1988ApJ. . .325. .798M The AstrophysicalJournal,325:798-819,1988February15 © 1988.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. Nazionali dellaRicerche. the upperboundsisessential for acorrectstatisticalanalysisof members fallinginthecombined fieldofview.(Knowledge compute upperboundsfor allofthenondetectedcluster more sensitivedetectionoffaintsourcesandpermitsus to makes useofallIPCexposurescontainingatleastoneHyades exposures containingHyadesmembers,thepresentsurvey ment ofpreviousworkontheHyadescluster(Sternetal with theEinsteinObservatory.Thisworkrepresentsarefine- the finaldatareductionanalysissystemusedhereallows a did nothaveavailableallimagingproportionalcounter(IPC) et al1981)waslimitedtothecentralregionofcluster and activity asseenatX-raywavelengths. data fortheclustermembersandexamineX-rayproperties paper, wetakeadvantageofthereadyavailabilityoptical original versionoftheEinsteindatareductionsoftwaresystem, star. Moreover,whiletheSternetalsurveywasbasedon the of allstarsintheHyadesregionskyusingdataobtained stellar samplesavailableforstudyingactivity.Inthis Pleiades cluster(CaillaultandHelfand1985;Micelaei a/. studied intheopticalwaveband,providesoneofbest 1985), constitutesastudyofthetemporalevolutionstellar 1981), andwhentakentogetherwithrecentstudiesofthe 2 1 AlsoHarvard-Smithsonian CenterforAstrophysics. Whereas thepreviousEinsteinsurveyofthiscluster(Stern The Hyadesclusterregion,whichhasbeenextensively PresentlyatIstitutoApplicazioni Interdisciplinari dellaFisica-Consiglio o imaging proportionalcounterexposurescoveringtheHyadesclusterregion(atotalof63Iximages). functions forfieldstarsingivenspectraltyperangeanddiscussadiscrepancywhichemergesthedGstars. expected numberpredictedonthebasisofBahcall/SoneiragalaxymodelandknownX-rayluminosity function ofstellarcolor.WealsocomparethenumberX-raysourcesidentifiedwithfieldstars groups ofcoevalstarssuggeststhatthedependencelevelX-rayemissiononstellarageisitselfa parison oftheX-rayluminosityfunctionsHyadesstarswithanalogousderivedforother functions fortheHyadesstarsinselectedspectraltyperanges,usingbothdetectionsandupperlimits.Acom- studies byusingrefinedX-raysourcedetectionalgorithmsandthecompletesetofEinsteinObservatory upper limitsforallthenondetectedHyadesmembersandhavederivedmaximum-likelihoodX-rayluminosity Hyades membersintheoriginalEinsteinsurveyofcentral5°cluster.Wehavealsocomputed3o Using asomewhatmoreextensiveandcompletecompilationofopticalcandidates,wehavedetected66out Subject headings:clusters:openstars—stars:X-rays 121 Hyadesmembersfallinginthecombinedfieldsofview,comparedwithadetectionrate48out85 © American Astronomical Society • Provided by the NASA Astrophysics Data System We reporttheresultsofanextensiveX-rayinvestigationHyadesregionandimproveuponprevious THE EINSTEINOBSERVATORYSURVEYOFSTARSINHYADESCLUSTERREGION I. INTRODUCTION 12 G. Micela,S.Sciortino,andVaiana Max PlanckInstitutfurExtraterrestrischePhysik Harvard-Smithsonian CenterforAstrophysics F. R.Harnden,Jr.,andRosner Received 1987March4;acceptedJuly29 Lockheed PaloAltoResearchLaboratory Osservatorio AstronómicodiPalermo J. H.M.Schmitt ABSTRACT R. A.Stern AND 798 ferring theHansonevaluation whenavailable.Wealso mag inthecentralregion of theHyades(«4°x4°),we greater than20%byHanson (1975)orvanAltena(1969),pre- with completenessdownto9th mag;forstarsfainterthan9th covering theentireregionof theHyadescluster(«20°x20°) catalog (vanBueren1952;Pels, Oort,andPels-Kluvyer1975), The nucleusofourcompilationcomprisesthevanBueren (fraction ofarcmin)andinformationonclustermembership. logs available1985Junewithsufficientlyaccuratepositions marize ourresultsin§VI. for theHyadesmembersaccordingtospectraltype.Wesum- in §IV,forX-raysourcesidentifiedwithfieldstars,and § V, fluxes (AppendixA)andthetreatmentofpartiallyobscured which thissurveyisbased(§II).Ourdataselectionand X-ray observations. data andwillthereforeprovideausefulcomparisonforfuture present studythusrepresentsthemostcompleteanddefinitive retained allthosestarsassigned amembershipprobability probable Hyadesmembers,usingthepublishedopticalcata- sources (AppendixB).Ourresultsarepresentedanddiscussed dices, wherewediscussextensivelythedeterminationofsource analysis methodsarepresentedin§IIIaswelltheAppen- selection procedureusedtocompiletheopticalcatalogon Hyades surveywhichcanbebasedontheEinsteinObservatory the surveyresults;cf.Avnietal1980;Schmitt1985.)The We haveconstructedanopticalcatalogof323certainor Our paperisorganizedasfollows.Wefirstdescribethe II. CHARACTERISTICSOFTHEOPTICALCATALOG 1988ApJ. . .325. .798M from thePels,vanEueren,Altena, andGiclascatalogs. (Upgren 1974;UpgrenandWeiss1977;Upgren,Weiss, catalogs). more, weaddedstarstothefinallistfromRegion7of (1975) onthebasisofpropermotioncriteria,orbyJohnson, included theLeidenstars(Pelsetal)acceptedbyHanson catalog compilation(includingthePels,vanEueren,Altena,andGiclas metric criteria(Upgren1974;UpgrenandWeiss1977, DeLuca 1979;Weiss,DeLuca,andUpgrenWeiss basis ofpropermotioncriteria(Giclasetal.)andonphoto- Giclas, Burnham,andThomas(1962)surveywhicharecon- Upgren 1982)onthebasisofphotometriccriteria.Further- Mitchell, andIriarte(1962)Upgrencollaborators sidered tobecertainorprobableclustermembersbothonthe Fig. 2.—Magnitudedistributionofour totalopticalcatalog,includingdata Fig. 1.—SpatialdistributionoftheHyadesmembersinourtotaloptical 5.2 4.84.43.63.2 © American Astronomical Society • Provided by the NASA Astrophysics Data System RIGHT ASCENSION(1950) EINSTEIN SURVEYOFHYADES each methodwillbefoundinfield(i.e.,~1.8spurious (0.2-0.8 keV),hard(0.8-3.5andbroad(0.2-3.5keV); the more spurioussourcesinregionswithweakdiffuseemission. maps (Harndenetal.1984).Thelattermethodismoresensi- bership hasbeenrejectedbyHanson(1975)(cf.§IV). data for10stars(brighterthan9thmag)whoseHyadesmem- displayed inFigure3c.WelistTable2Brelevantoptical for the121certainHyadesmemberspresentinoursurveyare broad band,eitherbytheLocal detectortheMap acceptance criteriamorerestrictive thanthoseappliedbythe detections perfield).Thedetectionthresholdinthepresent no morethan0.3spuriousdetectionsforeachbandand for Gaussian “sigmas”)arechoseninsuchawaythat,onaverage, detection thresholds(expressedintermsofapproximate rately inthree(notentirelyindependent)energybands: soft tive thanthefirst(particularlyincrowdedfields),butfinds tions withrespecttothelocalbackground,whileMapdetect with twodifferentmethodsofdetection:theLocaldetectand variations intheIPCfieldofviewandanalyzeseachimage rithm takesintoaccountthespatialandtemporalgain cessing (REV-1)software.Thestandardsourcedetectionalgo- Hyades starsdetectedasX-raysources(dashedline;cf.§V)are algorithm; thischoicegivesour surveyahighlevelofinternal survey wasneverlessthan2.5 a. searches forfluctuationswithrespecttobackgroundreference the Mapdetectalgorithms.Localsearchesforfluctua- Hyades starsinoursurvey(solidline)andofthecertain summarized inTable2A;themagnitudedistributionsofall have beenobservedmorethantwice).Therelevantopticaldata than once(inparticular,allstarsinthecentralHyadesregion REV-1 system.Weretainonly X-raydetectionsobtainedinthe Figure 3b.Abouthalfofthesestarshavebeenobservedmore Table 1.TheskycoverageofoursurveyisshowninFigure3a, combined fieldsofview(whetherdetectedornot)areshownin and thepositionsof121Hyadesmembersobservedin list oftheseimageswithsomerelevantinformationisgivenin taining atleastoneHyadesmemberofourclustercatalog.The mag inthecentralregionofcluster. passing thatthevanAltenacatalogincludesstarsupto17th plete to9thmag(i.e.,earlyKattheHyadesdistance)because of thecompletenessvanBuerencatalog.Wenotein considerably inskycoveragesandselectioncriteria,our the entireHyadesregion,ourcatalogcanbeconsideredcom- optical catalogcannotbeconsideredcomplete.However,over in Figure2. references attheendofTable2). found intheLuytensurvey(Luyten,Hill,andMorris1981), Figures 1;thecorrespondingmagnitudedistributionisshown one HertzsprungstarinthePleiadesregionofsky, a handfulofstarsfromvariousothercatalogs:three et al,WeissandUpgren1982).Finally,weadded MCL star,andoneBDallquotedascertainmembers(see In theREV-1system,twomethodsareappliedsepa- All IPCimageshavebeenanalyzedwiththefinalpro- For thepurposeofHyades surveywehaveapplied We haveanalyzed63EinsteinObservatoryIPCimagescon- The spatialdistributionofclustermembersisshownin Since thecatalogsonwhichwebasedourcompilationdiffer III. DATASELECTIONANDANALYSIS a) DetectionMethod 799 1988ApJ. . .325. .798M a a a a a a a a a 13667. 13666.. 13665.. 13664.. 13663.. 13662.. 13528.. 13527.. 13526. 13525. 13524. 13523. 13522. 13519.. 13517. 13521. 13520.. 13518.. 13516.. 13515.. 13514.. 13513.. 13512. 13511.. 13510.. 13284.. 12683.. 11939.. 1867.. . 1351 ... 1350.. . 