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EVN 2015: Astrometry

EVN 2015: Astrometry. Parallaxes of Galactic sources. W3(OH) parallax. CH 3 OH and H 2 O masers observed with VLBA kinematic distance > 4 kpc. CH 3 OH (12 GHz). H 2 O (22 GHz).  = 0.514  0.01 mas D = 1.95  0.04 kpc (Xu et al. 2006).  = 0.489  0.017 mas

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EVN 2015: Astrometry

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  1. EVN 2015: Astrometry Parallaxes of Galactic sources

  2. W3(OH) parallax • CH3OH and H2O masers observed with VLBA • kinematic distance > 4 kpc CH3OH (12 GHz) H2O (22 GHz)  = 0.514  0.01 mas D = 1.95  0.04 kpc (Xu et al. 2006)  = 0.489  0.017 mas D = 2.04  0.07 kpc (Hachisuka et al. 2006) consistent within 2 

  3. W3(OH) motion • fit outflow model to the relative motion of H2O masers • get center of expansion (CoE) • proper motion of CoE: –1.47 ± 0.1 mas/yr in RA • –1.01 ± 0.1mas/yr in DEC • relative motions in CH3OH are smaller • no outflow model needed • average proper motion : –1.204 ± 0.02 mas/yr in RA • –0.147 ± 0.01mas/yr in DEC • for a flat rotation curve: 14 km/s slower and 17 km/s inwards

  4. Role of EVN • Methanol masers are better sources • 6.7 GHz transition found all over the Milky Way • Unique opportunity for the EVN to: • get accurate distances • locate spiral arms • constrain Galactic rotation • test models of spiral density • wave theory • GAIA/SIM can not see • whole Milky Way (dust)

  5. τ0sec(Z1) Δτ0 τ0 τ0sec(Z2) Z } Dominant source of error: - inaccurate zenith delay τ0 at each antenna in correlator-model - different phase-errors for different sources Δφ1= 2πν·Δτ0sec(Z1) Δφ2= 2πν·Δτ0sec(Z2)  degrade image quality

  6. ‘geodetic’ observations • Include geodetic-like observations - many quasars at different elevations - 8 IFs spanning 70 MHz (EVN) or 450 MHz (VLBA) • Allows determination of atmospheric zenith delay and clock errors • CLCOR to correct for this error • Improved image quality

  7. ‘geodetic’ observations • Include geodetic-like observations - many quasars at different elevations - 8 IFs spanning 70 MHz (EVN) or 450 MHz (VLBA) • Allows determination of atmospheric zenith delay and clock errors • CLCOR to correct for this error • Improved image quality

  8. The EVN 2015 • higher bandwidth • - higher signal-to-noise ratio • - using weaker (closer) calibrators • better calibration • - improved ionospheric models (e.g. GPS) • - improved troposheric models (e.g. WVR, more accurate delays) • - multiple beams? • more long baselines • flexible scheduling

  9. high correlation between parallax and proper motion

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