Splinter Session

Angular momentum evolution of cool stars:
toward a synthesis of observations and theory before and after the ZAMS

Organizers: Søren Meibom (Harvard-Smithsonian CfA), Ansgar Reiners (University of Göttingen), Jonathan Irwin (Harvard-Smithsonian CfA)


Part I (3:00pm - 4:15pm):

Break (4:15pm - 4:45pm)

Part II (4:45pm - 5:30pm):

Discussion (5:30pm - 6:00pm)


Observations of the angular momentum evolution of cool stars continue to challenge and improve our understanding of star formation and stellar evolution. As cool stars contract onto the main-sequence (MS) they are expected to reach breakup spin rates as a consequence of angular momentum conservation. Yet, observations of surface rotation periods for cool coeval pre MS (PMS) stars at various ages show large dispersions from near breakup rates to a small fraction thereof. Recently, a picture has emerged in which cool PMS stars exhibit a mass-dependent period distribution that is bimodal at higher masses and in which the rotation of lower mass PMS stars is on average faster. The origins of the morphology in the PMS rotation period distributions are still controversial. A variety of mechanisms for removing angular momentum from PMS stars have been proposed, including magnetic star-disk interaction (i.e. disk braking or magnetospheric ejections), accretion-powered stellar winds, and scaled-up solar-type coronal mass-ejections. Numerous studies have been carried out to look for correlations between stellar spin rate, and mass, indicators for circumstellar disks, X-ray flux, and active accretion - and some have been established. However, the observational support for the proposed mechanisms and the capacity of the mechanisms to explain the observations remains debated.

In comparison, by the time cool stars reach the ZAMS (100-200Myr), the observational picture is more clear, with evidence for two distinct sequences of fast and slow rotators in the mass vs. period plane, presumably tracing the lower and upper envelopes of stellar rotation periods at the ZAMS. Observations in yet older open clusters show a clear convergence in the angular momentum evolution for all FGK dwarfs towards a single, well-defined, and mass-dependent rotation period by the age of the Hyades (~625Myr). The early MS evolution has been attributed primarily to differences and changes in the stars' internal structure as a function of mass. However, the explanations for the origins and convergence of the two rotational sequences are also controversial, and the paradigm for ZAMS and early MS evolution has yet to be extended back to the PMS stage.

Presumably, the rotational dichotomy observed at the ZAMS and the relationships between stellar mass and surface rotation period must develop at some stage during the PMS. Extensive kinematic data in the ZAMS clusters have shown that the divide between slow and fast surface rotation is not between single and close binary stars and should likely be understood in the context of the early angular momentum evolution of individual stars. Traditionally, PMS and MS angular momentum evolution have been addressed separately. We would like to take advantage of the abundance of new observational data on stellar rotation as well as of recent advances in our theoretical understanding of cool PMS and MS stars, to examine relations between PMS and MS rotational behaviors, what their origins might be, and whether or not a unifying framework can be established linking angular momentum evolution across the ZAMS divide.

Session goal and format

The session will begin with three 15+5 minutes invited talks to set the stage with up-to-date information about empirical and theoretical progress and shortcomings during the PMS and MS. The invited talks will be followed by five 10+2 minutes contributed talks to present new observational or theoretical results and new ideas relevant to the interpretation of the data and to the comparison with theoretical predictions. The goal of the splinter is to get the participants engaged in a discussion centered on the following specific questions: