When Collaborators Are a Couple: Globular Clusters Provide Case Study

Fred Rasio (l.) and Vicky Kalogera. Photo by Randall Lamb.

He was an assistant professor at MIT; she was a postdoctoral fellow at Harvard. They met at one scientific conference, and married at another (the latter, at least, in scenic Aspen). Both now hold endowed chairs at the same university, Northwestern; and both are theoretical astrophysicists whose principal scientific interests include globular clusters — aggregates of old stars, which were the subject of the 2009 program “Formation and Evolution of Globular Clusters” that drew Fred Rasio and Vicky Kalogera to the KITP.

They emphasize that because they both had valid scientific reasons for participating in the same program, coming to KITP for an extended stay posed none of the customary hurdles couples may face when one wants to visit while the other is deterred by employment commitments tied to place of residence. And the KITP Family Fund, established two years ago by Ann Rice in memory of her husband Myron, enabled a grateful Kalogera and Rasio to accommodate duties as parents of a toddler to the exigencies of intense participation in a program Rasio served as coordinator.

Interviewing them together on the progress of the “Globular Clusters” program and listening afterwards to a recording of that interview revealed something of the couple’s style in their approach to scientific collaboration. Instead of interrupting one another or talking over one another — as male theoretical physicists collaborating at KITP sometimes do — Rasio and Kalogera simply took turns addressing questions. And each did not tune out while the other was speaking, but instead listened attentively to the other, as demonstrated by frequent elaborations and clarifications and qualifications of each other’s assertions.

Said Kalogera, “We could have collaborated on every single research project, but each of us tried consciously not to cross that line all the time.”

“It doesn’t add to the intellectual vitality of an academic department,” said Rasio, “if two of its faculty are doing the same research.” Both also noted that the intellectual vitality of the individual also requires some separation of interest.

So Kalogera and Rasio have diversified. In addition to globular clusters, Rasio also focuses on extra solar planets, among the “hottest” of topics in astronomy, as evinced by the 2010 KITP program on “The Theory and Observation of Exoplanets.” And Kalogera is a member of the LIGO scientific collaboration that seeks to detect gravitational waves.

Globs Enable Death Star Collisions

Those waves and their detection are relevant to globular clusters because these globs of old stars are likely to contain within them the exotic binaries of stellar end products — neutron stars and black holes whose collision affords the most probable scenario for the production of gravitational waves detectable by the two-stage upgrades to basic LIGO.

“Cluster,” with respect to “globular,” means a large number of stars, ranging from as few as 10,000 stars to as many as seven million in a single gravitationally-bound, roughly-spherically-shaped entity within galaxies. The number of globular clusters within a given galaxy ranges from a few hundred in the Milky Way to many thousands in big elliptical galaxies. Some globular clusters exist in the galactic disk, where almost all stars in a given galaxy reside, but there are also — uncharacteristically for star location – many globular clusters in the galaxy halo. Their key feature is the density of their star components; that density means that clusters look like bright round entities to the observer, hence the adjective “globular.”

“When we are able to resolve them,” said Kalogera, “we see that the center is a lot brighter than the outskirts,” which is an indication of the typical “mass segregation” within a cluster whereby not only more and more stars, but also the more massive ones congregate via gravitational attraction towards the center of the cluster.

Program participants split roughly into two groups. Said Rasio, “People like me and Vicky want to understand the details of what is going on inside these systems. There are a lot of reasons why they are very interesting. In particular the high densities lead to all kinds of exotic interactions between stars that never happen anywhere else.” The other group, he said, focuses on what clusters reveal about how galaxies assembled and evolved, especially through mergers. Because the massing of stars makes clusters so bright, they can be detected within galaxies at time scales pertinent to cosmology.

As these two communities have tried to make progress and answer questions in more and more detail, they have realized the interdependence of the two research foci. Said Rasio, “Part of what determines the evolution of globular clusters within their host galaxies is also what’s going on inside of them.” The program was designed to bring the two perspectives together so that each could inform the other and thereby foster productive collaborations utilizing the expertise of the two points of view.

One particularly interesting intersection focuses on the question, “How are globular clusters made?” Studies of their current character indicate that they formed 12 to 13 billion years ago and are, therefore, among the oldest objects in a universe thought to be some 13.7 billion years old.

A key indicator of cluster age is the mass of the stars that now shine. They are about 80 percent the Sun’s mass, indicating that enough time has gone by since the inception of the clusters that all the more massive stars have devolved into white dwarfs or neutron stars or black holes (depending on the size of the initial star).

That low stellar mass signature, said Kalogera, also indicates that star formation is not an ongoing process within clusters. All the stars, in other words, were made about the same time a long time ago in contrast to galaxies as a whole, where star formation is ongoing. The key reason for the difference is that the clusters are not as a whole massive enough to exert a gravitational force sufficient to contain the gestational gas wherein new stars form.

The stars within globular clusters are also metal poor, again indicating that they formed long ago, before the interstellar medium had been enriched with heavier elements through successive supernova explosions of short-lived, super-massive stars. But some clusters are less metal poor than others, and one of the big puzzles the program participants tried to solve was why these two types of metal poor clusters tend to exist in two clumps rather than along a continuum of metallicity.

“It looks,” said Kalogera, “as if there were multiple epochs of cluster formation in the same galaxy. We ask what could trigger multiple epochs and conjecture that the cause is galaxy mergers. We debated at length in the program whether the oldest globular clusters are telling us something about the first mergers of little entities in the universe” and, hence, the birth pangs of galaxies."


KITP Newsletter, Winter 2009