The Age of Old Magellanic Cloud Clusters. I: NGC 2257

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Mon. Not. R. Astron. So c. 000, 000{000 (1994)

1 2 3 3;4

V. Testa , F.R. Ferraro , E. Bro cato , V. Castellani

Dipartimento di Astronomia, Universita di Bologna, Bologna, Italy

Osservatorio Astronomico, Bologna, Italy

Osservatorio Astronomico di Col lurania, Teramo, Italy

Dipartimento di Fisica, Universitadegli Studi, Pisa, Italy

ABSTRACT

Deep CCD photometry down to V 24:5 is presented for the old globular cluster NGC

2257 in the Large Magellanic Cloud. The main results of this study can b e summarized

as follows: i) the Red Giant Branch(RGB) Bump has b een detected at V =18:70:1,

and it is compatible with determination of this feature in Galactic Globular Clusters;

ii) using the currently accepted metho ds of dating Globular Clusters, NGC 2257 seems

to b e 2-3 Gyrs younger than a typical Galactic Globular; iii) the Helium abundance

has b een estimated to b e Y =0:21 0:03 using the R-metho d as calibrated by Buzzoni

et al. (1983).

Key words: Globular Cluster { Stellar Evolution { Magellanic Clouds

1 INTRODUCTION of the Universe. This is not the case for the MC, where

one nds globulars as young as 10 million years, if not lower

This is the rst of a series of pap ers devoted to investigating (Barb ero et al. 1990). However, the MC do have some glob-

the ages of old globular clusters in the Magellanic Clouds ular clusters whose characteristics closely resemble those of

(hereafter MC), in order to allow a comparison with the GGCs, with CMD morphologies quite similar to that typical

Galactic Globular Clusters (GGCs) system. of their galactic counterparts. The determination of age and

The age of stellar systems is one of the most imp ortan- abundances for these MC clusters will provide fundamental

t astrophysical parameters. As is well known, the galactic information ab out the very early history of the MC system

halo is mainly p opulated by old clusters, whose ages should and ab out p ossible interactions b etween the MC and the

provide not only fundamental information on the early evo- Galaxy, showing in particular whether or not the MC start-

lution of the Galaxy, but also severe constraints on the age ed making clusters at the same cosmic era and in similar

2 V. Testa et al.

initial conditions. the age of the cluster. Conclusions are presented in Sect.7 .

Among these MC old clusters, wecho ose as a rst tar-

2 OBSERVATION AND REDUCTIONS get NGC 2257, which app ears of particular interest since

according to Stryker 1983, Nemec Hesser and Ugarte 1985

2.1 Observations

(NHU) and Walker 1989 (W89) it is probably the oldest

A set of B,V CCD frames was secured during one photomet-

MC cluster. NGC 2257 lies 9 NE to the LMC bar, in a

ric night in Dec. 1990 using the NTT 3.5m ESO telescop e,

p o orly eld-contamined area and it has b een recognized as

equipp ed with EMMI. The chip wasaTektronix 1024x1024

intermediate Oosterhof typ e (NHU). Available metallicity

px, 0.44 arcsec/px sized. The cluster was centered in the

estimates, as discussed by W89, give[Fe/H] = 1:8 0:1

NE quarter of the CCD frame in order to have, in the same

and indications that the reddening E(B-V) is not greater

frame, a eld sky region far enough from the cluster center

than 0.04 are provided by the same author.

to obtain information on the eld p opulation. We obtained

Photometric results available in the literature concern-

short exp osures (V 15s, B 30s) to study the bright p ortion

ing either the bright stellar p opulation of the cluster or accu-

of the CMD, intermediate exp osures (V 120s, B 360s) for

rate determination of magnitude and p erio ds for cluster RR

the region b etween the Horizontal Branch (HB) and the up-

Lyrae (see W89 and references therein), already disclose that

p er Subgiant Branch (SGB), and deep exp osures (V 300s,

NGC 2257 shows a marked similarities with typical galac-

B 900s) to reach the Main Sequence (MS). All the frames

tic globulars of intermediate metallicity like NGC 5897 and

were secured during go o d seeing condition (0:8 FWHM).

