A Chandra X-ray census of the interacting binaries in old open clusters - Collinder 261

2 MB·

by  Smriti Vats

DraftversionFebruary1,2017
PreprinttypesetusingLATEXstyleAASTeX6v.1.0
ACHANDRAX-RAYCENSUSOFTHEINTERACTINGBINARIESINOLDOPENCLUSTERS–COLLINDER261
SmritiVats1andMaureenvandenBerg2,1
1AntonPannekoekInstituteforAstronomy,UniversityofAmsterdam,SciencePark904,1098XHAmsterdam,TheNetherlands;[email protected]
and
2Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138,USA;[email protected]
7
1
0 ABSTRACT
2
WepresentthefirstX-raystudyofCollinder261(Cr261),whichatanageof7Gyrisoneoftheoldestopen
n
clusters knownin the Galaxy. Our observationwith the ChandraX-ray Observatory is aimed at uncovering
a
J thecloseinteractingbinariesinCr261,andreachesalimitingX-rayluminosityofLX 4 1029ergs−1(0.3–
≈ ×
1 7keV)forstarsinthecluster. We detect107sourceswithintheclusterhalf-massradiusrh, andweestimate
3 thatamongthesourceswithL &1030ergs 1, 26areassociatedwiththecluster. Weidentifyamixofactive
X −
∼
binaries and candidate active binaries, candidate cataclysmic variables, and stars that have “straggled” from
]
E the main locus of Cr261 in the colour-magnitudediagram. Based on a deep optical source catalogue of the
H field,weestimatethatCr261hasanapproximatemassof6500 M ,roughlythesameastheoldopencluster
⊙
. NGC6791.TheX-rayemissivityofCr261issimilartothatofotheroldopenclusters,supportingthetrendthat
h
theyare more luminousin X-raysper unitmass thanold populationsof higher(globularclusters) and lower
p
- (thelocalneighbourhood)stellardensity.Thisimpliesthatthedynamicaldestructionofbinariesinthedensest
o environmentsisnotsolelyresponsiblefortheobserveddifferencesinX-rayemissivity.
r
st Keywords:openclustersandassociations: individual(Collinder261);X-rays: binaries;binaries: close;stars:
a activity;cataclysmicvariables
[

1
1. INTRODUCTION isaboutasoldastheSun(4 0.5Gyr;Dinescuetal.1995),
v ±
1 OpenclusterswithagesinexcessofafewGyrarerelatively revealed a large number of X-ray sources among the clus-
8 ter members(Bellonietal. 1993). Manyof these turned out
rareintheGalaxy(e.g.Kharchenkoetal.2013). Someaspect
8 to be close, tidally interacting binaries where the stellar ro-
oftheirproperties(perhapstheirlargeinitialmassortheirlo-
8
tation is locked to the orbital period, and therefore kept at
0 cationoutoftheGalacticplane,wheretheyavoidinteractions
. with large molecular clouds or the disruptive pull of exter- a level that can sustain magnetically active coronae. Sub-
1
sequent XMM-Newton (Gondoin 2005; Giardinoetal. 2008;
0 nal gravitational forces) helped them survive until old age.
7 Studies of old open clusters, with their well-developed sub- Gosnelletal. 2012), andChandra(vandenBergetal.2004,
1 2013;Giardinoetal.2008)observationsofoldopenclusters
giantand giantbranches, have been a cornerstoneof stellar-
v: evolution theory for many decades, thanks, in part, to their havedetected manysuch active binaries(ABs). ABs can be
i binaries of two detached stars, or they can have a contact
X accuratelymeasuredagesanddistances.
orsemi-detachedconfigurationsuchasinWUMaandAlgol
From the X-ray point of view, old open clusters are in-
r
a teresting for a number of reasons. First, X-ray observations binaries, respectively. In terms of number of sources, ABs
are the most prominentX-ray source class in old openclus-
efficiently detect different classes of close, interacting bina-
ters,butotherclassesofinteractingbinaryarerepresentedas
ries, enabling the study of processes such as tidal coupling
well. In cataclysmic variables (CVs), the X-rays are the re-
and the link between X-rays and rotation. The X-ray lumi-
sult of accretion from a late-type main-sequencedonoronto
nosity of late-type stars stronglydependson stellar rotation.
a white dwarf. In fact, the first ROSAT observationof M67
Assingle starsage, theyspin downdueto magneticbraking
wasaimedatstudyingtheX-raysfromaCVthatwasdiscov-
(Pallavicini1989).Asaresult,theirX-rayemissiondecreases
ered in the optical (Gillilandetal. 1991). The origin of the
accordingly. An old star like our Sun ( 4.5 Gyr) has an X-
ray luminosity of about 1026 to 1027 er∼g s 1 (0.1–2.4 keV; X-ray emission from more exotic open-cluster binaries, like
−
blue stragglers, is less well understood, but in X-rays they
Peresetal.2000). Evenwiththedeepestexposuresofasen-
aremoresimilartotheABsthantothemass-transfersources
sitive X-ray telescope like the Chandra X-ray Observatory,
(vandenBerg2013).
thisisnearlyimpossibletodetectexceptfortheneareststars.
A secondmotivefor studyingoldopenclustersin X-rays,
Nevertheless, an early ROSAT observation of the old open
isthattheir stellar densitieslie in betweenthose of thesolar
clusterM67,whichliesat 840pc(Pasquinietal.2008)and
∼
2
neighbourhood ( 0.1M pc 3) and dense globular clusters canonicalratio A /E(B V) = 3.1, and a neutral hydrogen
− V
( 104M pc 3).∼This a⊙llows an investigation of the effect column density N = 1−.9 1021 cm 2 (Predehl&Schmitt
− H −
≥ ⊙ ×
of stellar dynamics on the clusters’ close-binary population, 1995). The Galactic coordinates of Cr261 are l = 301.7 ,
◦
in a poorly studied density regime. With the growing sam- b = 5.5 ; due to its low Galactic latitude and location to-
◦
−
ple ofoldopenclustersstudiedin X-rays, itis nowpossible wards the bulge, the number of fore- and background stars
to do simple statistics regarding the number of sources de- projected onto the cluster is high. Cluster membership is
tected in each sourceclass. It was foundthat the numberof poorlyconstrainedforthemajorityofstarsinthefield.Cr261
CVsinM67andNGC6791scalewiththepresent-daycluster is includedin the star cluster catalogue of Kharchenkoetal.