1303.. . a Sequencecontainsmergeddataacquiredatepochsspanningmonths. Sequence Number © American Astronomical Society • hms 4 1705 4 1705 4 1705 4 1705 4 3136 4 1705 4 2255 4 2548 4 2843 4 2843 4 2548 4 2255 4 2000 4 2000 4 2000 4 2000 4 2255 4 2031 4 2255 4 3136 4 3136 4 2548 4 2548 4 2843 4 2000 4 1624 4 30 4 2924 4 30 4 30 41036 3 5610 R.A. Decl. (1950) 21 0124 17 3112 16 4236 14 1724 15 0600 14 4124 17 3112 17 0636 16 4236 15 0600 14 4124 15 5400 14 1724 13 5324 16 1836 17 0636 15 0600 15 5400 16 4236 15 3000 16 1836 15 3029 16 1836 15 5400 15 3000 10 1732 17 3800 10°22'00" 14 4124 5 1500 5 1500 5 1500 1980 Mar2 1980 Feb20 1980 Feb10 1980 Feb10 1980 Feb10 1980 Feb10 1980 Feb10 1980 Feb20 1980 Mar2 1980 Mar2 1980 Mar2 1980 Mar2 1979 Sep11 1980 Feb10 1980 Feb10 1980 Feb10 1980 Feb20 1980 Mar2 1980 Feb20 1979 Sep9 1980 Feb20 1979 Sep9 1980 Feb19 1979 Sep9 1980 Feb10 1979 Sep9 1980 Feb19 1979 Sep9 1979 Mar7 1979 Mar7 1979 Mar7 1979 Mar7 1979 Sep9 1979 Sep9 1980 Feb20 1979 Mar7 1980 Feb20 1979 Sep9 1980 Feb20 1979 Sep9 1979 Sep11 1979 Sep11 1979 Sep9 1979 Sep9 1980 Mar2 1979 Sep11 1979 Sep10 1979 Sep10 1979 Sep10 1979 Sep10 1979 Feb28 1979 Feb28 1980 Feb10 1980 Feb10 1979 Aug15 1979 Aug15 1981 Jan31 1981 Jan31 1979 Aug30 1979 Aug15 1979 Mar9 1979 Mar8 1979 Mar6 1979 Mar6 Start Date Stop Date Characteristics ofObservations Live Time (ksec) 30.3 14.2 6.6 2.9 4.0 2.7 2.4 2.7 2.0 2.4 2.7 3.2 2.6 2.0 3.3 3.4 2.4 2.0 2.6 0.8 2.1 2.0 2.1 1.9 3.5 1.2 1.9 1.7 1.4 1.9 1.3 1.2 TABLE 1 Provided bythe NASA Astrophysics Data System a a 110434. 110412. 110410. 110075. 110069. 19919.. 19917.. 19006.. 19005.. 19004.. 19003.. 19002.. 19001.. 19000.. 18978.. 17630.. 17512.. 17434.. 17433.. 17408.. 17247.. 16735.. 16311. 14516.. 14476.. 14451.. 13988.. 13819.. 13818.. 13816.. 13668. Sequence Number 4 2037 4 2807 4 3154 3 5848 4 2755 4 2543 4 2330 3 4630 4 2122 4 1954 4 2348 3 4630 4 1529 4 0945 4 4621 4 1050 4 1904 4 2900 4 5856 4 3152 4 2255 4 3303 4 0635 5 0643 3 5020 4 3054 4 2604 4 2342 3 5611 4 1804 4 2000 R.A. Decl. (1950) 20 3109 24 2310 23 3000 24 3000 22 5731 10 1600 16 3507 14 4910 23 3600 24 2251 15 5110 15 3023 14 5625 26 1652 16 3808 17 0805 11 1645 10 0512 10 0811 19 2505 18 0000 13 5149 10 1732 15 3400 16 2437 12 0950 18 0632 19 25 17 5512 16 0230 14 38 1981 Feb16 1981 Feb16 1981 Feb10 1981 Feb10 1981 Feb10 1981 Feb10 1981 Feb2 1981 Feb2 1981 Feb7 1981 Feb7 1981 Feb8 1981 Feb7 1981 Feb8 1981 Feb8 1981 Jan31 1981 Jan31 1981 Jan31 1981 Jan30 1980 Sep19 1980 Sep19 1981 Jan30 1981 Jan30 1981 Jan30 1981 Jan30 1980 Sep19 1980 Sep18 1980 Sep19 1980 Sep19 1981 Feb1 1981 Jan31 1981 Mar8 1981 Mar8 1980 Aug26 1980 Aug26 1981 Feb10 1981 Feb10 1980 Feb9 1980 Feb9 1981 Feb10 1981 Feb10 1981 Jan31 1981 Jan31 1980 Mar4 1980 Feb22 1980 Feb22 1981 Feb16 1980 Mar4 1980 Mar4 1979 Sep10 1979 Sep10 1980 Mar2 1980 Mar2 1980 Feb10 1980 Feb10 1980 Feb15 1980 Feb15 1980 Feb15 1980 Feb15 1980 Feb15 1980 Feb15 1980 Feb20 1979 Sep9 Start Date Stop Date Live Time (ksec) 10.8 10.6 2.0 2.7 2.8 9.5 4.7 5.8 6.1 6.1 9.4 9.1 1.7 8.4 5.1 2.4 5.0 3.6 2.5 6.4 2.0 2.6 1.9 2.7 1.7 1.2 5.6 3.2 1.5 1.3 1.8 1988ApJ. . .325. .798M o stars detectedasX-raysources(dashedline). stars inthepresentsurvey(solidline)andmagnitudedistributionofHyades observed inthepresentX-raysurvey,(c)MagnitudedistributionofHyades the centralregionofcluster,(b)SpatialdistributionHyades stars discarded byourcriteria,only twocouldbeidentifiedwith the softbandonly,orin hardbandonly,andtherefore Hyades clustermembers;hence ourchoiceisnotinefficient. consistency inthedetectioncriteriausedandreduces sta- Moreover, wenotethatof65 would-bedetectionsobtainedin 3, i.e.,from114to38;hence ourchoiceisalsoconservative. tistically expectednumberof spurious detectionsbyafactorof I xrepresentsoneormoreIPCfields.Notethecrowdingoffields in Fig. 3.—(a)SkycoverageofthepresentHyadessurvey.Eachsquare of © American Astronomical Society • Provided by the NASA Astrophysics Data System Fig. 3c EINSTEIN SURVEYOFHYADES 6 3 45 where theacceptancecriterionshouldberelaxeddueto identified withaHyadesmemberismorethan2'fromthestar for thepresentsurvey,namelythatopticalobjectliewithin loged fieldobjects:61,withstars,and11,nonstellar nation. reduced instrumentalaccuracyinsourcepositiondetermi- 2' oftheX-raysourcelocation.Onlyonesources companion. Thesedetectionshavebeen countedtwice,bothintheFstarand members ofopticallyresolvedbinary systemscontaininganFstarwithaG dwarfs ofspectraltypeA. present inthetotalfieldofview,andoneoutthreewhite detected sixofninedAstars,1618dF1315 dG source isneartheedge(distancefromIPCcenter39!2), optical position;inthissinglecase,wenotethattheX-ray and Rao(1972)foropticalcounterpartsofdetectedX-ray region, forwhichthemastercataloghasfewentries,wehave (Harris andIrwin1984)thatincludesthecatalogofHyades the Gstarsamples. stars, 16of30dK1446dMallthreegKstars tions with66distinctHyadesstars.Morespecifically,wehave sources inthisregion. searched, aposteriori,theOrionpopulationcatalogofHerbig used inthepresentsurveyarepointedTaurus-Auriga built forthisspecificsurvey.Becausesomeoftheexposures the Einsteinmastercatalog,acollectionof60distinctcatalogs tification foreachX-raydetectionwhosepositionlieswithina the sampleofMstars.TheX-rayemission isverylikelyduetotheMstar. 3' radiusofacatalogedcelestialobject;thisprocessisbasedon 4 6 5 3 Seenote3. Seenote5. We haveadoptedamorestringentidentificationcriterion In addition,wehaveidentified72X-raysourceswithcata- The REV-1systemautomaticallyproposesatentativeiden- ThewhitedwarfhasapossibleM companion whichhasbeencountedin TwooftheX-raysourceshaveoptical counterpartswhicharethemselves As canbeseenfromTable3,weidentify116X-raydetec- b) IdentificationsofHyadesSources 801 1988ApJ. . .325. .798M VA 288/GH7-190/ VA 282/GH7-189 VB 46/VA279/VR5/0226 VB 173/VA276/L35/GH7-185/ VA 275/GH7-187/ VB 45/VA272/HD27749/ VB 43/DM+19708 VA 262/LP415-35 VA 260/LP415-367 VB 41/VA256/HR1373/ VB 40/VA249/HD27691/ VB 39/VA248/HD27685/ VA 242/L41/GH7-179/ VA 241 VB 38/VA229/HR1368/ VA 216 VB 37/VA215/HD27561/ VA 213 VB 36/HD27534/ VB 35/HD27524/ VA 203/LP475-9 VB 34/VA201/HR1358/ VB 33/HR1356/HD27459/ VB 31/HD27406/ VB 29/VA179/HD27383/ VB 28/VA175/HR1346/ VA 162/HoB29/GH7-165/VR1 VB 27/VA156/HD27282/ VA 135/L30/J253/GH7-160/ VB 24/HR1331/HD27176/ VA 131/LP414-159 VA 127 VA 122/GH7-156 VA 118/GH7-154 VA 95 VB 19/HD26784/ VB 15/DM+23649 H14 L14/J233/GH 7-105 VB 8/HD25102/ VB 170/L6/DM+23571/J211.. L7 Hz 2411/MLC20/ VR 6/HoA78 J173/0217/VR 4/HoA55/ HD 27771/DM+14691 KA 91/RaA17/DM+15616. LP 475-37 HR 1376/DM+16586 HD 27697/DM+17712 DM +14690 DM +16585 LP 415-27 HD 27628/DM+13668 DM +14687 DM +18629 DM +20740 HD 27483/DM+13665 DM +14682 DM +18623 DM +16579 HD 27371/DM+15612 DM +17707 DM +17704 DM +21618 DM +10551 HR 1233/DM+9524 LP 357-4 © American Astronomical Society Name hms R.A. (1950)Decl. 4 2100 4 2051 4 2042 4 2034 4 2034 4 2032 4 2027 4 2021 4 2011 4 1954 4 1952 4 1945 4 1941 4 1914 4 1845 4 1845 4 1843 4 2003 4 1838 4 1834 4 1806 4 1804 4 1746 4 1718 4 1703 4 1657 4 1631 4 1615 4 1529 4 1525 4 1521 4 1516 4 1506 4 1501 4 1354 4 1149 4 1132 4 0714 4 0540 3 56 3 4940 3 4804 34642 14 4826 15 4559 14 3320 15 3854 14 1847 16 3945 16 3237 15 3552 15 0646 16 4032 17 2537 14 5625 20 5523 18 0913 15 4428 13 5739 14 3441 14 1733 16 4639 18 02 14 4431 13 4447 14 5838 19 0654 21 2732 16 2414 15 30 14 1149 17 2419 17 1805 23 2703 17 1744 14 2531 15 0622 25 3924 23 4518 16 3858 10 3435 24°09'54" 12 2018 12 0336 10 1123 15 5839 Hyades StarsinOurSurvey 13.34 14.82 10.51 14.94 15.85 16.71 13.01 15.36 15.64 15.46 16.70 12.79 10.0 16.19 16.19 15.00 17.07 15.55 15.63 11.28 11.15 10.23 14.21 Provided bythe NASA Astrophysics Data System 9.12 TABLE 2A 9.40 5.64 3.77 6.99 7.86 5.72 6.61 6.80 6.80 6.17 5.26 7.47 6.99 3.66 8.46 5.65 7.14 8.09 6.37 802 B— V 0.86 0.30 0.91 0.98 0.56 1.55 1.55 0.68 1.23 1.59 0.32 0.96 0.41 1.77 1.66 0.44 0.44 0.46 1.51 0.22 1.84 0.57 0.56 0.99 0.72 1.62 0.28 0.51 2.05 0.66 1.49 1.10 0.42 1.63 1.55 1.33 1.30 1.16 1.62 Ref. F7 +G3 KO III M2-3 F8 IV M2-3 Kl V K5 V KO III K2V GO V Aim M0.5 GO V M3e F6 V F5 V A9 V G8 V A3m G3 V FO V M2 M8 M6 G5 M2 M3 M5 M4e Ml F4 M5 M3 F6 M M3 M3 K8 K7 K5 F5 Sp. F5 M K 15 Ref. 10 14 14 13 10 14 14 11 10 9 6 6 7 6 9 9 6 9 9 6 9 9 9 6 9 18, 8.5 <12 <12 1.99 125 3.44 40 90 15 70 12 12 12 10 12 10 13 8 50 8 7 19, 18 20 22 Ref. 15 22 15 15 15 15 15 15 19 19 18 10 19 19 12 15 19 18 15 l Var. ôScuti SB1 60Tau 58 TauSB1 Susp. Bin. Susp. Bin. 61 ôTau Var? Bin. Susp. Bin. 54 yTau 63 Tau 51 Tau Flare SB(3) Bin? Var? Notes Var SB1 SB3 Var. SB2 SB1 SB3 Var 1988ApJ. . .325. .798M VA 292/EG36/VR7/GH7-191/ VA 297/VR8/GH7-194/ VA 294/L34/GH7-193/ VB 47/VA301/HR1380/ VB 50/VA308/HD27836/ VB 49/H307/HD27835/ VB 174/VA310/L39/GH7-196/ VB 51/VA315/HD27848/ VA 326 VA 321/GH7-198/ VA 334/VR11/RaA47/0300/ VA 329 VA 351/L42/GH7-203/ VB 175/VA342/L36/GH7-201/ VB 56/VA355/HR1389/ VB 57/VA360/HD27991/ VB 176/HD27990/ VA 366/GH7-208/ VA 362/LP415-71 VA 382/0433/GH7/218/ VA 368 VA 383/0373/GH7-212/ VB 59/VA384/HD28034/ VB 64/VA400/HD28099/ VB 63/VA389/HD28068/ VB 141/VA388/HD28052/ VA 410 VA 444/VR39/GH7-223/ VA 420/GH7-221/ VB 65/VA446/HD28205/ VB 179/VA459/L52/RaA151/ VB 189/VA475/L53/HoA426/ LP 415-46/HoA82/RaB28 VR 9/HoB61/0251/1262 GH 8-74/J260/RaB30/HoA89.... DM +16589 HD 27819/DM+17714..... GY 54/0271/KA113/HoB66/ DM +14693 DM +17715 DM +16591 DM +16593 HoA 134/GH7-200 LP 415-59 LP 415-65 DM +15621 HD 27962/DM+17719 DM +17721 RaB 73/HoA213 LP415-414/HoA 320/RaA117.... LTT 11425/GH8-39/KA189/ LP 475-68/HoA239/RaB85 DM +16598 DM +15624 DM +16601 HR 1394/DM+15625 0469/RaA 142/KA205/HoA363. HoA 339 GH 7-225/0484/KA212/ DM +15627 GH7-230/O514/VR 14/ DM +14699 GY 78/KA215 HoA 389/GY77/VR13/1274/ © American Astronomical Society • Provided by the NASA Astrophysics Data System Name R.A.