M 3. Stryker (1983) presented photometry results down to

Because of the crowding conditions, we cut o a circle of

V 22, nding indication for the turn o (TO). However,

60 px radius around the cluster center on the intermedi-

we still lackofinvestigation clearly detecting the TO lumi-

ate frames, and one of 110 px on the deep ones, while the

nosity which is the only reliable way to obtain indication on

bright p ortion was measured up to the cluster centre. All

the cluster age.

the frames were referred to the master intermediate one (V

In this pap er we present B,V photometry down to

120s), where the cluster centre is lo cated at the co ordinates

V 24:5, i.e. ab out 2 mag b elow the TO p oint, with

X = 851;Y = 816 px. The X and Y co ordinates are in

c c

the aim of estimating the age of the cluster on the basis

pixels starting from the lower left (SW) corner of the eld

of the various pro cedures currently available. The observa-

(see Figure 1).

tions and reductions are describ ed in Sect. 2. The overall

Bias exp osure has b een obtained from the median aver-

prop erties of the CMD are discussed in Sect. 3. In Sect. 4

age of 9 individual frames. Flat- eld exp osures were taken

we derive an estimate for the distance of NGC 2257. Sect.

at the b eginning and the end of the night in order to correct

5 is devoted to some comparisons with NGC 5897. In Sect.

the detector resp onse to uniform sensitivity. The level of

6we discuss the detection of the Turno (TO) region and

at eld exp osures was 15000-20000 ADU. Both B and V

Age of Old MC clusters: NGC 2257 3

Figure 1.Map of the observed eld in NGC 2257. Some stars have b een identi ed with the numb er they haveinTables 1a,b.

Table 2. Mean Ridge Lines for NGC 2257.

The adopted reduction pro cedure has b een widely reviewed

RGB 0.690 20.00 0.478 21.82

in Ferraro et al. (1990). According to this pro cedure the re-

1.770 16.30 0.680 20.10 0.436 21.88

1.620 16.36 0.670 20.20 0.391 22.00

duction package ROMAFOT (Buonanno et al. 1979, 1983),

1.460 16.42 0.670 20.30 0.372 22.10

1.290 16.49 0.670 20.40 0.358 22.20

1.200 16.59 0.660 20.50 0.351 22.30

in the version optimized for undersampled images (Buonan-

1.120 16.78 0.670 20.40 0.348 22.40

1.050 17.03 0.660 20.50 0.350 22.50

no and Iannicola 1988) was used to detect stars on each

0.960 17.54 0.650 20.60 0.355 22.60

0.870 18.14 0.650 20.70 0.360 22.70

frame, to transform the instrumental magnitudes and co or-

0.800 18.73 0.650 20.80 0.365 22.80

0.770 19.10 0.640 20.90 0.370 22.90

dinates to the frame assumed as reference and, nally,to

0.760 19.20 0.630 21.00 0.389 23.00

0.750 19.30 0.620 21.18 0.475 23.82

get an homogeneous set of CCD instrumental magnitudes,

0.740 19.40 0.600 21.41 HB

0.730 19.50 0.580 21.55 -0.015 19.42

0.720 19.60 0.560 21.62 0.056 19.15

colours and p ositions.

0.710 19.70 SGB{MS 0.140 19.07

0.710 19.80 0.538 21.72 0.319 18.98

Since a reliable sequence of photo eletric primary stan-

0.700 19.90 0.526 21.76 0.531 18.89

dards was not available for that night, we used a set of 350

stars in common with W89 in order to calibrate our data.

frames have b een attened to 0:2% p eak to p eak.

2.2 Magnitudes, colours, and p ositions

4 V. Testa et al.

The following relations have b een obtained:

B = b 0:05(b v )+30:35

V = v +30:41

where B and V are the magnitudes listed by W89 and

W W

b and v are our instrumental magnitudes.

In Table 1a we list nal magnitudes, colours and p o-

sitions for the 3169 non variable stars in the cluster region

(R<400px : 176"), whereas Table 1b rep orts the nal

magnitudes, colours and co ordinates for the 1787 stars in

the outer regions (R>400px). We present in Figure 1 a

computer map of some reference stars from the list of Table

1. Colour Magnitude Diagrams (CMD) obtained from stars

listed in Tables 1a and 1b are shown in Figure 2a,b resp ec-

tively. These CMDs are assumed as representative of the

cluster and the eld p opulation.

2.3 Photometric errors and eld contamina-

tion

The transformations of instrumental magnitudes at various

exp osure times to an homogeneus scale o ers the p ossibil-

Figure 2. (a) - Cluster CMD; the detected stars listed in Table

ity to compute the frame to frame scatter of magnitudes

1a lying at R 400px from the cluster centre; the horizontal line

separates the bright amd the faint samples; (b) - CMD of the

eld: stars listed in Table 1b all stars lying at R>400px form

for each individual star. Thus the internal accuracy of our

the cluster centre.