mass, pointingat a primordialorigin. For ABs, that propor- (2013), which lists structural parameters such as the overall
tionality is not so obvious, raising the issue of whether dy- sizeoftheclusterandtheradiusofitscentralregion. Inthis
namical interactions that break up or create binaries, play a work,we presentanestimate forthehalf-massradiusr and
h
role (vandenBergetal. 2013). The expectedlow encounter the approximate mass of Cr261, which, to our knowledge,
ratesinopenclustersdonotseemtofavourthelatterexplana- havenotbeenreportedintheliteraturebefore.Theseparame-
tion. Nevertheless,therearecluesthatdynamicalencounters tersfacilitateauniformcomparisonwiththeX-rayproperties
shapethepropertiesofatleastsomebinaries.N-bodymodels ofotheroldGalacticclusters.
ofM67(Hurleyetal.2005)suggestthatprimordialbinaries WepresenttheX-rayandopticalobservations,andthedata
anddynamicalencountersarenecessaryto explaintheblue- reduction in Sect. 2. In Sect. 3 we describe the analysis,
stragglerpopulationofM67. Someindividualsystems,such which includes the creation of the X-ray and optical source
asthe likely tripleS1082in M67 (vandenBergetal. 2001; catalogues, their cross-correlationto identifycandidateopti-
Sandquistetal. 2003) is also difficult to explain without in- calcounterpartstotheChandrasources,andthederivationof
vokingencounters.Therefore,theoriginoftheX-raysources the structuralpropertiesof Cr261. Sect. 4 is focusedon the
ofoldopenclustersmaynotbesolelyprimordial. X-raysourceclassification. InSect.5we discussourresults
TheX-rayemissivity,ortheintegratedX-rayluminosityper in the context of the X-ray emission from other old stellar
unitofmass,ofglobularclustersislowerthanthatofM67af- populations,andwesummariseourfindingsinSect.6.
terremovingthecontributionfromluminouslow-massX-ray
2. OBSERVATIONSANDDATAREDUCTION
binaries(LMXBs; e.g.Verbunt2001). Geetal.(2015)com-
pared the X-ray emissivities of more diverse environments 2.1. X-rayObservations
including dwarf elliptical galaxies and the local neighbour-
Cr261 was observed with the Advanced CCD Imaging
hood,andfoundthatoldopenclustersalsohavehigherX-ray
Spectrometer(ACIS;Garmireetal.2003)onboardChandra
emissivities than other old stellar populations. Various ex-
starting 2009 November 9 14:50 UTC for a total exposure
planationshavebeensuggested,relatingtoeithertheoverall
timeof53.8ks(ObsID11308).Theobservationwasmadein
mass-losshistoryoftheclusters,differencesindynamicalen-
VeryFaint,Timedexposuremode,withasingleframeexpo-
counterrates,ortheprocessesunderlyingtheX-rayemission.
suretimeof3.2s. Kharchenkoetal.(2013)estimatethatthe
Morestudyisneededtodeterminewhichofthesefactorsare radius1 of Cr261 is 14.1. This is considerably larger than
′
responsible. ∼
a single ACIS chip (8.4 8.4); therefore, we placed the cen-
′ ′
InordertoimprovethecensusofX-raysourcesinoldopen tre ofthe cluster (α ×= 12h38m06s.0, δ = 68 2201 ;
2000 2000 ◦ ′ ′′
clusters, we are undertakinga survey with Chandraof open Kharchenkoetal. 20132) close to the I3 aimpo−int, so that a
clusterswithagesbetween3.5and10Gyr. Theobservations
largercontiguouspartoftheclustercouldbeimaged(seeFig-
are designed to reach a limiting luminosity of L 1030
X ure1).TheCCDsusedwereI0,I1,I2andI3fromtheACIS-
≈
erg s 1 (0.3–7 keV), or better, at the distance of the clus-
− Iarray,andS2andS3fromtheACIS-Sarray.
ters. As part of this survey, we have carried out the first X-
Westartedthedatareductionwiththelevel-1eventfilepro-
ray study of Collinder 261 (Cr261), and we present the re-
ducedby the data processingpipelineof the ChandraX-ray
sults of our efforts in this paper. With an estimated age of
Center and used CIAO 4.5 with CALDB 4.5.5.1 calibration
6–7 Gyr (Bragaglia&Tosi 2006), Cr261 is one of the old-
filesforfurtherprocessing.Tocreatethelevel-2eventfilewe
est open clusters in the Galaxy, being superseded in age by used the chandra_reproscript. A background light curve
NGC6791(8–9Gyr)andBerkeley17(8.5–10Gyr)only.The
in the energy range 0.3–7 keV was created with the CIAO
clustermetallicityisclosetosolar(Drazdauskasetal.2016), dmextract routine using source-free areas on the ACIS-I
andreportedvaluesforthedistanceandreddeningliebetween chips, and was analysed with the lc_sigma_clip routine.
2.2–2.7 kpc and E(B V) 0.25 0.34, respectively (see
Nobackgroundflareswithmorethan3σexcursionsfromthe
− ≈ −
e.g.Gozzolietal.1996,Carraroetal.1999,Bragaglia&Tosi
averagebackgroundcountratewereobserved,hencethetotal
2006),withahighervalueofthereddeningconsideredmore
plausible(Frieletal.2003). Inthispaper,weadoptadistance 1 Here we refer to the Kharchenkoetal. (2013) parameter r2, which is
defined as the distance from the cluster centre where the projected stellar
of2.5kpcandE(B V)=0.34,unlessstatedotherwise. The densitydropstotheaveragestellardensityofthefield.
−
latter correspondsto a V-band extinction A = 1.05 for the 2TheclustercentreisredeterminedinSect.3.3.
V
AChandrastudyoftheoldopenclusterCollinder261 3
Basicdatareductionstepsofbiassubtractionandflat-fielding
wereperformedusingthebias,anddome-andsky-flatimages
takenwithinonedayofthescienceexposures.WiththeMSC-
CMATCHroutine,thegeometricdistortionoftheimageswas
removed, and the eight individualchips of a given exposure
werecombinedinto a single image. We createda master V-
band image by stacking the two individual, slightly offset,
240-s V-band exposures. As a result, in the stacked V im-
agethespacebetweentheindividualchipsoftheWFImosaic
(23 widealongthelengthofthechips,and14 widealong
′′ ′′
theirwidth)islargely,butnotcompletely,filledin(seeFigure
1). ThestellarprofilesinoneoftheB-bandimagesofCr261
areverydistorted,whichpreventedusfrommodellingagood
PSF.Sincethisdegradesthequalityofthederivedphotome-
try, we opted to discard this low-qualityimage and use only
asingle240-sBexposureforouranalysis. Therefore,ourB-
bandcatalogueoftheCr261fieldhasnocoverageinthechip
gaps.
