It has recently been suggested that the presence of multiple populations showing various amounts of helium enhancement is a common feature among globular star clusters. In this scenario, such a helium enhancement would be particularly apparent in the enhanced luminosity of thc blue horizontal branch (HB) stars compared to the red HB stars. In this Letter, wc test this scenario in the case of the Galactic globular cluster M3 (NGC 5272), using high-precision Stromgren photometry and spectroscopic gravities for blue HB stars. We find that any helium enhancement among the cluster's blue HB stars must be significantly less than I%, thus ruling out the much higher helium enhancements that have been proposed in the literature

Catelan, M.

Cortes, C.

Grundahl, F.

Sweigart, A. V.

Valcarce, A. A. R.

NASA Technical Reports Server

APJ (LETTERS), TO BE SUBMITTED 
Preprint typeset using ETEX style elnulataapj v. 03/07/07 
CONSTRAINTS ON HELILW ENHANCEMENT IN THE GLOBULAR CLUSTER M3 (NGC 5272): 
THE HORIZONTAL BRANCH TEST 
M. CAT ELAN"^'^, F. G R U N D A H L ~ ,  A. V. SWEIGART', A. A.  R. VALCARCE', AND C. CORTES~" 
ApJ (Letters), to he sz~bnsitted 
ABSTRACT 
It has recently been suggested that the presence of multiple populations showing various amounts of helium 
enhancement is a common feature among globular star clusters. In this scenario, such a helium enhancement 
would be particularly apparent in the enhanced luminosity of thc blue horizontal branch (HB) stars compared to 
the red HB stars. In this Letter, wc test this scenario in the case of the Galactic globular cluster M3 (NGC 5272), 
using high-precision Stromgren photometry and spectroscopic gravities for blue HB stars. We find that any 
helium enhancement among the cluster's blue HB stars must be significantly less than I%, thus ruling out the 
much higher helium enhancements that have been proposed in the literature. 
Szibject headings: stars: abundances - Hertzsprung-Russell diagram - stars: evolution - stars: horizontal- 
branch - globular clusters: general - globular clusters: individual (M3 = NGC 5272, 
MI3 = NGC 6205) 
1. INTRODUCTION 
Globular star clusters (GC's) have traditionally been as- 
sumed to be excellent approxin~ations to so-called "simple 
stellar populations," which are idealized systems in which all 
stars were formed at precisely the same time, from a chem- 
ically homogeneous cloud. However, recent observations, 
both photometric and spectroscopic, have cast some serious 
doubt on this long-standing paradigm. 
d Centauri (NGC 5139) was the first GC where the pres- 
ence of large abundance anomalies were identified. In this 
cluster, not only such light elements as C, N, 0 ,  F, Na, Mg, 
and Al, but also the Fe-peak, s-process, and r-process ele- 
ments, are seen to vary by large amounts from one star to 
the next (e.g., Johnson et al. 2008, and the extensive list of 
references provided therein). Many of these abundance pat- 
terns seem to extend all the way down to the main sequence 
(e.g., Stanford et al. 2008). This strongly suggests that such 
abundance anomalies owe their origin to n~ultiple star for- 
mation episodes within the cluster, each accompanied by a 
corresponding enrichment of the intracluster medium by the 
ejecta of the massive and intermediate-mass stars which were 
formed in the prior stellar generations. Note that, while a 
large spread in Fe-peak abundances has only been detected in 
i ~ .  Cen, many of Cen's abundance anomalies have also been 
found in other clusters, albeit often at a (nluch) less dramatic 
level (see, e.g., Gratton et al. 2004, for a recent review). 
