Mass segregation in the diffuse outer-halo globular cluster Palomar 14

0.57 MB·English

by  Matthias J. Frank

Mem.S.A.It.Vol.0,0
(cid:13)c SAIt 2008 Memoriedella
Mass segregation in the diffuse outer-halo
globular cluster Palomar 14
3 MatthiasJ.Frank1,2,EvaK.Grebel1,andAndreasH.W.Ku¨pper3
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1 Astronomisches Rechen-Institut, Zentrum fu¨r Astronomie der Universita¨t Heidelberg,
2
Mo¨nchhofstrasse12-14,D-69120Heidelberg,Germany
n 2 Landessternwarte,Zentrumfu¨rAstronomiederUniversit¨atHeidelberg,Ko¨nigsstuhl12,
a D-69117Heidelberg,Germany;e-mail:[email protected]
J 3 Argelander-Institutfu¨rAstronomie,AufdemHu¨gel71,D-53121Bonn,Germany

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Abstract.Wepresentananalysisoftheradialdependenceofthestellarmassfunctionin
]
A thediffuseouter-haloglobularclusterPalomar14.Usingarchival HST/WFPC2dataofthe
cluster’scentral39pc(correspondingto∼ 0.85×r )wefindthatthemassfunctioninthe
G h
massrange0.55≤m/M ≤0.85iswellapproximatedbyapower-lawatallradii.Themass
⊙
. function steepens withincreasing radius, froma shallow power-law slope of 0.66±0.32
h
inthecluster’s centreto aslope of 1.61±0.33 beyond the core radius, showing that the
p
clusterismass-segregated.Thisisseeminglyinconflictwithitslongpresent-dayhalf-mass
-
o relaxation time of ∼ 20Gyr, and with the recent finding by Beccari et al. (2011), who
r interpretthecluster’snon-concentrated populationofbluestragglerstarsasevidencethat
st dynamicalsegregationhasnotaffectedtheclusteryet.Wediscussthisapparentconflictand
a arguethattheclustermusthaveeitherformedwithprimordialmasssegregation,orthatits
[ relaxationtimescalemusthavebeenmuchsmallerinthepast,i.e.thattheclustermusthave