Figure1. Stacked WFI V-band image of Cr261. The four 3. ANALYSIS
black squares show the ACIS-I field of view with chip IDs 3.1. X-raySourceDetectionandSourceCharacterisation
marked. The solid white circle marks the core radius of
WelimitedtheX-rayanalysistothedatafromchipsI0,I1,
the cluster (r = 157 16 ), centred on the cluster centre
c ′′ ± ′′ I2andI3. TheS2andS3chipsliefarfromtheI3aimpoint,
(markedbytheredcross)asdeterminedbyus(seeSect.3.3).
givingrise to largepositionalerrorson anysourcesdetected
The dashed white circle marks the half-mass radius of the
on them. Such large errors make it hard to identify optical
cluster (r = 384 38 ). Blue rectangles show the off-
h ′′ ± ′′ counterparts,andthustoclassifythesources.
setfieldusedfordeterminingthebackgroundstellardensity.
Sourcedetectionwasdonein a soft(0.3–2keV),hard(2–
Small white rectangles are regionsof zero optical exposure.
7keV),andbroad(0.3–7keV)energyband.TheCIAOsource
Northisup,easttotheleft.
detectionroutinewavdetectwasrunforeightwaveletscales
exposurewasusedforfurtheranalysis. ranging from 1.0 to 11.3 pixels, each increasing by a fac-
tor of √2. Larger scales are better suited for more off-axis
2.2. OpticalObservations sources, where the PSF is wider or more distorted. Expo-
suremapswerecomputedforanenergyvalueof1.5keV.The
WeretrievedopticalimagesofCr261intheBandV bands
wavdetectdetectionthreshold(sigthresh)wassetat10 7.
fromtheESOpublicarchive.Thesedataweretakenaspartof −
The corresponding expected number of spurious detections
theESOImagingSurvey(EIS;programID164.O-0561).The
per wavelet scale is 0.42 for all four ACIS chips combined,
observationsof Cr261 were made using the Wide Field Im-
ager(WFI),mountedonthe2.2mMPG/ESOtelescopeatLa or 3.35 in total for all wavelet scales. We ran wavdetect
forthethreedifferentenergybandsandthencross-correlated
Silla,Chile. TheWFIhasafieldofviewof34 33 covered
′ ′
× theresultingsourceliststoobtainamasterX-raysourcelist.
byadetectorarrayofeight2k 4kCCDswithapixelscaleof
0.238pixel 1. TheCr261data×weretakenfrom2001June27 We detected113distinct X-raysources. To checkif we had
′′ −
missed any real sources, we ran wavdetectagain for a de-
23:55UTCto2001June2800:38UTC,withatotalexposure
tectionthresholdof 10 6, which increasesthe expectedtotal
time of 510 s in the B and V filter each. In each filter, two −
number of spurious detections to 33.5. We found a total of
exposures of 240 s were taken, supplemented with a single
151 distinct X-ray sources with more than two counts (0.3–
shortexposureof30stogetphotometryforthebrightstars.
7 keV) in this case. The positions of seven of the extra 38
Weonlyusedthelongexposuresforouranalysis. Theseeing
sourcesarefoundtomatchthoseofshort-periodbinariesdis-
duringtheobservationswas 1.15.
′′
∼ coveredby Mazuretal. (1995) (see Sect. 3.4). Close, inter-
ForreducingtheopticalimagesweusedtheImageReduc-
tion and Analysis Facility (IRAF3) v2.16, supplemented by acting binaries are plausible real X-ray sources and indeed
theexpectednumberofchancealignmentsbetweentheChan-
theMSCREDpackageforhandlingandreducingmosaicdata.
dradetectionsandthebinariesintheMazurcatalogueisvery
3 IRAF is distributed by the National Optical Astronomy Observatory, low(Sect.3.5). Itisthereforelikelythatatleasttheseseven
which is operated by the Association of Universities for Research in As-
additionalsourcesarereal;butgiventhe 34spuriousdetec-
tronomy(AURA)underacooperativeagreementwiththeNationalScience
∼
Foundation. tionsthatareexpected,wedonotbelievethattherearemany
4
E E
50 50
0.6 1 2 3 4 5 k eV 0.6 1 2 3 4 5 k eV
2.5 2.5
a. b.
V) 2.0 2.0
e
k
3
0.
E-75
V)/( 1.5 1.5
e
k
3
0.
E-25
3 ( 1.0 1.0
0.5 0.5
0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8

0.6 1 2 3 4 5 k eV 0.6 1 2 3 4 5 k eV
2.5 2.5
c. d.
V) 2.0 2.0
e
k
3
0.
E-75
V)/( 1.5 1.5
e
k
3
0.
E-25
3 ( 1.0 1.0
0.5 0.5
0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8
log(E /0.3 keV)/log(7 keV/0.3 keV) log(E /0.3 keV)/log(7 keV/0.3 keV)
50 50
Figure2. Quantile diagrams with model grids representing a MeKaL plasma (left) and a power-law spectrum (right). The top
panels show sources without, and the bottom panels show sources with candidate optical counterparts (see Table 2 for their
classification). The plasma temperaturekT or photonindexΓ, and the columndensity N can be estimated from the location
H
ofasourcewithrespecttothegrid: bluecurvesrepresentlinesofconstant N normalisedinunitsof1022 cm 2 (N , where
H − H,22
N 0.19cm 2istheclustervalue),whereasorangecurvesarelinesofconstantkT (labeledinunitsofkeV;left),andyellow
H,22 −
≈
curvesarelinesofconstantΓ(right). ThemedianenergyE canbereadofffromthetopx-axis. Hereweshowsourceswith
50
twentynetcounts(0.3–7keV)ormore;errorbarsareshownonlyforthesourceswiththehighestandlowestnumberofcounts
inagivenpanel. FilledcolouredsymbolsmarkX-raysourcesforwhichwehavefoundcandidateopticalcounterparts. Among
them,greencirclesrepresentABsandcandidateABs(Sect.4.1),olivetrianglesareforcandidateCVsorAGNs(Sect.4.2),yellow
four-pointstarsforcandidateBSSs,palereddownwardtrianglesforcandidateSSGs,maroondownwardtrianglesforcandidate
YSS (Sect. 4.3), pale blue squaresforlikely non-membersof the cluster (Sect. 4.4), and deep blue diamondsforsourceswith
uncertain classification (Sect. 4.5). Furthermore, sources CX18 and CX27 that have close-binary counterparts (Mazuretal.
