CONTENTS

UNCERTAINTIES CONCERNING GLOBULAR CLUSTERS

"A key outstanding problem regarding globular clusters is understanding how they form. This would be give us important insight into the early stages of galaxy formation. While the oldest globular clusters have ‘erased’ their initial conditions through dynamical evolution, there are massive LMC clusters young enough to preserve information about their formation process. Observations of their stellar content and dynamical state will allow us to determine things like the timescale for star formation in the progenitor cloud, the extent of primordial mass segregation, and so on. In the coming decades new telescopes and instrumentation will let us push deeper down the cluster’s mass functions and further into their crowded cores, just as HST has allowed us to do this decade."

"In my feeling we are too often in the situation of attempting to give quantitative results on the basis of rather uncertain observational data, as given by different authors with different telescopes, different calibrations an so on. What we needed now is a solid database of CM diagrams of galactic globulars, as one can obtain with a good telescope and with a solid and repeatible observing (and reducing) procedure. This alone can be solve many open questions such as the relative ages of clusters, or cluster age as the second parameter governing the HB features. Much more in general, I would suggest that when dealing with the request for quantitative results, as we actually do, one should start to consider errors. The large amount to of astrophysics given without errors is a curious anomaly in the more general field of current physics."

One of the key problems in globular cluster research is to obtain very accurate and detailed measurements of chemical abundances at all phases of evolution in a cluster. This will indicate how surface abundance changes (or does not change) with evolutionary state. Abundance ratios of unstable (radioactive elements would enable ages to be estimated for comparison with ages derived from evolutionary considerations. Such a comparison would be very important in telling us whether our age determinations are satisfactory or not. Measurements of this kind require telescopes of very great light gathering power. Thus I would expect that this would be an area in which the Gran Telescopio Canarias could make a major contribution."

"Many of the uncertainties surrounding the distances and ages of globular clusters boil down to understanding their chemical compositions in much more detail. For that, we need high-resolution spectra of lots of stars, and this is where the new large telescopes come in. For clusters in other galaxies, large samples of high-quality velocities will allow us to study the dynamics of the halo. In a broader sense, however, the biggest outstanding problem is to understand how clusters form. For this, we need modelling which employs gas dynamics at many scales, as well as star formation theory, in far greater detail than we have on hand today."

"Better understanding of what the initial mass functions were, and how and why they have changed. Also, better understanding of chemical composition and where it came from. With a big telescope, any number may play this game."

"I think the key issue is the second parameter. The Yale group strongly support age as second parameter; however, other "second" parameters are also needed to explain observations. There are a number of likely related effects, ranging already mentioned the discrepancies between different distance scales: here the main uncertainties are related to possible scale errors in colours (due to reddening and colour transformations) and metallicity. While photometry in cluster crowded fields is certainly done better using HST, there are several crucial spectroscopic observations to be done with ground-based 8 meter class telescopes like abundance analysis of main sequence stars or estimates of rotational velocities and of reddening.

"Globular clusters are an important rung in the 'cosmic distance ladder' and yet cluster distances are still subject to large systematic uncertanties. These uncertainties, in turn, impact a host of other issues. For example, to achieve 10% precision in absolute age dating a cluster requires that it’s distance be known to 5% accuracy. In the next few decades, however, new generations of astrometric telescopes, like the Space Interferometry Mission and GAIA, will hopefully follow the success of the HIPPARCOS mission and will deliver trigonometric parallaxes - the most direct and reliable way to obtain distances - for all of the Galactic globular clusters. This will truly be an important achievement, since we cannot yet determine a parallax for a single globular cluster! Only a few nearby OPEN clusters have been studied in this way, and even here the results remain controversial.

While trigonometric parallaxes will be obtained for Galactic clusters, it will also be possible in future decades to use large space-based and adaptive optics-corrected ground-based telescopes to obtain color-magnitude diagrams for, and spectra of stars in, increasingly distant extragalactic globular clusters. By comparing the Galactic and extragalactic clusters, the entire distance scale will be improved. In addition more direct comparisons of the chemodynamical properties of Galactic and extragalactic cluster systems will be possible, with great promise for understanding the pathways of galactic evolution. Finally, large telescopes will enable us to obtain color-magnitude diagrams of Galactic clusters down to the level of the white dwarf sequence, so that the stellar evolution timescales used to age date clusters may be compared to the ages derived from white dwarf cooling theory. This technique is now in its infancy with Space Telescope data."

"I think the most important problem relating to globular clusters concerns their role in the formation of galaxies. Are we dealing with condensations of matter that preceded and gave rise to the galaxies, or did the globulars form later? Were they contained in haloes of dark matter that fused to form galactic haloes? I believe that the new methods of observation will, over the following decades, bring to bear the necessary data for answering these questions. Already we can detect and investigate globular clusters in fairly remote galaxies. As we probe greater distances, from the present Universe to the epoch of galaxy formation, it will become possible to trace the history of the clusters, to see whether they appear only in galaxies below a given redshift or whether, to the contrary, their presence extends beyond galaxies with morphologies similar to those of the present."