An HST/WFPC Survey of Bright Young Clusters in M31. II. Photometry of Less Luminous Clusters in the Fields

Abstract

We report on the properties of 89 low mass star clusters located in the vicinity of luminous young clusters (blue globulars) in the disk of M31. 82 of the clusters are newly detected. We have determined their integrated magnitudes and colors, based on a series of Hubble Space Telescope Wide Field/Planetary Camera 2 exposures in blue and red (HST filters F450W and F814W). The integrated apparent magnitudes range from F450W = 17.5 to 22.5, and the colors indicate a wide range of ages. Stellar color-magnitude diagrams for all clusters were obtained and those with bright enough stars were fit to theoretical isochrones to provide age estimates. The ages range from 12 Myr to >500 Myr. Reddenings, which average E(F450 - F814) = 0.59 with a dispersion of 0.21 magnitudes, were derived from the main sequence fitting for those clusters. Comparison of these ages and integrated colors with single population theoretical models with solar abundances suggests a color offset of 0.085 magnitudes at the ages tested. Estimated ages for the remaining clusters are based on their measured colors. The age-frequency diagram shows a steep decline of number with age, with a large decrease in number per age interval between the youngest and the oldest clusters detected. Comment: 20 pages, 9 figures

Full text

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An HST/WFPC Survey of Bright Young Clusters in M31.
II. Photometry of Less Luminous Clusters in the Fields

P. W. Hodge1, O. K. Krienke2, M. Bellazzini3, S. Perina3, P.Barmby4, J. G. Cohen5, T. H.
Puzia6, and J. Strader7

1Department of Astronomy, University of Washington, Seattle, WA 98195-1580, USA
2 Seattle Pacific University, Seattle, WA 98119, USA
3 INAF-Osservatorio di Bologna, via Ranzani 1, 40127 Bologna, Italy
4 Department of Physics and Astronomy, University of Western Ontario, London, ON
N6A 3K7, Canada
5Palomar Observatory, Mail Stop 105-24, California Institute of Technology,
Pasadena, CA 91125, USA
6Herzberg Institute of Astrophysics, 5071 West Saanich Rd., Victoria, BC, V9E 2E7,
Canada
7 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, 01238, USA

Key words: galaxies: individual (NGC 224, M31, Andromeda) - (galaxies:) Local Group -
galaxies: star clusters - galaxies: stellar content


ABSTRACT. We report on the properties of 89 low mass star clusters located in the
vicinity of luminous young clusters (“blue globulars”) in the disk of M31. 82 of the
clusters are newly detected. We have determined their integrated magnitudes and colors,
based on a series of Hubble Space Telescope Wide Field/Planetary Camera 2 exposures
in blue and red (HST filters F450W and F814W). The integrated apparent magnitudes
range from F450W = 17.5 to 22.5, and the colors indicate a wide range of ages. Stellar
color-magnitude diagrams for all clusters were obtained and those with bright enough
stars were fit to theoretical isochrones to provide age estimates. The ages range from 12
Myr to >500 Myr. Reddenings, which average E(F450 – F814) = 0.59 with a dispersion
of 0.21 magnitudes, were derived from the main sequence fitting for those clusters.
Comparison of these ages and integrated colors with single population theoretical models
with solar abundances suggests a color offset of 0.085 magnitudes at the ages tested.
Estimated ages for the remaining clusters are based on their measured colors. The age-
frequency diagram shows a steep decline of number with age, with a large decrease in
number per age interval between the youngest and the oldest clusters detected.

1. INRODUCTION

This paper reports on the study of open (disk) star clusters in M31 (NGC224) detected on
images from the Hubble Space Telescope, obtained as part of a program designed to

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determine the nature of 19 luminous star clusters that were originally classified as
globular clusters, but which have blue measured colors. The first paper of a series that
report on the results of that program concerns the highly luminous young cluster vdB0
(Perina et al. 2009). Here we present a survey of less-luminous (“open”) clusters in M31,
similar to those of Krienke and Hodge (2007, hereafter KHI), who reported results from
archival images obtained with the Wide Field/Planetary Camera 2 (WFPC2), and Krienke
and Hodge (2008, hereafter KHII), who reported results from archival images from the
Advanced Camera for Surveys (ACS).

