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MNRAS459,880–901(2016) doi:10.1093/mnras/stw640 AdvanceAccesspublication2016March17 Open clusters in Auriga OB2 Amparo Marco1,2‹ and Ignacio Negueruela1‹ 1DFISTS,EPS,UniversidaddeAlicante,CarreteraSanVicentedelRaspeigs/n,E03690SanVicentedelRaspeig,Spain 2DepartmentofAstronomy,UniversityofFlorida,211BryantSpaceScienceCenter,Gainesville,FL32611,USA Accepted2016March14.Received2016March11;inoriginalform2016January7 ABSTRACT WestudytheareaaroundtheHIIregionSh2-234,includingtheyoungopenclusterStock8, D to investigate the extent and definition of the association Aur OB2 and the possible role of ow n triggeringinmassiveclusterformation.WeobtainedStro¨mgrenandJ,H,KS photometryfor loa d Stock 8 and Stro¨mgren photometry for two other cluster candidates in the area, which we e d confirmasyoungopenclustersandnameAlicante11andAlicante12.Wetookspectroscopy fro m of ∼33 early-type stars in the area, including the brightest cluster members. We calculate a h common distance of 2.80+−00..2274 kpc for the three open clusters and surrounding association. ttp://m We derive an age 4–6 Ma for Stock 8, and do not find a significantly different age for the n otherclustersortheassociation.ThestarLSV+34◦23,withspectraltypeO8II(f),islikely ras .o themainsourceofionizationofSh2-234.Weobserveanimportantpopulationofpre-main- xfo sequencestars,someofthemwithdiscs,associatedwiththeB-typememberslyingonthemain rdjo u sequence.Weinterprettheregionasanareaofrecentstarformationwithsomeresidualand rn a verylocalizedongoingstarformation.Wedonotfindevidenceforsequentialstarformation ls.o onalargescale.TheclassicaldefinitionofAurOB2hastobereconsidered,becauseitstwo rg a/ mainopenclusters,Stock8andNGC1893,arenotatthesamedistance.Stock8isprobably t U N locatedinthePerseusarm,butothernearbyHIIregionswhosedistancesalsoplacethemin IV thisarmshowquitedifferentdistancesandradialvelocitiesand,therefore,arenotconnected. ER S ID Keywords: stars:early-type–stars:evolution–Hertzsprung–Russellandcolour–magnitude A D diagrams–openclustersandassociations:individual:Alicante11–openclustersandassoci- D E ations:individual:Alicante12–openclustersandassociations:individual:Stock8. A L IC A N T E o distance.Awell-studiedexampleofsuchprocessesistheW3re- n 1 INTRODUCTION O gion,whichcontainsseveralsmallembeddedclusterssurrounding c to Stellarassociationsarelargeandloose,comovingstellargroupsof theopticallyvisibleclusterIC1795(Biketal.2012;Kiminkietal. b e low-densitystarsoftenassociatedwithsmalleropenclustergroups. 2015 ; Roma´n-Zu´n˜iga et al. 2015). A more dispersed population r 2 1 OBassociationscontainO-and/orearlyB-typestars.Theymustbe ofearly-typestarsseemstoconnectitwithIC1805,theionizing , 2 young (≤30 Ma) because they are unstable against Galactic tidal clusterofW4,whichincludesafewearlyO-typestars.Thenearby 01 6 forces.Therefore,mostoftheirlow-massmembersarestillfoundin clusterIC1848,ionizingW5,isalsolikelyconnected.Overallstars thepre-main-sequence(PMS)phase.Theyareexcellenttargetsfor traditionallyconsideredtobelongtoCasOB6covermorethan6◦ detailedstudiesoftheinitialmassfunctionandthestarformation onthesky.Otherexamplesoflargestar-formingregionscontaining history because they represent a place where the star formation moderatelymassiveclustersincludetheG305region,centredonthe processhasbeencompletedrecently.Theirspacedistributionalso Danks1and2clusters(Hindsonetal.2013,andreferencestherein) providesusefulinformationaboutthespiralstructureoftheGalaxy and the Carina Nebula (e.g. Preibisch et al. 2011). Other GMCs, (Blaauw1964;Preibisch&Zinnecker2007). suchasW33(Messineoetal.2015)orW51,lackthecentralmas- OBassociationsarethenaturaloutcomeofstarformationpro- siveclusters,andwillverylikelyevolveintodispersedassociations, cessesingiantmolecularclouds(GMCs)inmostMilkyWayenvi- similartoCygOB2(Wrightetal.2014). ronments.Theselargecloudsdemonstratehierarchicalstructurein ThePerseusarmhasbeenproposedasoneofthetwomainspi- bothtimeandspace,resultingintheformationofstarclustersthat ralarmsoftheGalaxy(Churchwelletal.2009).Itemergesfrom arenotboundgravitationallytooneanother,butshareacommon behindobscuringcloudsintheLocalarmaroundl=75◦andover asignificantfractionofthenorthernsky,itisclearlydelineatedby early-typeluminousstars(asfirstrealizedbyMorgan,Whitford& (cid:2)E-mail:[email protected](AM);[email protected](IN) Code1953).Betweenl=100◦and140◦,thePerseusarmcontains (cid:5)C 2016TheAuthors PublishedbyOxfordUniversityPressonbehalfoftheRoyalAstronomicalSociety OpenclustersinAurOB2 881 both large active star-forming regions, and more evolved associ- a variable reddening within the cluster region, going from ations,withagesbetween∼10and∼20Ma.Amongthem,some E(B − V) = 0.40–0.60 mag, and a distance of 2.05 ± 0.10 kpc. aredispersedandcontainsmallclusters(CasOB2/OB4/OB5/OB7), Theydetectedasignificantnumberofyoungstellarobjects(YSOs) whileothershavemassivecentralclusters,suchasthetwinclusters insidetheclusterandinaNebulousStreamtowardstheeasternside NGC869/884atthecoreofPerOB1orNGC663andNGC654, ofthecluster(seeFig.1).TheYSOslyingintheNebulousStream definingCasOB8.However,beyondl=140◦,stellarandmolec- werefoundtobeyoungerthanthestarsinStock8.Theyconclude ulartracersofthePerseusarmsuddenlybecomeveryrare.Cloud thatthemorphologyoftheregionseemstoindicatethattheioniza- complexes identified with the Perseus arm are seen again in the tion/shockfrontcausedbytheionizingsourceslocatedinsideand thirdGalacticquadrant(alreadyatl≈215◦),butitsstellartracers westofStock8hasnotreachedtheNebulousStreamandthestar are still scarce (Va´zquez et al. 2008). Partly because of this ab- formationactivityinbothregionsmaybeindependent. senceoftracers,westilllackacompletepictureoftheextentofthe In this paper, we study a much larger area of Perseusarmtowardsthethirdquadrant,andtheconnectionbetween ∼40arcmin × 40arcmin, where we find Stock 8 and two theLocalFeatureandthemajorspiraldesign(Va´zquezetal.2008; candidates to young clusters, as well as a large number of see a recent discussion in Foster & Brunt 2015). Recently, Choi early-typestarsspreadoverthewholearea.Weaimtodetermine etal.(2014)haveusedtrigonometricparallaxesforwatermasersin accurately the distance to all these objects and analyse how the D massivestar-formingregionstotracethePerseusarminthesecond stellarformationprocesshasdeveloped.Thepaperisorganizedas o w andthirdquadrants.TheyfindanumberoftracersofthePerseus follows.InSection2,wepresentthephotometricandspectroscopic n arm around l=190◦, with distances clustered around 2 kpc, but datausedtomakethesubsequentanalysis.Thisanalysis,including loa d again no tracers are found between l=140◦ and l=180◦. Over the determination of spectral types for individual stars, and the ed this longitude range, there are a number of young open clusters reddening, distance and age for the three clusters studied, is fro m withdistancesaround∼4kpcatl=150◦and∼5kpcatl∼180◦ developed in Section 3. In Section 4, we comment the impact of h (dNeleignueaetreuealapo&orMlyaprcoopu2l0a0te3d,asnpdirarlefaerrmen,cbeusttthheerieridni)s.taTnhceeyissecelmosetor oMuirlkryesWulatsyoonfSthtoechkis8toarnydosfursrtoaurnfodrinmgast.ioFninaanlldy,twheepeonsuimtioenraitneothuer ttp://m n than expected for the position of the continuation of the Cygnus conclusions. ra s (+II)arminmodelssuchasthoseofValle´e(2015) .ox Tohelpcastinglightontheseissues,theaimofthisworkistoclar- 2 OBSERVATIONS AND DATA ford ifytheextentanddefinitionofwhathastraditionallyknownasasso- jo u ciationAurOB2.AurOB2wasidentifiedasadistantandcompact rn associationlyingbehindAurOB1,withboundariesl=172◦–174◦ 2.1 Opticalphotometry als.o andb=−1◦.8tob =+2◦.0(Humphreys1978).Thestar-forming We obtained Stro¨mgren photometry of three fields in the area rg a/ openclusterNGC1893,containingfiveO-typestars,wasconsid- of study with the 1-m Jacobus Kapteyn Telescope (JKT) at the t U ered the core of this association, but most modern studies based Roque de los Muchachos Observatory (La Palma, Spain) during N IV on accurate photometry (Fitzsimmons 1993; Marco, Bernabeu & a run in 2003 February, 9–16. The telescope was equipped with E Negueruela2001)andspectroscopy(Negueruelaetal.2007)agree the 2048 × 2048 SITe1 chip CCD and the four Stro¨mgren uvby RS onadistancearound5kpcforNGC1893,incompatiblewiththedis- andthenarrowandwideHβ filters.Thecameracoversafieldof ID A tancedeterminedtootherpresumedmembers(Humphreys1978). 10arcmin×10arcminandhasapixelscaleof0.33arcsecpixel−1. D D Visually,IC410,theHIIregionilluminatedbyNGC1893,seems The first field was the area traditionally assigned to Stock 8, E A toformpartofasinglecomplexofilluminatedcloudstogetherwith whichwasobservedonthenightsof2003February,9–10.Foreach L IC 417 and IC 405. This is just an illusion, as IC 405 is a com- frame,weobtainedsevenseriesofdifferentexposuretimesineach IC A pletelyunrelatednebula,associatedwithaforeground(d≈450pc) filtertoachieveaccuratephotometryforabroadmagnituderange. N T cloud illuminated by the runaway star AE Aur, believed to have Thecentralpositionfortheclusterandtheexposuretimesusedare E o beenejectedfromtheOrionCloud(Franceetal.2004,andrefer- presentedinTable1andTable2,respectively. n O encestherein).IC417,alsoknownasSh2-234,istheHIIregion Theothertwofieldsobservedwerecentredontwostellaraggre- cto illuminatedbytheopenclusterStock8.ItsGalacticcoordinatesare gatesthatweconsideredopenclustercandidates.Subsequentlyto be