Title: Astrometric VLBI and Celestial Reference Frames
Author: Prof Patrick Charlot
Laboratoire d’Astrophysique de Bordeaux
Université de Bordeaux
Measuring the positions of objects in the sky and building a grid with such positions has been a fundamental objective of astronomy from the beginning. Such a grid (or celestial reference frame) may be based on stars in our galaxy or extragalactic objects located at cosmological distances. Alternately, a celestial frame may be defined in a dynamical way, in which case the underlying objects that serve for this purpose are solar system objects. Since 1998, the official celestial reference frame (i.e approved by the International Astronomical Union) has been based on the positions of extragalactic radio sources measured with the Very Long Baseline Interferometry (VLBI) technique. This frame has been called ICRF, i.e. International Celestial Reference Frame. The first generation ICRF comprised 608 sources, while the most recent one (ICRF3), adopted in 2018, includes 4536 sources. Coordinate accuracies in ICRF3 are at the 100 µas level on average, with a noise floor of 30 µas. Just about at the same time as ICRF3 was produced, the Gaia space mission released the first-ever celestial frame in the optical domain. This frame has a similar level of accuracy as the ICRF3 but has two orders of magnitude more sources. Comparison between the two frames is essential to unveil potential systematic errors but also to learn about source physics from the relative optical and radio positions. The ICRF has been useful for many applications: navigation of spacecrafts in the solar system, observation of weak objects in the vicinity of the reference sources, monitoring of the Earth’s orientation, measuring crustal deformations on Earth…
More about the author:
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 6000 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 at present through the project JUMPING JIVE. 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.