(1950)Decl.mB-VRef.Sp.VNotes v 4 2107 4 2105 4 2104 4 21 4 2113 4 2122 4 2123 4 2129 4 2147 4 2142 4 2137 4 2157 4 2220 4 2207 4 2236 4 2246 4 2253 4 2251 4 2300 4 2258 4 2311 4 2314 4 2315 4 2330 4 2332 4 2348 4 2423 4 2411 4 2441 4 2445 4 2457 4 2519 16 1423 13 5615 17 1947 16 3666 16 1554 17 5321 15 4614 16 5754 15 4541 15 4746 13 4853 16 5218 17 0919 17 4855 17 5427 15 4943 15 2433 17 2557 14 5325 17 003215.591 14 554612.141.454 15 2444 14 3839 15 3023 16 4430 15 1618 16 3808 17 0753 15 18 15 2844 14 1827 16 2141 TABLE 2A—Continued 10.90 12.55 14.22 14.93 15.35 16.62 10.28 11.64 13.21 15.87 16.27 12.30 13.02 16.13 11.07 11.11 4.80 9.98 8.24 7.62 6.97 4.30 9.01 6.46 4.48 0.252 7.49 0.542 8.06 0.632 8.12 9.48 7.42 0.542 803 -0.02 0.16 0.59 0.60 0.44 1.28 1.49 0.05 0.49 1.04 0.94 1.87 1.58 1.41 1.54 1.03 1.73 0.66 1.58 1.45 0.81 0.93 1.49 1.69 1.35 A7.5 V dM5.5 A8 Vn Gl V M2-3 K4 V K4 V K2 V F6 V K5.5 A3 V G5 V F7 V G6 V KO V MOe F8 V FO V K5 V F8 V MO GO M0 DA M2 Ml MO M7 M4 M2 Ml Ml M6 MO K2 K 16 17 14 10 15 15 13 17 9 6 9 8 9 6 9 7 6 5 6 9 9 9 6 7 * 7 6 6 7 <30 <6 160 35 30 18 15 13 11 10 6 7 6 6 5 20 11 19 19 19 15 15 12 19 21 19 18 11 18 11 19 2 3 Var <5Scuti 64 ôTau 68 ôTau Susp. Bin. 70 Tau 71 Tau 76 Tau Flare Bin? Var? Var Var? Bin. SB3 SB3 Bin. Bin. Var SB Var Var 1988ApJ. . .325. .798M L74 VB 92/VA692/HD28805/ VB 91/VA684/HD28783/ VA 674 VA 673/L70/EG38/MLC50/ VB 90/HR1436/ VA 657/LP415-187 VB 191/VA645/L62/GH7-253/ VA 638/LP415-176/ LP 415-175 VA 627/L63/GH7-250/ VA 622/L69/GH7-249/ VA 617 VB 87/HD28593/ VA 610/LP415-165 GH 7-246 VB 182/VA587/L61/0666/ VB 82/VA584/HD28527/ VA 575/LP415-148 VB 180/VA560/L56/GH7-246/ VA 559/GH7-244/ VB 181/VA548/L55/RaA213/ VB 78/VA544/HD28406 VB 76/HD283704/ VA 529/VR19/GH7-238/ VA 512/GH7-236 VB 75/VA511/HD28363AB/ VA 502/VR18/GH7-235/0558/ VB 190/VA500/L54/GH7-234/ VB 72/VA491/HR1412/ VA 490/EG37/0545/VR16/ VB71/VA 489/HR1411/ VA 486/L68/GH7-232 VB 68/VA485/HR1408/ GH 7-258/0816/DM+15647.... DM +15646 HZ 9/GH7-255/LP415-186 HD 28736/DM+5674 RaA 297 0749/HoA 766/VR25/KA283/ LP 415-380/GY102/J286/ GH 7-252/0740/HoA753/VR23/ RoA 291/WOR16 DM +17744 0727/MLC 49/HoB105/J285/ DM +19733 MLC 48 DM +15638 KA 259/VR22/RaA248/ HD 28545/HoA640/GY97/ HR 1427/DM+15637 GY 93/RaA222/DM+16609.... 0662/HoA 579/VR21/KA242/ HoA 576/RaB212 DM +15634 0595/GH 7-241/HD285805/ GY 91/KA238/HoA542/VR20/ DM +26722 HoA 521/RaB186 LP475-97/LP 415-378/ DM +15633 HoA 489/MLC46/RaA194 HoA 484/GY84 RaA 192/G556/VR17/KA229/ HD 28319/DM+15632... HoA 465/RaB169 GH 7-233/LP415-115/ HD 28307/DM+15631 HD 28294/DM+14702 © American Astronomical Society Name 4 3022 4 3008 4 2958 4 2935 4 2930 4 2925 4 2914 4 2901 4 2853 4 2853 4 2830 4 2843 R.A. (1950)Decl. 4 2835 4 2819 4 2819 4 2744 4 2743 4 2742 4 2730 4 2705 4 2703 4 2639 4 2636 4 26 4 2621 4 2608 4 2608 4 2601 4 2559 4 2548 4 2547 4 2543 4 2535 4 2533 23 5312 15 4253 15 5405 17 4759 17 3845 17 3334 15 2338 15 3125 15 31 17 3615 16 3042 16 1722 5 1815 14 3824 15 37 16 0512 17 2333 16 3355 20 0137 16 4823 26 3350 17 3646 16 0812 17 4518 15 0955 16 14 16 0301 15 5222 16 1048 16 5140 15 5110 17 3512 14 3753 15 4542 TABLE 2A—Continued 12.62 15.66 Provided bythe NASA Astrophysics Data System 13.97 15.25 11.08 12.17 14.56 11.94 17.01 15.13 14.68 14.45 12.75 10.32 8.66 6.40 12.44 8.94 14.28 10.71 11.96 14.02 -0.09 9.54 12.16 8.58 8.94 4.78 9.07 6.92 9.18 6.59 3.41 0.18 3.85 5.90 B— V 0.74 0.88 0.33 0.41 1.52 1.39 0.98 1.57 1.30 1.47 0.74 0.85 1.44 0.17 1.26 0.86 1.58 1.56 1.55 0.45 0.77 1.48 1.15 0.53 1.47 1.32 0.96 1.53 0.32 1.44 1.48 Ref. 2 5 5 1 2 2 4 1 2 4 5 2 4 1 DA +dMe F7 +G0 G8 V Kl V K5 V A6 Vn M0-1 F3 V A6 IV K7 V G8 V A7 IVn M0.5 M2-3 K7 Ve Kl V M0-1 K5 V M2 A7 III Ml-2 K0 III M0-1 F6 V Ml K9 M2 Ml K8 V K2 M4 Ml K7 K0 F0 V Ml M0 G8 DA Ml Sp. 16 17 17 Ref. 10 17 15 13 6 6 17 8 14 7 17 16 14 9 6 7 7 6 6 9 8 7 9 6 <30 15 130 75 135 21 20 2.4 19 19 21 Ref. 12 12 SB1 22 12 2 1 Var ôScuti Susp. Bin. 78 dTau 77 dTau Susp. Bin. Var? Bin? Flare Var Bin. Notes Var SB1 4 Bin? Bin. SB1 SB Bin? 1988ApJ. . .325. .798M viously reportedasX-raysourcesby Feigelson andDeCampli(1981). sources areidentifiedwithTTauri stars intheTaurus-Aurigacomplexpre- (15) Kraft1965;(16)EggenandGreenstein1965a,b;(17)Buscombe1977,1980,1981;(18)Soderblom1983;(19)Duncanetal1984;(20)Horan1979; (21)Abt1975; Johnson etal1962;(3)Hanson1975;(4)vanAltena1969;(5)Stauffer1982;(6)Pesch1968,1972;(7)Herbig(8)Blanco1970;(9)Morgan andHiltner Abt andMoyd1973;(22)GrayEndal1982. van Bueren1952;VR:RhijnandRaymond1934;VYS:Vyssotsky1956;WOR:Worley1962. Kapteyn anddeSitter1904;L:Pelsetal1975;LP:Luyten1981;MLC:Murray1966;O:Osvald1954;Ra:Ramberg1941;VA:van Altena1969;VB: Hanson 1975;Ho:Holmberg1944;Hz:Hertzsprung1947;HTK:LuytenasquotedbyHerbig1962;HZ:HumasonandZwicky;J:Johnson etal1962;KA: 1965; (10)Cowleyetal1969;(11)Lockwood1984;(12)Treanor1960;(13)Wilson1963;(14)HoffleitandJasheck1982;Hoffleit,Saladiga, Wlansk1984; Not Identified Nonstellar Other Stellar Hyades VB 89; VB 83; VB 85; VB 80; VB 79; VB 77; VB 73; VB 52; VB 30; VB 22; VB 97/VA748/HD28992/ VB 183/VA747/HD28977/ VB 96/VA727/DM+14721 VB 95/VA725/HR1444/ VA 717 VB 93/VA712/HD28878/ a VB 122/HD30810AB/ VB 99/VA778/HD29159/ VB 184/L76/DM+16630 VA 763/LP415-382 VA 751/0913/DM+17751 L104/J348 NineteenX-raysourcesareidentified withPleiadesmembers.SixX-ray Note.—The starnamesarecodedaccordingtothefollowingconventions:EGEggenandGreenstein1965a,b;GHGiclasetal1962GYGyllenberg 1931;H: Totals 318 * Spectraltypescomputedfromcolorindexorrotationalvelocityinferredmodulationofchromosphericemission. References.—{1) Upgren1974;andWeiss1977;Upgren,Weiss,DeLuca1979;DeLuca, 1982;(2) Sources 114 Sources 14 Sources 74 Members 116 0855/DM +16620 DM +15651 KA 324/GY110/DM+15650. VR 28/HoA914/RaA363/ HD 28910/DM+14720 DM +10654 DM +15654 VA 644. VA 597. VA 569. VA 536. VA 319. VA 182. VA 108. VA 589. VA 547. VA 495. © American Astronomical Society • Provided by the NASA Astrophysics Data System Name Name Number ofPointedSerendipitousDistinct Detections Sources Summary ofDetections TABLE 3 a 8 611 6 6861 5 11166 R.A. (1950)Decl. hms R.A. (1950)Decl.mB-V v 4 3054 4 3045 4 5846 4 4826 4 3314 4 3308 4 3237 4 3148 4 3144 4 3139 4 3108 4 3100 4 2900 4 2748 4 2717 4 2755 4 2638 4 2627 4 2555 4 2136 4 1708 4 17 299 236 114 98 EINSTEIN SURVEYOFHYADES Potential HyadesStarsinOurSurvey 0 15 4356 15 0337 14 4428 15 3230 16 3932 13 5142 10 5903 15 3300 16 2624 15 1752 17 3846 15 2408 15 4445 16 0230 15 3505 15 3149 17 4706 17 2611 17 1035 16 4619 13 5458 1649'35" TABLE 2A—Continued TABLE 2B 18.37 11.39 10.83 16.13 4.66 9.40 9.37 8.51 6.02 6.51 6.76 7.95 7.94 5.48 5.58 8.96 7.05 7.85 7.80 5.59 8.34 9.69 7 (six) arepointedtargetsoftheimages considered.In§IV,wewillconsiderthe X-ray propertiesofthenon-Hyadesfield stars. pointed targets.Ontheotherhand, only asmallfractionofthestellarobjects given inFigure5. down intospectraltypesofthedetectedmain-sequencestars is diagram oftheclusterstarsin63IPCimages,andabreak- nal regionoftheHyadescluster;six areserendipitoussourcesandfive either aspuriousdetectionorreal,unrelatedsourceappears X-ray detectionswithHyadesstars.Mistakescanoccurwhen graphical forminFigures4-5.Figure4showsthe H-R survey area(i.e.,thetotal ofall2'radiuscirclesaround near thepositionofaHyadesstar.Letpberatio the sources inthe63surveyedfields.Ourresultsaresummarized in objects. Thusweareleftwithatotalof98nonidentifiedX-ray 7 B —V 0.34 0.43 0.26 0.32 0.83 0.61 0.50 0.60 0.28 0.77 Themajorityofnonstellaridentifications areinimagestakentheexter- We nowconsidertheproblemofmistakenlyidentifying 0.40 0.63 0.91 0.84 0.24 0.89 0.54 0.87 1.66 1.36 1.26 1.62 Ref. Ref. KO IV-V F7 + A8 Vn Gl V K2 V Kl V K2 V Sp. M6 M5 K8 K7 F2Vn F4 G8V G1V G1V K0V F5V F0V F7V F0V Am Sp. Ref. 14 9 9 9 9 9 ♦ * * Ref. 125 <6 109 134 109 3.5 4 21 25 15 7 8 Ref. Ref. 11 15 18 18 21 15 19 11 19 Var ôScuti SB1?, Var? Susp. Bin. 86 pTau SB1, Var Notes Notes Var? SB2 Bin. Bin. Var Var Bin. SB1 SB1 SB 805 1988ApJ. . .325. .798M 8 932 806 lated sources;thus,weobtainnomistakenidentificationofa field ofview,pisalsotheprobabilityforamistakenidentifica- detections andunrelatedsourcesappearuniformlyoverthe Hyades stars,ofwhichthereare250withrepetitions)tothe binary systemcontainingadMstar. two mistakenidentifications. the probabilityofobtainingnomistakenidentificationis Hyades star,withaprobability(1-p).Sincep~1.4x10“, tion. Weexpect~38spuriousX-raydetectionsand~55unre- total areaofallIPCfields.Ifweassumethatthespurious discussed in§IVindicateanexpected totalof~15fieldstarsinourcombined extrapolated tothe3<7detectionlevel, andtakinggalacticabsorptioninto field ofview. account, wepredict~40extragalactic sourcesinoursurvey.Thecalculations note thatbecauseofthespectraldependencemirror each detectedsource,togetherwithitsstatisticalerrors. We account inordertogivetheeffectivebroad-bandcountratefor IPC entrance-aperturesupportsstructureweretakeninto which onlytheMapmethodwasused,wehaveinstead used the broaddetectioncell.Hence,formajorityofLocal, response andoftheIPCpointfunctioncorrection, by aninnerradiusof5'andouter6';fortheLocal ground countsinanannulus(centeredonthesource)defined by collectingsourcecountsinacircleof3'radiusandback- unobscured X-raydetectionswehaveevaluatedthecount rate stellar spectra,~30%ofthesourcecountsfalloutside the sizeofbroad-banddetectioncell,andrelatively soft the standard(REV-1)netcountratesincreasedby25%. We obscured X-raydetections,andforthosedetections for ~0.27. Wethereforerecognizethatwemayhavemadeoneor 8 F. 4.