1995)aremarkedwithalargerblackopencircle.
morerealsourcesamongtheextradetections.Weflaggedthe tine provides us with the source positions, but is not op-
sources that are only found for sigthresh=10 6, but kept timised to measure source counts. We determined the net
−
theminthemastersourcelist. sourcecountsusingACISExtract(Broosetal.2010,version
For computing the positional uncertainties, required for 2013mar6).Alleventsbetween0.3and7keVwereextracted
cross-correlationwithothersourcecatalogues,weusedEq.5 from regions enclosing 90% of the PSF at 1.5 keV. ACIS
∼
fromHongetal.(2005),whichgivesthe95%confidencera- Extractalso performsvariability characterisation based on a
diuson the wavdetectposition, P . The wavdetectrou- Kolmogorov-Smirnov(K-S)testontheeventarrivaltimesfor
err
AChandrastudyoftheoldopenclusterCollinder261 5
sources with five counts or more that spend more than 90% In order to characterise the spectral properties of our X-
ofthetotalexposuretimeontheACIS-Ichips4. Forexample, ray sourceswe use quantile analysis, which is optimisedfor
asourcenearachipedgecouldeffectivelyhaveashorterex- sources with few counts (Hongetal. 2004). In this method,
posuretimeifthetelescopedithermotionoccasionallymoves the median energy, E , and 25% and 75% quartile energies
50
it off the detector. There were 76 sources with no evidence (E and E , respectively)of the sourceevents’ energydis-
25 75
forvariability(0.05< P );foursourceswhichshowedpos- tributionareusedtodeterminespectralhardnessandspectral
KS
sible variability(0.005 < P < 0.05; CX9, CX13, CX64, shape. ConventionalX-ray hardness ratios use fixed energy
KS
CX93) and four which were likely variable (P < 0.005; values for defining hard and soft energy bands, and give re-
KS
CX63, CX91, CX120, CX137), where P is the proba- sultsoflittlemeaningifalleventslieineitherthesoftorthe
KS
bility for a source to have a constant count rate. The X-ray hard energy band. Details of the source properties are pre-
lightcurvesofthevariablessuggestflare-likebehaviour,with sentedinTable1,whilequantilediagramsareshowninFig-
alargefractionofthetotaleventsarrivinginarelativelyshort ure2forsourceswith 20netcounts(0.3–7keV).
≥
time interval. The brightest of these sources is CX63, for
3.2. OpticalSourceCatalogue
whichthirteenofseventeeneventsarriveinthelast3.5hofthe
observation (and nine of seventeen events in a single hour). The absolute astrometry of the optical images was tied
Fortheotherthreesources,80%ormoreoftheeventsarrive to the International Celestial Reference System (ICRS). We
within2.5–3h. X-rayflaresarecommonlyobservedinactive did this by computing an astrometric solution based on the
late-typestarsorbinaries. Thisisconsistentwith ourclassi- positions of unsaturated stars in the field that are also in-
ficationofCX120(aWUMabinaryandlikelynon-member cluded in the USNO CCD Astrograph Catalog 4 (UCAC4;
ofthecluster)andCX91andCX137(likelyforegroundlate- Zachariasetal. 2013). For the V image we used 1912 un-
typedwarfs).TheclassificationofCX63islesssecure,butit saturated stars and obtained rms residuals of 0.141 in right
′′
couldbealate-typestarorbinaryaswell. Thesesourcesare ascensionand0.166indeclinationinthesolution. Forthe B
′′
furtherdiscussedinSects.4.4and4.5. image,weused1773unsaturatedstarsandobtainedresiduals
Onlyfivesourcesinourcataloguehavemorethan100net of0.157inrightascensionand0.173indeclination.
′′ ′′
counts(0.3–7keV),withthebrightestsourcehaving475net Forperformingphotometry,weusedtheDAOPHOTpackage
counts. For the majority of our sources the spectrum of the in IRAF. Aftercreatinga sourcecatalogueforthe B andthe
X-ray emission is therefore poorly constrained. We calcu- V imageseparately,wecross-matchedeachofthemwiththe
latedunabsorbedfluxvaluesinthe0.3–7keVband,F ,for Gozzolietal.(1996)catalogueinordertoconvertourinstru-
X,u
each source from its net count rate and local rmf and arf mentalmagnitudestotheGozzolietal.calibratedmagnitudes
response files using Sherpa. We assumed a 2 keV MeKaL intheJohnsonsystem. TheGozzolistudycoversaregionof
model (xsmekal) attenuated by a neutral hydrogen column radius3.5aroundtheiradoptedclustercentre.Wefound2018
′
density N = 1.9 1021 cm 2 (the value for Cr261) using matchesforthesourcesintheBcataloguewithinacalibrated-
H −
×
thexstbabsmodel.TheMeKaLmodeldescribestheemission magnituderange13.7< B<24.0,and2276matchesfortheV
froma hot, diffuse gasor optically thin plasma, as is appro- cataloguewithinarangeof13.0<V <22.5.Wemanuallyin-
priate for ABs. Since the nature of our sources is unknown spectedallthematchedsourcesandfoundthatnoneappeared
a priori, and the number of counts is too low to do any de- to be blended or saturated. Over these magnitude ranges, a
tailedspectralfitting,weexploredtheeffectofusingdifferent constant offset provides a good transformation from instru-
spectralmodelsonthederivedvaluesof F . We compared mental to calibrated magnitudes. The resulting WFI source
X,u
theunabsorbedfluxvaluesobtainedusingthe2keVMeKaL listsfortheentirefieldhavea calibratedmagnituderangeof
model with those obtained using a 1 keV MeKaL model, a 12.9–23.5inVand13.7–24.6inB.Finally,wecross-matched
10keVthermalbremsstrahlungmodel(xsbrems)andapower- the BandV sourceliststomakeamasteropticalsourcelist.
law model (xspowerlaw) with a photon index, Γ, set to 1.4; SomesourcesinthemastercatalogueweredetectedintheV
thexstbabsmodelwasusedinallcases. Thefluxvaluesob- bandbutwerenotpresentinthesingleBimageduetoitschip
tainedusingthesemodelswereabout6%smaller,40%larger, gapsandashorterexposure.
and80%larger,respectively,thanthefluxvalueobtainedus- The colour-magnitudediagram (CMD) of Figure 3 shows
ingthe2keVMeKaLmodel. TheX-raysensitivitylimit, as ourBandV photometryofstarsinsider (seenextsection).
h
definedby theunabsorbedflux ofthe faintestdetection, was
foundto be 6 10 16 ergcm 2 s 1 forthe 2 keV MeKaL 3.3. EstimatefortheHalf-massRadiusandMassofCr261
− − −
∼ ×
modeland assumed cluster N , which correspondsto an X- OneofouraimsistocomparethenumberofX-raysources
H
ray luminosity of L 4 1029 erg s 1 (0.3–7 keV) at the in Cr261 with those detected in other old open clusters
X −
≈ ×
adoptedclusterdistance(2.5kpc). and globular clusters. Making a uniform comparison be-
tweenclustersrequiresanestimatefortheirmassesandstruc-
4basedontheevaluationoftheFRACEXPOkeywordgeneratedbymkarf tural parameters. An estimate for the King-profile (King
inCIAO 1962)coreradiusr ofCr261wasderivedbyFroebrichetal.