CCD measurements can b e estimated from the rms frame-

to-frame scatter of the instrumental magnitudes of each in- of the nal mean magnitude. Only errors for stars brighter

dividual star computed according to the formula given in than V>17 and B>17 have b een plotted since the esti-

Ferraro et al. (1991), where the weights have b een comput- mate of the errors for the brightest stars, measured almost

ed taking into account the quality of the CCD frame, seeing in the central zone, where only one frame p er lter was mea-

conditions etc. sured, has b een made using the scatter of the magnitudes

The rms-values ( )ineach 0.5-mag bin, for the two l- obtained during the simulation of arti cial stars to deter-

ters, are plotted in Figure 3a,b resp ectively as a function mine the completeness factor, obtaining 0:01 in b oth

Age of Old MC clusters: NGC 2257 5

Figure 4. Colour distribution of the stars in Tab el 1a in the

range 22:0

Table 3. Contaminating Field Stars p er square arcmin.

B V<0 0BV<1 BV1

Figure 3. Internal photometric errors of the measurements; the

V18:0 0.0 0.6 0.2

mean values of the frame-to-frame scatter computed in bins of

18:0 V 20:5 0.0 2.0 0.7

0.5 mags. are plotted versus V (panel (a)) and B (panel (b)) nal

20:5 V 22:0 0.0 5.2 1.0

magnitude.

V 22:0 2.0 36.4 5.0

Table 2 presents the values we will adopt as normal

p oints for the various branches. The contamination from

lters. However, it is imp ortant to p oint out that the nal

background and foreground stars can b e evaluated dividing

errors include the combination of two more contributions:

the CMD into cells and computing the numb er of eld stars

the intrinsic error involved in the calibration pro cedure and

exp ected in each cell, using the CMD of p ortion of the frame

the uncertainties in the various transformations.

assumed as eld and scaling the area of the eld to that of

A preliminary insp ection of the CMDs presented in Fig-

the cluster. In Table 3 we rep ort the exp ected number of

ure 2 reveals that the eld contamination seems not to af-

contaminating ob jects, p er square arc min in eachinterval

fect signi cantly the de nition of the ridge lines of the main

of magnitude and colour, in the cluster area.

branches, at least at the brighter luminosity.At the lower

luminosity, one nds that - for each given interval of lumi-

2.4 Completeness

nosity - the distribution of star colours shows a steep max-

To test the completeness achieved in our measures following imum around a quite well de ned colour values, which can

the pro cedure describ ed in Ferraro et al. (1990), we ran- b e fairly assumed as ridge line. Figure 4 shows an example

domly added to the original frames a set of stars (N )of of the pro cedure followed, relatively to the magnitude bin

sim

known magnitude and colours, and re-reduced each frame. 22:0

6 V. Testa et al.

and for this reason we considered only a selected sample of

stars with the b est photometry. The ab ove pro cedure yields

only \ rst order" corrections (Mateo 1988), but we b elieve

that the residual incompleteness still a ecting our samples

should not bias our conclusions.

3 THE COLOUR MAGNITUDE DIAGRAM

3.1 The CMD overall morphology

The CMD plotted in Figure 2 shows that the photometry

presented here reaches V 24:5, increasing by ab out t-

wo magnitudes the previous photometries. Our CCD frame

Figure 5. Completeness curves for the inner region (panel (a),

R<60px) and the intermediate region (panel (b), 60 R<

covered a large region of the cluster so that we are con -

110px).

dentwe sampled a large fraction of the light of the cluster

making the presented CMD a reliable \picture" of the star

The output of the pro cedure is the ratio of the number of

p opulation in NGC 2257.

stars detected (N ) with resp ect to N .

out

sim

The main characteristics of the diagram can b e summa-

In order to test the dep endence of the completeness level

rized as follows:

from the crowing conditions, we p erformed the pro cedure in

1. The giant branch app ears fairly well de ned and three annuli at di erent distance from the cluster center, up

p opulated also in the very upp er part; to V =20:3:

2. The HB is p opulated mainly in the blue side of the annulus (a) - r<26:4 i:e : r<60px

00 00

instability strip, with the p ossible o ccurrence of few very hot annulus (b) -26:4

stars at its blue end; annulus (c) - r>66 i:e: : r>150px.