—H-RdiagramfortheHyadesstarsinpresentsurvey.ThecrossesindicatedetectedasX-raysources.Notethatwhite dwarfisa Usingthemedium-sensitivitysurvey resultsofMaccaroetai(1982) Vignetting, detectioncellefficiency,andshadowingbythe IG c) CountRate,Flux,andLuminosityComputation © American Astronomical Society • Provided by the NASA Astrophysics Data System MICELA ETAL. 1-2 further detailsanddiscussion,seetheAppendices). evaluated inthedetectioncellwithLocalmethod(for have appliedthesamecorrectiontoupperboundsthatare 0.16-4.0 keVenergybandusingaconstantconversionfactor allow easycomparisonofourresultswithpreviouslypublished of 2.0x10“ergscountcm.Thisvaluewasusedto this X-raysurvey{solidline),andthose starsdetectedasX-raysources{dotted line). optical catalog{dashedline),thosestars whichfellintooneoftheIPCfields Hyades stars.Theplotshowsthe spectral distributionsof:thecomplete § 60 Ixl tr Z> We haveconvertedfromcountratestoX-rayfluxesinthe Fig. 5.—Spectraltypebreakdownof thedetectionsofmain-sequence 120 100 40 80 20 0 dA dFdGdKdM Vol. 325 1988ApJ. . .325. .798M 6 9 30 -1 19-2 No. 2,1988 data andiscorrectforasourcetemperatureof~3x10K, with negligiblehydrogencolumndensityabsorption. extremely goodagreementbetweenthetwodeterminations. simple singleconstantconversionfactorapproach(giveninordinate).Notethe IPC spectra(giveninabscissa)andofX-rayluminositiesdeducedfromthe discussion isdeferredtoaforthcomingpaper,allowusverify thermal, solar-abundance,plasmaemission(Raymondand (Hanson 1975)forallclustermembers,is4.86x10ergs can beseenfromFigure6.Theconversionfactorcount responding valuesderivedwithacompletefittingprocedure,as osity derivedusingtheconstantconversionfactorandcor- a posterioritheagreementbetweenvaluesofX-raylumin- count. Wenotethatadetailedanalysisofthespectra rate toluminosity,assumingacommondistanceof45pc Smith 1977)for27distinctX-raysources.Thesefits,whose component thermalplasmamodelsdonotfitappropriatelythe three strongHyadesX-raysourcesdemonstratedthatsingle- nificantly affectthecomputationoffluxconversion. IPC pulse-heightspectra(Stern,Antiochos,andHarnden for the2.5osources),systematicerrorsininstrument cali- is afactor~2andconsistsofstatisticalerrors(lessthan40% uncertainties), andthe3<7upperlimitsinferredfrom our brations (<10%,Harndenetal1984),errorsduetotheindi- tionable. TheestimatedoverallerrorintheX-rayluminosities and inTable4Bforthe10starswhosemembershipisques- survey, arelistedinTable4A,forthecertainHyadesmembers, if used,wouldonlyincreasetheconversion factorby~2%. of starsintheHyadesregion,quoted a valueof1-2x10cmforNwhich, identified withX-raysources intheHyadesregionsuggested errors inconvertingcounttofluxduetheassumedsource vidual clustermemberdistance(<100%),andsystematic Recently, Westergaardetal.(1985), using broad-bandEXOSATobservation temperature (<40%). 1986). However,thesemoresophisticatedmodelsdidnotsig- H 9 Fig. 6.—ComparisonofX-rayluminositiesdeducedfromdetailedfittothe We wereabletoperformspectralfitsusingamodelof The meanX-rayluminosities(alongwiththeirstatistical Thislatterassumptionisjustifiedby themodestHyadesmemberdistance. The largenumberoffieldstars (i.e.,non-Hyadesmembers) IV. X-RAYSOURCESIDENTIFIEDWITHFIELDSTARS © American Astronomical Society • Provided by the NASA Astrophysics Data System IN THEHYADESREGION EINSTEIN SURVEYOFHYADES o certain PleiadesclustermembersandeightX-raysourcesiden- ing discussion19X-raysourcesthatwehaveidentifiedwith is inagreementwithexpectations.Weexcludefromthefollow- each field,and(c)thebestcurrent(1986spring)estimatesof has beencomputedusing(a)theGalaxymodelofBahcalland tified withTTauristarsintheTaurus-Aurigacomplex. that weinvestigatewhetherthenumberoftheseX-raysources can befoundinFavataetal(1988). X-ray luminosityfunctionsforeachspectraltypeofthenearby central 5'(toremovethebiasintroducedbytargeted dwarf stars.Adetaileddescriptionoftheprocedureemployed IPC fieldofviewinsidetheribs(32'x32')andexcluding Soneira (1980),(b)thevaluesofmeanlimitingsensitivity with theactualnumberofX-raydetectionsidentifiedeach different IPCfields,andthestarsfallingintheseregions,more sources). WehavealsotakentheIPCfieldsasindependentof distinct spectraltype,indicatedanexcessforthedGstars:We than once.Theresultsofthesecalculations,whencompared one another;thatis,wehaveconsideredtheoverlapregionsof identified withGstarsisfive.Wealsoperformedasimilar predict 0.85dGstars,whiletheobservednumberofdetections (Although theinclusionofregionoutsidewindow comparison byconsideringthefullIxIPCfieldofview. inside theribsby~30%.Thusintotalfieldofviewwe conservatively evaluatedincreasingtheexpectationcomputed The expectednumberofdetectionsfortheentirefieldcanbe lower sensitivityofthisregionfartherfromthetelescopeaxis.) effective apertureisincreasedbyalesseramountduetothe furnish afurtherindependentcriterionforclustermembership support structurenearlydoublesthegeometricaperture, in analogytotheproposalofKraftandGreenstein(1969)for identified withdGstarsandsuggestthatX-rayemissioncan predict ~1.1,whilethenumberofdetectionsidentifiedwithG membership. AcloserexaminationofthissampledGstars stars is12. using thepresenceofCanemissionasacriterionforPleiades information forassessingtheirHyadesclustermembership.) membership. (Fortheremainingstars,thereisnodefinitive stars (i.e.,atleasteightof12detectionsinvolvingfivedistinct shows thatforthemajorityofdetectionsidentifiedwiththese (1969). (Thetwoauthorsinvestigatenearlyidenticalregions of catalog) havebeenacceptedasclustermembersbyvanAltena authors anddifferentmethodologiesforestablishingcluster stars), thereissubstantialdisagreementbetweendifferent Hanson (1975)(andbecauseofthatnotincludedinouroptical (assuming thattheyareattheHyadesclusterdistance) and G starswehavecomputedanX-rayluminosityfunction the sky.)Thissampleof45rejectedstarscontains10 G from thatoftheHyadesGstars. have verified(usinganonparametricstatisticaltest,cf. § V) stars, fiveofwhichappearinthepresentsurvey.Forthese five luminosity functionofMaggio etal(1987)forthenearby X-ray flux,followingthemethod ofFavataetal(1988).The that theluminosityfunctionforthissampleisindistinguishable sources associatedwithsolar-like surveystarsasafunctionof prediction (solidline)compared tothe12observationsof(five) solar-like starshavebeenadopted. InFigure7weshowthe Bahcall andSoneira(1980)model ofourGalaxyandtheX-ray The predictednumberofdetectablefieldstarsinthissurvey We haveperformedourcomputationusingtheareaof The abovecomparisonsshowanexcessofX-raydetections We noteinthiscontextthat45ofthe248starsrejectedby We havealsocomputedtheexpectednumberofX-ray 807 1988ApJ. . .325. .798M VA 362. VA 275. VB 43.. VA 262. VB 41.. VB 40.. VB 39.. VA 241. VB 39.. VA 213. VA 203. VB 31.. VB 170. VB 176. VB 50.. VB 49.. VB 47.. VA 297. VA 294. VA 288. VA 282. VB 46.. VB 173. VB 45.. VA 260. VA 242. VB 38.. VA 216. VB 36.. VB 35.. VB 34.. VB 33.. VB 29.. VB 28.. VA 162. VA 135. VB 24.. VA 131. VA 127. VA 122. VA 118. VA 95.. VB 19.. VB 15.. Hz 2411 VB 56.. VB 175. VA 334. VA 329. VA 326. VA 321. VB 51.. VB 174. VA 292. VB 27.. H14 .... VB8 .... L7 L14 VB 57.. VA 351. Name © American Astronomical Society • <0.36 <0.47 <0.51 <0.49 <0.49 <0.22 <0.37 <0.32 <0.70 <0.58 <0.33 <0.45 <0.89 <0.21 <0.40 <0.36 <0.54 <0.38 <0.34 <0.37 <0.40 <0.54 <0.81 <0.56 <0.51 <0.66 <0.67 <0.68 <0.31 <0.69 1 w [ergs sJ 0.10 0.69 0.69 0.58 0.31 0.18 0.36 0.21 0.66 0.11 0.13 0.15 0.35 0.55 0.15 0.14 0.74 0.13 0.50 0.19 0.11 0.13 0.18 0.76 0.55 0.58 0.11 0.41 0.35 0.35 0.15 30 29 30 x 10 X 10 x 10 29 29 29 29 29 29 29 29 30 30 30 29 29 29 29 30 29 29 30 30 30 30 29 29 30 30 29 29 29 29 29 29 30 29 29 29 29 29 29 29 29 29 29 30 29 30 29 30 29 29 29 29 29 29 30 30 29 30 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 29 29 29 29 28 28 29 29 29 29 29 29 29 29 29 29 29 29 