“Open” or “disk clusters” in M31 have been recognized since Hubble’s pioneering work.
He identified the cluster subsequently known as vdB0 as an open cluster, as shown in the
frontispiece of his book, “The Realm of the Nebulae” (Hubble 1936). Most subsequent
studies of such clusters have dealt with the more luminous examples, especially those
mistaken for globulars; see an excellent history of the subject of M31’s luminous blue
clusters in Caldwell et al. (2009).

As in Paper I, we adopt a distance modulus for M31 of (m – M)0 = 24.47 +/- 0.07.

2. OBSERVATIONS

1 The Images

The observations, obtained with the Wide Field Planetary Camera 2 (WFPC2) of the
Hubble Space Telescope (HST), were described in detail in Perina et al. (2009). The
images were obtained with blue (HST F450W) and red (HST F814W) filters,
approximately in the traditional B and I bands. Exposures were relatively short (2x400
seconds per filter). The scale of the WF fields is 0.099 arcsec/px and for the PC fields it is
0.045 arcsec/px. While the main program dealt with the bright globular-like clusters on
the PC images, we searched both the PC and the WF images, identifying star clusters,
measuring their integrated properties and carrying out stellar photometry of their member
stars. Figure 1, in a color version produced by one of us (TP), reproduces a sample WF
field showing several open clusters. The total area covered by the survey is 48.1 arc
minutes2.

2 Cluster Identification

The clusters included in the survey range from large, very luminous clusters to small
objects that are barely resolved in our rather short exposures. The brightest disk clusters
in this sample have absolute magnitudes of M(F450)0 = -8, while we were able to identify
a few clusters as faint as M(F450)0 = -2.5. Thus our brightest clusters are equivalent to
the mean absolute magnitudes of M31’s globular clusters (though bluer and less
massive), while our faintest are fainter than the faint limit of most cluster catalogs for
nearby galaxies.

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Figure 1. A sample WF image, containing several recognizable star clusters. This figure
demonstrates how clusters are distinguished by their resolution, high stellar density and
blue color, compared to the background of the M31 disk stars.

The disk of M31 presents a dense star field, in which low-density star clusters are
difficult to detect even with special statistical techniques. For that reason we chose to
select only conspicuous objects for which there would be little or no question of their
being physical clusters (see examples in Figure 2). Our cluster identification criteria
included:
1. a conspicuous spatial concentration
2. a centrally-peaked radial distribution
3. detectability in both colors
4. recognition of more than four well-resolved stars above an unresolved
background

Page 4

5. a normal luminosity distribution (number increasing with magnitude)
6. a color-magnitude diagram that shows a distribution different from that of the
background

Two of the authors (PH and OKK) searched the frames independently in both colors,
varying brightness and contrast. We categorized objects as definitely clusters or as
candidates, and for borderline cases, we met, discussed images, and reached agreement.
As a final test, we asked each other whether we could defend an object against being
classed as an asterism, background galaxy or other type of non-cluster. Figure 2 provides
F450 images of 12 of the clusters.





Figure 2. Images of 12 of the brightest clusters in the sample. Each small field is 7 arcsec
on a side, except for cluster 12, for which the sides are 14 arcsec. The images are from
the F450W filter and the WF camera.

3. DATA REDUCTION




3.1 Integrated Photometry

We determined integrated magnitudes and colors of the clusters using a photometric
program written by Krienke in IDL and described in detail in KHI (2007). Magnitudes in
the HST photometric system were calibrated according to the results of Holtzman et al.
(1995). The program determines the cluster properties within a contour chosen to include
most of the light, but omitting any bright foreground stars. The critical feature of the

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photometry is determining the background surface brightness (the “sky”). Because many
of the clusters have both a low surface brightness and a significant size, the M31
background is often a significant fraction of the measured signal. Our program measures
both a probable background level and determines the uncertainty of it by sampling
several (10 to 24) similarly-dimensioned fields on the image These data are refined by
Chauvenet criteria, rejecting samples with less than 0.02 probability of belonging to the
set. The average of the remaining values of the background is then flux subtracted from
the total flux within the cluster contour. The correction to the magnitudes due to the
background subtraction was usually several tenths of a magnitude, but in some cases,
where the cluster surface brightness was especially faint compared to the background, it
reached values as large as 2 magnitudes (see Figure 3). Clearly, the background
correction is an important element in this photometry and it is essential that it and its
uncertainty be evaluated carefully. The photometric uncertainties provided in Figure 4
and Table 1 include that of the background, which in some cases dominates the
uncertainty.



Figure 3. The background corrections plotted against the corrected integrated F450
magnitudes of the clusters. Magnitudes are not reddening-adjusted.