3. The region of the TO and the upp er p ortion of the Figure 5a,b shows the ratio = N =N as a function

out

sim

Main Sequence is de ned and well p opulated at least 2 of the V magnitude in the (a) and (b) annuli obtained from

mag b elow the cluster TO. the whole pro cedure. Star detection losses increase at fainter

4. A puzzling features is that, b elow V ' 17 the giant magnitudes, but the completeness degree is always greater

branch b ecomes steep, as exp ected in metal p o or clusters, than 85 % at V<19:5. Annulus (c) is almost complete

whereas the brighter p ortion of the giant branch attens as ( 1) even at the fainter magnitudes.

in typical metal rich clusters. However, no completeness estimate has b een made b e-

The eld CMD ( g. 2b) do es not allow one to sim- low V =20:3, b ecause, at faint magnitudes, wewere in-

ply mark the attened stars as no cluster memb ers. We terested only to the determination of the mean ridge lines

Age of Old MC clusters: NGC 2257 7

note that the star 895 corresp onds to star 491 in W89

(V=15.74, B-V=1.61), di ering from our measurementby

V =0:49; (B V )= 0:18. This may b e and indica-

tion of variability, but no rm conclusions can b e drawn.

Apart from star 895, the attening of the RGB is present

also in Walker's diagram, and S83 considers the p ossibility

that the reddest stars may b e incorrectly measured or b e

variables (see refs. in S83 and followings sections). It may

b e noted that on our frames there is no sign of saturation,

nor of severe crowding a ecting these stars.

Certainly, small numb er statistics may also play a sub-

stantial role and the lackofvery red standard stars for cal-

ibration may b e resp onsible of the unesp ected attening.

However, we cannot provide a de nitive explanation on this

change of slop e of the RGB, further observational investiga-

tion are the required using near IR lters or sp ectroscopic

measurements.

Figure 6. Observed RGB luminosity function: Panel (a) shows

3.2 The Giant Branch and metal abundance in-

the di erential LF, the arrow indicates the lo cation of the RGB{

Bump; panel (b) shows the integral LF, the dashed lines are the

linear ts to the regions ab ove and b elow the RGB-Bump.

dicators

All the available estimates of the reddening in NGC We can measure suitable quantities on the RGB in order to

2257 (listed by W89) indicate a lowvalue. W89 nally get a photometric estimate of the parameters V (Sandage

adopted E (B V )=0:04 though some metho ds suggest- and Wallerstein 1960), S (Hartwick 1968) and (BV)

o;g

ed an even lower value. In the following we will adopt (Sandage and Smith 1966), and use their calibration in terms

E (B V )=0:04. of [Fe/H] to obtain indication of the metallicity of the clus-

The measure of V in NGC 2257 can b e obtained by ter. A summary of these relations can b e found in Ferraro,

lo oking at the histogram of the star counts in the HB vs the Fusi Pecci, Buonanno (1992, hereafter FFB) (see their Ta-

V magnitude. We assume as magnitude of the ZAHB, the ble 5). In order to determine these parameters let us adopt

magnitude of the lower b ound of the histogram's \b ell" of a gure for the interstellar reddening E (B V ) and the

the HB (0:20

HB HB

8 V. Testa et al.

0:05, which is in go o d agreement with the value obtained by gel and Elias 1988 (their Fig. 1), who discuss the red vari-

W89 who found V =19:12 0:05 (from V =19:03 ables in GCs, concluding that, at intermediate metal p o or

HB RR

0:05 and considering that the di erence b etween the mean abundances, the red variables have b olometric magnitudes

magnitude of the RR Lyrae and the ZAHB level is 0.09 mag). slightly less luminous than the RGB tip; in this case NGC

Using this determination and assuming E (B V )= 0:04 we 2257 should presentaV brighter than what measured on

tip

derive our CMD, and so the value of V increase to ab out 2.9,3.0

thus giving a metal abundance calibration compatible with

V = 2.60:1

the other two calibrations, and with the value found by W89.

(B V ) =0:73 0:01

o;g

More data are necessary to establish rm conclusions.

Since stars at the RGB tip are at least less reliable

S =5:90:1

(low statistic, susp ected variability,mayb e eld stars) than

Adopting the relations listed in Table 5 of FFB we obtain

the lower part of the RGB we prefer [F e=H ]= 1:7as

the mean values of metallicity for each observable:

\photometric" measure of the cluster metal abundance ne-

gleting the V metho d. The errors we asso ciate with the

[F e=H ] = 1:48 0:05

metallicity estimations are formal errors since some photo-

[F e=H ] = 1:79 0:04

(B V )

0;g

metric estimates of the cluster metallicity are, of course,

sensible functions of the assumed reddening. Would E(B-

[F e=H ] = 1:72 0:06:

V) b e decreased to 0:00, we had [F e=H ] =0:2,

(B V )

0;g

It is interesting to note that, as exp ected, the estimate

[F e=H ] =0:07., while [F e=H ] is formally indep en-

V S

obtained with V metho d disagrees with the two others;

dent on the reddening but is heavily a ected by other un-

this is the consequence of the attening of the RGB in its

certainties in the photometry (as, for example, a uncorrect

bright p ortion, but we cannot claim a high metal contentin

application of the colour eq., etc). For this reason we adopt

the cluster to justify this result. W89 states that these stars

a conservativevalue of 0:2 for the error in the metallicity.

are subluminous for a metal-p o or giant branch, so that the

In the following we will adopt [Fe/H]= 1:7 0:2.

memb ership to the cluster is questionable and no rm con-

clusion can b e established. Moreover, Olszewski et al. 1991

3.3 The RGB Bump

made sp ectroscopic measurements of two luminous giants,

namely stars LE7 (our 862) and LE8 (our 866), to get an in- In order to study the LuminosityFunction of the RGB down

dication on the metallicity and velo city of the cluster. They to the SGB we selected stars in the magnitude range 16:0 <

nd that the two stars are relatively more metal-rich than V<20:3 lying from the cluster ducial line listed in

the cluster ([F e=H ] 0:4). On the other hand, the ve Table 2 ( ranging from 0.05 up to 0.2 at di erent magnitude

reddest stars could b e variable AGB stars, according to Fro- level).

Age of Old MC clusters: NGC 2257 9

In Figure 6a,b the RGB LFs, di erential and integral,

have b een plotted as a function of the V magnitude, in order

to detect the existence of the RGB{bump. As emphasized

byFusi Pecci et al. 1990 the di erential and integral LF-

s o er two indep endent to ols to identify the RGB{Bump.

The plot shows this feature, visible in b oth LFs, lo cated at

V =18:70:1. This feature marks the evolutionary stage

at which the H{burning shell passes through the chemical

discontinuity left by the maximum p enetration of the con-

vectiveenvelop e (see Renzini and Fusi Pecci 1988 and refs.

therein). The RGB{bump has b een identi ed in several G-

Figure 7. Observed values of as a function of

B ump

GCs (Fusi Pecci et al. 1990, Ferraro et al. 1994a), thus con-

sinh (Z=0:00025). Op en circles are from Fusi Pecci et al. 1990,

Ferraro et al. 1994a and the lled circle is NGC 2257. The solid

rming the theoretical exp ectations, though with the known

line represents the relation found byFusi Pecci et al. 1990.

discrepancy in the exact zero p oint of the relation.

Coupled with the HB magnitude, V , the value of

describ e the b ehaviour of NGC 2257 we adopt the param-

V can b e used to compute the parameter V =

bump

eter de ned in Lee (1990) (LZ =(BR)=(B+V+R))

V V de ned byFusi Pecci et al. 1990. The value

bump HB

where B and R are the HB stars bluer and redder than the

we obtain for NGC 2257 is V = 0:40 0:12. Figure

bump

instability strip resp ectively and V the variables, and com-

7 shows V as a function of the hyp erb olic arcsine of

bump

pare it with other Galactic Globulars. The counts have b een

the metallicity for all the clusters for which this parameter

corrected for the completeness factor obtained as describ ed

has b een determined. Op en circles refer to the data listed by

ab ove, and the value we obtain is LZ =0:47, typical of a

Fusi Pecci et al. 1990 and Ferraro et al. 1994a for Galactic

normally \blue" HB. In order to determine the number of

Globulars, and the solid line is a b est t for those data,

stars N , N , N to insert into the ratios, we

AGB RGB HB

HB 1

0:68: V =0:33sinh

bump

followed the assumptions made by Buzzoni et al. (1983).

0:00025

In Table 4 we give the numb er of stars we counted in

As can b e seen, the result for NGC 2257 (the lled circle)

each branch. A small correction for incompleteness was ap-

ts the relation well.

plied, and the ve reddest stars ((B V ) > 1:5) close to

3.4 HB morphology and Helium abundance

the RGB tip were attributed to the RGB. The ratios com-

puted on the basis of these p opulations are in quite go o d The HB-morphology of NGC 2257 shows a concentration on

agreement with the results found for GGCs (see Buzzoni et the blue side, but it is not as extended in the blue as oth-

al 1983, Ferraro 1992a), R1=N =N =0:16 0:05, er GGC with similar metallicity. In order to quantitatively

AGB RGB

10 V. Testa et al.

Table 4. Star Counts in the various Branches.

value rises to (m M ) =18:50:2ifwe consider E (B

RGB blue HB RR HB red HB AGB

V )=0:0. From these values a mean distance of d =

134 101 37 25 21

50 10Kpc has b een assumed for NGC 2257.