29 28 29 29 29 29 29 29 28 29 28 29 29 0.37 x10 0.18 x10 0.19 x10 0.17 x10 0.92 x10 0.68 x10 0.20 x10 0.28 x10 0.20 x10 0.27 x10 0.14 x10 0.39 x10 0.13 x10 0.22 x10 0.44 x10 0.27 x10 0.31 x10 0.20 x10 0.32 x10 0.88 x10 0.16 x10 0.44 x10 0.39 x10 0.40 x10 0.17 x10 0.50 x10 0.83 x10 0.43 x10 0.88 x10 0.31 X10 0.16 X10 1 gerror on L y Summary ofX-RayLuminositiesforCertainHyadesMembers ab Flux FlagRECO WM WM WM WM WM WM WM WM WM WM WM MB MB MB MB MB MB MB MB MB MB MB MB MB MB MB LC LC LC LB LB LB LC LC LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y TABLE 4A Provided bythe NASA Astrophysics Data System VB 64 VB 59 VA 368..... VB 189 VB 179 VB 65 VA 444 VA 420 VA 410 VB 63 VB 141 VA 383 VA 382..... VA 366 VB 122 VB 99 VA 751 VB 96 L194 VB 184 VA 763 VB 97...... VB 183 VB 95 VA 717 VB 93 VB 92 VB 91 VB 90 VB 191 VA 638 VA 622 VA 617 VA 610 VB 82 VA 575 VB 181 VB 78 VB 76 VA 529 VA 502 VB 72 VA 674 VA 673 VA 657 VA 627 VB 87...... GH 7-24.... VB 182 VB 180 VA 559 VA 512 VB 75...... VB 190 VA 490 VB 71 VA 486 VB 68 L74 LP 415-175. Name <0.21 <0.32 <0.45 <0.38 <0.76 <0.63 <0.71 <0.19 <0.32 <0.36 <0.64 <0.51 <0.61 <0.31 <0.54 <0.26 <0.12 <0.19 <0.49 <0.47 <0.66 <0.67 <0.19 <0.22 <0.38 A [ergs s] 0.59 0.87 0.18 0.11 0.18 0.13 0.28 0.32 0.29 0.54 0.27 0.80 0.12 0.83 0.30 0.69 0.16 0.73 0.53 0.24 0.33 0.11 0.82 0.24 0.39 0.64 0.72 0.16 0.15 0.26 0.97 0.32 0.71 0.18 0.72 L x 29 29 30 29 29 29 29 30 30 31 30 29 29 29 29 30 29 29 29 30 30 29 29 29 29 29 30 29 29 29 29 29 29 29 30 29 29 29 29 29 30 29 29 29 29 29 30 29 29 29 29 30 29 30 29 29 30 30 29 29 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 lO 10 10 10 lO lO 10 10 10 10 10 lO 10 10 10 10 10 10 10 lO 10 10 10 10 10 lO 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 29 28 29 29 29 28 29 29 29 29 29 29 29 29 29 29 29 29 29 29 29 28 29 28 28 29 29 29 29 28 29 29 29 29 29 0.12 x10 0.73 x10 0.20 x10 0.13 x10 0.27 x10 0.88 x10 0.11 x10 0.27 x10 0.17 x10 0.10 x10 0.11 x10 0.44 x10 0.24 x10 0.26 x10 0.27 x10 0.22 x10 0.24 x10 0.10 x10 0.15 x10 0.12 x10 0.22 x10 0.73 x10 0.12 x10 0.68 x10 0.92 x10 0.12 x10 0.17 x10 0.14 x10 0.25 x10 0.68 x10 0.22 x10 0.43 x10 0.19 x10 0.28 x10 0.11 x10 1 ererror on L y 8b Flux FlagRECO WM WM WM WM WM WM MB MB MB MB MB MB MB MB MB MB MB MB MB MB MB MB MB MB LC LC LC LC LC LB LB LB LC LB LC LC LC LC LC LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB LB Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y 1988ApJ. . .325. .798M bership isquestioned(dashedline). number ofnon-Hyadessolar-likestars butexcludingthosestarswhosemem- ing thestarswhosemembershipis questionable (dottedline),andobserved sensitivity (solidline),observednumber ofnon-Hyadessolar-likestarsinclud- conclude thattheseresultsstronglysupportourhypothesis that thesefiveGstarsareHyadesmembers. Hyades membershipalsocompletelyremovestheexcess.We with thefiveGstarsforwhichthereisdisagreementabout 29 first case,theremovalofeightX-raydetectionsidentified 3028 29 29 29 3029 While theexcesswithrespecttopredictionisevidentin which thereisnoconclusivedataontheirHyadesmembership. 3029 G fieldstars,andtothefourobservationsof(four)starsfor 3029 29 c3129 Stern etal1983). shadowing (cf.Appendix). sources observedmorethanonce. 25% (onlyforupperlimits);WM=weightedmeanofthecorrectedvalues with the“Local”method;LB=fromcellcountrateincreasedby LC =fromthe“Local”countrateina3'radiuscircleforsourcesdetected sources partiallyobscuredordetectedonlywiththe“Map”method; MB =fromthe“Map”cellcountrateincreasedby25%(seeAppendixB)for VB 890.43x100.17MB VB 850.32x100.37LC VB 83<0.42x10LBY VB 800.74x100.29MBY VB 79...... 0.88x100.17MBY VB 770.33x100.49MBY VB 730.13x100.28LC VB 520.12x100.34LC VB 30<0.63x10LBY VB 220.28x100.54LC Fig. 7.—Expectednumberofsolar-like fieldstarsasfunctionofthelimiting c b a ThisvalueoftheX-rayluminosityhasbeenderivedduringaflare(cf YmeansthatX-rayluminosityisderivedinpresenceofpotentialsource FlagindicatinghowtheX-rayluminosityisevaluated(cf.Appendix): -1ab Summary ofX-RayLuminositiesforPotentialHyadesMembers Name (ergss)onLFluxFlagRECO x © American Astronomical Society • Provided by the NASA Astrophysics Data System L 1gerror x TABLE 4B FLUX EINSTEIN SURVEYOFHYADES -1 be explainedeasilybythepresence ofadGcompanionbecause m =SAmag.However,theintensity ofX-rayemissioncannot as aG4Vstar(withanaccuracy ofonespectralsubtype)with occultations, resolvedthesystem andidentifiedthecompanion star 71Tauisalsoamember of aspectroscopicbinarysystem; X-ray emissioninanyothermemberoftheôScuticlass. The motivated bythe71Taudetection,however,failedtodetect Peterson etal(1981),using thetechniqueoflunar fundamental periodof3.9hr(Horan1979);anunpublished (v sini=160kms,Abt1975)anda<5Scutivariablewith a survey ofsixotherÔScutivariablesperformedwithEinstein may berelatedtothestarbeingaveryfastrotator discussed earlierbySternetal(1981),thebehaviorof71 Tau cluster (CaillaultandHelfand1985;Micelaetal1985). As v surveys (CashandSnow1982;Golubetal1983;Schmitt al emission isquiteintensewhencomparedtotheX-ray [log (LJ=30.12],detectedinthesurveysofPleiades of anyotherHyadesstarortotheAstarsreportedinprevious 30.11]. Thestar71TauappearspeculiarbecauseitsX-ray 1985); theonlyAstarcomparableto71TauisHzII1384 71 TauisthestrongestX-raysourceinHyades[log(L)= (B— V)=0.81-1.45,dKstars;and>1.45,dMstars. 9 early dFstars;(B—V)=0.51-0.80,solar-like distinct colorintervals,namely:(B—V)=0.0-0.5,dAand stars (CaillaultandHelfand1985;MicelaetalSmith, et al1987;Bookbinder1985)andofothercoevalgroups Pravdo, andKu1983),wewillconsiderthedwarfstarsinfour surveys ofnearbydwarfstars(Schmittetal1985;Maggio these latterauthors. x Wilcoxon testinthefollowing,wemeandescribedby form forthevarianceasderivedbyLatta(1981)andapplied Feigelson andNelson(1985).Whenwerefertothegeneralized distribution, wehaveappliedthenonparametrictwo-sample minimize thedependenceoftestoutcomesoncensoring Peto-Prentice generalizedWilcoxontest,usingthefunctional two-sample testsisstrictlyspeakingnotappropriate(cf. Schmitt 1985;FeigelsonandNelson1985).Hence,inorderto use ofthepermutationalvarianceinapplyingnonparametric than forthefieldstars.Insuchacaseof“censoreddata,” for thePleiadesthanHyades,andlargerHyades example, theminimumdetectableX-rayluminositiesarelarger distances forthesampleswehaveconsidered,distributions (Schmitt 1985;FeigelsonandNelson1985). of thelimitingX-rayluminositysensitivitiesaredifferent.For comparisons ofsamplepropertiesusingnonparametrictwo- sample testsinpresenceofdatacontainingupperbounds and upperbounds(Avnietal1980;Schmitt1985),aswell tion functionswhichtakeintoaccountbothfluxmeasurements realm toconstructmaximumlikelihoodluminositydistribu- advantage oftechniquesrecentlyappliedtotheastronomical to compareourresultswiththoseofpreviousstudies,wetook X-ray emissionlevelsonstellarageandspectraltype.Inorder § II).Ourparticularinterestliesinthedependenceofstellar Hyades clustermembers(usingtheidentificationcriteriaof V. X-RAYSOURCESIDENTIFIEDWITHHYADESCLUSTERMEMBERS The A8Vn(Cowleyetal1969)orF0V(Hoflleit1982)star To compareourresultswiththoseofpublishedX-ray We emphasizethatbecauseofthedifferentrangesstellar In thissectionweconsidertheX-raysourcesidentifiedwith a) Main-SequenceStars—LuminosityFunctions i) dAandEarlydFStars 809 O'!00 r- J 810 MICELA ET AL. Vol. 325 CM

log (Lx) (erg/sec) Fig. 8c Fig. 8—Integral X-ray luminosity functions for (a) the early A stars of the Hyades {solid line), the Hyades excluding 71 Tau {dotted line), and the field stars in the same range of B—V (data from Schmitt et al 1985 ; dashed line); {b) the late A and early F stars of the Hyades {solid line) and the field stars in the same range ofB — V (data from Schmitt et al 1985; dashed line); (c) the solar-like stars of the Hyades {solid line), the Pleiades (data from Micela et al 1985; dotted line), and the field stars in the same range of B— V (from Maggio et al 1987; dashed line); {d) the K stars of the Pleiades {dashed curve [a]), the Hyades {solid line), the young disk nearby population {dotted line), and the old disk nearby population {dashed curve [d]); (c) the M stars of the Hyades (so/id line), the young disk nearby population {dotted line), and the old disk nearby population {dashed line).

the observed luminosity is too intense even for a typical To compare our results with those of the X-ray survey of Hyades G dwarf. nearby A and early F stars by Schmitt et al. (1985), we have We have detected X-ray emission from 16 of 21 dwarf stars separately considered two B—V intervals (0.00-0.30) and in the B-V range between 0.00 and 0.50; for the remaining (0.31-0.50), and have taken care to exclude all Hyades five stars, we have computed 3 a upper limits. This compares members sampled in the original survey of Schmitt et al with the 17 out of 25 stars of spectral type A2-F8 detected in (1985). the cluster central region survey (Stern et al 1981) with a Our sample of stars in the range B—V = (0.00-0.30) consists median of log (Lx) ~ 28.7. of eight stars (five detections and three upper bounds), all