Page 6

Figure 4. Photometric errors derived from the measurements of the integrated
magnitudes, uncorrected for reddening. Filled symbols are for the F450 data and open
symbols are for the F814 data.



3.2 Stellar Photometry

We carried out two independent programs of stellar photometry of the clusters. In one
case, the entire WFPC2 images of each field were measured at Bologna as part of the
luminous young clusters program. The details of that photometry are given in Paper I
(Perina et al. 2009). For this paper we have extracted from the Bologna database the
magnitudes and colors of stars within our outline of a cluster’s boundary. Following the
practice of Perina et al. (2009), we provide HST Vega magnitudes as measured in the two
filters, which we refer to in the following as “F450” and “F814”.

A second photometric program was carried out in Seattle using a program developed by
one of us (OKK), based on DAOPHOT (Stetson 1994) and written within IDL. It was

Page 7

adjusted to allow us to measure stars in the more crowded central areas of clusters, where
there often are bright stars, frequently including the brightest main sequence stars in the
cluster. Without at least approximate photometry of these stars, we would be missing
important information about the ages of the clusters. Zero points were adopted from
Holtzman et al. (1995). PSFs were derived from several bright, well-separated stars in the
field.

A comparison of the magnitudes and colors of the two sets of photometry showed good
agreement. We identified stars in common by using both magnitudes and positions,
finding that most bright stars were easily identified, while for faintest stars there was
sometimes an ambiguity. For stars with F450 magnitudes brighter than 23.0 the mean
differences (Bologna-Seattle) were -0.12 ± 0.05 magnitudes in F450 and -0.13 ± 0.11
magnitudes in F814. At fainter magnitudes, where the photometry is strongly affected by
crowding and by the short exposures of the images, the dispersion is larger. We have
adjusted the Seattle photometry to the Bologna system by using the above offsets


4. PROPERTIES OF THE CLUSTERS

4.1 The Cluster Catalog

Table 1 provides the positions, integrated magnitudes and integrated colors of the
clusters. Five` of the clusters were found to have been identified previously according to
the Revised Bologna Catalog of M31 Globular Clusters (Galleti et al. 2004, hereafter
RBC). One of them, DAO84, was identified as a possible galaxy by Caldwell et al.
(2009), but our images show a clearly-defined star cluster. Additionally, one coincides
with an open cluster identified in KHI (2007) and one to a cluster

Table 1
Star Clusters of the Survey
Name RA (J2000) Dec. F450 err F450 – F814 err
KHM31-
22 9.99416 40.59044 20.36
1 10.00226 40.59630 20.00
B319 10.01277 40.56638 17.77
WH 10.03147 40.58568 20.75
2 10.05996 40.47970 21.10
3 10.06724 40.46574 20.87
4 10.07673 40.46278 20.23
5 10.08475 40.47733 21.29
6 10.09359 40.46366 22.10
7 10.10565 40.61191 21.23
8 10.12093 40.60816 20.31
9 10.12172 40.62505 20.68
10 10.12880 40.62470 20.26
11 10.13828 40.61543 21.08
12 10.14448 40.61308 18.00
13 10.15506 40.65390 19.36
0.03
0.04
0.01
0.05
0.05
0.07
0.03
0.05
0.04
0.13
0.03
0.08
0.04
0.10
0.08
0.02
1.38
1.48
0.89
0.64
0.11
0.72
0.93
0.81
0.50
-1.01
0.67
0.30
0.01
1.06
1.42
1.47
0.07
0.05
0.04
0.09
0.12 *y
0.11 y
0.06 y
0.10 y
0.13 y
0.18 *y
0.07 y
0.13 *y
0.11 *y
0.15
0.09 y
0.05 y