Analyses of the multiple main sequences found in the deep 
color-magn~tude diagrams (CMD's) of u! Cen strongly sug- 
gest that the helium abundance in the cluster may some- 
how also have changed dran~atically from one star formation 
episode to the next (e.g., Norris 2004; D'Antona et al. 2005; 
I John Simon Guggcnhcim Mcn~orial Foundation Fellow 
' Departamento de :\stroncnia :Y Asrr-ofislzb, Poi~tificia tiniversidad 
Catolica dc Chile. Av. Vxuiia Mackcnna 4560. 782-0436 Maall, Santiago, 
Cllile - --~.. 
'On sabbatical lcavc at Catholic University of America, Dcpartn~ent of 
Physics. 200 Hannan Hall, Washington, DC 20064 
.' Department of Physics & Astronomy, University of Aarhus. Ny 
Munkcgadc, 8000 Aarhus C, Denmark 
NASA Goddard Space Flight Center. Exploration of the Universe Divi- 
sion. Code 667. Greenbelt, MD 20771 
" Departamento dc Fisica Teorica e Expc~imcntal, Universidnde Fcdcral do 
Rio Grande do Norte, Campus Uni~rs i t i r io ,  59072-970 Natal, RN, Brazil 
Piotto et al. 2005). Recent evidence suggests that other clus- 
ters may also show sizeable Y variations. According to the 
deep CMD analysis ofNGC 2808 by Piotto et al. (2007), mul- 
tiple populations with different helium abundances are also 
present in this cluster. Strong arguments have also been raised 
in favor of helium enhancements among at least some of the 
stars in NGC 6388 and NGC 6441 (see, e.g., Catelan et al. 
2006; Caloi & D'Antona 2007, and references therein). In- 
terestingly. these four figure among the most massive of all 
Galactic GC's. 
Very recently, it has been suggested that multiple star for- 
mation episodes in GC's, accompanied by widely different 
levels of helium enrichment, are in fact not the exception, 
but instead the rule among GC's (D'Antona & Caloi 2008). 
In the particular case of M3 (NGC 5272), such a claim had 
previously been made also by Caloi & D'Antona (2008), on 
the basis of an analysis of the period distribution of M3's RR 
Lyrae variables and the color extension of the HB blueward 
of the instability strip. Due to the recognized need to as- 
sume a very sharply peaked mass distribution to account for 
the sharply peaked shape of the RR Lyrae period distribution 
in M3 (Rood & Crocker 1989; Catelan 2004; Castellani et al. 
2005), D'Antona & Caloi suggested that the mass distribu- 
tion of HB stars is a21vays sharply peaked, the color spread 
routinely observed in GC CMD's being instead due to inter- 
nal variations in the helium abundance Y. It is important to 
emphasize that such a spread in Y is not strictly required to 
explain the observed period distributions: the case for helium 
enhancement, at least in the case of M3-like clusters, rests 
alnlost entirely on the presence of color spreads among HB 
stars. 
If confilmed, this scenario would not only hme major im- 
plications for our understanding of how GC's fornl, but also 
represent a major deviation fkom the canonical paradigm, 
dominant since the late-1960'sl'early-1970's (Castellani et al. 
1969; Iben & Rood 1970; Faulkncr 1972; Rood 1973), which 
ascribes the color spread seen among HB stars to the stochas- 
tic nature of mass loss on the red giant branch (RGB). In the 
canonical scenario. blue HB stars lose significantly more mass 
as they climb up the RGB than do their red HB counterparts. 
In contrast, in the D'Antona & Caloi (2008) scenario the bluer 
HB stars owe their bluer colors to a higher initial Y. 
https://ntrs.nasa.gov/search.jsp?R=20090006873 2019-08-30T06:05:47+00:00Z
M. Catelan et 
- - - ~ - - - - T - ! ~ -  
- 0 5  1 1 
L a 7 
al. 
