Letters of Intent received in 2019

LoI 2021-2074
Astrometry for 21st century astronomy

Topics

Modern astrometry review talk

Astrometry science highlights (covering a broad range of topics)
- Gaia EDR3
- GRAVITY
- VLBI
- AO-assisted narrow field astrometry
- ...

Future astrometric surveys
- GaiaNIR, JASMINE, WFIRST, Theia/MAP, LSST, ...
- VLBI, SKA, ...
- Research and development needs for achieving sub-muas astrometry
- Opportunities for coordination

Dense and accurate reference frames to leverage extreme telescopes and large surveys
- Astrometry with extremely large telescopes
- Optical-radio synergies
- Space-ground synergies
- ...

Astrometric techniques
- Global astrometry, lessons learned from Gaia
- Narrow-field high precision astrometry (GRAVITY, ELT, etc)
- Astrometry from ground based surveys (LSST, DES, Pan-Starrs, etc)

Complementary surveys
- Spectroscopy (WEAVE, 4MOST, ...)
- Photometry (LSST, ...)

Rationale

The data from the Gaia mission have enabled spectacular progress in the
characterization and understanding of the complex structure and formation
history of the Milky Way, mostly driven by the availability of distance and
kinematic information over unprecedented volumes of Galactic phase space. The
astrometric data from the Gaia mission have impacted all astronomy disciplines
as evidenced by the 2000 papers having appeared since April 2018 that use Gaia
DR2 astrometry in one way or another. In the field of stellar astrophysics it
is now natural for astronomers to look up parallax and proper motion
information on any star in the Gaia catalogue; the field of white dwarfs is
profiting from a dramatically expanded knowledge of them in the volume around
the sun, leading to the first unequivocal evidence of the crystallization of
their interiors; the field of exoplanets has profited from a much more detail
understanding of the planet host stars, while the studies of proto-planetary
disks are free of the distance uncertainties from the past. Current and future
Gaia data releases hold the promise of closing in on a per cent level
measurement of the Hubble-Lemaitre Constant, while at the other end of the
astronomical spectrum studies of asteroids in the solar system will be
revolutionized by the availability of hundreds of thousands of extremely
accurate orbits.

In parallel the GRAVITY instrument at the VLT has revolutionized narrow field
astrometry, producing measurement accuracies at the tens of micro-arcsecond
level which led to the definitive measurement of the distance to the Galactic
centre, high accuracy studies of the motions of stars around the central black
hole, and the first direct detection of an exoplanet by optical
interferometry. The VLBI astrometry community has continued to refine their
techniques and routinely deliver parallax and proper motion measurements at
accuracy levels equal to or better than Gaia. Both techniques allow to probe
regions throughout the Milky Way disk that are obscured to optical astrometry,
such as star forming clouds, the spiral arms, and the Galactic centre region.

This breadth of applications of astrometric data from Gaia, GRAVITY, and VLBI
illustrates the extent to which 21st century astronomical research is
underpinned by the access to highly accurate positions, parallaxes, and proper
motions for a dense network of sources out to faint magnitudes, as well as
high astrometric precision over small and possibly obscured or crowded fields.

With this focus meeting we aim to bring together the producers and users of
high accuracy astrometric data in order to reflect on recent scientific
successes founded on astrometry; to present and exchange ideas on proposals
for future astrometric surveys; to learn about astrometric techniques across
different wavelength ranges and instrument concepts; and to have a discussion
on the synergies between the different astrometric techniques and surveys,
with for example the dense optical reference frame provided by Gaia feeding
into high precision astrometry conducted with future extreme telescopes.

In particular we aim at bringing together the various astrometry communities
(Gaia, VLBI, ground-based surveys, specialized astrometric instruments) which
often work in relative isolation from each other. The complementarity of the
various techniques and surveys can be leveraged through coordinated strategies
in defining future directions for astrometry.

To ensure a broad coverage of astrometric topic, the scientific organizing
committee of this focus meeting will consist of representatives from the
various astrometry communities mentioned above, taking account of geographical
and gender balance.