undergoneasignificantexpansion.
1
v
Key words. Galaxy: globular clusters – Globular clusters: individual: Palomar 14 –
0
Galaxies:stellardynamics–Stars:formation–Galaxy:halo
1
0
5
. 1. Introduction masssegregation,i.e.moremassivestarsshow
1
a more concentrated radial distribution than
0
Almost all Galactic globular clusters (GC)
3 lower-mass stars, and the cluster appears de-
have present-day half-mass relaxation times
1 pletedinlow-massstars.
shorterthantheirages(e.g.Harris1996,2010
v: edition). In these clusters, two-body relax- Oneofthefewexceptions,withapresent-
i ation has already altered the distribution of day half-mass relaxation time exceeding the
X
Hubbletime,istheouter-haloGCPalomar14
stars: massive stars, losing kinetic energy to
r (Pal14). According to Sollima et al. (2011),
a lower-mass stars sink into the cluster’s cen-
Pal14hasaprojectedhalf-lightradiusofr =
tre, whereas low-mass stars gain energy al- h
46pc, making it the most extended Galactic
lowing them to populate orbits further away
GC in the Milky Way. Its low mass and large
from the cluster’s centre. This is observed as
radius implies a half-mass relaxation time of
Sendoffprintrequeststo:M.Frank ∼20Gyr.Therefore,nomasssegregationisin-
Frank,Grebel&Ku¨pper:MasssegregationinPalomar14 1
tuitivelyexpectedinPal14.Inagreementwith
this expectation, Beccari et al. (2011) found 18
that the cluster’s population of blue straggler
stars(BSS)isnotcentrallyconcentratedcom-
paredtoredgiant(RGB)andhorizontalbranch
20
(HB) stars. BSS in such a diffuseclusterhave
most likely formed via mass-transfer in pri-
mordial binary systems that have larger to-
tal masses than individual RGB or HB stars. g]22
a
m
Hence, these systems would segregate most [
W
quicklyinaGC,suchthatBSSareexpectedto 5
5
5
tracethissegregationprocess.Beccarietal.in- F
24
terprettheirfindingsasevidencethattwo-body
relaxationhasnotaffectedPal14yet.
Ontheotherhand,Jordietal.(2009)found
that the mass function of main sequence stars 26
inPal14isdescribedbyapower-lawdN/dm∝
m−α with a slope of α = 1.3 ± 0.4, i.e. the
cluster is significantly depleted in low-mass
starscomparedtoaKroupa(2001)initialmass 0.4 0.6 0.8 1.0 1.2 1.4 1.6
function (IMF; α = 2.3 in this mass range). F555W-F814W[mag]
This wouldreadily be understood,if the clus-
Fig.1. Observed CMD of Pal14 obtained from
terweremass-segregated,oralternatively,ifit
archivalWFPC2datausingtheHSTPHOTphotom-
formed with a IMF already depleted in low-
etry package (Dolphin 2000). Artificial star tests
massstars(Zonoozietal.2011). wereusedtoestimatephotometricuncertainties(er-
In this contribution we present evidence rorbarsontheright)andcompletenesslimits(light
that Pal14 is mass-segregated based on an anddarkgreylinesatthefaintandbrightendscor-
analysis of radial dependence of the cluster’s respondto80%and50%completenesscontours,re-
stellar mass function (Section 2) and discuss spectively).Theisochrone(thickgreycurve)corre-
two possible scenarios that can reconcile our spondstoanageof11.5Gyr,[Fe/H]=−1.5dexand
results with the cluster’s large relaxation time [α/Fe]=+0.2dexassumingadistanceof71±1.3kpc
andreddeningof E(F555W−F814W) = 0.06mag
scaleanditsnon-segregatedpopulationofBSS
(Jordi et al. 2009). To calculate the mass function
(Section3).
we used stars within the colour limits represented
bythingreycurvesonbothsidesoftheisochrone.
2. DataandAnalysis
We used deep V and I band archival observedmagnitudesofthesestarsinorderto
HST/Wide-Field Planetary Camera 2 infertheirmasses. We correctedforthe radial
(WFPC2) imaging of Pal14 (program variation of the photometric completeness, as
GO 6512, PI: Hesser) to obtain the colour- well as the inhomogeneous coverage of the
magnitude diagram (CMD) shown in Fig. 1. clusterbytheWFPC2pointing,andcalculated
The overlaid isochrone is taken from the the maximum likelihood power-law represen-
Dotter et al. (2008) library and corresponds tation of the mass function in different radial
to an age of 11.5 Gyr, [Fe/H]= −1.5dex and ranges. The cluster’s observed mass function
[α/Fe]= +0.2dex. To derive the stellar mass inradialbinscontainingeachonefourthofthe
function, we followed the basic procedure observed stars is shown in Fig. 2. The dotted
described in Frank et al. (2012): we selected curves correspond to raw star counts in ten