R2=N =N =0:13 0:04, R = N =N =

AGB HB HB RGB

1:22 0:21, R = N =(N + N )=1:05 0:17.

HB AGB RGB

5 A TWIN-METALLICITY GGC: NGC 5897

The last two ratios can b e used to estimate the primordial

helium abundance using equation 11 and 13 by Buzzoni et al

The shap e of the RGB of NGC 2257 closely resembles that

1983. From the quoted values we get Y =0:21 0:03 and

of NGC 5897. The RGB parameters and the derived metal-

Y =0:21 0:03. These values are slighlty greater than

licity computed in Section 3.2 are in quite go o d agreement

those found by W89 (Y =0:19 0:02). As stated in Section

with those found in NGC 5897 by FFB: (B V ) =0:74,

0;g

3.2 the ve reddest stars could b e AGB stars, considering

S =5:9 and from which a metallicityof[F e=H ]=1:75

this p ossibilitywe found R1=0:20 0:06, R2=0:16 0:04,

has b een derived. In Figure 8 the mean ridge lines of NGC

R =1:26 0:21 and R =1:05 0:17, values still quite

5897 have b een overplotted on the diagram of Figure 2. A

compatible with what found in GGCs.

shift of (B V )=0:06 in colour and of V =2:75

in magnitude have b een applied to NGC 5897 ridge lines

4 THE DISTANCE OF NGC 2257

in order to takeinto account the di erent reddening and

As well known, one can use various di erent scenarios to get

distance mo dulus (since for NGC 5897 E (B V )=0:10

the absolute magnitude of the HB at the colour interval of

and V =16:35). The NGC 5897 mean ridge line repro-

the RR Lyrae variables, dep ending on the zero-p oint and

duces well the RGB in NGC 2257, except for the quoted

slop e of the adopted M vs:[F e=H ] { calibration (see for

very bright region where the RGB of 2257 attens (see Sec-

a discussion Buonanno, Corsi and Fusi Pecci 1989, hereafter

tion 3.1 and 3.2), supp orting the \photometric" indication

BCF) The absolute magnitude of the HB can vary from

of a comparable metallicity.

M =0:48 using the relation obtained byWalker (1992):

Concerning the HB morphology, in b oth clusters HB

M =0:15[F e=H ]+0:73

stars mainly p opulate the blue side of the instability strip,

even though the HB tends further blueward in NGC 5897. up to M =0:74 at [F e=H ]= 1:6 using the recent re-

In particular, NGC 2257 shows 15% of the HB stars on sults byLayden, Hanson and Hawley (1994). In the follow-

the red side of the instability strip and 23% are RR Lyrae, ing we assume the mean value b etween these two extreme

in the case of NGC 5897 almost the whole HB p opulation values M =0:61 0:13. The di erences in these two

is lo cated on the blue side with only a few RR Lyrae. We values are indicative of the uncertainties that still a ect the

can use the LZ parameter de ned in Section 3.4 in order to determinations of the distances of GGC.

quantify the di erences in the HB morphology of the two Considering that V =19:10 and E (B V )=0:04

clusters. The LZ parameter is LZ =0:87 in NGC 5897 we obtain a distance mo dulus (m M ) =18:40:2. This V

Age of Old MC clusters: NGC 2257 11

6 THE AGE OF NGC 2257

In the previous section we obtained some preliminary clues

which indicate that NGC 2257 is a few Gyrs younger than

its twin-metallicity GGC NGC 5897. Since the TO p ointis

well de ned in our CMD, in this section we will try to get

a quantitative estimate of the age of NGC 2257 appling the

currently accreditated metho ds to date GGCs.

6.1 The V metho d

The luminosity of the TO p oint is considered the \classical"

clo ck of the stellar evolution. BCF used the TO luminosity

Figure 8: Mean ridge lines of NGC 5897 shifted to match the

CMD of NGC 2257. Shifts of (B V )=0:06 and V =2:75

referred to the HB level in order to obtain a new parameter,

have b een applied.