l=173◦.37andb=−0◦.18.Inthispaper,wepresentaphotometric ourobservations,theseopenclustershaveappearedintheliterature r 2 1 andspectroscopicstudyofStock8,thediffusepopulationofOB asclustercandidateswiththenamesofFSR777(Froebrich,Scholz , 2 0 stars generally assigned to Aur OB2 and two other young open &Raftery2007)andKronberger1(Kronbergeretal.2006).These 16 clustersthatweidentifyinitsvicinity. twoobjectsareconfirmedasyoungopenclustershereandnamed TherehavebeenafewpreviousstudiesofStock8andsurround- Alicante11andAlicante12,respectively.Theframes,takenonthe ings.However,theextentoftheclusteritselfhasnotbeenclearly nights of 2003 February, 15–16, were centred on the coordinates defined.ThenameStock8hasbeentraditionallygiventoastrong displayedinTable1.Weobtainedsevenseriesofdifferentexposure stellarconcentrationthatseemstobeembeddedinthenebulaas- timesforAlicante11andthreeseriesforAlicante12.Theexposure sociated with Sh 2-234. Mayer & Macak (1971) carried out an timesusedarepresentedinTable2.SinceStock8ispartiallyem- spectroscopicandUBVphotoelectricphotometrystudyof11bright beddedindarknebulosity,exposuretimesforthisfieldwerelonger starsinitssurroundings,findingacommondistanceforStock8and thanthoseintheothertwofields.Ourintentionwastoreachanab- someoftheOBstarsinitsvicinity(afewarcminutestothewest;see solutemagnitudethatwouldallowustotracethewholesequence Fig.1),namely,adistancemodulus(DM)of12.36mag(2965pc). ofBtypes.Aswillbeseenlater,thiswaseffectivelyachieved. Malysheva(1990)usedUBVphotographicphotometryof∼66stars Standard stars were observed throughout the run in the clus- brighterthanV=16insideanangulardiameterof20arcminfrom tersNGC869,NGC884,NGC1039,NGC1502,NGC2169and Stock8,andobtainedadistancemodulusof11.39mag(∼1897pc) NGC 2244, using the exposure times suitable to obtain good andanageof∼12Ma.Joseetal.(2008)tookUBVI CCDphotom- photometricvaluesforallstarsselectedasstandardsintheseclus- c etryofthecompactclusterandfoundaclusterradiusof∼6arcmin, ters.Thecentralpositionsfortheclustersandtheexposuretimes MNRAS459,880–901(2016) 882 A.MarcoandI.Negueruela D o w n lo a d e d fro m h ttp ://m n ra s .o x fo rd jo u rn a ls .o rg a/ t U N IV E R S ID A D D E A L Figure1. MapoftheareashowingthefullfieldstudiedandthethreeareasforwhichweobtainedStro¨mgrenphotometry.Theimagehasbeendownloaded IC A withALADINandrepresentsafalse-colourcompositeofWISEbands.Thepinkishregionsrepresentextendeddustemission.Theredsquaresindicatestarswith N T spectra.Thecirclesinblue,greenandyellowarestarswithopticalphotometryinthethreeareas(Stock8,Alicante11andAlicante12).TheNebulousStream E ofJoseetal.(2008)canbeseenjusttotheeastofStock8.TheclustercandidateCC14isthesmallclumpofIRsourceslocatedwherethestreammeetsthe on gridlinecorrespondingtoRA=05h29h.Thesizeoffieldis59.18arcmin×47.82arcmin.Northisupandeastisleft. O c to b e Table 1. Clusters observed from the JKT in 2003 Febru- usedare presentedinTable1andTable2,respectively. Ifanyof r 2 ary.Thetoppanelcontainsthetargetfields,whilethebot- theselectedstandardsturnedouttobesaturated,werepeatedthe 1, 2 tompanelincludestheclustersthatcontainthephotomet- observationswithashorterexposuretime. 01 ric standards. The coordinates are those provided by the WereducedtheframesforallclusterswithIRAF1routinesforthe 6 WEBDAdatabase:http://www.univie.ac.at/webda/(Netopil biasandflat-fieldcorrections.Photometrywasobtainedbypoint- etal.2015),exceptforthetwonewlydefinedclusters. spread function (PSF) fitting using the DAOPHOT package (Stetson 1987) provided by IRAF. The apertures used are of the order of Name RA(J2000) Dec(J2000) the full width at half-maximum. In this case, we used a value of Stock8 05h28m08s.0 +34◦25(cid:7)42(cid:7).(cid:7)0 6pixelsforeachimageinallfilters.InordertoconstructthePSF Alicante11 05h27m36s.2 +34◦45(cid:7)19(cid:7).(cid:7)0 empirically, we automatically selected bright stars (typically 25 Alicante12 05h28m20s.3 +34◦47(cid:7)13(cid:7).(cid:7)5 stars). After this, we reviewed the candidates and we discarded NGC2244 06h31m55s.0 +04◦56(cid:7)30(cid:7).(cid:7)0 those that did not reach the best conditions for a good PSF star. NGC2169 06h08m24s.0 +13◦57(cid:7)54(cid:7).(cid:7)0 OncewehadthelistofPSFstars(≈20),wedeterminedaninitial NGC1502 04h07m40s.4 +62◦20(cid:7)59(cid:7).(cid:7)1 NGC1039 20h23m10s.6 +40◦46(cid:7)22(cid:7).(cid:7)4 NGC869 02h19m04s.4 +57◦08(cid:7)07(cid:7).(cid:7)8 NGC884 02h22m00s.6 +57◦08(cid:7)42(cid:7).