Stellar age estimation

Abstract: From the decades of the theoretical studies, it is well known that the formation of the bar triggers the gas funnelling into the central sub-kpc region and leads to the formation of a kinematically cold nuclear stellar disc (NSD). We demonstrate that this mechanism can be used to identify the formation epoch of the Galactic bar, using an N-body/hydrodynamics simulation of an isolated Milky Way-like galaxy. As shown in many previous literature, our simulation shows that the bar formation triggers an intense star formation for ~1 Gyr in the central region, and forms a NSD. As a result, the oldest age limit of the NSD is relatively sharp, and the oldest population becomes similar to the age of the bar. Therefore, the age distribution of the NSD tells us the formation epoch of the bar. We discuss that a major challenge in measuring the age distribution of the NSD in the Milky Way is contamination from other non-negligible stellar components in the central region, such as a classical bulge component. We demonstrate that because the NSD is kinematically colder than the other stellar populations in the Galactic central region, the NSD population can be kinematically distinguished from the other stellar populations, if the 3D velocity of tracer stars are accurately measured. Hence, in addition to the line-of-sight velocities from spectroscopic surveys, the accurate measurements of the transverse velocities of stars are necessary, and hence the near-infrared space astrometry mission, JASMINE, would play a crutial role to identify the formation epoch of the Galactic bar. We also discuss that the accuracy of stellar age estimation is also crucial to measure the oldest limit of the NSD stellar population.

Abstract: We present recent progress on quantitative estimation of stellar ages using indicators such as theoretical evolutionary tracks, rotation, rotation‐driven chromospheric and coronal activity, and lithium depletion. Our focus is on roughly solar‐mass and solar‐metallicity stars younger than the Sun. We attempt to characterize the systematic and random error sources and then derive “best” ages along with the dispersion in age arising among the various age estimation methods. Our main application of these techniques is to the evolution of debris disks.

Abstract: Understanding our Galaxy’s structure, formation, and evolution will, over the next decades, continue to benefit from the wonderful large survey by Gaia, for astrometric, kinematic, and spectroscopic characterization, and by large spectroscopic surveys for chemical characterization. The weak link for full exploitation of these data is age characterization, and stellar age estimation relies predominantly on mass estimates. The ideas presented in this White Paper shows that a seismology survey is the way out of this situation and a natural complement to existing and planned surveys. These ideas are strongly rooted in the past decade’s experience of the so-called Seismology revolution, initiated with CoRoT and Kepler. The case of red giant stars is used here as the best current illustration of what we can expect from seismology for large samples, but premises for similar developments exist in various other classes of stars covering other ranges of age or mass. Whatever the star considered, the first information provided by stellar pulsations is always related to the mean density and thus to the mass (and age). In order to satisfy the need for long-duration and all-sky coverage, we rely on a new instrumental concept which decouples integration time and sampling time. We thus propose a long (~1 year) all-sky survey which would perfectly fit between TESS, PLATO, and the Rubin Observatory (previously known as LSST) surveys to offer a time domain complement to the current and planned astrometric and spectroscopic surveys. The fine characterization of host stars is also a key aspect for the interpretation and exploitation of the various projects -- anticipated in the framework of the Voyage 2050 programme -- searching for atmospheric characterization of terrestrial planets or, more specifically, looking for a signature of life, in distant planets.