V = V V , free by uncertainties on reddening

TO HB

while in Section 3.4 we found LZ =0:45 for NGC 2257. In and absolute calibrations.

the scenario prop osed by Lee 1993, this di erence could b e In NGC 2257 the TO can b e lo cated at V =22:40

interpreted in terms of a di erence in age b etween the two 0:10, (B V ) =0:35 0:05. From this result and

clusters, NGC 2257 b eeing 2Gyr younger than NGC 5897. the gure for the HB we obtained in Section 3.2 we nd

This results is in agreement with Figure 9 byWalker (1992) V =3:30 0:15, whichislower than the mean value

in which the old LMC clusters app ear in the mean to b e 1 < 3:55 0:09 > obtained by BCF for a sample of 18 GGCs.

Gyr younger than the mean age of the GGCs. This is a clear indication that the old LMC cluster NGC

Another di erence b etween the mean lo ci of the two 2257 is younger than the average p opulation of clusters in

clusters is visible in the TO and SGB region, though the the Galaxy.

photometry in NGC 5897 is not deep enough to reach the In order to have a quantitative estimate of the age of

Main Sequence. From Figure 8 it can b e seen that the mean NGC 2257, with resp ect to a similar metallicity cluster in

ridge line of NGC 5897 do es not repro duce the shap es of the Galaxy,we follow the prescriptions presented by Buo-

the SGB and the TO in NGC 2257, since the SGB shap e nanno et al (1993), who prop osed to use V in a di er-

lo oks \smo other" in NGC 5897 than in NGC 2257 and the ential sense, (i.e. referring to another cluster, with compa-

TO of NGC 2257 seems to b e brighter and bluer than that rable metallicity,chosen as reference). They de ned the so-

HB HB

of NGC 5897, suggesting again that NGC 2257 is slightly called vertical parameter as = ( V V ) where

1 2

TO TO

HB HB

younger than NGC 5897 (see the discussion on the age in V and V are the parameters for the programme

1 2

TO TO

Section 6). and the reference cluster, resp ectively. They presented also

12 V. Testa et al.

Figure 9: The comparison with theoretical iso chrones by Straniero and Chie 1991. The mean ridge line for NGC 2257 (small circles)

is compared with two iso chrones at 10 Gyrs (solid line) and 14 Gyrs (short dashed line).

a calibration of this di erential parameter in terms of di er- In the previous section we identi ed NGC 5897 as the

ences in age, using the VandeBerg and Bell mo dels (1985): metallicitytwin of NGC 2257. Though the FFB's CMD of

NGC 5897 do es not reach the MS, the TO can b e roughly

logt =(0:44+0:04[F e=H ]) (1)

lo cated at V =19:85 0:10 giving V =3:50, so that,

where t indicate the age expressed in Gyr.

for the cluster pair NGC 2257-NGC 5897 the parameter is

An analogous \horizontal " parameter (B V ) has b een

=3:30 3:50 = 0:20. By using this value in the relation

prop osed byVandenBerg, Bolte and Stetson (1990) and

(1) we get logt = 0:074 which means t = 2:6Gy r

Sara jedini and Demarque (1990). This metho d uses the

(for t =15Gy r ). On the other hand, another well studied

colour di erence b etween the main sequence TO and the

GGC could b e used as reference cluster having almost the

base of the giant branch as age-indicator (see also Chie and

same metallicity as NGC 2257: M 3 (Buonanno et al 1994a),

Straniero 1989). Since our photometry do es not reach the

=3:52, for which[F e=H ]=1:66. In this case V

MS with the accuracy needed to safely apply this metho d,

and the parameter for the cluster pair NGC 2257-M 3

we will base our analysis only on the parameter used by

is=3:30 3:52 = 0:22, giving logt = 0:08, i.e.

BCF and Buonanno et al 1993.

Age of Old MC clusters: NGC 2257 13

t = 2:8Gy r . Both these results indicate that NGC 2257 ty estimates which suggest that NGC 2257 is an intermedi-

is ab out 2-3 Gyr younger than the mean clusters p opulation ate metal p o or cluster. This can b e further investigated by

with comparable metallicity in the Galaxy. comparing the CMD with available iso chrones for old stellar

In the recent past, some GGCs have b een found to b e y- p opulations.

ounger than the average globulars p opulation in the Galaxy: Adopting the distance mo dulus from Section 4 one nds

Rup 106, Buonanno et al 1990, Arp 2 Buonanno et al 1994b, that only iso chrones with [Fe/H] ' -1.7 are able to repro duce

IC 4499 Ferraro et al 1994b (in preparation), Terzan 7 Buo- the slop e of the ma jor p ortion of the red giant branchupto

nanno et al 1994c, (in preparation). In particular Arp2 and V'17, supp orting the adopted cluster metallicity.

