David Hobbs

David Hobbs

Senior lecturer

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Global Astrometry:

In 2007 I joined Lund observatory to work on the Gaia mission which is designed to measure the three dimensional distribution of stars in the Galaxy and their three dimensional velocity distribution. It will also be able to determine their astrophysical properties, such as surface gravity and effective temperature, to map and understand the formation, structure, and past and future evolution of our Galaxy. As noted in the Gaia mission paper, the Galaxy contains many different types of stars and planets, interstellar gas and dust, and dark matter. These components are widely distributed in age, reflecting their formation history, and in space, reflecting their birth places and subsequent motions. Objects in the Galaxy move in a variety of orbits that are determined by the gravitational force, and have complex distributions of chemical element abundances, reflecting star formation and gas-accretion history. Understanding all these aspects in one coherent picture is the main aim of Gaia.

Gaia's very accurate astrometric measurements will also allow the International Celestial Reference System to be improved by a few orders of magnitude. Several sets of quasars are used to define a kinematically stable non-rotating reference frame with the barycentre of the solar system as its origin. Gaia will also observe a large number of galaxies. Although they are not point-like, it may be possible to determine accurate positions and proper motions for some of their compact bright features.

Real time cosmology:

Various sources of proper motion are measurable by Gaia with cosmological implications: 1) Quasar photocentre variability. 2) The acceleration of the solar system barycentre, presumably, towards the Galactic centre. 3) The instantaneous velocity of the solar system with respect to the CMB causes extragalactic sources to undergo an apparent systematic proper motion. 4) Primordial gravitational waves could give rise to systematic proper motions over the sky. 5) An anisotropic expansion of the Universe would result in a distortion of distant objects on the celestial sphere. 6) The peculiar motion of a galaxy is its velocity relative to the Hubble flow. 7) Microlensing events can be detected by Gaia.

Exoplanets with astrometry:

The existence of an exoplanet can be inferred from the motion of its host star and Gaia can probe a poorly explored area in the parameter space giving an un-biased, volume-limited sample of Jupiter-mass planets in multi-year orbits and provide astrophysical parameters such as masses (rather than lower limits) not obtainable by other means. These are logical prime targets for future searches of terrestrial-mass exoplanets in the habitable zone in an orbit protected by a giant planet further out. The synergies between PLATO and Gaia data will unveil their level of coplanarity and reveal solar system analogues. Finally, the data of Gaia will provide detailed distributions of the giant planet -- brown dwarf transition regime as a function of stellar-host properties with unprecedented resolution.

Future missions:

A weakness of Gaia is that it only operates at optical wavelengths while much of the Galactic centre and spiral arm regions are obscured by interstellar extinction making it difficult to probe deeply. I successfully proposed the study of a future Gaia-like mission called GaiaNIR motivated by: 1) Near-Infra-Red (NIR) all-sky astrometry and photometry to penetrate obscured regions and for observing intrinsically red objects. 2) Proper motions with 14-20 times smaller errors than from Gaia alone when combining positions from two epochs at 20 year intervals opening new science cases. 3) The slowly degrading accuracy of the Gaia optical reference frame, which will be the basis for modern astronomical measurements, can be maintained. I have also been involved in other high precision astrometry mission proposals such as NEAT and Theia.

The glue that binds them:

Much of my work at Lund Observatory has been focused on developing the numerical solutions used for reducing the raw Gaia data to the scientifically usable quantities, position, parallax and proper motion mentioned above and for me this has been the great challenge of Gaia. The astrometric core solution of Gaia is a global noise dominated problem involving a very large number of unknowns. Similar problems face the LISA community where the problem is also global but is signal dominated. The numerical techniques used to uncover exoplanets, or proper motion patterns are similar to those used for gravitational wave detection. Synergies between missions like Gaia, PLATO and LISA give greater insight into the Universe and the glue that binds them are the innovative numerical methods that we are known for in Lund.

UKÄ subject classification

  • Astronomy, Astrophysics and Cosmology
  • Condensed Matter Physics


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