(cid:7)1 1IRAFisdistributedbytheNationalOpticalAstronomyObservatories,which areoperatedbytheAssociationofUniversitiesforResearchinAstronomy, Inc.,undercooperativeagreementwiththeNationalScienceFoundation. MNRAS459,880–901(2016) OpenclustersinAurOB2 883 Table2. Logoftheopticalphotometricobservationstaken PSFbyfittingthebestfunctionbetweenthefiveoptionsofferedby attheJKTin2003February. thePSFroutineinsideDAOPHOT.WeallowedthePSFtobevariable (of order 2) across the frame to take into account the systematic Exposuretimes(s) patternofPSFvariabilitywithpositiononthechip. Filter Longtimes Shorttimes Weneededtoperformanaperturecorrectionof26,25and30pix- Stock8 elsfortheu,v andbfilters,respectively.Theaperturecorrection u 1800 400 fortherestofthefiltersy,HβnandHβwwasof28pixels.Theat- v 500 175 mosphericextinctioncorrectionswereperformedusingtheRANBO2 b 200 100 programme,whichimplementsthemethoddescribedbyManfroid y 150 50 (1993). Finally, we obtained the instrumental magnitudes for all Hβn 1800 400 stars. Hβw 200 100 TheselectionofstandardstarshasbeenexplainedinMarco& Alicante11 Negueruela (2013). In this work, we used two new open clusters u 450 100 that fulfilled the requirements to provide standard stars. The list v 180 40 of newly adopted standard stars and their photometric data to be D b 100 20 usedinthetransformationsofCCDStro¨mgrenphotometryofopen o w y 50 8 clustersisgiveninTable3. n Hβn 400 100 Withthestandardstars,wetransformedtheinstrumentalmagni- loa Hβw A6li0cante12 15 WtudeeismtpolethmeesntatenddtahredfsoylslotewminugsiunvgbtyhteraPnHsOfToCrAmLaptiaocnkeaqgueaitnisoindse,IaRfAteFr. ded fro m u 450 − Crawford&Barnes(1970)andtheβtransformationequationafter h vb 118000 −− Crawford&Mander(1966): ttp://m y 50 − V =(−4.937+A)−0.110(b−y)+yi nra HHββnw 46000 −− ±0.009±0.035 (1) s.oxfo rd NGC2244 jo u 35 20 (b−y)=(0.231+B)+1.037(b−y)i urn a v 30 20 ±0.002±0.013 (2) ls.o b 15 10 rg y 20 5 a/ HHββnw 2150 −− m1 =(−0.668+C)+0.923m1i −0.159(b−y) t UNIV ±0.047±0.051±0.031 (3) E NGC2169 RS u 50 − ID vb 3200 −− c1 =(0.016+D)+1.020c1i −0.202(b−y) AD D y 20 10 ±0.011±0.011±0.030 (4) E A Hβn 50 − LIC Hβw 25 − β =(1.198+E)+0.703βi AN T NGC1502 E u 400 − R2 =0.97, (5) on v 200 − O where each coefficient is given with the error resulting from the c b 150 − to HyHββnw 3335000 1−−0 tpproraoinnvstifsdoewrdmhiaontsiTeoanvb.allTeuh4ees. avraeludeisffeorfenAt,fBor, eCa,chDnaignhdt.EThreepsreesveanlutezsearore- ber 21, 20 Thenumberofstarsthatcanbedetectedinallfiltersislimitedby 16 NGC1039 thelongexposuretimeintheufilter.Weselectedfortheanalysis u 50 − allstarswithgoodphotometry(photometricerrors≤0.05mag)in v 30 − allsixfilters.WeidentifythesestarsontheimagesinFigs2,3and b 20 − 4foreachfieldobserved. y 20 5 Hβn 50 − Wehaveobtaineduvbyβ CCDphotometryfor163starsinthe Hβw 25 5 clusterStock8,reachingamagnitudelimitV≈16.InTableA1,we listtheircoordinatesinJ2000.Thesecondcolumncross-references NGC869andNGC884 withthenear-infrared(near-IR)photometry,indicatingtheirnum- u 600 − ber in Table A2, for those stars that have values in our near-IR v 350 − b 200 − photometry.ForstarswithnomeasurementsinTableA2,thelast y 100 − columns show JHKs photometry from the Two Micron All Sky Hβn 600 − Survey (2MASS) catalogue (Skrutskie et al. 2006). The uvbyβ Hβw 120 − CCDphotometryisgiveninTableA3,whereNisthenumberof measurementsforeachmagnitude,colourorindexandσistheerror assignedtoeachvalue.Theerrorassignedisthestandarddeviation MNRAS459,880–901(2016) 884 A.MarcoandI.Negueruela Table3. Newadoptedstandardstarswiththeircataloguedvaluesandspectraltypestakenfromtheliterature. Star V b−y m1 c1 β Spectraltype NGC2244 114 7.590 0.207 −0.048 −0.081 2.608 O8.5V NGC2169 11 10.600 0.084 0.065 0.541 2.698 B8V 15 11.080 0.130 0.109 0.944 2.864 B9.5V 18 11.800 0.115 0.105 0.912 2.872 B9.5V Note. The data are taken from Crawford (1975) and Johnson & Morgan (1953) for NGC 2244 and Perry, Lee & Barnes (1978) for NGC 2169. Spectral types are taken from Walborn (1971) for NGC 2244, and Perryetal.(1978)forNGC2169. Table4. ValuesoftheconstantsA,B,C,DandEforeachnight. thephotometryfromthereducedframeswasthesameasdescribed inSection2.1fortheopticalimages. D o Night A B C D E The next step is to transformthese instrumental magnitudes to w n the2MASSmagnitudesystem(Skrutskieetal.2006).Forthis,we lo 20030209 18.079 0.174 0.816 0.431 2.105 a 20030210 18.036 0.173 0.818 0.429 2.104 selected those stars in our field having JHKS magnitudes in the ded 20030215 16.574 0.432 0.259 0.805 2.049 2MASS catalogue with photometric errors smaller than 0.03mag fro 20030216 13.784 0.805 0.156 0.478 1.687 ineveryfilter(about80stars).Alineartransformationwascarried hm outbetweeninstrumentaland2MASSmagnitudes.Nocolourterm ttp was needed, since a simple shift in zero-point results in a good ://m forstarswithmorethanonemeasurementandthephotometricer- transformation.Thiswascheckedbyplottingthetransformedmag- n ra rorsfromDAOPHOTforstarswithonlyonemeasurement,withthe nitudesagainst2MASSmagnitudesforallthestarswith2MASS s.o x