© American Astronomical Society • Provided by the NASA Astrophysics Data System O'!00 r" No. 2, 1988 EINSTEIN SURVEY OF HYADES 811 et al 1985) in the same range of B—V. Again using the non- parametric generalized Wilcoxon test, we find the two distribu- tions to be indistinguishable (null hypothesis rejection confidence level of only 61%). The values of the mean of log (Lx) and their 68% confidence level errors using a boots- trap technique are

Hyades = 28.99

Fie,d = 28.56 ^'*2 ii) Solar-like Stars It is useful to compare the data presented here for solar-like stars (0.51 < R—F < 0.80) with the results of an Einstein survey of the nearby solar-like stars (Maggio et al 1987) and with those of the Einstein Pleiades surveys (Caillault and Helfand 1985; Micela et al 1985). We have detected 17 of 19 Hyades members in the range of B—V under study, while Micela ei al (1985) report X-ray luminosities for 26 Pleiades stars (13 detections and 13 upper limits) in the same color interval.10 This compares with 11 out of 13 Hyades members of spectral type G0-G8 detected in the previous survey limited Fig. 8e to the central region of the cluster (Stern et al 1981), with a median of log (Lx) ~ 29.1. Using both the detections and upper members of multiple systems. We note that the majority of limits, we have plotted the integral X-ray luminosity functions these stars are Am or S Scuti variables, and that the peculiarity for both the Hyades and the Pleiades solar-like stars in of these stars could affect the comparison with the (presumably Figure 8c, where we display for comparison the luminosity normal) stars surveyed by Schmitt et al (1985). In Figure 8a, function for the nearby (d < 25 pc) solar-like field stars from the integral X-ray luminosity function of these early A Hyades Maggio et al (1987). Because, in general, there is not enough members is plotted (twice: solid line with 71 Tau included, information in the cases of the Hyades and Pleiades for assess- dotted line with 71 Tau excluded) together with the luminosity ing the multiplicity of the optical counterparts, we have com- function for the binary stars of Schmitt et al (1985), which bined the samples of single and binary stars in performing comprise 28 objects (five detections + 23 upper bounds). We these comparisons. With the Wilcoxon test, we find evidence have applied a nonparametric generalized Wilcoxon test to the that the samples of Hyades and Pleiades X-ray sources are first and last of these distributions and find them to be indistin- different (null hypothesis rejected at the 97.8% confidence guishable. (The null hypothesis that they are drawn from the level). The same test applied to a comparison of the Hyades same parent distribution can be rejected only at the 78% con- and the field stars indicates a more definite difference (rejection fidence level.) We have also computed the values of the mean of of the analogous null hypothesis at the 99.5% confidence level). log(L ): The mean values of log (Lx) for the three samples considered x above are + 0.16 pleiades = 29.54 ^ + 0.10 Field = 28.25 _0 l2 Hyades = 29.16 ^

Here, and in the following, the 68% (1 a) errors on are computed using the bootstrap technique (cf. Schmitt 1985 = 27.71 ^ and references therein) using 200 bootstrap replications. The FieId above Hyades values have been computed with 71 Tau This confirms the trend of decreasing with stellar included. To assess the effects of this source on the above x comparison, we have recomputed the value of age previously discussed by, among others, Vaiana (1983), x Stern (1983), Caillault and Helfand (1985), and Micela et al without this “peculiar” star, obtaining = 28.57 x (1985). [28.69, 28.46]. Thus, the inclusion of 71 Tau hardly matters, For the sake of completeness, we have also considered the and we conclude that the distributions of Hyades and nearby sample of Hyades dG stars augmented by the five dG stars field A stars are essentially indistinguishable, as expected from whose Hyades membership is questioned (cf. § IV). The the fact that the A stars in the Hyades, like nearby field A stars, are quite evolved. 10 In Figure Sb we show the integral X-ray luminosity function In the following, we reference the data of the Pleiades survey by Micela et al. (1985), because that survey—like the present one and the survey of field for Hyades stars (single and binary stars combined) with solar-like stars by Maggio et al. (1987)—was derived using the REV-1 system, B—V = (0.31-0.50), and compare it to the luminosity function while the Pleiades survey of Caillault and Helfand (1985) utilized an earlier for the nearby (d < 30 pc) single and binary field stars (Schmitt reduction system and did not take individual upper limits into account.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1988ApJ. . .325. .798M fident ofanydifferencebetweentheHyadesdKstarsand the find thesetwodistributionstoberatherdissimilar (null functions oftheHyadesdKstarsandnearbystars. We metric two-samplegeneralizedWilcoxontesttotheluminosity indicate acleardeclineofX-rayluminositywithstellarage. can inpartbeattributedto asensitivityeffect:theHyades comparable tooryoungerthantheHyadesages,weareled to hypothesis rejectedat99.3%confidencelevel)butareless con- where wequotethe1aerrorcomputedviabootstraptech- because thereductioninsensitivity thatcouldexplainthe an explanationisnotviable in thecaseofolddiskstars young fieldstarsurvey(dueto greaterHyadesdistance).Such upper limitsinX-rayluminosity arehigherthanthoseofthe suggest thatthedifferenceinfortwosamples of only91.7%).Notingalsothattheyoungdiskstarshave ages young diskdKstars(nullhypothesisrejectionconfidence level nique using200bootstrapreplications.Thesevaluesagain observed behaviorwouldbe too largetobecompatiblewith composition oftheopticalsamples. tion (withrejectionconfidencelevelsrangingfrom46%to population. Theresultsofatwo-samplenonparametricsta- compatible withtheirhavingbeendrawnfromthesameparent investigate possiblebiaseffects,wehavecheckedwhetherthe of theX-rayluminosityfunctionsforthesesamples their emissioncannotbeexplainedintermsofbiasthe B— Vdistributionscompatiblewithasingleparentdistribu- tistical testindicatethatthesamplesconsideredabovehave B—V distributionsofthethreesamplesunderexaminationare the HyadesandPleiadesclustersdoesnotaffectourresults.To demonstrate thattheincompletenessofopticalcatalog cluster. IncomparingtheresultsofHyadesdKstarswith integral X-rayluminosityfunctionshownas(b)inFigure8d. mean log(L)(=29.20[29.26,29.14])andaluminosityfunc- enlarged sample(with22of24dGstarsdetected)hasbotha 84%), andthatpossibledifferencesintheX-rayintensityof those ofthefielddKstarsandPleiadeswemust optical completenessofthecataloginouterregions the sampleofHyadesdKstarsisnot,duetolack stars ofMicelaetal(1985). binary, fromBookbinder1985),andfor(a)thePleiadesdK young diskand(d)theoldfielddKstars(single Also shownforcomparisonareluminosityfunctions(c)the both thedetectionandupperbounds,havecomputed restricted sample. tion whichareindistinguishablefromthoseofthemore x 812 x Continuing ourinvestigation,wehaveappliedthenonpara- On thebasisoftheseresults,wecancompareproperties While thesampleofHyadessolar-likestarsiscomplete, We havedetected16of30dKstarsinourcatalogand,using © American Astronomical Society • Provided by the NASA Astrophysics Data System <10g (E)>pierdes=29.30_qq^ x =27.71_ =28.18 x0da09? xYd iii) KStars + 0.05 + 0.10 + 0.06 MICELA ETAL. 1 for log(L): extremely similar(nullhypothesisrejectionconfidencelevelis hypothesis atthe96%level. have computedtheintegralX-rayluminosityfunction,shown log (L)<28.5.Usingbothdetectionsandupperbounds, we center-region survey(Sternetal1981)reporteddetection for distribution ofthefieldMstarswithin25pctothat as thesolidcurveinFigureSd,andfollowingmeanvalues log (L)=29.20.WenoteinpassingthattheHyadescluster we haveperformedacomparisonoftheX-rayproperties stars oftheHyadesandfoundthatthesecolordistributionsare be presentintheHyadescatalog,wehavecomparedcolor the fieldMstarswithdistanceslargerthan10pcshouldalso bias workingtolimitthecompletenessofopticalcatalog complete, whiletheinclusionofknowndMstarsbetween catalog ofthefielddMstarswithin10pcisknowntobe stars belongingtotheHyades;highestX-raydetectionhas Hyades Mstars.Wehavedetected14outofatotal46 test ontheX-raypropertiesofthesedifferentsamples.The sample ofthefieldMstarswithin25pcwith against thatofthenearbyMstarsbeforewecanperformany test thecolordistributionofHyadesMstarsinoursurvey these starsthanfortheKstars.Henceitisimperativethatwe completeness oftheopticalHyadescatalogismoreseverefor different luminositydistributions(theanalogousnullhypothe- sis isrejectedataconfidencelevelgreaterthan99.99%). and inthiscasetherecanbenodoubtthatthesestarshave lation canbemadewith99.8%confidence).Wehavealso applied