stars within the colour limits shown as thin evenly spaced mass bins from 0.54 to 0.82
grey lines in the CMD and interpolated the M ,correspondingtothe∼70%completeness
⊙
masses tabulated in the isochrone file to the limit at the faint end to the tip of the RGB.
2 Frank,Grebel&Ku¨pper:MasssegregationinPalomar14
Dashed curves represent the star counts after 0.00<r≤0.44arcmin
s
correctionforgeometriccoverage,solidcurves ar256
st
after additionally correcting for photometric of128
completeness. Thick grey lines show the er
b
m 64
best-fitting power-law. The mass function u
is well described by a single power law at N 32 α=0.66±0.32
0.44<r≤0.68arcmin
all radii and a trend of an increasing power s
ar256
law slope with increasing radius is apparent. st
This trend is seen more clearly in the finer of128
er
radial subdivision of Fig. 3, which shows the mb 64
best-fittingmassfunctionslopeαasafunction Nu α=0.47±0.32
32
of radius. The mass function steepens with 0.68<r≤0.99arcmin
s
increasing radius, ranging from α < 1 within ar256
st
the cluster’s core radius to a slope almost of128
compatible with the Kroupa α = 2.3 in the er
b
outermostradialbinatr =1.6arcminor33pc. um 64
Aconstantmassfunctionslopeasafunctionof N 32 α=0.86±0.33
0.99<r≤1.89arcmin
radiusisexcludedatthe98%confidencelevel. s
ar256
The lack of low-mass stars in the cluster’s st
centre compared to larger radii clearly shows of128
er
thattheclusterismass-segregated. mb 64
u
N α=1.61±0.33
32
0.52 0.56 0.60 0.64 0.68 0.72 0.76 0.800.84
3. Discussion m[M⊙]
Findingmass-segregationinPal14isinappar-
Fig.2. The mass function in four radial ranges in
ent conflict with its present-day half-mass re-
order of increasing distance from the cluster cen-
laxation time of ∼ 20Gyr. If the cluster had
tre. Dotted lines correspond to the raw observed
a similar structure and therefore a similar re-
star counts, dashed lines to the star counts after
laxationtime-scalethroughoutitslifetime,the correction for geometric coverage and solid lines
observed mass segregation would have to be to thestar counts additionally corrected forphoto-
primordial,aswassuggestedbyZonoozietal. metric incompleteness. The best-fitting power-law
(2011). In this case, it is likely that the clus- massfunctionsareshownasthickgreylinesandthe
terspentmostofitslifetimeinthelow-density best-fittingslopesαarereportedthebottomofeach
environmentofanonlyrecentlyaccreteddwarf panel.Inallradialrangesthemassfunctioniswell
describedbyapower-law.
galaxy(cf.Sollimaetal.2011;C¸alıs¸kanetal.
2012), or otherwise it is puzzling how such a
diffuse cluster can have survived in the tidal
fieldoftheGalaxy.Thattheclusterisaffected
bytheGalactictidalfield,evenatitscurrentre- orbit about the Galaxy, similar to the expan-
mote location (at a Galactocentric distance of sion of Palomar5 due to disk shocks that has
66kpc),isevidencedbyitstidaltails(Jordi& beensuggestedby(Dehnenetal.2004).Given
Grebel2010;Sollimaetal.2011). its present Galactocentric distance of 66kpc,
Alternatively, the cluster may have been Pal14 would have to be on a highly eccentric
significantly morecompact(by a factorof ∼2 orbitin orderto comesufficientlyclose to the
in the projected half-light radius rh) in the Galactic centre to be affected by tidal shocks.
past, implying a previously much shorter re- This scenario could not only explain the ob-
laxation time scale trh of a few Gyrs (trh ∝ servedmasssegregation,butalsothe cluster’s
r3/2;Spitzer&Hart1971).Suchanexpansion large physicalsize, whose light profile in this
h
could have been caused by tidal shocks dur- case maybesignificantlyinflatedbyunbound
ing pericenter passages of the cluster on its stars(Ku¨pperetal.2010).
Frank,Grebel&Ku¨pper:MasssegregationinPalomar14 3
2.5 havebeenexpelledfromtheclustercentre,re-
rc Kroupa sulting in a more extended radial distribution
2.0 ofBSSinthecluster.
α
e
p
slo1.5 Acknowledgements. This work was partially sup-
on ported by Sonderforschungsbereich 881, “The
ncti1.0 Milky Way System” (subprojects A2 and A3) of
u
f the German Research Foundation (DFG) at the
s
as0.5 University of Heidelberg. M.J.F and A.H.W.K.
m
kindly acknowledge support from the DFG via
0.0 Emmy Noether Grant Ko 4161/1 and project KR
1635/28-1,respectively.
0.0 0.5 1.0 1.5
radius[arcmin]
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