Page 8

14
15
10.15727 40.66958
10.17087 40.65345
10.25410 41.10937
10.25739 41.12103
10.26360 41.11692
10.27091 41.11649
10.27805 41.12904
10.31100 41.11747
10.32247 41.11345
10.32486 41.10686
10.32638 41.09547
10.40369 40.72710
10.40514 40.68031
10.41120 40.73322
10.41445 40.67577
10.41904 40.72756
10.42279 40.66916
10.42782 40.71453
10.43303 40.71460
10.43314 40.71762
10.43358 40.71122
10.43870 40.72325
10.44996 40.71653
10.45031 40.69453
10.45168 40.69946
10.45521 40.72142
10.45635 40.73367
10.46038 40.70244
10.51435 40.76969
10.51689 40.74818
10.52399 40.77104
10.52901 40.76606
10.52987 40.76940
10.53052 40.77541
10.53562 40.77516
10.55479 40.82819
10.57024 40.81240
10.57764 40.81500
10.57851 40.81922
10.63578 41.36173
11.10224 41.25305
11.11621 41.23792
11.12238 41.23356
11.22630 41.88489
11.23180 41.91120
11.23438 41.89684
11.23474 41.89572
11.23536 41.88171
11.23619 41.91635
11.24062 41.89716
20.83
20.96
19.60
21.01
21.11
20.42
19.41
22.03
20.69
21.40
21.88
21.31
20.56
18.55
19.81
21.63
20.19
19.66
21.08
21.09
20.89
20.38
20.79
21.05
19.16
20.66
21.08
20.58
20.14
21.25
21.15
20.84
19.17
20.95
19.70
20.63
20.76
22.11
19.89
19.41
20.34
20.96
22.21
21.28
20.29
22.04
20.11
20.45
20.41
22.12
0.04
0.06
0.02
0.05
0.04
0.02
0.12
0.08
0.10
0.09
0.05
0.04
0.07
0.02
0.01
0.03
0.03
0.02
0.04
0.05
0.04
0.04
0.04
0.06
0.02
0.04
0.26
0.04
0.03
0.03
0.04
0.07
0.02
0.04
0.02
0.04
0.04
0.06
0.06
0.03
0.02
0.02
0.04
0.04
0.02
0.07
0.06
0.04
0.05
0.10
1.71
0.74
1.63
1.14
1.15
1.16
1.23
1.23
1.95
1.18
1.60
-0.36
1.42
0.21
1.11
-0.95
0.92
0.69
0.12
1.22
2.04
1.33
0.70
0.59
0.38
0.27
0.06 y
0.11 *
0.04
0.09 y
0.08
0.04 y
0.21 y
0.14 y
0.16 y
0.12 y
0.10
0.12 *y
0.10 y
0.07 *y
0.03 y
0.12 *y
0.07 y
0.07 y
0.11 *
0.09 y
0.09
0.08 y
0.11 y
0.10 y
0.07 y
0.10 *y
B014D
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
B061D
52
53
54
55
56
57
58
59
60
61
y
0.69
1.53
0.82
1.22
1.58
0.71
0.37
0.71
1.20
1.22
1.08
1.35
0.67
1.73
2.11
1.86
0.19
0.92
2.31
1.15
2.06
1.80
1.38
0.09 y
0.08
0.09 y
0.09 y
0.09 y
0.07 y
0.10 *y
0.07 y
0.09
0.10 y
0.11
0.09
0.09 *
0.05
0.03
0.08
0.10 *
0.04
0.12
0.11 y
0.05
0.08
0.14 y

Page 9

B256D
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
DA084
11.24448 41.91018
11.24560 41.89819
11.24637 41.91047
11.24650 41.91050
11.24744 41.89167
11.24854 41.90391
11.24969 41.93580
11.24973 41.90117
11.25109 41.90682
11.25216 41.88646
11.25366 41.88541
11.25606 41.89460
11.25914 41.91537
11.26204 41.89759
11.26219 41.90101
11.26942 41.89441
11.28053 41.90742
11.28957 41.91235
11.29089 41.91942
11.43302 41.72510
11.45692 41.71174
11.45853 41.70832
11.46799 41.71365
17.57
20.09
19.05
18.87
21.55
20.21
20.85
21.32
21.06
20.48
19.87
21.76
19.97
20.52
20.38
20.02
21.67
21.56
20.10
19.63
22.35
22.23
19.59
0.02
0.06
0.02
0.03
0.07
0.09
0.06
0.13
0.09
0.04
0.04
0.13
0.04
0.09
0.08
0.06
0.06
0.06
0.05
0.03
0.07
0.06
0.06
1.58
0.84
1.93
1.88
-0.84
1.43
0.78
1.06
1.17
1.17
0.85
0.76
1.31
0.67
-0.08
1.03
0.83
0.61
1.50
0.55
1.85
1.61
0.81
0.03
0.10 y
0.02
0.05
0.13 *y
0.12 y
0.10 y
0.17 y
0.19 y
0.10 y
0.08 y
0.17
0.07
0.12 y*
0.16 *y
0.11
0.11 y
0.10 y
0.07 y
0.08 y
0.11
0.13
0.14


NOTES: Objects with asterisks have uncertain colors because of a low ratio of signal to galaxy
background in the F814W image. Objects with “y” have CMDs indicating young ages, less than
~5X108 years.

discovered by Williams and Hodge (2001b). Only two of the previously-identified
clusters, B319 and KH22, had published magnitudes in B and only B319 had previously-
published magnitudes in both B and I. We transformed our magnitudes to Johnson-
Cousins B and I for comparison. The average difference (previous – this paper) in B was
found to be 0.16 mag. and the difference in I is 0.18 mag.