, u - ~ B  .d I ,*----------------------- WdUQ 
4 
,-- 1 
1 / // ;;:----- - - ------I 
ZAHB 
-I f 
1 
i 
Y,, = 0 23 
3 
black I~nes. Z = 0 002 i 
I 1d.L 
z = 0.002 
black lines: Y,, = 0.23 
i Y,, = 0.23 black lines: 2 = 0.002 
FIG. I .- Pone1 u- comparison between fiducial HB l o c ~  (ZAHB. MAHB, 90.4HB. TAHB: see definit~ons in $3) for two dityerent inetallic~tles (hluck lines: 
Z = 0.002: gr~xv lirzes: Z -  0.001), in the .q. (b-~.)(, diagram. To produce this plot, the Z = 0.001 loci were sliiftcd in by +0.075 mag. Pailel b: as in panel 
a. but for a fixed metallicity and two difirent helium abundances (block lir~es: Gs = 0.23; grc{v lines: yvS = 0.28). and without applying any shifts to the CMD 
positions. Panels c and d: as in a and b, hut in the logg, log plane. and without applying any shifts to the data. 
Fortunately, the Y enhancement scenario can be tested, us- 
ing a variety of photometric and spectroscopic tools. In partic- 
ular, it is well known that HB stars with higher Y are brighter, 
at a given T,ft; due to their stronger H-burning shells (e.g., 
Siwigari & Gross 1976). Therefore, in the D'Antona & Ualoi 
(2008) scenario bluer HB stars should be brighter than their 
redder counterparts. While this may help account for the slop- 
ing natures of the HB's in the moderately metal-rich GC's 
NGC 6388 and NCC 6441 (e.g., Busso et al. 2007, and ref- 
erences therein), such a CMD test has never being carried out 
for nlost of the GC's that, according to D'Antona & Caloi, 
present n~ultiple populations with increased Y. The purpose 
of the present Letter is accordingly to present a first test of 
the helium enhancement scenario. in the specific case of M3, 
by conlparing high-precision photometry for the cluster in the 
Stromgren (1963) system, as well as spectroscopically de- 
rived gravities. with theoretical models computed for a variety 
of Y values. 
2. OBSERVATIONAL DATA 
The CMD data used in this paper were taken from Grun- 
dahl ct al. (1998, 1999), to which the reader is referred for 
additional details regarding the calibration of our photome- 
try. In summary, our M3 photometry is based on a series of 
images obtained on the Nordic Optical Telescope, using the 
Constraints on Helium Enhancement in M3 
I " "  I 
I - - - - TAHB 
A
(b-y), 
FIG 2 - Compar~son &&teen fiducial HE3 sequences and the empir~cal data tor M3, in the X(, (b-J.)O planc (panel cr). ,w,, (b-y)o pldnc (panel b), ,L(, (h-1 )o 
plane (panel L ) .  and IM,, (b-v)o plane (panel 4 In each panel, the cmplr~cal data were shlfted \ert~cally so as to produce a satisfactory match to the thco~ctical 
red ZAHB 
thinned AR-coated 2048 x 2048 pixel HiRAC CCD camera, 
with 0 1 1 I' pixel size, thus covenng approximately 3 75 ' on 
a side. T\+o overlapping fields \\ere obsened. ~ l t h  one field 
centered on the cluster center to ensure a large sample of HB 
and red giant branch (RGB) stars 
in! add~tion to the CMD data, we also used the gravit~es and 
teinperatures derived by Behr (2003), on the basis of spectro- 
scopic obse~atlons using the HIRES cross-dispersed echelle 
spectrograph on the Keck I telescope 
3 THEORETICAL MODELS 
In the present paper, we use the evolutionary tracks com- 
puted by Catelan et al. (1998) and Sweigart & Catelan (1998) 
for Z = 0.001 and Z= 0.002, respectively. The Z= 0.001 evo- 
lutionary tracks assume a main-sequence helium abundance 
of YMs = 23% by mass and scaled-solar heavy element abun- 
dances, whereas Z = 0.002 tracks were computed for several 
different Y ~ s  values, namely 23%, 28%, 33%,38%, and 43%. 