errorsinthecolourandindicescalculatedthrougherrorpropaga- magnitudes in the field, finding that the best fit corresponds to a fo tion.InFig.2,weindicatewiththeyellowopencirclesstarswith straightlineofslope1inallthreefilters. rd jo opticalphotometry.Theopenbluesquaresarestarsthatwecould Wehavenear-IRphotometryfor589stars.InTableA2,welistthe urn detectinournear-IRphotometry.Thefilledredsquaresarestarsfor numberofeachstar,theirRAandDeccoordinatesinJ2000,andthe als whichweobtainedspectra.InTablesA4andA6,welistthenumber valueofJ,HandKSwiththephotometricerrorforeachmagnitude. .org and the coordinates in J2000 for stars in the Alicante 11 and the There is only one measurement for each filter. The completeness a/ Alicante12fieldswithopticalphotometry.Inthesetables,weshow limitsinthe2MASSstandardsystemareJ∼18,H∼17andK ∼ t U S N theirJHKsphotometryfromthe2MASScatalogue(Skrutskieetal. 18. IV E 2006)too.InTablesA5andA7,wepresenttheresultingvaluesfor Astrometricreferencingofourimageswasmadeusingpositions R V, b − y, m1, c1 and β for stars in Alicante 11 and Alicante 12, ofanumberofstarsfromthe2MASScatalogue.ThePSFsofthese SID together with the number of measurements for each magnitude, starsarenotcorruptedbytheCCDoversaturationeffects.Weused A D colourorindexandtheerrorassignedforeachvalue.Theseerrors IRAFtasksccmap/cctranfortheastrometrictransformationof D E are calculated as above. The designation of each star is given by the image. Formal rms uncertainties of the astrometric fit for our A the number indicated on the images (Figs 3 and 4). We obtained imagesare<0(cid:7).(cid:7)10inbothrightascensionanddeclination. LIC opticalphotometryfor57starsinAlicante11andfor52starsin A N Alicante12. T E o n 2.3 Spectroscopy O c 2.2 Near-IRphotometry to SpectraofthebrighteststarsintheareaweretakenwiththeAure´lie be Near-IRJHKimageswereobtainedintheareaoftheopencluster spectrographonthe1.52-mtelescopeattheObservatoiredeHaute r 2 1 Stock8usingNICS(NearInfraredCameraSpectrometer)onthe Provence(OHP)duringtwodedicatedrunson2002January18–22, , 2 0 TelescopioNazionaleGalileo(TNG)intheLaPalmaobservatory and2002February25–28(whenonlythefirstnightwasusefulbe- 16 on 2011 May 14. NICS was equipped with an HgCdTe Hawaii causeoftheweather).Thespectrographwasequippedwithgrating 1024 × 1024 array, providing a pixel scale of 0.25 arcsec pixel #3(600lnmm−1)andtheHorizon20002048×1024EEVCCD and a field of view of 4.2arcmin × 4.2arcmin in its large field camera(seeGilletetal.1994foradescriptionoftheinstrument). (LF)mode. Theobservations werecarriedoutunderphotometric Intheclassificationregion,thisconfigurationgivesadispersionof conditions. 0.22Åpixel−1(resolvingpowerofapproximately7000),covering These near-IR observations consisted of series of several inte- awavelengthrangeof≈440Å.Itis,therefore,necessarytoobserve grations repeated at dithered positions on the detector. The total twowavelengthregionstocovertheclassicalclassificationregion. effectiveexposure,resultingfromtheco-additionofthejitteredex- Forthisprogramme,thetworegionsselected(identifiedasPosi- posures,is60sforlongexposuresand15sforshortexposuresin tion1and2inTable5)werecentredatλ=4175Åandλ=4680Å. allfilters Inprinciple,allobjectswereintendedtobeobservedinbothregions, Thestandardreductionofnear-IRdataincludesthedarksubtrac- resultingincoverageintheλλ3950–4900Årange,withasmallgap tionandflat-fieldcorrectionofindividualframesthataremedian- (∼60 Å)aroundλ4425Å.Nostrongphotosphericlinesarefound combined to build a sky image. The sky image is subsequently inthegap,butthestrongdiffuseinterstellarlineatλ4428Å,which subtractedfromtheindividualframesthatarethenshiftedandfi- isagoodindicatorofthereddening, islost.Thecomplete logof nally stacked into one single image. The procedure for obtaining observationsatthe1.52-mOHPisgiveninTable5. MNRAS459,880–901(2016) OpenclustersinAurOB2 885 D o w n lo a d e d fro m h ttp ://m n ra s .o x fo rd jo u rn a ls .o rg a/ t U N IV E R S ID A D D E A L IC A N T Figure2. FindingchartforstarswithphotometryintheclusterStock8.Theimage,downloadedfromALADIN,isaDSS2reddigitization.Thedarkareasthus E o mapnebularHαemission.Theredfilledsquaresrepresentstarswithspectra.Thecirclesinyellowarestarswithopticalphotometryandthebluerectangles n O arestarswithTNGnear-IRphotometry.NumbersandcoordinatesinJ2000arelistedinTablesA1andA2.Thesizeoffieldis12.28arcmin×10.67arcmin. c to Northisupandeastisleft. b e r 2 1 ThesespectroscopicdatawerereducedwiththeStarlinkpackages wehadthespectraoftargetsandarcs,weusedthepackageIDENTIFY , 2 0 CCDPACK(Draper,Taylor&Allan2000)andFIGARO(Shortridgeetal. to perform the wavelength calibration. We used ThAr arc lamp 16 1997)andanalysedusingFIGAROandDIPSO(Howarthetal.1998). spectra,takenbetweentheexposures.Thermsforthewavelength