A New Threshold of Precision, 30 micro-arcsecond Parallaxes and Beyond
Abstract
The ESA Gaia mission is poised to dramatically tighten the distancescale for all stellar types, with a billion Milky Way parallaxesreaching 10 microarcseconds for V<12 mag, 20 microarcseconds atV=15, and 200 microarcseconds at V=20. These data will have enormousimpact on nearly any investigation that makes use of stellarastrophysics, including stellar evolution, galactic archeology,exoplanet characterization, and physical cosmology. Measurements thisrevolutionary demand a number of independent tests for the presence ofsystematic errors. We have developed a method that can measureparallaxes of the best-observed stars to 30 microarcseconds with WFC3using spatial scanning (Riess et al. 2014). We propose to obtain 4 newepochs of spatial-scanning measurements for 9 previously observedfields in order to collect 150 stellar parallaxes and improve thesample mean precision to 30 microarcseconds, sufficient for ameaningful test of Gaia. The proposed doubling of the temporalcoverage for these fields will deliver (1) a 40% improvement in theprecision of the HST parallaxes which otherwise limit the precision ofthe comparison, (2) the ability to weed out relevant astrometricbinaries which could otherwise pollute a number of parallaxmeasurements and the comparison to Gaia, and (3) a significant overlapin time of the HST and Gaia measurements, insuring that all parallaxesare subject to similar orbital motion in the event of undetectedbinarity, thus improving the accuracy of the comparison. We propose tofollow the old Russian proverb - trust but verify.
- Publication:
-
HST Proposal
- Pub Date:
- October 2015
- Bibcode:
- 2015hst..prop14206R