The theoretical models were transformed to the Stran~gres 
(1963) z~vby system by  sing the color transforlnations and 
boloinetric correction tables provided by Clem et al. (2004). 
The same procedures were also adopted in the recent work by 
Catelan & Cortes (2008) and Cortes & Catelan (2008), where 
the period-color and period-luminosity relations of RR Lyrae 
stars in the Stromgren system were presented. 
As noted, our helium-enhanced models were computed 
only for a metallicity Z = 0.002, which is arguably too high for 
4 M. Catelan et al. 
(b-Y 10 
F I G  3 As in Figure 2, but plotting only the ZAHB's. for tile several differcnt irlues indicated The dh so/idlii~es indicatc thc interpolated ZAHB loci 
for Yvs valucs between 24% and 27%. in intervals of 19'0. 
M3, even in the Carretta & Gratton ( 1  997) scale (which favors 
a metallicity for the cluster closer to 1.4 x However, 
since we are priniarily interested in constraining the change 
in Y when going from the red to the blue HB, the exact choice 
of Z value turns out to be of minor relevance for our pur- 
poses. This is better rhowr? ir, Figure !a,  ivherc our rnodcls 
for Z = 0.001 and Z = 0.002 are coinpared in the M,., ( b - ~ ) ~  
diagram. 'To produce this plot, n.e have first regislered the 
low-metallicity zero-age HB (ZAHB) sequence to its higher- 
n~etallicity counterpart. This was achieved by shifting the for- 
mer by +0.075 mag in M,,. This clearly leads to an excel- 
lent match over the entirerange of ZAHB colors, except (as 
expected) at the red extreme (which is irrelevant for our pur- 
poses). 
We next apply the same shift in M,. also to the evolvcd HB 
sequences for Z = 0.00 1. We then derived several f ducial loci 
representing evoloved HB stars, as follows: i) MAHB, stand- 
ing for the micIdIe-age HB. or HB ridgeline, which gives thc 
average position occupied by all HB stars, assuxning a uni- 
fo~n i  Inass distriburion along the entlre ZAHB; ii) 90AHB, 
or 90%-age HB, which is approximately the locus below 
which one should tind -- 90% of all HB stars;' iii) TAHB, 
or terrniilal-age HB, which is simply the He exhaustion lo- 
cus. As also shown in Figure la,  once the red ZAHB's are 
' Note that. SIIICC the distribution of HB luminosities is not Gaussian, thc 
MAHB is in fact slightly Inore lutiiinous than the corresponding '-5036-age 
HB" locus, which is thc locus occupied by HB stars of different masses whicll 
have co~npletcd 50% of their HB cvolution. 
Constraints on Helium Enhancement in M3 5 
, 
, 
b) Y, = 0.28 
, 
, 
3.0 
, 
, 
i 
___( M3, Behr (2W3) 
4.0 - j 0 Ml3 ,  Behr (2003) i 
- _ _ _ _ _ _ A  
i , , l , , , t l l i i ,  
4.05 4.W 3.95 3.90 
FIG. 4.--- Co~nparison between predicted and observed loci in the logg, log & plane, for four differcnt helium abundance values: 23% (panel u), 2804 (panel 
b), 33% (panel c). and 38% (panel d). Empirical data are provided for M3 (black circles) and M13 b u y  circles), from Behr (2003). 
matched, also the MAKB's, 90AHB7s, and TAHB's agree 
quite well. That these loci do all depend strongly on Y can 
be easily inferred from Figure 16. We conclude that our pro- 
posed test, which relies on the difference in brightness be- 
tween red and blue HB stars, is basically insensitive to our 
choice of Z. 
Panels c and d in Figure 1 reveal the effects of ~netallicity 
and Y, respectively, on the logg-logTeRplane. As can clearly 
bc seen, metallicity plays but a minor role in defining the po- 
sition of a star on this plane, compared with Y. 
Bearing these comparisons in mind, in what follows we 
shall use the models computed for Z = 0.002 in our compari- 
son with the empirical data. 
