Table Of ContentMem.S.A.It.Vol.75,282
(cid:13)c SAIt 2008 Memoriedella
Globular clusters seen by Gaia
E. Pancino1,2,M. Bellazzini1,andS. Marinoni2,3
1 IstitutoNazionalediAstrofisica–OsservatorioAstronomicodiBologna,ViaRanzani1,
I-40127Bologna,Italy
3
2 ASIScienceDataCenter,I-00044Frascati,Italy
1
3 IstitutoNazionalediAstrofisica–OsservatorioAstronomicodiRoma,ViadiFrascati33,
0
2 Monteporzio(RM),Italy
e-mail:[email protected]
n
a
J
Abstract.Wepresentasimulationoftwelveglobularclusterswithdifferentconcentration,
8
distance,andbackgroundpopulation,whosepropertiesaretransformedintoGaiaobserv-
1
ableswiththehelpofthelatesGaiascienceperformancesprescriptions.Weadoptsimplified
crowdingreceipts,basedonfiveyearsofsimulationsperformedbyDPAC(DataProcessing
]
A and Analysis Consortium) scientists, to explore the effect of crowding and to give a ba-
sic idea of what will be made possible by Gaia in the filed of Galactic globular clusters
G
observations.
.
h
Keywords.Spacevehicles:instruments—Galaxy:globularclusters—Methods:miscel-
p
laneous
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o
r
t
s 1. Introduction external parts. With all Gaia scientistists busy
a
inpre-launchactivities,weattemptheretouse
[ GaiaisacornerstoneESAastrometricmission
the results of 5–10 years of stellar blending
whichisgoingtobelaunchedinOctober2013.
1 simulations with GIBIS (Babusiaux 2005) to
v Itwillprovide6Dpositionandvelocityinfor- derive simplified receipts, and to give a basic
6 mationonGalacticstars,solarsystemobjects, ideaonwhatwillbemadepossiblebyGaiain
6 unresolvedgalaxies,withexquisitequalityand
thefieldofGalacticglobularclustersobserva-
3 additional astrophysical information such as
tions.
4 E(B–V),astrophysicalparameters,objectclas-
1. sification, chemical tagging, for objects down
0 to V(cid:39)20 mag. A deep discussion about the
2. Simulations
3 Gaia expected scientific harvest can be found
1 inmanypapers(seee.g.,Mignard2005). TwoclustersweresimulatedwiththeMcluster
v: Gaia will certainly be able to provide in- code (Ku¨pper et al. 2011) — including the
i terestingdataforglobularclustersaswell,but prescriptions by Hurley et al. (2000) — dif-
X
crowdingisthegreatunknown,itsknowledge fering only in their concentration parameter,
r deciding whether we will be able to pierce one having c=1.0 and r =5 pc and the other
a h
throughthecentralregionsorbelimitedtothe c=2.5 and r =4 pc. Both clusters were spher-
h
ical, non-rotating, and did not include bina-
Sendoffprintrequeststo:E.Pancino ries.Theunderlyingstellarpopulationwasold
Pancino:GGCseenbyGaia 283
Fig.1.AGIBIS(Babusiaux2005)simulationcomputedbyG.Giuffrida(2012,privatecommunication)of
aglobularclusterobservedthroughtheGaiaspectro-photometers.Eachstarhasacygnus-likeshape,and
greenrectanglesarethewindowsassignedtobrightstars(G≤20mag)bytheon-boarddetectionalgorithm.
The750000windowslimitimposesthatnonewwindowisassignedtoanewstar(enteringfromtheleft)
untiloneofthepresentlyassignedwindowsisfreedbyastartransitingoutofthefield(fromtherightside).
Morecrowdedregionshaveahigherprobabilityofhavingawindowassigned,thus,theclustercenterhas
morewindowsassignedthanthetwoexternalregionsaboveandbelowit.Thisisratherunusualcompared
totypicalground-basedobservationsofcrowdedregions,andhasimplicationsforthecompletenesslevels
ofthephotometry(courtesyofG.Giuffrida).
