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

Program

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

• Sydney Barnes: "A rotational dichotomy among young cool stars, and certain significant connections"

Abstract: Over the last decade, the dispersion in rotation rates of young main sequence cool stars was resolved using rotation period measurements into two relatively distinct groups of fast and slow rotators. This distinction seems to persist for a mass-dependent timescale before it goes away. This forces us to confront (again?) the question of what the difference between these stars is, and where the boundary between them is. I will interrogate these observations, and examine how they have disproved key components of our models for main sequence stellar rotational evolution. I will argue that the remaining components can still be used to cobble together a scenario for the main sequence, including making key connections to activity phenomena, but it is by no means obvious how to extend this backwards to the PMS. The hope is that discussions at the Splinter could lead to the construction of a coherent scenario.


• Kevin Covey: "Observations of rotation for cool pre-main sequence stars and correlations with evolutionary diagnostics"

Abstract: Multi-epoch surveys of the youngest stellar clusters are providing detailed portraits of the mass-period plane for optically revealed PMS stars. These studies have revealed correlations between stellar masses, rotation rates, and observable proxies for evolutionary status (e.g., the presence or absence of a circumstellar disk) that must emerge within the first few Myrs of the PMS phase. These correlations remain statistical in nature, and one-to-one, predictive relationships between these parameters remain elusive. I will summarize the evidence for correlations between masses, rotation rates, and evolutionary states, which is heavily dependent on rotation periods measured from photometric surveys at optical wavelengths. I will describe recent work that has refined our understanding of these correlations, incorporating improved optical monitoring capabilities, refinements to period detection techniques, or detailed descriptions of the star-disk system from focused SED modeling. I will conclude by highlighting the near-term potential of infrared observations for extending our understanding of stellar rotation well into the protostellar phase.


• Sean Matt: "The Physics of Stellar Spin Evolution"

Abstract: In order to make sense of the observed spin rates of young stars, we need to understand (a) how stellar spin rates can evolve in time and (b) how some stars do this differently than others, producing the observed distributions at a given age. I will briefly describe the physical ingredients necessary for modeling the spin rate of a star as a function of time. I will review the different processes (e.g., star-disk interaction and winds) that are known to exert a net external torque on stars and discuss the similarities and differences likely to exist before and during the main sequence phase. These physical mechanisms suggest that differences in (e.g.) stellar magnetic properties, mass accretion rates, and/or internal angular momentum transport could explain the existence and evolution of wide distributions of spin rates. Many of these ideas will be addressed by subsequent speakers.


• Aurora Sicilia-Aguilar: "The different ways of disk dispersal and accretion evolution"

Abstract: The short lifetime of protoplanetary disks poses strong constraints to the formation of planetary systems - but fully dispersing a massive disk in a short time can also be a problem. Multiwavelength observations reveal a large variety of disks in clusters of any age, ranging from diskless stars in young regions, to rare disks with strong accretion and little evidence of evolution at 10 Myr. Objects found to be in an intermediate state between disked and diskless systems (transition disks) show also differences in disk and accretion properties. This suggests an interplay of different effects in the disk dispersal, where the system's initial conditions probably play an important role.

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

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

• Rob Jeffries: "Linking cool star accretion history, luminosity, and rotation"

Abstract: We present an analysis of the rotation of young stars in four PMS associations and discover a link between rotation rate and luminosity which cannot be reconciled simultaneously with disc braking scenarios and the interpretation of luminosity spreads as a spread in stellar ages. We discuss the implications of this result for the origin of the period distribution seen in PMS stars.


• Julien Morin: "Evidence for a bimodal distribution of magnetic fields in cool stars"

Abstract: The first spectropolarimetric study of a sample of active late M dwarfs (M5-M8) carried out with ESPaDOnS@CFHT has revealed the co-existence of two distinct types of large-scale magnetic fields among stars having similar masses and rotation rates. A possible explanation for this unexpected discovery is the genuine existence of two dynamo branches in this parameter regime. I will present the theoretical interpretations that start to emerge in order to account for these observations as well as ongoing observational efforts that will allow us to further explore this behavior in both MS and PMS stars. I will conclude by briefly discussing how this bimodal distribution of magnetic fields can affect the magnetically-driven physical processes that set the angular momentum evolution of cool stars.


• Ana Palacios: "Modeling angular momentum evolution from the PMS to the TAMS in low-mass stars"

Abstract: With the scope of providing self-consistent evolutionary models of low-mass PMS and MS stars including MHD processes, and in particular all those that impact the transport of angular momentum during these phases, we present a first exploration of the parameters space for rotating models including meridional circulation, shear turbulence and internal gravity waves.


• Ansgar Reiners: "A common explanation for angular momentum evolution in partially and fully convective stars"

Abstract: The standard model of angular momentum loss is unable to explain the full range of stellar rotation. In order to explain slow rotators like the Sun and faster rotators, like for example mid-M dwarfs, the model assumes two different angular momentum loss rates divided by a critical rotation rate. I show that the standard model follows from erroneous assumptions on the stellar magnetic field strength. Using an observationally justified parameterization of the field strength naturally leads an angular momentum loss law that strongly depends on radius. As a result, the new description can explain the main properties of angular momentum evolution both in fast and slow rotators, and explains the dichotomy through differences in radii existing particularly between partially and fully convective stars. The range of rotation rates can be explained without a need for a dynamo model that differentiates between partially and fully convective stars.

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

Motivations

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:

• Does the two-branch mass-period morphology present on the ZAMS become established during the PMS, and is there a unifying physical theory to connect the PMS evolution with the MS evolution?


• To what extent do observations of rotation in PMS and MS populations demand fundamentally different mechanisms for angular momentum evolution?


• Are there observational similarities that have gone unnoticed and are there theoretical mechanisms for PMS evolution that mirror those on the main sequence?