Open clusters in Auriga OB2

22 Pages·2016·4.9 MB·English

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
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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
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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)
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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
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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)
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FJ2i0g0u0rear3e.liFsitneddiinngTcahbalretAfo4r.sTtahresswiziethopfhthoetofimeeldtryisi1n8A.0l1icaarnctmei1n1×.T1h0e.7i5maargcem,ipnr.oNviodrethdibsyuApLaAnDdINe,aisstaisDleSfSt.2bluedigitization.Numbersandcoordinatesin a.org/
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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
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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
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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)
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Figure7. Left:theV/(b−y)diagramforallstarsinStock8.Theredcirclesrepresentstarsspectroscopicallyobservedandgreensquares,non-memberstars. ad
Right:theV/c1diagramforallstarsinStock8.Theredcirclesrepresentstarsspectroscopicallyobservedandthegreensquaresnon-memberstars. ed fro
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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
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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)