(12Gyr),metal-poor(Z=0.003),andtheother CCDsreadinTDI(TimeDelayedIntegration)
clusterpropertieswererathertypicaloftheav- mode, as described in many Gaia technical
erage Milky Way globular clusters (for exam- documents (but see also Cacciari 2011; Jordi
ple:M =–7.6mag,M =3.6105 M ,Kroupa 2011; Pancino 2012). Crowding thus has an
V tot (cid:12)
IMF). Each cluster was projected at three dis- impact which deserves in depth studies to
tances (5, 10, 15 kpc) and onto two back- be fully understood. Many groups used the
grounds, simulated with the Besanc¸on model various Gaia simulators to better understand
inahalodirection(l=150,b=80)andacrowded crowding both for deblending and for image
bulge field (l=5,b=5). Thus, in total, we sim- reconstrunction.Hereweusetheirworktode-
ulated twelve clusters with different combi- rivesimplifiedreceiptsthatcanbeusedinour
nations of concentration, distance, and back- simulations of globular clusters. In particular,
groundcrowding. weclassifyblendsasfollows:
We transformed the simulated properties
of each star into Gaia observables and their
uncertainties (injecting Gaussian noise) using Hard blends (or classical blends). They
the latest version of the Gaia science perfor- occur when two or more stars are closer than
mances1andrelatedpublishedtransformations the FWHM in the astrometric field (AF) of
(e.g.,Jordietal.2010;Kordopatisetal.2011). Gaia ((cid:39)0.53”); they could in principle be de-
blendedusinginformationowingtothediffer-
entorientationofeachGaiatransit,andtothe
3. Gaiaandcrowding
radial velocity variations in the radial veloc-
ity spectrometer (RVS), but not having suffi-
Gaiaisacomplexinstrument,withalargefo-
cient technical literature, for the moment we
cal plane hosting three instruments and many
justflaggedthesecasesinoursimulations;we
1 http://www.rssd.esa.int willtreatthematalaterstage.
284 Pancino:GGCseenbyGaia
Fig.2.Thecolor-magnitudediagraminGaiacolorsinanareaof0.7×0.7degaroundthesimulated“diffi-
cult”cluster(seetext).Blacksymbolsinbothpanelsrefertofieldstars,yellowsymbolstodeblendedstars
(seetextforadescription),greensymbolstodecontaminatedstars,andbluesymbolstocleanstars.The
left panel shows the full catalogue, while the righ panel shows only bona-fide cluster members selected
by proper motions and distance from the cluster center; in this panel two cluster stars were accidentally
rejected and are colored in magenta, while only a handful of the otherwise dominating field population
remain after the selection. The number of well measured member stars is small, but they all have space
qualitymeasurements(oftheorderofammagprecisionandafew%calibrationforthebrightstars).
Blends. Blends occur when two or more Contaminated stars. When two stars are
starsarecloserthantheshortsideoftheread- not blended, but are closer than the long side
out window assigned in the AF and in the oftheassignedwindow(3.54”inBP/RP),they
BP/RP instruments (Blue and Red spectro- will be assigned the same window in some
Photometers),whichis2.12”.Inthiscase,the transit and separarate windows in some other
stars will always be assigned the same win- transits, depending on the orientation of the
dow (thus be compressed to one single 1D Gaiascandirectiononthesky.Thustheywill
spectrumbeforegroundtransmission),nomat- beeasiertodisentanglethanblends.However,
ter the orientation on the sky. However, the brightstars(V≤15mag)willhaveenoughflux
spectra will overlap differently for different outside their windows to contaminate neigh-
orientations in the sky, and will have differ- bouringstarssoavariable“size”dependingon
ent radial velocities, measurable on RVS (for theirmagnitudeneedstobedefined(Marrese,
V<17 mag). All these effects can be modeled 2008,privatecommunication)totakethisinto
andthestarswilleffectivelybedeblendedwith account. In general, various experiments on
residuals of (cid:39)3–5% according to a test made two stars in isolation showed that they can be
byG.GiuffridawithGIBIS(Babusiaux2005). decontaminated with residuals better than 1–
Thetestwasmadewithouttakingintoaccount 3%.WeconservativelydegradedtheGaiasci-
CTI (Charge transfer inefficiencies), however, ence performances of (de)contaminated stars
sowedegradedtheGaiascienceperformances by 5%, proportionally to the flux contamina-
of(de)blendedstarsby10%,proportionallyto tion.
thefluxcontamination.
Pancino:GGCseenbyGaia 285
Table1.Systemicpropertiesoftwoclusters(seetext)
Property Easycluster Difficultcluster true(input)value
#ofstars 16838 3513 —
µ –4998.7pm0.8µas/yr –4993±3µas/yr –5000µas/yr
RA
µ –5000.2pm0.7µas/yr -4994±3µas/yr –5000µas/yr
Dec
π 199.7±0.7µas 101.2±1.4µas 200/100µas
D 5.007±0.007kpc 9.997±0.017kpc 5/10kpc
4. Resultsandconclusions difficult cluster has c=2.5, d=10 kpc, and
bulge-likebackgoundandisalsoshownin
In general, as was largely expected, we see
Figure2)2.
that crowding has a large effect on the central
areas of clusters, generally making Gaia per-
Acknowledgements. Thisworkusessimulateddata
formances rather poor (for a space telescope) providedbytheSimulationUnit(CU2)oftheGaia
inside the half-light radius of a cluster. Also, DataProcessingAnalysisConsortium(DPAC),run
Gaia does not go very deep (V<20 mag) and withGIBISatCNES(Centrenationald’e´tudesspa-
thus clusters farther than 15 kps are not sam- tiales).
pled down to their turnoff point. However, for
thosestarsthataremeasured,Gaiagrants:
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