Abstract: Cataloguing positions of celestial bodies to study their absolute motions in the sky has been a long quest ever since the beginning of astronomy. For this cataloguing, a proper coordinate frame, in which positions of celestial objects can be referred to in a consistent way over time, is needed. Celestial reference frames may be defined dynamically, that is based on the laws of mechanics, or kinematically, through a grid of accurately-known fiducial directions in the sky. Assuming that the Universe is non-rotating, the two definitions are equivalent.Until the 1980s, celestial reference frames relied on optical measurements and were based on positions of stars. The advent of the Very Long Baseline Interferometry (VLBI) technique thereafter, changed the picture, allowing for positions to be determined with miiliarcsecond accuracy in the radio domain, hence surpassing any ground-based optical techniques. This led to the construction of the first-ever extragalactic reference frame, which became known as the International Celestial Reference Frame (ICRF) after its adoption by the IAU in 1998. Thanks to the accumulation of data, two further ICRF realizations were produced, ICRF2 in 2009 and ICRF3 in 2018, each with a larger number of sources (quasars) and an increased positional accuracy. The current version of the frame, ICRF3, is based on nearly 40 years of VLBI data at the standard geodetic and astrometric frequencies (2.3 and 8.4 GHz) together with additional data collected at higher frequencies (24 GHz and 8.4/32 GHz) over the past 15 years. It includes positions for more than 4500 sources with a noise floor in the individual coordinates of 30 microarcseconds. The ICRF is important in various areas, from geodesy to Earth’s rotation studies, from measuring Galactic proper motions to spacecraft navigation.The advent of the Gaia space mission and the subsequent release of an extragalactic frame in the optical domain with a positional accuracy similar to that of the ICRF3, and with two orders of magnitude more sources, has recently brought another giant step in the field. The availability of positions at four frequencies for some sources (three in the radio and one in the optical), not always coincident, brings interesting prospects for studying the source physics.
Bio: Patrick Charlot has worked in the field of geodetic, astrometric and astrophysical VLBI for more than 30 years, originally at Paris Observatory (where he obtained a Ph. D. in 1989) and since 1998 at the University of Bordeaux in the southwest of France. He also stayed as a postdoc fellow at the Jet Propulsion Laboratory (USA) in 1989-1990.Patrick pioneered studies of radio source structure from geodetic and astrometric VLBI data, including the effect of such structures on the VLBI observables and the celestial reference frame. Following up on his early work, he introduced the now widely-used structure index, an indicator of source quality, and pushed imaging of the reference frame sources. This led to the Bordeaux VLBI Image Database, which now incorporates more than 7000 images from extragalactic radio sources. Additional contributions are concerned with studies of the Earth’s precession and nutation, the estimation of the link between the dynamical and extragalactic frames, and astrophysics of active galactic nuclei, in particular from VLBI observations.Patrick chaired the ICRF3 Working Group (2015-2018) who delivered the current celestial reference frame, adopted by the International Astronomical Union in August 2018. He has also long been connected to European VLBI, e.g. as chair of the European VLBI Network Program Committee from 2003 to 2008 or more recently through the JUMPING JIVE project. Patrick is a member of the JIVE council, representing France, and a member of the IVS Directing Board, representing the International Astronomical Union. His interest also extends to the Gaia space mission for linking the radio and optical frames and understanding the underlying source physics. He is a member of the Data Processing and Analysis Consortium for Gaia. Patrick was Director of the Laboratoire d’Astrophysique de Bordeaux (formerly Bordeaux Observatory) from 2008 to 2015.