thistesttoKandsolartypestarswithintheHyades, that thetwosamplesaredrawnfromsameparentpopu- (Micela etal1985),whilethecorrespondingrangefor 10 pctothatoftheMstarsHyades; thetestpermitsustorejectnull 40%). them toberatherdissimilar(rejectionofthenullhypothesis the KstardistributionsofHyadesandPleiades,wefind Hyades isfrom29.20to28.35.ApplyingtheWilcoxontest that oftheKHyadesmembers. value offortheyoungKfieldstarswithrespectto ity components(U,V)(Upgren1978),contaminationofthese 17 of50KandMstarswithanupperlimittothemedian 10 and25pcmakestheopticalcatalogincomplete.Because Pleiades rangesbetween29.77andanupperboundof29.20 stars. Thiseffectcanpartiallyaccountfortheslightlylower and increasingthevalueforolddiskpopulation the truevalueforyoungdiskpopulationstars two samplesispossibleandwouldhavetheeffectoflowering disk isbaseduponstatisticalconsiderationsofthespaceveloc- since thecharacterizationofagivenfieldstarasyoungorold the typicalexposuretimesofHyadessurvey.Moreover, x x x x x x 11 Wehavealsocomparedthecolor distribution ofthefieldMstarswithin Due totheintrinsicfaintnessofMstars,problem Satisfied thatthecolordistributionsarenotundulybiased, We notethatthevalueoflog(L)forKstarsin x =27.14 =28.02^ x0d xYd iv) MStars Vol. 325 1988ApJ. . .325. .798M 2 2 from thevolume-limitedsamplesofnearby stars,andanalysisofsamplescoevalstars(includingthepresent Hyades survey). with RtakenfromAllen1973)isconstant isdrawnforcomparison;(b)summaryofthebehavioryear, anditsfluxlevelisalsoconstant,theconstancyof observed threetimes,ontimescalesrangingfrommonthstoa between 4and7x10ergss^ThestarEG38wasobserved data, donotsupportthesuggestionofBahúnasandcollabo- although inconclusiveduetothepaucityandqualityof three timesanddetectedonceatalevelofL=3.3x10 ergs rators thatthelargespreadinemissionlevelsofthesestars for theobservedemission.This isfurthersubstantiatedbythe (Eggen andGreenstein1965a, b)thatcouldwellberesponsible s; itstwoupperlimitsarecompatiblewiththedetection. fact thattheX-rayluminosity inferredfromthisdetectionis However, thisstarprobablyhasanunresolveddMcompanion is duetothepresenceofcyclicvariability. compatible withtheresultsof Vaianaetal.(1981)andofFon- taine, Montmerle,andMichaud (1982),whosurveyedasample ranging between 26.5and28.9. of whitedwarfsandreportvalues of,orupperlimitsto,log(L) x x Due tothelargeuncertaintiesindistancedeterminations(up In anattempttoclarifythereason(s)forthesedifferencesin In thepresentsurveyonlythreeof11confirmedHyades © American Astronomical Society • Provided by the NASA Astrophysics Data System -5 =6 =5 (fx/fv)oi =3.17(±0.12)x10 (fjfv)s 4.61(±0.11)x10~ (fx/fv)y 1-31(±0.10)X10- c) WhiteDwarfs MICELA ETAL. (1981, 1982),andWalter1982)thevariouscorrela- most previousstudieshavemadeuseofstarsdiffering,notonly magnetic fieldrelatedphenomenaonotherstars.However, previously beeninvestigatedby,forexample,Pallavicinietal. from thepointofviewrotationalvelocities,butalsointerms tions foundhavebeeninterpretedasevidenceforsolar-type than explicitlyassumed.Moreover,thereisevidencethatthe of ages,i.e.,thespaceparametersinvolvedwaslarger in astudyofnearbysolar-likestars. et al.(1981),whoconsideredstarsinthespectralrangeF7to Pleiades Kstars(CaillaultandHelfand1985;Micelaetal. reducing thedimensionalityofparameterspace).More- neous becauseallstarshaveapproximatelythesameage(thus expected forcoevalstarsiftherotationrateisdeterminedonly velocity distributionoftheHyadesstarsinrangeF5-K3is, over, accordingtoBenz,Mayor,andMermilliod(1984)the M5, andwhoseresultswereconfirmedbyMaggioetal.(1987) by age.ArelationbetweenLandvsiniwouldthenpredict at agivencolor,consistentwithDirac¿-function;thisis that allstarsinthissamplehavethesameactivitylevel.In was consistentwithaquadraticdependenceofLonv. previous Hyadesclustersurveylimitedtothecentralregion, 1985) donotfollowthequadraticlawfoundbyPallavicini log (vsini)forthesubsampleofsolar-likestarswhichboth average equatorialrotationvelocity,vconcludingthatthe limits inoneparameterortheotherareindicatedwitharrows. X-ray luminosityandrotationrateareknown(17stars);upper average X-rayluminosityfortheHyadesGstarstheydetected Stern etal.(1981)discussedtherelationbetweenLand We alsoshowtherelationdeducedbyMaggioetal.(1987), who consideredasampleofthenearby(i.e.,d<25pc)solar- x xe e9 x a volume-limitedsampleofnearbysolar-like stars. stars. Overlaidistherelationdeduced byMaggioetal.(1987)fromanalysisof Correlations betweenX-rayemissionandrotationratehave The Hyadessampleofthesolar-likestarsisquitehomoge- Figure 10showsthescatterplotoflog(L)versus Fig. 10.—Scatterplotoflog(L)versus log{vsini)fortheHyadessolar-like x x 0 I 2 3 d) CorrelationwithRotationforSolar-likeStars log (vsini) Vol. 325 1988ApJ. . .325. .798M individual measurements. is consistentwithaGaussian, suggestingthatthedispersionofpointscanbeexplained intermsofrandomlydistributederrors of 0.20,anddemonstratesthat thetwomethodsgiveaconsistentcountrateevaluationwithin thestatisticalerrors.Thedistribution rates forallsourcesdetectedwith bothmethods.Thisdistribution,showninFigure11,has ameanof0.95,withstandarddeviation for oursurveysources.Forthis purposewehavederivedthedistributionofratio Local-computedtoMap-computedcount possible inconsistencies.Wehave investigatedtowhatextentwecanfreelyinterchange count ratesderivedwiththesetwomethods detections, with137detectionscommontobothmethods. present survey,theMapmethodwasrunfor56of63fields, andweobtained318detections:275Mapdetections180Local The Localmethod,whichisusedonallfields,somewhat lesssensitivethantheMapmethod;butinordertopreventspurious estimated directlyfromtheimageinimmediatevicinity ofthedetectedsource,andMapdetectinwhichcomparisonwitha background referencemap(properlyscaledtothegivenimage) isusedtoestablishtheexistenceofsourcesinobservedimage. count ratesfortheeffectsofmirrorvignetting,detectorspatial response,andshadowingbytheIPCentranceapertureedges Map detectionsinthevicinityofstrongsourcesordiffuse emission,theMapalgorithmisnotrunonsomeoffields.In support structure(ribs).Itemploystwodistinctalgorithms todetectX-raysources:Localdetect,inwhichthebackgroundis following majorconclusions. members studiedinthepresentsurveyallowsustodraw cluster, inparticularamongsolar-typestars. survey amongtheubiquityofstellaractivityinHyades substantial agreementwiththeconclusionofprevious which wereunavailableforthepreviouswork.Thuswefind extensive useofsophisticatedstatisticalanalysistechniques version ofIPCdataprocessingsystem,andthatwehavemade analyzed inthepresentsurvey,thatwehaveusedfinal earlier surveywaslimitedtoonly27outofthe63fields (Stern etal.1981)showsagoodagreementinbothqualitative for theHyadesstarsgivencolorindex(B—V)ranges.An and quantitativeresults,notwithstandingthefactsthat those ofaprevioussurveythecentralregioncluster puted maximumlikelihoodintegralX-rayluminosityfunctions overall comparisonoftheresultspresentsurveywith members oftheHyadesstellaropenclusterandhavecom- Hyades stars(Zolcinskietal.1987). factor of2).Anotherexplanationwouldbelong-andshort- bility hasbeendemonstratedonlyforacoupleofthesolar-like term variabilityoftheX-rayemission,butevidenceforvaria- measurements. Forexample,themassofthesestarscannotbe partially accountfortheobservedspread(say,approximatelya responsible forthespreadinX-rayemissionbecauserange no strongdependenceofLon(B—V).Multiplicitycanonly recently demonstratedthatinarestrictedB—Vrangethereis stars canbeexplainedbycomparablestatisticalerrorsinthe neither theobservedspreadinX-rayluminosity(aboutone magnitude) canbeexplainedintermsofrandomorientation observed spreadinL.Furthermore,Maggioetal.(1987) have tobeverysteepinordertakeintoaccountthe order ofmagnitude)norvariationsinotherpropertiesthese the rotationalaxes(Benz,Mayor,andMermilliod1984),but like stars.Thespreadinvsini(lessthanoneorderof No. 2,1988 than afactor2),sothatthedependenceoíLonB—Vwould of massesinthespectralrangeF7toKOisquitesmall(less x x x In principle,thetwodetectionmethodsmayyielddifferent countratedeterminationsforthesamesource,thusproducing The standardREV-1processingsystem(Harndenetal.1984) computescountratesforthedetectedsourcesbycorrectingraw Our analysisoftheX-rayemissionfrom121Hyades We haveinvestigatedcoronalX-rayemissionfromcoeval © American Astronomical Society • Provided by the NASA Astrophysics Data System VI. CONCLUSIONS COUNT RATEDETERMINATION EINSTEIN SURVEYOFHYADES APPENDIX A by theLockheedIndependentResearchProgram. (Italy), andbytheMaxPlanckGesellschaft(J.H.M.S.). IAIF-CNR forsupport.R.A.S.wasalsosupportedpartially C.N.R. fellowship,andG.S.V.wishestothanktheItalian lowship. G.M.wishestoacknowledgesupportbyanItalian the SmithsonianVisitorsProgramandanItalianC.N.R.fel- One oftheauthors(S.S.)wishestoacknowledgesupport Investigator grantstoR.andA.S.,bythePSNMPI supported byNASAcontractNAS8-30751,Guest S. Serio,andH.Tananbaum.Thisworkhasbeenpartially Maggio, andtheusefulcommentsofF.Seward,P.Gorenstein, luminosity functionsandspatialdistributioninourGalaxy. field starsisconsistentwithourknowledgeoftheirX-ray With theexceptionofsolar-likestars,numberdetected can thereforebeusedasacriterionforclustermembership. membership hasnotbeenproperlyassessed:X-rayemission stars canbeexplainedasarisingfromHyadeswhose present inthissurvey. by comparingthevaluesoff/fforthreeHyadesgiants has anunusuallyhighandanomalousX-rayemissionlevel. which isa<5ScutivariablewithwideG4Vcompanionand sample ofnearbyAstars,withtheexceptionstar71Tau, ie., L=(t,B—V). only afunctionofstellaraget,butisalsocolor, the correspondingsourcesofHyades. stars havemuchsmallerlevelsofX-rayemissionthanthose young diskKandMstars,whilethenearbyold that theX-rayluminositydistributionfunctionsofKand that fornearby,generallyolder,solar-likestars.Wealsoshow M starsoftheHyadesareconsistentwiththosenearby sion fortheyoungsolar-likeHyadesstarsismoreintensethan xv stellar X-rayemissionisdependentonage:emis- x 1 We wishtoacknowledgeextensivediscussionswithA. e. WeshowthatanexcessofX-ray-detected,solar-like,field d. Weconfirmthat0andyTauaremoreactivethanö c. WeconfirmthattheHyadesAstarsarequitesimilartoa b. Inaddition,weshowthattheX-rayluminosityLisnot a. WeconfirmtheearlierresultofSternetal.