Wetookmorespectrawiththe2.6-mNordicOpticalTelescope solutionis≈0.2pixels. (NOT,LaPalma,Spain)onthenightsof2011September1–2.The telescopewasequippedwiththeimagerandspectrographAndaluc´ıa 3 RESULTS FaintObjectSpectrographandCamera(ALFOSC).Spectraofsome ofthebrighteststarsinStock8,Alicante11andAlicante12were 3.1 Spectralclassification obtainedwithALFOSC.Inspectroscopicmode,weusedthegrism #16combinedwitha1arcsecslittoobtainintermediateresolution Tocharacterizethemorphologyofthewholeregion,weobserved spectroscopy. Grism #16 covers the 3500–5060 Å range with a spectroscopicallyalmostallthestarsintheareathathavebeenclas- nominaldispersionof0.8Åpixel−1.Theresolvingpowerforthis sifiedasearly-typestarsintheliterature.Thestarsobservedcover configurationisR∼1000.ThelogofobservationsattheNOTis aregionofapproximately40arcmin×40arcminthatincludesthe giveninTable6. three open clusters studied and a very significant diffuse popula- These spectroscopic data were reduced using IRAF routines for tionscatteredovermostoftheregion.Thesestarsarerepresented thebiasandflat-fieldcorrections.Theextractionofthespectrawas bytheopenredsquaresinFig.1andlistedinTables5and6.In doneusingtheAPALLpackageinsideAPEXTRACTinTWODSPEC.Once thesetables,allthestarsarelistedbytheiridentifierintheLSV MNRAS459,880–901(2016) 886 A.MarcoandI.Negueruela D o w n lo a d e d fro m h ttp ://m n ra s .o x fo rd jo u rn a ls FJ2i0g0u0rear3e.liFsitneddiinngTcahbalretAfo4r.sTtahresswiziethopfhthoetofimeeldtryisi1n8A.0l1icaarnctmei1n1×.T1h0e.7i5maargcem,ipnr.oNviodrethdibsyuApLaAnDdINe,aisstaisDleSfSt.2bluedigitization.Numbersandcoordinatesin a.org/ t U N IV E R S ID A D D E A L IC A N T E o n O c to b e r 2 1 , 2 0 1 6 Figure4. FindingchartforstarswithphotometryinAlicante12.Theimage,providedbyALADIN,isaDSS2bluedigitization.Numbersandcoordinatesin J2000arelistedinTableA6.Thesizeoffieldis18.01arcmin×10.75arcmin.Northisupandeastisleft. MNRAS459,880–901(2016) OpenclustersinAurOB2 887 Table5. StarsobservedfromtheOHP1.52-m.ThefirstcolumnindicatesthevolumeandnumberintheLuminousStarscatalogueorthenameinthe HenryDraperCatalogue.Thesecondandthirdcolumnsindicatethedateswhenthetwospectralpositionswereobservedand(betweenbrackets)the exposuretimeinseconds.Thederivedspectraltypesaregiveninthefourthcolumn.Spectraltypesmarkedwithan‘*’arelesssecurethantheaverage, becauseofpoorsignaltonoiseorpresenceofdoublelines.Referencesforthephotometricvaluesincolumn7are(1)Hiltner(1956)and(2)Mayer& Macak(1971). NameNumber Position1 Position2 Spectral V (B−V) Reference DM±0.5 KS type LSV+34◦22a 18/01(600) 19/01(700) O7.5V 8.58 0.23 2 11.5 7.89±0.02 LSV+34◦15 18/01(750) 19/01(900) B1V 10.02±0.05 0.20 1 11.6 9.37±0.02 LSV+34◦16 18/01(1000) 19/01(1000) B1V∗ – – – – 9.22±0.02 LSV+34◦18 18/01(750) 19/01(1200) O9.5V∗ 10.01 0.67 2 11.0 8.04±0.02 LSV+34◦21 18/01(1200) 19/01(1500) O9V∗ 10.70 0.94 2 10.9 7.88±0.02 LSV+34◦29b 20/01(1000) 25/02(1200) O9.7IV 8.89 0.18 2 12.0 8.32±0.02 LSV+34◦31c 20/01(1200) 25/02(1200) B0.5V∗ 9.87 0.19 2 12.0 9.23±0.02 LSV+34◦36d 18/01(600) 19/01(750) O8V(n) 8.78 0.26 1 11.4 8.02±0.03 D LSV+34◦23e 20/01(750) 25/02(900) O8II(f) 8.04 0.32 1 12.0 6.99±0.02 ow LLSSVV++3344◦◦2151fg 2180//0011((970500)) 1295//0012((715200)0) O9.5BV0+.5BV0.2V 9.–70 0.–17 2– 11.9 97..2579±±00..0022 nload HD281147 18/01(900) 19/01(1200) B1V – – – – 9.63±0.02 ed aHD35619=Alicante11-57;bBD+34◦1054=ST93;cBD+34◦1056=ST120;dBD+34◦1058;eHD35633;fHD35652=Alicante11-56(Eclipsing from binaryIUAur);fHD281150. http Table6. ClassificationspectraofstarsintheareaofStock8.ThesestarswereobservedwiththeNOT.Referencesforthephotometricvaluesincolumn ://m n 7are(2)Mayer&Macak(1971)and(3)Mayer(1964). ra s .o x Star LS Spectraltype Exposuretime(s) V (B−V) Reference DM±0.5 KS ford AAlliiccaannttee1112--1221 VV++3344◦◦1373 BB22VV 235000 –– –– –– –– 190..6166±±00..0032 journa Alicante12-24 – B7V 300 – – – – 12.82±0.03 ls.o AAlliiccaannttee1112--2290 V+3–4◦19 BB29VV 430000 –– –– –– –– 1101..7871±±00..0022 at Urg/ Alicante11-49 V+34◦20 B1.5V 400 – – – – 10.29±0.02 N – V+34◦14 B1V 300 – – – – 10.19±0.02 IV E ST101 – A3V 400 – – – – 11.39±0.01 R S ST97 V+34◦30 B1V 400 – – – – 10.45±0.02 ID ST61 V+34◦28a B1V 350 10.92 0.27 3 12.3 10.03±0.02 AD ST23 V+34◦27 B2IV 400 11.66 0.33 3 13.0 10.70±0.02 D ST25 – B5V 400 – – – – 11.63±0.02 E A – V+34◦35 B0.7V 200 – – – – 9.75±0.02 L – V+34◦12 B1V 300 10.91 0.30 2 12.2 10.01±0.02 ICA – V+34◦13 B2IV 400 – – – – 10.92±0.02 N T Alicante11-27 – B8V 300 – – – – 8.06±0.03 E o A–licante12-30 V+–34◦8 BB42IVII 340000 –– –– –– –– 1100..5679±±00..0022 n Oc – V+34◦24 B1V 350 11.24 0.40 3 12.2 10.05±0.02 tob – V+34◦10b B0.2V 200 9.43 0.16 2 11.7 9.01±0.02 er 2 – V+34◦37c O9.7V 200 9.22 0.19 2 11.5 8.80±0.02 1, 2 0 aBD+34◦1053;bHD281151;cBD+34◦1059. 