IC 4499 have metallicity( [F e=H ] 1:7) comparable with The reddening:

that assumed for NGC 2257 and are thus very interesting In order to get a complete overlapping of the data with

reference galactic clusters to investigate the age of our "old" the iso chrones wehave to assume a a reddening even lower

LMC cluster. Since Arp 2 has V =3:29 the vertical than the low one suggested by W89 E (B V )=0:04, so

parameter is = 0:01, corresp onding to logt =0:004 that our b est t is obtained assuming E (B V )=0:0. A

and an age di erence of 0.1 Gyr. In the case of IC 4499, value whichwould supp ort the value found by W89 on the

we obtain = 0:05 (since V =3:25) corresp onding to basis of RR Lyrae.

logt =0:019 and an age di erence of 0.6 Gyr. The age:

These last twoevaluations of the vertical parameter in- As discussed in the previous section there are some in-

dicate that the ages of the oldest globulars in the LMC are dications that NGC 2257 is younger than the mean GGC

similar to those of the p eculiarly young GGC subsample p opulation. On these basis gure 9 compares the mean lo ci

recently discovered by Buonanno et al and Ferraro et al. of NGC 2257 to two iso chrones computed for [Fe/H] = - 1.7

This evidence suggests a p ossible connection b etween GGC- and for the two alternative assumptions ab out the cluster

syoung subp opulation and the globulars in the LMC (Fusi age t = 10 and t = 16 billion year. One nds that the quot-

Pecci et al 1994, in preparation). ed distance and reddening drive theoretical prescriptions to

nicely overlap observation, constraining in particular the age

of the cluster within the already quoted limits and suggest-

6.2 The direct comparison to iso chrones

ingavalue t 13Gy r .

More light on the metallicity and age determination in NGC

2257 can b e done by direct comparison of the CMD to the

7 CONCLUSIONS

iso chrones.

The metal licity question: The analysis p erformed throughout this pap er allows us, for

As discussed in paragraph 3.3 the tip of the RGB shows the rst time, to reach a clear insight on the problem of

a p eculiarly low slop e, in spite of all the available metallici- the age of old MC clusters. NTT photometry going down

14 V. Testa et al.

2 magnitudes b elow the TO p oint gives indeed rather Buonanno R., Corsi C.E., Buzzoni A., Cacciari C., Ferraro F.R.,

Fusi Pecci F., 1994a, A&A, in press.

rm constraints at least on the age of the cluster relative

Buonanno R., Corsi C.E., Fusi Pecci F., Richer H.B., Fahlman

to Galactic Globulars, suggesting that this age should b e

G.G, 1994b, AJ, in press

safely estimated around 13 Gyrs (i.e. is ab out 2{3 Gyr

Buonanno R., et al, 1994c, in preparation

younger than typical Galactic Globulars with comparable

Buzzoni, A., Fusi Pecci, F., Buonanno, R., Corsi,C.E., 1983,

metallicity). Further investigations are obviously needed for

A&A, 123, 94

other MC clusters to address some fundamental questions

Chie A., Straniero O., 1989, ApJS, 71, 47.

like whether old MC clusters are really co eval and, in any

Ferraro F.R., 1992a, in Star Clusters and Stellar Evolution, E.

case, to compare the derived MC clusters' age with similar

Bro cato, F.R. Ferraro, G. Piotto eds., Mem. SAIt., vol. 63,

ages for the GGCs system.

n.1, p.95.

Ferraro, F.R., 1992b, in New Results on Standard Candles, F.

ACKNOWLEDGMENTS

Caputo and O. Straniero eds., Mem. SAIt., vol. 63, n.2, 491.

Ferraro, F.R., Clementini, G., Fusi Pecci, F., Buonanno, R.,

Weacknowledge Dr. A.R. Walker for helpul comment and

Alcaino, 1990, A&ASS, 84, 59

suggestions. This work was partially supp orted by the Ital-

Ferraro, F.R., Clementini, G., Fusi Pecci, F., Buonanno, R.,

ian Ministry for the University and the Scienti c Research

1991, MNRAS, 252, 357

(MURST). This pap er is based on observations obtained at

Ferraro, F.R., Fusi Pecci, F., Buonanno, R., 1992, MNRAS, 256,

the Europ ean Southern Observatory, La Silla, Chile.

376 (FFB)

Ferraro F.R., Fusi Pecci F., Guarnieri M.D., Moneti A., Origlia

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This pap er has b een pro duced using the Blackwell

Scienti c Publications T X macros. E