CONTENTS

Course: STELLAR POPULATIONS AND THE HISTORY OF THE GALAXY

Prof. Steven R. Majewski
University of Virginia U.S.A

STELLAR EVOLUTION

The stars in globular clusters were formed, just like their host clusters, by a process of gravitational condensation inside galaxies. Thus were born, virtually simultaneously associations of millions of stars of various masses. Each would shine in its own special way and be blue, yellow or red in colour, depending on its mass. The advanced age of the globular clusters in the Galactic halo, however, is a parameter that was often used to define these types of objects. Younger globular clusters were discovered in the eighties ("age dispersion") although it might be, as Prof. Steven Majewski now explains, that the youngest globular clusters evolved in nearby dwarf galaxies.

"Globular clusters have provided an essential empirical check on one of this century’s greatest astrophysical triumphs - the understanding of stellar evolution. By fortunate circumstance, the first intensive studies of globular cluster color-magnitude diagrams (with photographic data in the 1950’s) by Sandage, Arp and others coincided with the birth and developement of modern theories of stellar evolution and nucleosynthesis. Since then, refinements of stellar evolution theory have been driven by observational aspects of star cluster CMD’s, which provide almost the only real constraints on the models. Stellar evolution and nucleosynthetic theory, in turn, have given us the ability to age-date the clusters and read the levels of enrichment written both in the spectra of cluster members and in the shape of the cluster CMD. Age and abundance information from the clusters, in turn, have provided vital clues toward solving numerous other problems, from the formation of the Milky Way to setting the extragalactic distance scale and providing limits on the age of the universe.

In 1917, Harlow Shapley used the distribution of globular clusters to locate the true center and determine the relative size of the Milky Way. Globular clusters, by virtue of their luminosity and distinct morphology have continued to play a central role in mapping the shape of the Galaxy.

More recently, globular clusters have become pivotal in defining the chemodynamical attributes of old stellar populations in the Galaxy. This role will increase as we gather more data on the space motions and detailed abundance patterns of both globular and open clusters."

"It has long been suspected that there are age differences within the halo globular cluster system. Two decades ago, on the basis of the so-called "second parameter" spread in the morphology of the horizontal branches (HBs) of cluster color-magnitude diagrams, Searle & Zinn postulated that an age spread existed in the outer halo globular cluster system. They proposed that this was the result of slower development of stellar populations in subsystems ("fragments") left behind as the interior of the Galaxy collapsed and developed. In this scenario, the age spread, at least that found in the outer halo globular clusters, is attributable more to the timescales of collapse and star formation in the fragments left behind than on the duration of the collapse of the gas that formed the early flattened components of the Milky Way.

However, confidence that age is the sole second parameter of HB morphology has waxed and waned throughout the years; other possible contributors to the variance in HB morphology that have been suggested include cluster density, rotation, alpha-capture element abundances, oxygen abundances affecting mass loss rates, helium abundance and "deep mixing" of helium in highly convective stars, helium core mass at helium flash, and even the possibility of differences in the amount of swallowed up planets. One of the most exciting developments in the era of CCD photometry is our ability not only to photometer a number of cluster main sequences, but also to discern subtle differences in the shape of main sequence and red giant branch distributions that are the hallmarks of small age differences. Assuming a predominant contribution by age to the second parameter effect, the latest work seems to maintain that about a 3 Gyr age spread does exist in the halo cluster system, but this is still highly debated and the spread may be as low as 1 Gyr, depending on the stellar evolution model used for comparison. Interestingly, several of the second parameter (younger?) halo clusters appear to be associated kinematically with the satellite dwarf galaxies of the Milky Way. This suggests that the Milky Way may have accumulated such clusters through accretion processes - that is, through processes similar to those envisioned by Searle & Zinn.

Information on the timescale for the collapse of the protogalactic cloud that led to the bulk of the Milky Way, a process envisioned, for example, in the landmark Eggen, Lynden-Bell and Sandage paper of 1962, may be encoded in the properties of less remote, more flattened, old components that have been hypothesized to have formed early on. These components - the "lower halo" and the "thick disk" or "Intermediate Population II" - may actually be parts of a continuum of old populations formed during the primordial collapse. The associated globular clusters may be the so-called "disk" and "old halo" clusters, which, at present, indicate little age spread, although much more work is needed here."

"It has been theorized that stars may have formed in the early universe, before the era of galaxy formation, at redshifts higher than z = 1000. The Jeans mass at these times may have limited the mass function of "Population III" stars to masses larger than about 10 to 100 solar masses or so (a limit not yet certain), so these stars would have quickly evolved to supernovae and would have had a chance to dump their nucleosynthetic yield of heavy elements to pre-galactic gas. These first generation stars may have contaminated the primordial Big Bang nucleosynthesis yields with heavy elements to about one ten thousandth of the solar level. Interestingly, this is about the lower bound on metallicities seen in Galactic field star surveys, but two orders of magnitude less than the lower limit of enrichment in Galactic globular clusters."

"I am particularly intrigued by the possibility of gathering a detailed picture of the evolution of the Milky Way by tracing - via chemical and spatio-kinematical data - some clusters back to their site of origin in dwarf galaxies. We see that, for example, the Sagittarius dwarf galaxy is in the process of 'donating' its family of globular clusters to the Milky Way cluster retinue. It is unlikely that this process is unique, and there must exist some present Milky Way globulars that were originally part of smaller galactic subsystems that may or may not have yet totally succumbed to accretion processes by the Milky Way."

PROFILE

Steven R. Majewski, a native of Chicago, received a BA with honors from Northwestern University in 1983, with majors in physics, mathematics, and integrated science. Since his graduate work at the University of Chicago’s Yerkes Observatory, from which he received his Ph.D. in 1991, his research has concentrated on the evolution of galaxies and stellar populations, both from the perspective of studying extragalactic systems to high redshifts as well as through detailed study of the spatial, kinematical and abundance distributions of populations in the Milky Way and its satellite system.
In 1990 Majewski began postgraduate work, first as a Carnegie Fellow and then as a Hubble Fellow, at the Carnegie Observatories in Pasadena, CA. He is grateful to maintain connection with the Observatories as a Visiting Associate.
Since 1995, he has been on the astronomy faculty at the University of Virginia where his research group focuses on astrometry, photometry and spectroscopy of stellar systems in the Milky Way. One goal of the research is to understand the history of satellite mergers with the Milky Way. Soon the Virginia group hopes to turn some attention to preparatory science for the Space Interferometry Mission, which will be launched into orbit in 2005. In 1997 Majewski was awarded a David and Lucile Packard Foundation Fellowship and a National Science Foundation Career Award.