As a ground-based check on the HST photometry, one of us (JS) determined the
integrated magnitudes and colors of 16 of the brighter clusters from the SDSS database.
Measures were obtained in the SDSS system (u,g,r,i,z) and transferred to B and I in the J-
C system. All measures used a circular aperture with a radius of 4 arcsec. The measures
produced data that agreed fairly well, with mean differences (CfA-Seattle) of ΔB
= -0.24 +/- 0.39 and Δ(B – I) = 0.23 +/- 0.14. Experiments with HST photometry using a
4 arcsec aperture indicated that the differences are probably caused at least partly by
nearby bright stars that were avoided by the original HST photometry, which used
smaller apertures.


4.2 The Integrated Cluster Color-Magnitude Diagram

Page 10

Figure 5 shows the color-magnitude diagram (hereafter CMD) of the present sample (we
include in this diagram and in Figure 6 two clusters from the main target program, which
were found serendipitously on the WF frames). It closely resembles the two diagrams
published for similar samples of M31 clusters by KH I and II (2007, 2008), though with
different filter pairs. The mean absolute magnitude for the cluster sample plotted is
M(F450)0 = -4.59 and the mean unreddened color is (F450-F814)0 = 0.67.



Figure 5. The color-magnitude diagram for the integrated colors and magnitudes of
clusters in this survey. The plot shows observed values, before corrections for reddening.

The clusters are nearly uniformly distributed over the diagram, but with a mild
concentration at about F450 = 21 and F450 - F814 = 1. For reference, a cluster with
observed values of F450 = 21.0 and F450 – F814 = 1.0 will have an age of about ~70
Myr and a mass of 450 solar masses, assuming a Salpeter stellar luminosity function and
Giardi (2006) population models. But note that the age-color diagram is multi-valued at
these colors (see Section 5.2).

The mean size of the isophotal radii of all clusters was 1.61 arcsec (6.12 pc).

4.3 The Integrated Cluster Luminosity Function

The luminosity function of the clusters is shown in Figure 6, where the magnitudes are
corrected for extinction, assuming a mean reddening of F450 – F814 of 0.51 (see Section
6). The shape of the luminosity function is approximately Gaussian, with a maximum at

Page 11

M(F450)(0) = -4.2. All three samples show an enhanced frequency at the bright end,
compared to a symmetrical curve. Artificial cluster tests on the WFPC2 HST images in
KHI indicated that much of the turn-down at faint magnitudes results from detection
limits. It is not yet clear what the shape of the true luminosity function is at such faint
limits. While KHI suggested that the luminosity function may continue to rise, at least to
M(F450) = -1, similar HST searches for faint clusters in the SMC have produced contrary
results (Rafelski and Zaritsky, 2005). In any case, the luminosity function at the faint end
is a complicated product of selection effects, evolutionary fading rates and dynamical
disruption (Hunter et al. 2003).



Figure 6. The luminosity function for the clusters of this survey (solid line) compared to
that of KHI (2007) (dotted line) and KHII (2008)(dashed line). The latter two are
normalized to the total number of clusters in the present survey.

4.4 Individual Cluster CMDs

As described in Section 3.2, we measured stellar CMDs for all clusters. Most diagrams
looked reasonable, but not all of the clusters were well-enough resolved to allow
meaningful interpretation. Especially for the faintest clusters, the number of stars on the
F814 frame was often quite small, on the order of 5 to 10.

Page 12

Figure 7 shows the CMDs for 10 clusters for which the CMDs show a well-defined main
sequence. These clusters show a main sequence with F450 – F814 near 0.5 and with the
tip of the main sequence in the range with F450 magnitudes = 20 to 24. The CMDs in
Figure 7 have been adjusted for reddening (see Section 5.1).

Table 2 lists the clusters for which it was possible to determine age and reddening by
comparison with the Girardi models. The quoted uncertainties indicate the extreme limits
of acceptable fits judged by eye.

