthevaluesfrom thelow-resolutionspectrumare(cid:24)30%. 4.3.Radialvelocityandvsini OnNov.24alargeflareeruptedwhichwemonitoredsimul- taneouslyinspectroscopyandphotometry.Note,thattheevent We derived the RV and the projected rotational veloc- is not seen in the equivalentwidth curve shown in Fig. 5 be- ity (vsini) from the optical high-resolution spectra using a causethevaluesforWHα measuredduringtheflarearehigher cross-correlationanalysis with several template stars of spec- thanthedisplayedplotrange. tralclassK2,K4andK5.