Letters of Intent received in 2019

LoI 2021-2097
GA Focus Meeting - Runaway and hypervelocity stars in the age of Gaia

Date: 16 August 2021 to 27 August 2021
Location: Busan, Korea, Rep of
Contact: Stephan Geier (sgeier@astro.physik.uni-potsdam.de)
Coordinating division: Division G Stars and Stellar Physics
Other divisions: Division A Fundamental Astronomy
Division B Facilities, Technologies and Data Science
Division C Education, Outreach and Heritage
Co-Chairs of SOC: Patrick Neunteufel (University of Leicester)
Stephan Geier (University of Potsdam)
Chair of LOC: None (None)



1. Surveys for runaway and hypervelocity stars
2. Stellar dynamics and ejection in stellar clusters and the Galactic
3. Kinematics and chemical composition of runaway and
hypervelocity stars
4. Core-collapse and thermonuclear supernova ejection
5. Runaway neutron stars
6. ISM-interactions of runaway and hypervelocity stars



unaway stars, moving with notably high velocities compared to their surroundings, and hypervelocity stars, moving fast enough to be unbound from the Galaxy, are thought to be produced by dynamical interaction in various environments. A number of different production mechanisms for these objects have been proposed, including dynamical ejection from star clusters and the Galactic center, in binary supernova explosions, and from satellite galaxies. Runaway stars ejected from the Galactic center are linked to the nature and environment of the central massive black hole; the trajectories of these stars - which travel from the very center to the outer halo - are potentially unique probes of the Galaxy's mass and dark matter distribution.
The list of known run-aways includes a plethora of stellar types and classes, including early type main sequence stars, hot subdwarf stars, white dwarf stars and proto white dwarf stars with peculiar chemical compositions - the study of which would be of exceptional interest even ignoring their identity as runaways -, as well as neutron stars.

In 2016 and 2019, observers and theoreticians working on the diverse fields relevant to the understanding of run-away and hyper-velocity stars gathered for two successful workshops held in Bamberg and Potsdam, Germany (http://www.sternwarte.uni-erlangen.de/hvs2016 and http://www.astro.physik.uni-potsdam.de/~starsontherun).

One of the main conclusions of these meetings was that high-precision astrometry is a prerequisite for tracing the place of origin of these stars, whether a particular cluster, the center of the Galaxy, or elsewhere, and to put constraints on the various proposed acceleration scenarios.

The Gaia mission Data Release 2 (DR2) has provided astrometry of unprecedented accuracy for most of the run-away and hypervelocity stars known, and allowed the community to discover and study much larger samples - e.g. thousands of new runaway stars are to be found among Gaia data compared to hundreds known before. The aim of this two-day focus meeting is to confront theoretical predictions with the new observational results based on Gaia data.
The expected release schedule of forthcoming Gaia mission data suggests the fall of 2021 as an exceptionally convenient time for the proposed meeting. Early Data Release 3 (EDR3) of the Gaia mission, providing improved astrometry (positions, parallaxes, proper motions) and photometry, both of significant importance in the study of runaway stars, is expected to be made available in the third quarter of 2020, with the full DR3 to follow towards the end of 2021. This timing will give members of the community, both observers and theoreticians, ample time to work with Gaia EDR3 data, while the conclusions of the focus meeting will be immediately applicable to Gaia DR3.