Page 13

Figure 7. Color-magnitude diagrams for 10 young clusters with well-defined main
sequences, fitted by eye to Girardi (2006) isochrones for solar abundance and ages with
log(age) of 7.0, 7.6, 8.0, 8.25, and 8.7 years.

One of the clusters, B319 (also known as G44) has been studied previously using other
HST images (Williams and Hodge, 2001a). The present CMD is shallower and it covers
only the central region of B319, but the two CMDs are morphologically similar. We
cannot usefully make detailed comparisons because the Williams and Hodge data were
taken with different filters (F 336W, F439W and F555W).

A careful inspection of the CMDs of the clusters and their surrounding fields shows that
the degree of contamination of the cluster MS by field stars is negligibly low and does
not affect our estimates of age and reddening.

Table 2
Characteristics of Cluster CMDs with Well-Defined Main Sequences

cluster
no.
log
age(yrs) uncertainty
E(F450-
F814) uncertainty

Page 14

KH22
B319
3
5
8
11
12
13
18
34
37
45
B061D
58
62
68
74
75
80
7.6
7.6
7.5
8.0
7.5
7.3
7.6
7.1
7.1
8
7.9
7.8
7.8
7.6
8.0
7.8
8.1
7.8
7.1
0.35
0.5
0.45
0.6
0.35
0.6
0.6
0.5
0.35
0.45
0.35
0.3
0.6
0.2
0.2
0.3
0.3
0.5
0.45
0.4
0.5
0.8
0.5
0.55
0.5
0.55
0.85
0.5
0.65
0.5
0.5
0.5
0.8
0.25
0.82
0.65
0.5
0.75
0.15
0.25
0.2
0.3
0.2
0.2
0.25
0.8
0.2
0.25
0.25
0.15
0.15
0.15
0.15
0.15
0.15
0.25
0.15

5. AGES AND REDDENINGS

5.1.

For clusters with sufficiently well defined sequences of stars, especially young clusters
with narrow main sequences, it was possible to determine approximate reddenings and
ages. Based on the case for vdB0 (Perina et al. 2009), we assumed that these young
clusters are characterized by solar abundances. We compared the observations with
evolutionary model isochrones made available from the Padua webpage (Girardi 2006)
and determined the offset by eye, providing approximate values of age and reddening
(Table 3). Because of the faintness of the magnitudes, the crowding and the sparseness of
the CMDs, these values have fairly large uncertainties, as quoted in the table. Within the
accuracy of the fitting and if our assumption of solar abundances is correct, the fits
provide individual reddenings for the selected clusters, which range from E(F450-F814)
= 0.25 to 0.85, with a mean uncertainty of 0.23. The average reddening for this sample is
0.59 with a standard deviation of 0.21 magnitudes. Selection effects, of course, severely
limit our sample of clusters with bright main sequences to the youngest clusters in the
sample; most are younger than 200 million years.

For the remaining clusters in the sample, the color-magnitude diagrams are difficult to
interpret in terms of ages and reddenings except in approximate terms. Table 1 notes
those clusters that have significant numbers of stars in the blue section of their CMDs to
indicate that they are younger than a few times 108 years. Most of the remaining clusters
are older, as is also indicated by their integrated colors.

5.2 From the Integrated Cluster Photometry

From the CMDs

Page 15

Integrated colors of open clusters can be used to estimate cluster ages by comparison with
theoretical models. There are a number of problems with this procedure in our case:

a) the colors are intrinsically uncertain because of the spatially-variable brightness and
color of the M31 background, which is the major source of the photometric uncertainty.
b) the theoretical models show a dependence on the elemental abundances, which are
unknown.
c) for young small-mass clusters, the colors depend on small number statistics in the
presence or absence of the most luminous blue stars or a few red giants (see Frogel,
Cohen and Persson, 1983 and Cervino and Luridiana, 2004, for quantitative treatments of
this problem).
d) different theoretical models, even for the same abundances, give different relationships
for the age-color diagram.
e) for the colors used in this program (F450 and F814), the change with color for young
clusters (<2x108 yrs.) is multi-valued for some regimes and is generally smaller than the
measurement uncertainties (Figure 8).



Figure 8. Ages and reddening-corrected colors determined from MS fitting compared to
the theoretical age-color relationship for young clusters (Girardi 2006).

In spite of these difficulties, it is possible to estimate approximate ages from the colors
and, for the younger clusters, the average reddening. Figure 8 shows the colors of the