Stellar differential rotation from direct starspot tracking

On the Sun, the rotation periods of individual sunspots not only trace the latitude dependence of the surface rotation rate, but also provide clues as to the amount of subsurface fluid shear. In this paper we present the first measurements of stellar diffe

Mon.Not.R.Astron.Soc.000,000–000(0000)Printed1February2008

A(MNLTEXstyle lev2.2)

Stellardi erentialrotationfromdirectstarspottracking

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A.CollierCameron,J.-F.Donati,M.Semel1

2Laboratoire

arXiv:astro-ph/0111050v1 2 Nov 2001

SchoolofPhysicsandAstronomy,Univ.ofStAndrews,StAndrews,ScotlandKY169SS

d’Astrophysique,ObservatioreMidi-Pyr´en´ees,AvenueE.Belin,F-31400Toulouse,France

3DASoP,ObservatoiredeParis-Meudon,F-92196Meudon-Cedex,France

Accepted2001November2.Received2001October31;inoriginalform2001September20.

ABSTRACT

OntheSun,therotationperiodsofindividualsunspotsnotonlytracethelatitudedependenceofthesurfacerotationrate,butalsoprovidecluesastotheamountofsubsurface uidshear.Inthispaperwepresentthe rstmeasurementsofstellardif-ferentialrotationmadebytrackingtherotationofindividualstarspotswithsizescomparabletothelargestsunspots.Toachievethiswere-analysefoursequencesofdensely-sampled,highsignal-to-noiseechellespectraofABDoradusspanningseveralstellarrotationsin1996December.Usingspectralsubtraction,least-squaresdecon-volutionandmatched- lteranalysis,wedemonstratethatitispossibletomeasuredirectlythevelocityamplitudesandrotationperiodsoflargenumbersofindividualstarspotsatlowtointermediatelatitude.Wederivevaluesfortheequatorialrota-tionrateandthemagnitudeofthesurfaceareinexcellentagreementwiththoseobtainedbyDonati&CollierCameron(1997)fromcross-correlationofDopplerimagesderivedayearearlierin1995December,andwithare-analysisofthe1996databythemethodofDonatietal.(2000).Thedi erencesbe-tweentherotationratesofindividualspotsandthe tteddi erentialrotationlawaresubstantiallygreaterthantheobservationalerrors.Thesmallerspotsshowagreaterscatteraboutthemeanrelationthanthelargerones,whichsuggeststhatbu etingbyturbulentsupergranular owscouldberesponsible.

Keywords:stars:activity–stars:imaging–stars:individual:ABDor–stars:rotation–stars:spots

1INTRODUCTION

Observationalstudiesofdi erentialrotationonrapidlyro-tatingstarshaverecentlybeenpublishedbyanumberofDonati&CollierCameron(1997)andDonatietal.(1999)appliedalatitude-by-latitudecross-correlationanaly-sistopairsofsurfaceimagesoftherapidlyrotatingKdwarfABDoradussecuredseveraldaysapart.Theyfoundasolar-likerateofsurfaceshear,withtheequatorialregionrotatingfasterthanhigherlatitudesandabeatperiodofabout110daysbetweentheequatorialandhigh-latituderotationpe-riods.Barnesetal.(2000)obtainedasimilarresultfortheyoungKdwarfPZTel.Donatietal.(2000)usedapara-metricimagingmethodtoestablishthedi erentialrotationrateonthepre-mainsequenceGstarRXJ1508-4423(=LQLup).Petitetal.(2001)measuredthedi erentialrotationoftheKsubgiantprimaryoftheRSCVnbinaryHR1099bythissamemethod.Vogtetal.(1999)andStrassmeier&Bartus(2000)examinedtheevolvingstarspotdistribu-

E-mail:andrew.cameron@st-and.ac.uk

tiononHR1099fromlongsequencesofDopplerimages,andfoundevidenceofagradualpolewarddriftoflargespotgroupsontimescalesofmonthstoyears.

StudiesbasedonDopplerimagessu erfromanumberofdrawbacksforthiskindofwork.Whenthesamplingofthelinepro learoundthestellarrotationcycleissparse,theproblemofmappingthestellarsurface-brightnessdis-tributionbecomesanill-conditionedinverseproblem,neces-sitatingaregularisedχ2 ttingapproach.Theregularisingfunctionimposesapenaltyonthegrowthof ne-scalestruc-tureinthestellarimage.Thishasthewell-documentedef-fectofbiassingthecentroidsoflow-latitudefeaturesintheimagesawayfromthestellarequator,andmayevensup-pressthemaltogether.Ititlikelythatdi erentialrotationmeasurementsbasedonthesemethodsmaysu ersomedis-tortionasaresult.

SeveralrecentDopplerimagingstudiesofrapidlyro-tatingyoungmain-sequenceandpre-mainsequencestarshave,however,employedamuchdensersamplingstrategy.Withechellespectrographsandlarge-formatCCDdetec-torsitispossibletomonitorthepro lesofthousandsof

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