We report the preliminary results of a CCD surface photometry survey of a large fraction of all known Galactic globular clusters. About 1/5 of all surveyed clusters show a characteristic post-core-collapse (PCC) morphology. The PCC clusters are on average closer to the Galactic center than the King-model–like clusters

Djorgovski, S.

King, I. R.

Oren, A.

Penner, H.

Vuosalo, C.

Caltech Authors - Main

THE STRUCTURE OF GLOBULAR CLUSTERS 
S. Djorgovski, I.R. King, C. Vuosalo, A. Oren andH. Penner 
Astronomy Department 
University of California 
Berkeley, CA 94720, U.S.A. 
ABSTRACT. We report the preliminary results of a CCD surface photometry 
survey of a large fraction of all known Galactic globular clusters. 
About 1/5 of all surveyed clusters show a characteristic post-core-
collapse (PCC) morphology. The PCC clusters are on average closer to 
the Galactic center than the King-model-like clusters. 
We have completed a multi-color CCD surface photometry survey of some 
113 Galactic globular clusters. Many of the observed clusters had no 
previous photometry, or density profiles of any kind. The first 
scientific goal of this survey was to do a complete census of cluster 
cores, and determine the fraction of clusters which show a post-collapse 
morphology. Several good reviews of this topic can be found in the 
volume edited by Goodman & Hut (1985). 
This work is a follow-up to the earlier study (Djorgovski & King, 
1984). Most data for the present survey were obtained with the CTlO 
lo5-m telescope and RCA and GEC CCDs, and with the UBVRI filters. 
Some of the data were obtained with the Lick 1-m telescope and Tl and 
GEC CCDs, and with a special red filter. Several hundreds of cluster 
exposures were obtained. Most of the data are already reduced to one-
dimensional surface brightness profiles, but with little absolute 
calibration. The full account of the survey will appear in future 
papers. We supplement our data with reliable surface density profiles 
from the literature. 
There are 21 definite PCC clusters, 6 possible ones, 3 uncertain 
ones, 97 definite or probably King-model-like clusters. Thus, PCC 
clusters are about 1/5 of the total. 
The PCC clusters are generally not very rich; this may be an 
additional evidence for their advanced dynamical state: the central, 
stabilizing binary pumps the energy into the cluster, and thus speeds 
up its dissolution. 
One interesting result emerged from this study; the PCC clusters 
are on the average much more concentrated toward the Galactic center. 
For a' set of 20 definite or possible PCC clusters, the mean and the 
median galactocentric radii are 4.2 and 2.7 kpc respectively; for a 
2Rl 
1. B. Hearnshaw and P. L. Collrell (eds.), Instrumental;on and Research Programmes for Small Telescopes, 281-282. 
© 1986 by the IA U. 
2R2 S. DJORGOVSKI ET AL. 
set of 91 King-model-like clusters, the corresonding numbers are 14.8 
and 7.6 kpc. The difference is highly significant, and it suggests that 
the tidal shocks (from disk passages) play an important role in the 
internal dynamics of globular clusters, viz., by accelerating their 
dynamical evolution. This effect was predicted by Chernoff, Kochanek & 
Shapiro (1985, preprint). 
There is one remaining puzzle, however: there are a number of very 
concentrated clusters with very dense cores, and thus very short 
relaxation times (a few x 10 7 yr). They had plenty of time to collapse, 
but they look like perfect King models. Either they managed to recover 
completely from the collapse (faster than the current theories would 
predict), or they do,have collapsed cores, but they are hidden in the 
invisible heavy stellar remnants (neutron stars, stellar black holes). 
REFERENCES 
Djorgovski, S. and King, I.R. 1984, Ap. J. Lett. 277, L49. 
Goodman, J. and Hut, P., (eds.) 1985, Proceedings of the I.A.V. 
symposium #113~ "Dynamics of Sta:r> Clusters". Dordrecht: D. Reidel. 
2.5 
....... 
•• 
··a 
• 
• 
2 .. 
• 
Bl ~ 
tl() 1.5 r 
..8 II 
• 
• 1 
• 
a: 
.5 
0 
NGC 6388 I .. j ~~~~~~~~!-!~! ~I~!~!~!~!~!~! ~~ 
-.5 o .5 1 1.5 2 o .5 1 1.5 2 
log r (arcsec) log r (arcsec) 
Figure 1. Examples of cluster surface brightness profiles: NGC 6388 
(left) is an example of a highly concentrated, King-model-like cluster: 
note the flat core, and a steep envelope. Trz 2 (right) is an example 
of a PCC cluster: the light distribution is almost a pure -1 power law. 
Both profiles were derived from CCD images taken in a B band, at CTlO 
1.5-m telescope. 


The Structure of Globular Clusters

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The Structure of Globular Clusters

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