Taking the Measure of the Universe: Precision Astrometry withSIM PlanetQuest

Abstract

Precision astrometry at microarcsecond accuracy has application to a wide range of astrophysical problems. This paper is a study of the science questions that can be addressed using an instrument with flexible scheduling that delivers parallaxes at about 4 microarcsec (µas) on targets as faint as V = 20, and differential accuracy of 0.6 µas on bright targets. The science topics are drawn primarily from the Team Key Projects, selected in 2000, for the Space Interferometry Mission PlanetQuest (SIM PlanetQuest). We use the capabilities of this mission to illustrate the importance of the next level of astrometric precision in modern astrophysics.SIM PlanetQuest is currently in the detailed design phase, having completed in 2005 all of the enabling technologies needed for the flight instrument. It will be the first space-based long baseline Michelson interferometer designed for precision astrometry. SIM will contribute strongly to many astronomical fields including stellar and galactic astrophysics, planetary systems around nearby stars, and the study of quasar and AGN nuclei. Using differential astrometry SIM will search for planets with masses as small as an Earth orbiting in the 'habitable zone' around the nearest stars, and could discover many dozen if Earth-like planets are common. It will characterize the multiple-planet systems that are now known to exist, and it will be able to search for terrestrial planets around all of the candidate target stars in the Terrestrial Planet Finder and Darwin mission lists. It will be capable of detecting planets around young stars, thereby providing insights into how planetary systems are born and how they evolve with time. Precision astrometry allows the measurement of accurate dynamical masses for stars in binary systems. SIM will observe significant numbers of very high-and low-mass stars, providing stellar masses to 1%, the accuracy needed to challenge physical models. Using precision proper motion measurements, SIM will probe the Galactic mass distribution, and through studies of tidal tails, the formation and evolution of the Galactic halo. SIM will contribute to cosmology through improved accuracy of the Hubble Constant. With repeated astrometric measurements of the nuclei of active galaxies, SIM will probe the dynamics of accretion disks around supermassive black holes, and the relativistic jets that emerge from them.

Gravitational waves: Classification, methods of detection, sensitivities and sources

“…Long baseline optical interferometer with sub-micro-arcsecond and nano-arcsecond (nas) astrometry is technologically feasible. 114 With this kind of interferometer implemented, precision astrometry of quasar proper motions may possibly be improved by 4 orders of magnitude and reach nas yr -1 . In terms of energy, the precision of determining/constraining Ωgw(f) could reach a sensitivity of 2.4  10 −9 or better ( Fig.…”

Relativistic gravitational deflection of light and its impact on the modeling accuracy for the Space Interferometry Mission

“…A major objective of this paper is to show that, before microarcsecond-level astrometry will become a powerful tool for 21-st century astronomy, there is a need for an adequate modeling necessary to match this new frontier of astrometric accuracy. The prospect of new high precision astrometric measurements from space with SIM requires inclusion of relativistic effects at the (v/c) 3 level [6,21]. At the level of accuracy expected from SIM, even more subtle gravitational effects on astrometry from within the solar system will start to become apparent, such as the monopole and the quadrupole components of the gravitational fields of the planets [22] and the gravito-magnetic effects caused by their motions and rotations.…”

“…Space-based astrometry has brought about a renaissance in the entire field of astrometry that is perhaps the most fundamental, and oldest of all areas in astronomy [6]. The ESA Hipparcos mission, which operated from 1989-1993, yielded an astrometric catalog of 118,000 stars down to 12.5 magnitude, with positional accuracy of 1 mas for stars brighter than V = 11.…”

“…The European Space Agency (ESA) is now developing the Gaia mission as a next generation astrometric survey mission [7,8], which is expected to produce a catalog of ∼ 10 9 stars, with accuracy ≃ 20-25 microarcsec (µas) for stars brighter than V = 15. Precision astrometry remains a cornerstone of the field and is poised to make a major impact on many fields of modern astronomy, astrophysics, and cosmology [6].…”

“…This performance is about 1000 times better than existing capabilities on the ground or in space, and about 100 times better than the upcoming Gaia mission, for differential measurements. Such high accuracy will allow SIM to detect and measure masses of terrestrial planets around stars in our Galactic neighborhood (see [6] for review).…”