(1981)that x 815 1988ApJ. . .325. .798M central cell)subdividedin16subcells.TheRECO(RibsandEdgeshadowingCOde)valueprovidesanestimateofthenumber background anddetectionsubcellspotentiallyobscuredisnonzerowheneverasourcecouldhaveanyofitscentralorframe shadowing. source countrateisinprinciplesubjecttosystematicunderestimation.Inoursample,abouthalfofthesourcesareaffectedbysuch correct forsucheffectsusingtheRECOvalues,onlycompletelyreliableapproachistoregardcountratessourceswith ribs, anditneglectsspectraldependencies(cf.Harndenetal1984).Consequently,althoughaquantitativeattemptismadeto cally affectedbyanonzeroRECOcode.However,thisresultdoesnotprovethatcountratesderivedinthevicinityofaperture distributions ofcount-rateratios(Local/Map).Acomparisonthedistributionforallsources(Fig.11)withthatpotentially background isderivedfromamapandnotevaluatedinthelocalimageframework. nonzero RECOvaluesasindeterminate.NotethattheMapalgorithmmissesfewerdetectionsnearedgesorribsbecauseits subcells obscured.However,thisapproachnecessarilyemployssimplifyingassumptionsregardingtheshapeanddimensionsof 816 have excludedsourcesthatmaybevariable(cf.Zolcinskietal.1987).Inordertoestimatetheintrinsicscatterofcountratesdue mean valueis0.998±0.09,andtheintrinsicdispersionofdata-0.50.Fromtheseresultsweconcludethatpotential shadowing expectation, whiletheintrinsicdispersionofdatapoints,asmeasuredbytheirstandarddeviation,islessthanorequal to0.20. is simplyduetopoorercountingstatistics,sincehighRECOimpliestheutilizationoffewercounts(potentiallyshadowed subcells does notintroduceasystematicshiftinthecountrateestimationbutgreaterrandomerror.The dispersion potentially obscuredobservation;47measurementsmeetthisrequirement.Forthesesources,thecumulativedistribution ofthe “true” fluxistakentobethemeanvalue(cf.Fig.13,solidline).Theofratio0.995±0.10,inagreement withthe requirement. Usingthesesources,wehavecomputedthecumulativedistributionofratioobservedto“true”flux,where only tostatisticalerrors,wehaveconsideredthosesourcesobservedmorethanoncewithnoshadowing;44measurementsmeetthis obstructions canbeusedfreelywiththosederivedforsourcesunaffectedbyribsoredges. obscured sources(Fig.12)showsthattheyarevirtuallyindistinguishableanddemonstratesthecountratesnotsystemati- are notused)andhencegreaterstatisticalerror.Toverifythishypothesiswehavecomparedthecumulativedistributions ofthe ratio ofcountratedeterminedwithnonzeroRECOtothat=0isshowninFigure13(dashed line).The accounted forbytheintrinsicdispersionofconsideredsamples. statistical error40%)versusRECO0values(mean20%),i.e.,thevalueof50%fordatadispersion canbe sample; ontheotherhand,wehavecomputedavalueof50%fordispersiondistributionRECOdifferent from0(mean statistical errorsforthetwosamples.Theresultingmeanerroris—20%inRECO=0sampleand—40% intheother be systematicallyunderestimated,itcanarguedthatthedimensionsofwindowsupportstructureweretoo conservatively than orequaltoafactorof2.Althoughthisresultcontradictsthenaiveexpectationthatpotentiallyobscured-source fluxeswould of sources,althoughitdoesincreaseindividualmeasurementerrors.In98%thecaseswehaveexaminedsucherrors wereless (i.e, RECO^0). The Localalgorithmusesadetectioncellsubdividedintoninecentralsubcells(3x3)withbackgroundframe(aroundthe Further problemsarisewhentheX-raysourcesaredetectednearIPCedgesorribs;inthiscase,determinationoftrue To investigatewhetherthetwomethodsgiveconsistentresultsinpresenceofapertureshadowing,wehaveconstructed In addition,wehaveconsideredthosesourceswithatleastoneobservationwithoutpossibleshadowingand ormore Pursuing thisproblemfurther,wehaveconsideredsourcesobservedmorethanoncewithdifferentRECOvalues;however, In summaryweconcludethatproximitytoribsoredgesdoesnotsystematicallychangethestatisticalpropertiesofan ensemble Fig. 12.—Differentialdistributionof theratiosofLocalandMapcomputedcountratesforallX-raysources potentiallyobscuredbytheribsoredges Fig. 11.—Differentialdistributionof theratiosofLocalandMapcomputedcountratesforallX-raysources detectedwithbothmethods. © American Astronomical Society • Provided by the NASA Astrophysics Data System MICELA ETAL. Vol. 325 1988ApJ. . .325. .798M mean correctionfactor1.25obtained previously. disagreement withtheMapcount rates,weusethecirclecountratevalue. fluctuations, wecomputethemeanvalueusingallsources exceptthetwonearribs.Theresultingmeanvalueis1.25,witha standard deviationof0.20. inadvertently includedasintrinsicemissionofthesource. However, themeanvalueandstandarddeviationofthisdistribution are notstronglyaffectedbythepresenceofthesehigh values; becauseofthedifficultyscreeningsourceswithnearby cell countrate.Attheotherextreme,highervaluesare generallyobtainedforsourceswithnearby,unresolvedemissionthatis method. ThisdistributionisshowninFigure14.Thetwolow pointsareduetotwosourceswhosedistancefromtheribsissuchthat the ribspartiallyobscurecircle,butdonot detectioncell,affectingthecountrateincircleandnot requirements. large enoughtocollectalmostallcountsfromapointsource; wehavealsocorrectedforthepresenceofothersourcesnearerthan 6'.) Inthefollowingwewilldealonlywithsourcesidentified withHyadesstarsandRECO=0;40sourcesmeetthese broad detectioncell.Tocorrectforthiseffectwehaveconsidered thecountratesmeasuredinacircleofradius3'.(Thisis conservatively retainthelargestsuchvalue. If therearenounobscuredobservationsforasource,thenRECO#0measurementsused. best suitedfordetectingsuchaneffect,wediscernnosignificanteffect. overestimated bythedatareductionsoftwareandthatinpracticeonlyaveryfewsourceswillfallnearenoughto actual support structure tohavetheirobservedfluxessubstantiallyunderestimated.Inthesampleconsideredhere,whichisperhaps thesample with onlyoneobservationRECO=0asfunctionoftherationonzerocountratemeasurementto{dashed line). rate measurementtothemeancountvalueforgivensource{solidline),andintegraldistributionofHyadesX-raysourcesobserved morethanonce No. 2,1988 2. Forthetwosourcesdiscarded under(1)andforthe13sourcesonlyMapdetectedwe usetheMapcountratemultipliedby 3. ForthefoursourcesonlyLocal detectedweusethecirclecountrate. To correctthecountratesfor these spatialresponseeffects,weadoptedthefollowingprocedure : 1. Forthe40sourcesdetected bothwithMapandLocalmethods,excludingthetwosources inwhichtheratescircleare We haveconstructedthedistributionofratiocount ratemeasuredinthecircletocountwithMap Because ofthesizeIPCpointresponsefunction,weexpect thatacertainnumberofstellarX-raycountswillfalloutsidethe b. Forupperbounds,weusethelowest,availablevaluemeasuredwithRECO=0.Ifonly^0valuesareavailable, we Therefore, wehaveadoptedthefollowingproceduretocomputesourcecountrates: a. Fordetections,wetakeaweighted(byinversesquareofstatisticalerror),meancountrateallobservationswith RECO =0. Fig. 13—IntegraldistributionforX-raysourcesobservedmorethanonce,withRECO=0ineachobservation,asafunctionoftheratioindividual count © American Astronomical Society • Provided by the NASA Astrophysics Data System DETECTOR SPATIAL-RESPONSECORRECTION EINSTEIN SURVEYOFHYADES APPENDIX B 817 1988ApJ. . .325. .798M to detection-cellrates. .19656,A.J.,142,925. .1981,MKSpectralClassifications.FifthGeneralCatalogue(Evanston: .1980,MKSpectralClassifications.FourthGeneralCatalogue 818 Avni, Y.,Soltan,A.,Tananbaum,H.,andZamorani,G.1980,Ap.J.,238,800. Allen, C.W.1973,AstrophysicalQuantities(3d.ed.,London:Athlone). Abt, H.A.,andMoyd,K.1973,Ap.J.,182,809. Abt, H.A.1975,Ap.J.,195,405. with Hyadesstarsandnotaffectedbyriboredgeshadowing. Gyllenberg, W.1931,Medd.Lund.Astr.Obs.,Ser.II,No.57. Gray, D.F.,andEndal,A.S.1982,Ap.J.,254,162. Golub, L.,Harnden,F.R.,Jr.,Maxson,C.W.,Rosner,Vaiana,G.S., Cash, Giclas, H.,Burnham,R.,Jr.,andThomas,N.1962,LowellObs.Bull,5,257. Eggen, O.J.1969,Ap.J.,158,1109. 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