4. RESCLTS 
We compare the predicted loci for KVs = 0.23 with the em- 
pirical CMD data in Figure 2. To produce these plots, we 
have first corrected the empirical data for reddening using a 
E(B-V) = 0.0 1 (from Harris 1996) and the extinction coef- 
ficients for the Stromgren system summarized in Catelan & 
Cortes (2008). We then shifted the data vertically, as requirzd 
to nlatch the theoretical red ZAHB to the lower envelope of 
the observed distribution. 
Once we do this, it becomes readily apparent that both the 
red a77d the blue ZAHB's are fairly well described by the the- 
oretical ZAHB, without the need to invoke an increase in Y 
for the blue HB stars. In addition, all the other HB fiducials 
for a fixed Y ,  including the MAHB, 90AHB, and TAHB, ap- 
pear to describe the empirical data fairly well, without signif- 
icant evidence for an excessive number of overluminous blue 
HB stars, as would be expected in the helium-enhancement 
scenario (see Fig. I b). To be sure, the evolutionary lifetimes 
along the canonical tracks do not provide a perfect match to 
the CMD distribution (Valcarce & Catelan 2008); however, it 
appears extremely unlikely that any such disagreements may 
solnehow be due to internal variations in Y, since they seem to 
affect blue and rrd HB stars alike, as also noted by Valcarce 
& Catelan. 
Figure 3 allows us to provide a more q~~antitative limit on 
the increase in Y between the red HB and blue HB populations 
that ma) be allowed for b) our CMD data. This figure is 
similar to Figure 2, except that the evolved loci have been 
removed, and ZAHB sequences for different Y values added. 
As can clearly be seen, an enhancement in Y by more than 
1% would hardly be coinpatible with the data. As a matter 
of fact, in the Caloi & D'Antona (2008) scenario, one should 
expect to see an increase in Y towards bluer colors, but this 
is not supported by our data. Recall, from Figure 4 in Caloi 
& D'Antona (2008) and Table 1 in D'Antona & Caloi (20081. 
that the bulk of the blue HB stars in M3 are predicted, in the 
helium-enhancement scenario, to be enriched in helium by 
2% or more. Such a level of helium enhancelnent is clearly 
ruled out by our data. 
Finally, Figure 4 colnpares the empirical and predicted po- 
sitions of blue HB stars in M3 in the logg, logTeff plane, for 
four different helium abundances, ranging from 23% (panel 
a)  to 38% (panel d). To produce this plot, we restrict our- 
6 M. Catelan et al. 
selves to temperatures cooler than 11,500 K, to avoid the of helium-enriched populations in GC's. Our results strongly 
well-known complications that are brought about for hotter suggest that any populations that may have formed after a 
HB stars (Grundahl et al. 1999). Consistent with the CMD main initial burst in M3 likely preserved the saillc helium con- 
analysis, the empirical gravities again seem entirely consis- tent as in the cluster's primordial gas, with a maximum level 
tent with a uniform value of Y among the blue HB stars in of helium enhancement most likely not higher than 1%. In 
M3. Interestingly, the available data also suggest that at least future papers, we plan to apply similar tests to several other 
the redder blue HB stars in M13 (NGC 6205) have a similar GC's (Valcaree et al., in preparation). 
helium abundance as in M3. 
5. CONCLUSIONS Support for M.C. is provided by Proyecto Basal PFB- 
In this Letter, we have shown that high-precision, well- 06/2007, by FONDAP Centro de Astrofisica 15010003, by 
populated en~pirical CMD's, along with spectroscopic grav- Proyecto FONDECYT Regular #1071002, and by a John Si- 
ities, can be used to pose strong constraints on the presence mon Guggenheim Memorial Foundation Fellowship. 
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Constraints on Helium Enhancement in the Globular Cluster M3 (NGC 5272): The Horizontal Branch Test

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