c
6
7
100
30 129
40
95
73
123
18
74
132
130
9
86
136
67 12
31
58
120 15
151
24 10
38 91
8 54 77
53
4 44 137
20
38
70
Figure3. Colour-magnitudediagramofCr261basedontheWFIphotometry. Thedifferentsymbolsandcolourshavethesame
meaningasinFigure2. Furthermore,sourceswithclose-binarycounterpartsarecircledwithalargerblackopencircle;forthese
stars,theBVphotometryisobtainedfrom(Mazuretal.1995,seeSect.4.1).Solidlinesrepresentisochrones(Bressanetal.2012;
Chenetal.2014;Tangetal.2014;Chenetal.2015)fortheupperandlowerlimitsoftheclusterreddening(redforE(B V) =
low
−
0.25,andblackfor E(B V) = 0.34). Dottedlinesarethesameisochronesbutshiftedupwardby 0.75magtoindicatethe
up
− −
limitforunresolvedphotometricbinaries.Thedashedlinerepresentsazero-agemain-sequenceisochrone.Errorbarsaresmaller
than the symbols for some sources. The combineduncertainty in the location of the isochrone due to reddeningand distance
uncertaintiesisshownintherectangleatthetopright.Forclarity,theABsanduncertainclassificationshavenotbeenlabeled.
(2010)andKharchenkoetal.(2013), whofoundtwosignifi- motionsin the field of Cr261 (used by Kharchenkoet al. to
cantlydifferentvalues,viz.52 and192 48 , respectively. weedoutpossible non-members)haverelativelylargeerrors
′′ ′′ ′′
±
Thesevaluesarebasedonthe2MASSnear-infraredcatalogue ( 9masyr 1,onaverage)anddonotdisplayacleardistinc-
−
∼
(Skrutskieetal.2006),and(incaseofKharchenkoetal.only) tionbetweenclusterstarsandfieldstars.Wedecidedtoderive
the optical PPMXL catalogue (Ro¨seretal. 2010). Both star ourownestimateofr andr ,withoutmakinguseofthePP-
c h
listsarerelativelyshallow,reaching.1magbelowtheCr261 MXLpropermotions.
main-sequenceturnoff.Atthesametime,thePPMXLproper In order to estimate r for Cr261, we assumed that the
h
AChandrastudyoftheoldopenclusterCollinder261 7
Bellazzinietal. (2008). We calculated the integrated mag-
nitude of stars inside r (i.e. I (V)) by summing the V-band
h h
fluxes of the stars inside r that satisfy the same magnitude
h
andcolourrestrictionsasoutlinedabove. Again,photometry
of the offset fields provides a correction for the flux density
of foregroundand backgroundstars. We converted I (V) to
h
theabsoluteintegratedV magnitudeofstarsinsider ,which
h
resulted in M (V) 3.6. Next, we compared this value
h
≈ −
with the theoretical curves for the evolution in time of the
absoluteV magnitudeofsolar-metallicitystarclustersofvar-
ious,constant,masses(Bellazzinietal.2008;Bragagliaetal.
2012). The age of Cr261 (7 Gyr) combined with our esti-
mateforM (V),yieldsanapproximatevalueforhalftheclus-
Figure4. Projected stellar density profile of Cr261 after cor- h
ter mass of 4000–5500 M . The uncertaintystems fromthe
rectionforthecontributionfromforegroundandbackground
⊙
rangespannedbythe theoreticalcurvescomputedfordiffer-
stars (points connected with a dashed line). The solid blue
ent initial-mass functions. As a final step, we have reduced
line shows the best-fitting King profile (King 1962), which
theinferredtotalmass(about8000–11000 M )withanem-
has a central stellar density f = 0.0057 0.0007 stars per
arcsecond2andacoreradiusr0 =157 1±6 . piricalscalingfactor. Thiswasmotivatedbyo⊙urfindingthat
c ′′ ′′
± the above method overestimates the masses of the old open
clustersM67(byafactor1.1–1.7)andNGC188(byafactor
stars are symmetrically distributed about the cluster centre of1.3–1.9),forwhichaccuratevirialmasseshavebeendeter-
according to a King profile. In Figure 4 we plot the pro- mined(Gelleretal. 2008, 2015). Afterscaling, ourestimate
jectedstellardensity f(r)versusradialoffsetfromthecluster forthetotalmassofCr261isabout5800–7200M .
⊙
centre r, computed in 50 -wide annular regions around the Obviously, our mass estimate should be consideredas ap-
′′
centre. Stars were selected from the region between two 7- proximate, only: we assumed that the total cluster mass is
Gyrisochronesofsolarmetallicity(Z = 0.019;Bressanetal. containedwithinKharchenkoetal.’sclusterradiusr ,wehave
2
2012). One is modified for a distance of 2.5 kpc and red- no comprehensivelist of members, and the evolutionary se-
denedbyE(B V)=0.34;thesecondisochroneisthesame, quencesfor M(V)fromBellazzinietal.(2008)maynotbea
−
but shifted upwardin the CMD by –0.75mag. This is done perfectmatchtoCr261(inmetallicityormassfunction).For
to include the contribution from unresolved photometric bi- ourpurposes,though,thisestimateisgoodenough.
naries (see Figure 3). To correct for the contribution from
starsthatareunrelatedtothecluster,weestimatedthedensity 3.4. OpticalandX-rayCross-matching
of stars within the same magnitude and colour limits, from
Thepossible errorin the alignmentof Chandra’sabsolute
a catalogue we created from offset fields in our WFI image
astrometry to the ICRS is small5, but still allows for a sys-
(the blue rectangles in Figure 1) that lie outside the cluster
tematic offset between the X-ray positions and our ICRS-
radius r (Kharchenkoetal. 2013). We fitted the function
2 calibrated optical positions. This systematic offset, or bore-
f(r) = f (1+(r/r )2) 1 to this background-correctedradial
0 c − sight, canbe comparablein size to the randomerrorsonthe
profile; here f is the central projected stellar density. This
0 X-raypositions(P ),andcancomplicatethesearchforop-
function is the limit of the King profile for the assumption err
ticalcounterpartsif notcorrectedfor. To calculatethe bore-
that the tidal radius r is much larger than r . In the case
t c sight, we used the 45 short-period (P < 3 d) close binaries
of Cr261, the values of r 0.8 and r 22.2 as derived
c ≈ ′ t ≈ ′ that were discovered by Mazuretal. (1995) in an optical-
by Kharchenkoetal. (2013), support this assumption. Fit-
variability study of the Cr261 field. The reason for using
tingthe abovefunctionto the radial-densityprofile, givesus
theshort-periodvariablesforcalculatingtheboresightisthat
a centre for the cluster that is about 1.5 different from the
′ close binaries are plausible X-ray emitters and hence, there
one given by Kharchenko et al., viz. α = 12h38m07s.1,
2000 isalowerchanceofspuriousdetectionsthatcouldaffectthe
δ = 68 2333 ,withaformaluncertaintyofabout16 .
