Baltic Astronomy, vol. 6, 3 - 10, 1997.
SOME INTERESTING COLOR-MAGNITUDE DIAGRAMS
Peter B. Stetson
Dominion Astrophysical Observatory, Hertzberg Institute of Astrophysics, National
Research Council of Canada, 5071 W. Saanich Rd., Victoria, B. C., Canada
Received July 2, 1996
ABSTRACT. I present color-magnitude diagrams that some of my colleagues
and I are currently working on.
Key Words: globular clusters: individual - open clusters: individual galaxies: individual: dwarf spheroidal - techniques: photometric
1. INTRODUCTION
Figs. 1 - 7 present some CCD color-magnitude diagrams for a few open
clusters, globular clusters and dwarf spheroidal galaxies in the Local Group.
M y colleagues and I are grateful to the TACs of CFHT, CTIO, DAO and
KPNO for granting us the time to do these observations.
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Peter Β. Stetson
4
Ο
0.5
1
1.5
V-I
2
2.5
Fig. 1. This calibrated V, I color-magnitude diagram of the open cluster
N G C 7789 was derived by University of Victoria student Munhwan Gim based
on observations made on the D A O 1.8 m telescope by himself and by David
Zurek, formerly a student at UVic. The giant branch and core helium-burning
clump of this rich two Gyr-old cluster are particularly well defined. The cluster
main-sequence disappears into an ocean of thick-disk turnoff stars for 1.0 r ^
V —I <, 1.5, but may be perceived again near V « 20; we hope to obtain
proper motions to establish membership probabilities.
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Some Interesting Color-Magnitude Diagrams
Π
8
1
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I
1
5
I
I
I
I
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NGC 2 8 1 8
10
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16
18
20
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0
1
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b-r
Fig. 2. This instrumental b, r color-magnitude diagram for the open
cluster N G C 2818 is based on observations that I made with the GTIO 0.9 m
telescope. N G C 2818 is the only open cluster known with a planetary nebula
as a probable cluster member. I have scanned many archival plates of this
cluster with the D A O PDS, and hope soon to derive proper-motion
probabilities that will help to distinguish between true cluster members and
fore- and background disk stars. An improved estimate of the cluster distance
and turnoff mass should help constrain models of planetary nebulae.
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6
Peter Β. Stetson
Fig. 3. This calibrated Β, I color-magnitude diagram for the globular
cluster Μ 13 = NGC 6205 is based on observations made by Don VandenBerg
and me with the prime focus camera at CFHT. A 2 χ 2 mosaic of 2k χ 2k
images permitted the measurement of 105 stars in an area some 13y on a side.
Here are plotted only 22,591 stars, selected to lie more than 2.4y from the
cluster center, and to have σ(Β-Ι) < 0.05 mag. The observed thickness of the
giant branch suggests a reddening gradient across the field of order ΔΕ(Β-I) =
0.004 mag/arcmin =» ΔΕ(Β - V) ~ 0.002 mag/arcmin. The asymptotic giant
branch seems unexpectedly sparse. The brightest giant branch tip stars are not
plotted because they are saturated in the CCD images.
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Sowie Interesting Color-Magnitude Diagrams
7
Fig. 4. This instrumental b, i color-magnitude diagram for Μ 92 - N G C
6341 comes from the same observing run as Fig. 3. A mosaic of 2K χ 2K images
permitted the measurement of some 5 χ 104 stars. Here are plotted only stars
with a(b - i ) < 0.05 mag, at all clustocentric distances. The fringe of stars above
the turnoff and subgiant branch at ν — 17.5 probably consists of blended stars
near the cluster center.
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8
Peter Β. Stetson
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0.5
B-V
1
15
2
Fig. 5. This calibrated Β, V color-magnitude diagram for the dwarf
spheroidal galaxy in Draco is based on observations made by me and many
collaborators (in alphabetical order: M. Bolte, B. Carney, W. E. Harris, J.
Hesser, R. D. McClure, P. Seitzer and D. A. VandenBerg) with the CFHT and
the KPNO 4 m and 0.9 m telescopes. Unlike the case with Μ 92, crowding is
not an issue here. Therefore it is quite possible that the semi-detached fringe of
stars above the subgiant branch represents a secondary burst of star formation
a Gyr or more after the primary burst. What of the stars extending brightward
and blueward of the turnoff? Are they "ordinary" blue stragglers, or could they
represent a third (and fourth?) generation of stars? Note that there are a number
of stars lying above the red end of the horizontal branch, just where the core
helium-burning members of this seemingly younger population would be
expected to lie.
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Some Interesting Color-Magnitude Diagrams
9
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0
0.5
1
B-R
1.5
2
Fig. 6. This calibrated B, R color-magnitude diagram of Carina is derived
from observations made by Tammy Smecker-Hane and me at the prime focus
of the CTIO 4 m telescope. This diagram shows clear evidence of a significant
burst of star formation some 13 - 15 Gyr ago, followed by a much stronger
burst some 6 - 7 Gyr ago, and perhaps some trailing off of the star formation
rate after that time. There is a clear gap between the two subgiant branches,
suggesting only minor star formation, if any, between 13 and 7 billion years
ago. A strong clump of core helium-burning stars belonging to the younger
generation(s) is seen above the red end of the horizontal branch.
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10
Peter Β. Stetson
18
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22
24
0
1
2
B-R
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Fig. 7. This calibrated B, R color-magnitude diagram for the dwarf
spheroidal galaxy in Fornax was derived by Tammy Smecker-Hane from
observations that she made in collaboration with Jim Hesser, Doug Geisler and
me, at the prime focus of the CTIO 4 m telescope. This galaxy, too, shows that
star formation took place from 13 - 1 5 Gyr to only a few Gyr ago. Unlike
Carina, Fornax shows no evidence for distinct bursts of star formation; instead,
stars appear to have formed more or less continuously over the whole period.
Note the apparent extension of the zero-age main sequence to B - R < 0, Ύ '
- 19. This suggests that non-trivial star formation may be taking place at the
current epoch (at least within the last several χ 107 yrs), even though no
interstellar gas has yet been discovered in Fornax!
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