Microarcsecond Radio Astrometry

Abstract

Astrometry provides the foundation for astrophysics. Accurate positions are required for the association of sources detected at different times or wavelengths, and distances are essential to estimate the size, luminosity, mass, and ages of most objects. Very Long Baseline Interferometry at radio wavelengths, with diffraction-limited imaging at sub-milliarcsec resolution, has long held the promise of micro-arcsecond astrometry. However, only in the past decade has this been routinely achieved. Currently, parallaxes for sources across the Milky Way are being measured with ∼ 10 µas accuracy and proper motions of galaxies are being determined with accuracies of ∼ 1 µas y −1 . The astrophysical applications of these measurements cover many fields, including star formation, evolved stars, stellar and super-massive black holes, Galactic structure, the history and fate of the Local Group, the Hubble constant, and tests of general relativity. This review summarizes the methods used and the astrophysical applications of micro-arcsecond radio astrometry.

SUBMILLIMETER H2O MEGAMASERS IN NGC 4945 AND THE CIRCINUS GALAXY

“…Though the absolute astrometric precision for ALMA observations is typically limited to ∼0.05 arcsec without taking special calibration steps (Reid & Honma 2014;Remijan et al 2015), the relative uncertainty in point-source position within the same primary beam (as a fraction of the half-power beam width) is inversely proportional to the S/N (see, e.g., Fouque et al 1990). Thus future high-resolution ALMA observations should have little difficulty mapping the 321 GHz masers in Circinus.…”

Geodetic VLBI with an artificial radio source on the Moon: a simulation study

“…Both approaches imply that positioning is only possible relative to a reference source and often a nearby phase calibrator is needed. In the latter case, relative measurements are also affected by position uncertainties of the reference source (Fey et al 2015;Reid and Honma 2014). In order to avoid relative positioning or phase-referencing, one might try to position a target in an absolute sense, i.e., expressed directly in the celestial reference frame.…”

FIRST PARALLAX MEASUREMENTS TOWARD A 6.7 GHz METHANOL MASER WITH THE AUSTRALIAN LONG BASELINE ARRAY—DISTANCE TO G 339.884−1.259.

“…This is being undertaken using the NRAO Very Long Baseline Array (VLBA) to measure position changes of methanol and water masers in the Galactic disk, with respect to distant background quasars. Phase referencing of one of the quasar and maser emission data to the other (Alef et al 1988;Beasley et al 1995;Reid & Honma 2014), combined with careful calibration of atmospheric effects, allows the change in the relative separation between the masers and quasars to be accurately measured. Repeated observations timed to maximize the measured amplitude of the parallax signature can measure the parallax to an accuracy of up to 10 μarcsec and simultaneously determine the source proper motions to ∼1 μarcsec yr −1 (Reid & Honma 2014).…”

“…Phase referencing of one of the quasar and maser emission data to the other (Alef et al 1988;Beasley et al 1995;Reid & Honma 2014), combined with careful calibration of atmospheric effects, allows the change in the relative separation between the masers and quasars to be accurately measured. Repeated observations timed to maximize the measured amplitude of the parallax signature can measure the parallax to an accuracy of up to 10 μarcsec and simultaneously determine the source proper motions to ∼1 μarcsec yr −1 (Reid & Honma 2014). Thus far the combination of all astrometric VLBI observations (including the European VLBI Network (EVN; e.g., Rygl et al 2008Rygl et al , 2010Honma et al 2012;Sakai et al 2012;Imai et al 2013;Chibueze et al 2014;Reid & Honma 2014) has yielded more than 100 parallax and proper motion measurements for star forming regions across large portions of the Milky Way visible from the northern hemisphere.…”

“…Repeated observations timed to maximize the measured amplitude of the parallax signature can measure the parallax to an accuracy of up to 10 μarcsec and simultaneously determine the source proper motions to ∼1 μarcsec yr −1 (Reid & Honma 2014). Thus far the combination of all astrometric VLBI observations (including the European VLBI Network (EVN; e.g., Rygl et al 2008Rygl et al , 2010Honma et al 2012;Sakai et al 2012;Imai et al 2013;Chibueze et al 2014;Reid & Honma 2014) has yielded more than 100 parallax and proper motion measurements for star forming regions across large portions of the Milky Way visible from the northern hemisphere. Having determined the position at a reference epoch, parallax and proper motion of the masers, the complete threedimensional location and velocity vectors of these sources relative to the Sun can be found.…”