2000 − ◦ ′ ′′ ′′ boresightmeasurement(seealsoSect.3.5). Withthefinding
We haveused this new cluster centrefor allpurposesin this
charts in Mazur et al., we were able to identify all 45 vari-
study.Thebest-fittingKingprofilehasr =157 16 ,con-
c ′′± ′′ ablesamongtheWFIsources.TheirWFIpositionswerethen
sistent with the value from Kharchenkoet al. Adoptingthis
cross-matchedwiththeX-raycatalogue,whereweadopteda
valueof r andassumingthatthe total massof the cluster is
c 95% match radius that combinesthe error in the optical po-
containedwithinr ,weusedEq.A3inFreireetal.(2005)to
2
computer . Wefindthatr =384 38 .
h h ′′± ′′ 5 For ACIS-I, the 95% confidence radius on the alignment
We estimated the mass of Cr261 from the integrated V
is 0.9–1 within a distance of 3 from the aimpoint; see
magnitude I(V) of the cluster, following the method of http:∼//c′x′c.ha′r′vard.edu/cal/ASPECT/celmon ′
8
sitions6 and the random error on the X-ray positions (P ) which of the energy ranges considered in the Kim study is
err
in quadrature. To account for errors in the alignment, we closest to our broad band (0.3–7keV). To convertcounts to
also add the 95% confidence radius on Chandra’s absolute fluxes we adopted a power-law spectrum with Γ = 1.4 and
astrometry. Fifteen candidate counterparts were thus found, N =2.3 1021cm 2,i.e.equaltothetotalintegratedGalactic
H −
×
whichwerethenusedtocalculatetheboresightfromtheav- columndensityalongthelineofsight(Marshalletal.2006).
erageX-ray–opticalpositionaloffsets. AfterupdatingtheX- We calculated N forr < r , where mostX-raysourcesthat
B c
raypositionsforthisinitialboresight,thecross-matchingwas aretrulyassociatedwithCr261areexpectedtolie. Therea-
repeated until the net boresightconverged. This method for son is that closer to the centre, the density of cluster stars is
calculatingandcorrectingfortheboresightisoutlinedinde- simply higher; in addition, mass segregation makes the ra-
tail in Sect. 3.3.1 of vandenBergetal. (2013). We found a dialdistributionofbinaries(andthuspotentialX-raysources)
small boresight that is consistent with zero, viz. 0.06 0.07 moreconcentrated. For a 5-countdetectionlimit, we expect
′′ ′′
±
inrightascensionand0.09 0.08indeclination. N 9.5 3.1versus22sourcesactuallydetected. Fora10-
′′ ′′ B
± ≈ ±
AftercorrectingtheX-raysourcepositionsforthe(almost countlimit, itisexpectedthat 5.7 2.4ofthetensources
∼ ±
negligible)boresight,wematchedourX-raysourcelistwith detectedareextra-galactic.Intheregionr <r r ,45.7 6.8
c h
≤ ±
the entire optical source list, again using 95% match radii. ofthe58sourcesdetectedabove5counts,or27.3 5.2ofthe
±
For89uniqueX-raysources,wefound124opticalmatches; 38sourcesdetectedabove10countsareexpectedtobeextra-
ofthelatter,104arepresentinboththeV andBimageswhile galactic. These numbers indicate that we do detect a popu-
for20weonlyhaveaV orBdetection.Wealsoinspectedthe lationof, mainly faint, X-raysourcesthatis truly associated
areaaroundeachX-raysourceintheWFIimagesbyeye,and withthecluster.
discoveredthatfivemoreX-raysourceshavecandidateopti- GiventhelowGalacticlatitudeofCr261,afewforeground
calcounterpartsthataresaturatedandthereforemissingfrom X-ray sources are also expected to contaminate our sample.
our optical catalogue. Finally, we added to the list of can- TheexactnumberishardtoestimatesincethereisnoGalactic
didatecounterpartssixopticalsourcesthatliejustoutsidethe X-raysourcedensitydistributionforthislatitudethatreaches
95%matchradius,butinsidethe3-σradius.Intotal,98ofthe downtoourdetectionlimit. WehaveusedthelogN logS
−
151uniqueX-raysourceswerethusmatchedtooneormore curvesfromFigure9inEbisawaetal.(2005)forthesoftband
opticalsources. Foracompletelistofcandidatecounterparts (0.5–2keV),andreadoffthelogN forafluxlimitthatcorre-
andtheiropticalpropertieswerefertoTable2. spondstoa5-countdetectionemittinga2keVMeKaLspec-
trum and N = 1.9 1021 cm 2. We expect 2.0 Galac-
H −
3.5. FalsePositivesTest,BackgroundGalaxies,andGalactic × ∼
tic sources in the region inside r and 8.2 sources in the
c
Sources ∼
r < r < r annulus; this must be an upper limit since the
c h
To estimate the number of spurious matches between our EbisawafieldliesrightintheplanewhileCr261isafewde-
X-ray and optical sources, we calculated the surface density grees off. Other factors, such as the difference between our
of optical sources. Within r , the average density is 0.029 andEbisawa’ssoftband,anduncertaintiesintheX-rayspec-
c
sourcesarcsec 2,whilebetweenr andr itdropsslightlyto tralmodelandN ,alsoaffecttheaccuracyofthisnumber.
− c h H
0.024 sources arcsec 2. Multiplying the optical source den-
−
sities with the totalarea coveredby the 95%errorcircles of 4. RESULTS
theX-raysourcesinthetworegions,weexpect2.4spurious
We usedthreecriteriatoclassifyourX-raysources. First,
matchesamongthe23matchesthatwefindinthiscentralre-
weconsideredthehardnessoftheX-rayspectrumasinferred
gion,and11.6spuriousmatchesamongthe47matchesinthe
from the energy quantiles. Coronally active stars and bina-
outerregion.Similarly,weusethenumberofMazurvariables
rieshavethermalX-rayspectrawithplasmatemperaturesthat
per arcsec2 to estimate that the number of spurious matches
generallydonotexceed3 4keV(e.g.Gu¨del2004). Thein-
between X-ray sources and variables is 0.021 out of seven −
tegrated Galactic column density in the direction of Cr261
matchesin the inner region,and0.022outof sevenmatches is 2.3 1021 cm 2; Galactic X-raysourceswithoutanyin-
−
fortheouterregion(oneX-ray–detectedMazurvariableslies ∼ ×
trinsicabsorptionshouldthereforehaveanN notlargerthan
H
outside r ). Therefore, all Mazur binaries that match with a
h this. AsaresultofthesetemperatureandN constraints,the
H
Chandrasourcearelikelyrealcounterparts.
expectedE valuesforcoronalsourcesarenotmuchhigher
50
In order to estimate the number of background galaxies than 1.5 keV7. On the other hand, accreting binaries with
N amongourX-raydetections,we usedtherelationforthe ∼
B compactobjects,andAGNsoftenhaveintrinsicallyharderX-
cumulative number density of high–galactic-latitude X-ray
ray spectra, and sometimes are observedthroughadditional,
sourcesaboveagivenfluxlimitS (Eq.5inKimetal.2007).
localisedobscuringmaterial;inbothcases, theexpected E
50
WeadoptedthelogN logS relationforthe0.3–8keVband,
B ishigherthan 1.5keV.
