Letters of Intent received in 2015

LoI 2017-271
Advances in stellar ages

Topics

The state of stellar models and their limits
Ages of pre-main sequence stars and age spreads
Ages of evolved stars and applications to the Galaxy and Local Group
Ages of main sequence stars
Calibrating ages and age scales
The oldest stars of the Galaxy
Eclipsing binaries as tests of models
Ages from asteroseismology
The relation between rotation and age for solar-type stars
The ages of Galactic components and the Galaxy’s star formation history
Ages of white dwarfs
Ages of star clusters in the Galaxy and Local Group

Rationale

In 2008, IAU Symposium 258 was held in Baltimore on “The Ages of Stars.” Motivated partly by that meeting and partly by other advances, the subject of determining stellar ages has made great progress since then and it is more than time to convene again.

Much of astrophysics deals implicitly with age because we wish to determine time scales for physical processes such as angular momentum loss, nucleo-synthetic processing, changes in magnetic fields, and the like, or we wish to compare objects or groups of objects. Stellar and Galactic evolution cannot be understood without some consideration of ages. In studying Galactic processes in particular we are drawn to the population of low-mass stars because at one solar mass and below the Galactic disk contains stars from all epochs of formation. If we could pin ages on those stars we could determine the Galaxy’s star formation history and we could understand the physics of low-mass stars much better. The well-studied spin-down of stars like the Sun and the concomitant decline of observed activity indices makes it possible to estimate ages of individual stars, but the scarcity of older clusters makes calibrating and testing the activity-age relation uncertain. However, recent extended surveys of stellar asteroseismological data open new possibilities for field stellar age determination. Moreover, more and more precise parameter determinations for binary stars, some in clusters, help to ocnstrain stellar models and thus age determination. Evolved stars, on the other hand, can be observed over nearly the entire Galaxy, and several projects are underway to study them on wholesale basis as part of “Galactic archaeology." Related to this is the need to increase the precision and accuracy of the ages of clusters in the Galaxy and Local Group.

Age has been a slippery enough topic that it has been relatively poorly examined in and of itself over the years. Before IAUS258, only one international meeting had specifically addressed the ages of stars, and that was held in 1972 (see below). The topic has come up in other contexts but has rarely been the focus of a dedicated symposium, which is truly amazing given the fundamental importance of age.

Now is a very appropriate time to re-examine the problem of stellar ages in detail. We now understand models of stars and systems of stars much better than even a decade ago. Our understanding of the physics that goes into those models has greatly improved, including such aspects as opacities,nuclear reaction rates, diffusion, effects of rotation and gravity waves, and the influence of magnetic fields. At the same time observations have tested the models, including now the detection of white dwarf cooling sequences in globular clusters, and evidence for multiple generations of stars within those clusters. At the other end of the age scale, we want to understand the mechanisms and duration of planet formation, but the stars around which we find circumstellar material have highly uncertain ages. We would like to be able to tell if the youngest star clusters and associations undergo multiple waves of formation, or if higher mass stars form at different times from those of lower mass.

We propose to bring together astronomers from the around the world to discuss the current state of the problem of estimating ages of individual stars and populations, where the advances are now being made, and what the near future offers.