16 (Hardorp,Theile&Voigt1965)catalogueexceptforHD281147, LSV+34◦23,LSV+34◦12,LSV+34◦21,LSV+34◦13,LSV whichismissinginthiscatalogue.Cross-correlationwithournum- +34◦14,LSV+34◦11,LSV+34◦16,LSV+34◦18,LSV+34◦15, bering system is provided for stars with photometry. Other more LSV+34◦24,LSV+34◦10andLSV+34◦8. usualidentifiersofthebrighteststars,suchasHDnumbers,arealso (iii) TwostarstothenorthofStock8,clearlydetachedfromthe given. cluster,LSV+34◦35andLSV+34◦36. Thestarsobserved,accordingtotheirdistribution,canbegrouped (iv) A more distant star to the NW, HD 281147 and another asfollows. isolatedstartothenorthofAlicante12:LSV+34◦37. (v) FourstarsinthecoreofAlicante12:20,21,24and30. (i) ThetwobrighteststarsilluminatingtheHIIregion,atthecore (vi) Six stars in the core of Alicante 11: 12, 27, 29, 49, 56 of Stock 8, LS V+34◦29 (ST93) and LS V+34◦31 (ST120), and and57. asetoffivestarswithinthelimitsgivenforStock8byJoseetal. (2008):ST23,ST25,ST61,ST97andST101. The spectra obtained were used for spectral classification by (ii) Agroupof12starsspreadtothewestofStock8thathavebeen comparison to spectra of MK standard stars observed at simi- assumedtobesomehowconnectedtoStock8bypreviousauthors: lar resolution, following the standard classification procedures of MNRAS459,880–901(2016) 888 A.MarcoandI.Negueruela Figure5. OHPspectraofthetwobrighteststarsinStock8,andthenearby Figure6. OHPspectraofotherO-typestarsinthearea.HD35619andHD HD35633thatis,verylikely,themainsourceofionizationinIC417.The 35652arepartofAlicante11.BD+34◦1058liesnorthofStock8. D smallgapisduetothetwodifferentsettingsexplainedinthetext. o w n lo Thesevaluesareinsurprisinglygoodagreementwiththemasses a Wteranlablorcnon&sisFteitnzcpyatbriyckco(m19p9a0ri)n.gInalaltsheecospnedcstrtaepo,bwtaeincehdeacmkeodngins-t M(1919=4).2D1.r3ecMhs(cid:10)eleatnadl.M(1299=4)1a4l.s4oMid(cid:10)entdifeyriavethdirbdyboDdryecihnstehlisetsyasl-. ded fro themselves. In principle, the much higher resolution of the OHP m spectra allows a much more accurate classification, but in some otefmth,ewliitghhta.mSianscseMa2th=ird1s7p.8ecMtru(cid:10)m,iwshniocthsaececno,uDnrtescfhosrel∼e2t0alp.e(1r9c9e4n)t http cases the signal-to-noise ratio in the 4000–4500Å region is very supporttheideathatthethirdbodyisaclosebinaryconsistingof ://m low, resulting in inaccurate classifications, marked as such in Ta- n twoB2–B3stars. ra ble5.SpectraobservedwithALFOSCcanbeconsideredaccurateto (iv) LSV+34◦22(Alicante11-57)isthebrighteststarinAli- s.o ±1Aslulbthtyepest.ars in the LS catalogue (Hardorp et al. 1965), as ex- cante11.Onthebasisofstandardcriteria,weclassifyitasO7.5V. xford Sotaetal.(2014)giveamoreaccurateclassificationO7.5V((f))z, jo pected,turnouttobeofearlytype.Theonlyobjectthatdoesnot u lbaetleofnogrttohethceatOalBoggureo,uapndistoLoSoVld+to34b◦e8lotnhgatt,oatthBe4saImIIe,ipsoapubliattitoono bas(evd) LonShVig+he3r4q◦3u6al,ictylasspseificetrda.asO8nnbyMorgan,Code&Whit- rnals.o ford(1955)doesnotshowverybroadlinesinourspectrum(Fig.6). rg auslutshe(craelsctuolafttehdefsrtoamrstihneth2eMrAegSioSnd,aetvae)nisthroeuadgihlyitscodmisptaanticbelem.oAdl-l The object, though, is a double-lined spectroscopic binary. The at U/ combined spectrum has spectral type O8 V, but all the lines are N thestarsobservedwithintheclustershavespectraltypescompati- broad and asymmetric, clearly showing the presence of a second IV bSolfTet1hw0e1itshipnemScettrmoacbaker8ers,dhaiinspcdeuAxscsleiecdpatnbfteoelro1wt1w.-o27e.aNrlyo-tetywpoertfhoyreagsrpoeucntdsoofbsjeocmtse, acoremOpa-tnyipoen.sGtairvse.nSitnhceewHidethIIo4f68H6eÅII4is54a2bnÅo,rmthaelltywowecaokm,paotnleenatsst ERSIDA oneofthemcouldbeofhigherluminosity. D (i) LS V +34◦29 (ST 93) is the brightest star in Stock 8. The (vi) HD 281147 is given as O8nn in SIMBAD, after Garmany DE spectrumsuggestsitisamoderatelyfastrotator,butthelinesappear & Conti (1984). This is almost certainly due to a confusion with AL veryshallow(Fig.5).Formally,thecriterionSiIII4553Å(cid:9)HeII BD+34◦1058,whichistheobjectobservedbyIUE.Ourspectrum ICA a44r56e4826quÅÅitiednedwfiiecnaaektsess(SptheiecIVntraa4l1lu1tym6pÅienoOiss9itn.y7o.ctlTaehsvseenIrVes.leaHetionvw)e.ewAveellra,kmtnheeetsaSslilioIcVfllHiinneeeIssI LgiSv(evVsiia+)sT3p4he◦ec2tP1raoalsritetyiploienttlB1e1smpVoe.rcetrtahoanfLnoSisVe,+an3d4◦t1h6er,eLfoSrVeth+e3ir4s◦p1e8catrnadl NTE on O look too weak and shallow, and this suggests that this object is typesarebasedonaverylimitedspectralrange,andmustnotbe c to reallyaspectroscopicbinarywhosespectraltyperesultsfromthe consideredaccurate. be combinationofalate-Oandanearly-Bstar. r 2 (ii) LS V +34◦23 has a spectral type O8, rather earlier than ThedistributionofspectraltypesintheregionisshowninFig.17. 1, 2 previouslyassigned(Fig.5).IthadbeenclassifiedasB0.5IVby Noobviouspatternemerges. 016 Morgan et al. (1953), most likely as a consequence of misidenti- fication.TheNIII4634–41–42complexisinemission,whileHeII λ4686Å is very weak and flanked by emission components. The 3.2 HRdiagrams object, therefore, presents a rather high luminosity. Just short of 3.2.1 OpticalphotometryforStock8 being a supergiant according to the criteria of Sota et al. (2011), we classify it as O8 II(f), while noting the complete absence of Asafirststep,weanalysedtheuvbyβCCDphotometryobtainedfor anyabsorptionNIIIlines.Ifthestarisnitrogendeficient,itcould theclusterStock8(TableA3)todeterminethephysicalparameters probablyhaveahigherluminosity. of the cluster: reddening, distance and age. Initially, we plotted (iii) LS V +34◦25 (Alicante 11-56) is a well-known spectro- the diagrams V/(b − y ) andV/c for all the stars in the field. In 1 scopicbinary(IUAur),whichhasreceivedseveralverydiscrepant Fig.7,theredcirclesarestarswithspectraandthegreensquaresare classifications.Althoughthetwocomponentsarenotverywellre- starsthatappearmisplacedinbothdiagramsand,therefore,were solvedinthePosition1spectrum,theyareclearlyseparatedinthe removedforthefollowinganalysis. Position 2 one (Fig. 6), and we derive spectral types of O9.5 V Thenextstepintheanalysisistheestimateofmembershipfor andapproximatelyB0.2Vforthetwocomponents,basedmainly thestarsmeasuredinthefield.Giventhedepthofourobservations, ontherelativestrengthsoftheHeII4686ÅandCIII4650Ålines. andthepresenceofO-typestarsconfirmedbyspectroscopy,most MNRAS459,880–901(2016) OpenclustersinAurOB2 889 D o w n lo Figure7. Left:theV/(b−y)diagramforallstarsinStock8.Theredcirclesrepresentstarsspectroscopicallyobservedandgreensquares,non-memberstars. ad Right:theV/c1diagramforallstarsinStock8.Theredcirclesrepresentstarsspectroscopicallyobservedandthegreensquaresnon-memberstars. ed fro m h ttp ://m n ra s .o x fo rd jo u rn a ls .o rg a/ t U N IV E R S ID A D D E Figure8. Left:the[c1]−[m1]diagramforthestarsobservedintheopenclusterStock8.ThesolidlinesrepresenttheaveragelocioffieldB-type(magenta AL line),A-type(brownline)andF-type(cyanline)main-sequencestars(Perry,Olsen&Crawford1987).Right:theβ-[u−b]diagramforthestarsobserved IC intheopenclusterStock8.Thecolourconventionisasintheleft-handpanel,withtheaveragelocioffieldstarsalsotakenfromPerryetal.(1987).Inboth A N diagrams,theredsquarescorrespondtospectroscopicallyobservedstars.TheorangecirclesarestarsfallingclosetothepositionofB-typestarsinthetwo T E diagrams;thegreendotsareA-typestarsandthebluedotsareidentifiedasF-typestars. o n O c to clustermembersinoursamplemustbeB-typestars.Now,wecan Afterthis,weusethosestarsfallingontheB-typebranchinboth be calculatethereddening-freeindices[m ],[c ]and[u−b],where diagramstomakethefollowinganalysis. r 2 1 1 1 First,wecalculateindividualreddenings.Wefollowtheproce- , 2 [m1]=m1+0.32(b−y) (6) duredescribedbyCrawford,Glaspey&Perry(1970):weusethe 016 observed c to predict the first approximation to (b − y) with 1 0 theexpression:(b−y) =−0.116+0.097c .Thenwecalculate [c ]=c −0.20(b−y) (7) 0 1 1 1 E(b − y) = (b − y) − (b − y) and use E(c ) = 0.2E(b − y ) to 0 1 correctc forreddeningc =c −E(c ).Theintrinsiccolour(b− 1 0 1 1 [u−b]=[c1]+2[m1]. (8) fyo)0r(ibsn−owy)c.aTlchurleaeteitderbaytiorenpslaarceinegnocu1gwhitthorce0aicnhtchoenavbeorgveenecqeuiantitohne 0 We plot the [c ]−[m ] and β−[u − b] diagrams for all stars process.Naturally,thisprocedureonlyresultsinphysicallymean- 1 1 chosenintheV−(b−y)andV−c diagrams(seeFig.8).Wecan ingfulvaluesforB-typestars.Aswecanseeinthemaps(seeFigs1 1 dividethestarsaccordingtotheirspectraltype.Inthisfigure,we and2),theareaofStock8isstillembeddedintheparentalcloud, canseethatthe starsspectroscopically classified(redsquares)as and therefore, we expect to find an important degree of differen- B-type stars fall on the B-type star standard line, confirming the tialreddeningamongstmembers. E(b−y)spansvalues between validityofthisrelationship.TheonlystarclassifiedasA3Vfalls 0.3 and 0.8. In Fig. 9, we plot their surface distribution that can ontheA-typestarstandardline.Wecanobservethatthemajorityof be compared with the extent of the cloud. Most of the stars with thestarsareinthesameregioninbothdiagrams.Therefore,using relativelylowreddening(markedinred)fallalonganarrowstrip, thesestandardrelationships,weassignaspectraltypeforeachstar. almostvertical,thatseemstocoincidewithagapinthedarkcloud. MNRAS459,880–901(2016)

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Open clusters in Auriga OB2. Amparo A10. Messineo M. et al., 2015, ApJ, 805, 110. Morgan W. W., Whitford A. E., Code A. D., 1953, ApJ, 118, 318.
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