−
∼
6 Theerrors onthe optical positions that areadopted here arethe 1-σ
errorsintheastrometriccalibrationgiveninSect.3.2,scaledtoa95%confi- 7 IntheMeKalgridofFigures2aand2c,thelocationkT 4keVand
denceradiusassuminga2-Dgaussianerrordistribution NH ≈2×1021cm−2correspondstoE50≈1.5keV(seetopaxe≈s).
AChandrastudyoftheoldopenclusterCollinder261 9
Secondly, we looked at the ratio of the unabsorbedX-ray
toopticalflux,orthelimitsthereonforsourceswithoutcan-
didateopticalcounterparts. We calculatedthisratiowith the
equationlog(F /F ) = logF +V /2.5+5.44,wherethe
X V u X,u 0
lasttermisthelogarithmoftheV-bandfluxforsourceswith
V =0. Weadopteda2keVMeKaLmodeltocalculateX-ray
fluxes,assumedN = 1.9 1021 cm 2 tocorrectforabsorp-
H −
×
tion, and used V = V A = V 1.05. We caution that
0 V
− −
formostsources,N isunknown;iftheadoptedN islower
H H
(higher) than the actual N , the flux ratio is overestimated
H
(underestimated). Like E , thefluxratioismostlyusefulto
50
distinguish between coronal and accretion-poweredsources.
Theformertypicallyhavelog(F /F ) . 1,withthemost
X V u
−
active late-type dwarfsreaching valuesof about−0.5, while Figure5. The median energies E50 and X-ray–to–opticalflux
the latter have log(FX/FV)u & −1 (Stockeetal. 1991). In- ratioslog(FX/FV)u showa trendoflowerflux ratiosforsoft
deed, foroursources, theaveragefluxratio islower forsoft sourcesthanfor hardersources. For sourceswithoutoptical
(E50 .1.5keV)thanforhard(E50 &1.5keV)sources(Figure counterparts,thelowerlimitonlog(FX/FV)u(shownasopen
5).AnopticalsourceinsidetheX-rayerrorcircleisnotneces- circles)was calculatedforthe detectionlimitV = 23.5. For
sarilythetruecounterpart,butcanbeaspuriousmatch.Find- sourceswithmultiplecounterparts,therangeonlog(F /F )
X V u
ing a relatively hard X-ray source with a low log(FX/FV)u is indicated with a vertical line. Error bars on the flux ratio
value,cansignalsucharandomalignment. onlyincludestatisticalerrors,notanysystematicerrorsresult-
For sources with candidate optical counterparts we also ing from uncertainties in the adopted X-ray spectral model.
tookintoaccountthepositionofthesematchesintheCMD.In CX150 is an outlier and may be spuriously matched to the
mostcases,thisworksreasonablywelltoseparateABsfrom starinitserrorcircle. Onlysourceswithσ <1areplotted.
E50
AGNs (which can lie far off the cluster sequence) and CVs
(which typically are blue). The position in the CMD doesa
butwitherrorsonE thataretoolarge(&1keV)tomeaning-
poorjobinseparatingclusterstarsfromfore-orbackground 50
fullyconstraintheirX-rayspectra;thesesourceswereclassi-
stars. As can be seen in Figure 3, and also in the CMDs in
fiedas“AB?”. CX25isanuncertainABbecauseitsposition
Gozzolietal.(1996),theclusterstarsdonotclearlystandout.
inthequantilediagramsuggestsanN thatisenhancedwith
The lack of membershipinformationfor stars in the field of H
respecttotheGalacticcolumn,whichisnotexpectedfortypi-
Cr261limitstheclassificationofourChandrasources,aswe
calABs. Finally,CX41andCX57haveE =2.2 0.5 keV
discussbelow.Inthefollowing,X-rayfluxesandluminosities 50 ±
and 2.5 0.5 keV, respectively; this is on the high side for
refertothe0.3–7keVband. ±
ABs,butgiventhelargeerrors,wealsoputthesetwosources
4.1. ActiveBinariesandCandidateActiveBinaries in the “AB?” category. With B V = 1.34 at V = 20.15,
−
CX41 is relatively blue, but not as offset from the main se-
ForidentifyingpossibleABsinCr261,weselectedsources
quenceasthesourcesdiscussedinSect.4.2;however,itisnot
with candidate optical counterpartsthat lie along the cluster
inconceivablethatthissourceisanAGNoraCV.Weexpect
main sequence or sub-giant branch; if a source has multi-
thatasignificantnumberof“AB”and“AB?”sourcesarefore-
plematchesthatallsatisfythiscondition,itisalsoclassified
andbackgroundactivestarsorbinaries.
as a (candidate) AB. We allowed for the possible contribu-
Ten ABs are matched to Mazur variables. CX27/V42,
tiontothelightbyabinarycompanion,andforuncertainties
CX89/V11, CX97/V38, and CX138/V21 are (semi-
in the reddening, as indicated by the pairs of black and red
)detached eclipsing binaries. The first three have periods of
isochronesinFigure3. Theuncertaintyintheclusterdistance
1.3d orshorter,while thelightcurveofV21showseclipse-
( 350pc,basedontherangeofdistancesreportedintheliter-
∼ like eventswith an unconstrainedperiod. Themaximumor-
ature),andinourabsolutephotometriccalibration(Sect.3.2)
bitalperiodthatcanbetidallycircularisedin 7Gyr(i.e.the
arealsosourcesofsystematicerror(seetheerrorbarinthetop ∼
age Cr261) is 15 d (Mathieuetal. 2004). Since the time
rightofFigure3).Therefore,weclassifiedcandidatematches ∼
scale for tidal synchronisation is shorter than that for cir-
that are only a little bit off the main sequence or sub-giant
cularisation (Hut 1981; Zahn 1989), it is perfectly plausi-
branchasABs,too.
blethatatleastV42,V11,andV38containrapidlyrotating,
Atotalof33Chandrasourcessatisfythephotometriccrite-
and therefore X-ray–active, stars. CX93/V30, CX126/V13,
riaoutlinedaboveandhaveE valueswithin1σof1.5 keV
50 CX133/V108, CX144/V24, CX147/V25, and CX149/V33
or lower; all have log(F /F ) . 1.4. We classified them
X V u
−
as “AB” in Table2. Fouradditionalsources(CX62, CX80, 8CX133ismatchedwithtwostarsonthemainsequence;V10isthemore
CX115, CX125) have similar optical and X-ray properties, likelycounterpartofthetwo.
10
arecontactbinariesoftheWUMatype. ForWUMa’s,adis- andX-rayproperties. However,since veryfewCVs inopen
tanceconstraintcanbederivedfromtheknowncalibrationof clusters have been found, it is worthwhile to highlight any
theabsolutemagnitudesintermsoforbitalperiodandB V candidates. Follow-up optical spectroscopy can confirm or
−
orV I colour(seee.g.Rucinski&Duerbeck1997). Mazur disprovewhetherasourceisaCVnornot.
−
et al. thus found that the distances to V30, V13, V25, V33,
4.3. CandidateBlueStragglers,YellowStragglers,and
andlikelyV24,arecompatiblewiththatofCr261;theseare
Sub-subgiants
themostreliableclusterABsinoursample.Mazuretal.were
inconclusiveregardingthedistancetoV10. Some of the brightest X-ray sources in old open clusters
ThetimespanoftheWFIobservationsis 0.75h,withthe are membersthat lie off the main locus of the cluster in the
∼
B data taken first. For CX149/V33, with a period of 6.96 h CMD. These systems challengeour understandingof binary
andalarge-amplitude( 0.8mag)lightcurve,thisspans 0.1 evolution,and,insomecases,wedonotunderstandwhythey
∼ ∼
in orbital phase. If our observations happened to be timed emit X-rays (vandenBergetal. 1999). Therefore, they de-
around eclipse ingress (something we cannotcheck because servespecialattention.
theephemerisisnotknownwithsufficientprecision),thiscan Blue stragglerstars (BSSs) are bluerandbrighterthanthe
explain why we find a much bluer colour (B V = 0.72) main-sequenceturnoff(MSTO)ofacoevalpopulation.Their
−
thanMazuretal., whoreport B V = 1.09,i.e.rightonthe formationscenariosmustexplainhowthesestarsmanagedto
−
mainsequence.Wefoundsimilarcolourdifferencesforafew continuecore hydrogenburningfor a longer time than clus-
other variables. Mazur et al. adopted a method that makes ter stars of similar mass. Mass transfer in a binary, direct
their colours much less sensitive to non-simultaneous mea- collisions, and the merger of the close inner binary in a hi-
surements. Therefore, in Figure 3, we plotted the variables erarchical triple driven by the Kozai-Lidov mechanism, are
withtheirMazurphotometry(seeTable3),ifavailable. the three proposed formation channels (Davies 2015). For
mostBSSs,itisnotclearwhich(ifany)ofthesechannelsap-
4.2. CandidateCataclysmicVariablesorAGN
plies. The detection of X-rays in a bona-fide cluster BSS is
Our mass estimate for Cr261 (5800–7200 M ) is similar asignofongoingbinaryinteractionandthusprovidesaclue
⊙
to the mass of NGC6791 (5000–7000 M ). If CVs in open tothecurrentsystemconfiguration. Thereisnostrictbright-
⊙
clusters are primordial, Cr261 would host a similar number nesslimitwithrespecttotheMSTOthatwecanusetoselect
of CVs as NGC6791, i.e. 3 to 4 (vandenBerg 2013). CVs candidateBSSsinCr261.InM67,whichhasoneofthebest-
typicallylietotheblueofthemainsequenceduetothelight studiedBSSspopulations,thebrightestBSS(F81; Leonard
from the accretion disk, and possibly from the white dwarf 1996)lies 2.7magabovetheMSTOintheV band.Wetake
∼
(although in the B band, the blue excess is not always that the equivalent location in the CMD of Cr261, i.e. V 14,
≈
obvious, see e.g. Bassaetal. 2008). For eleven sources, the as a (somewhat arbitrary) limit, and consider brighter stars
candidate optical counterpart(s) are blue with respect to the to be non-members. We thusfind eightmatcheswith candi-
main sequence, and ten of them are possibly CVs: CX4, date BSSs: CX18/V45, CX67, CX73, CX74, CX86/V12,
CX8, CX20, CX22, CX24, CX38, CX44, CX54, CX70, CX130, CX132, andCX136/V22. Exceptfor CX67, these
andCX1519. Theotherbluesource,CX120,isnotamem- sources are soft (E . 1.4 keV) and all have log(F /F )
50 X V u
berofthecluster(seeSect.4.4). Weconsiderasourcetobe between –3.9 and –2.8. This is consistent with the proper-
blueif it lies to theleftof the isochronethatis reddenedfor ties of ABs, and their X-rays are therefore likely the result
thelowestpossibleclusterreddening. Inaddition,werequire of magnetic activity. Indeed, three sources are matched to
thebluewardoffsetfromthisisochronetobeatleast0.13mag, (semi-)detachedeclipsing binaries with periods between 1.1
i.e.theerrorsontheabsolutephotometriccalibrationinVand and 2.1 d. V12 and V22 show Algol-typelightcurves. The
Baddedinquadrature(seeSect.3.2). ideaofapossiblelinkbetweenAlgolsandBSSswasalready
ThetensourceslistedabovehaveE valuesbetween 1.5 putforthbyMcCrea(1964). InanAlgolbinary,theoriginally
50
∼
and2.8keV,log(F /F ) between–2.5and+1.0,andL be- lessmassivestarisnowobservedtobethemoremassiveone
X V u X
tween5 1029and3 1031ergs 1. Thisisconsistentwitha asaresultofthemassitreceivedfromitsRoche-lobefilling
−
× ×
CV classification, althoughlog(F /F ) 2.5forCX151 companion—herewemaybeseeingaBSS“inthemaking”.
X V u
≈ −
isonthelowsideforaCV;thismaysuggestadifferentsource It would therefore be particularly interesting to determine if
class or the presenceofa randominterloperin the X-ray er- V12andV22areclustermembers.
ror circle. We label these sources as candidateCVs (“CV?” ThecandidatecounterpartsofCX9andCX123liebetween
in Table 2). Confusionwith other classes in this partof the theBSSsandredgiants.StarsinthisregionoftheCMDhave
CMDismainlywithAGNs,whichoutnumbertheCVsinthe beendubbedyellowstragglersandmaybeBSSdescendants.
field observed (Sect. 3.5) and can have similar blue colours All yellow stragglersin M67 are solid cluster membersand
X-ray sources (Bellonietal. 1998). Their X-ray properties
9TheopticalmatchtoCX22,andoneofthematchestoCX24,areonly
detectedinB,butthedetectionlimitinVimpliestheymustbeblue(B V. point at the presence of magnetic activity, and the same ap-
0.7). − pearstobethecaseforCX9andCX123.