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  =  0.01 mas D = 1.95  0.04 kpc (Xu et al. 2006) CH 3 OH (12 GHz)  =  mas D = 2.04  0.07 kpc (Hachisuka et al. 2006) H 2 O (22 GHz) consistent within 2 

W3(OH) motion fit outflow model to the relative motion of H 2 O masers get center of expansion (CoE) proper motion of CoE: –1.47 ± 0.1 mas/yr in RA –1.01 ± 0.1 mas/yr in DEC relative motions in CH 3 OH are smaller no outflow model needed average proper motion : –1.204 ± 0.02 mas/yr in RA –0.147 ± 0.01 mas/yr in DEC for a flat rotation curve: 14 km/s slower and 17 km/s inwards

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)

Dominant source of error: - inaccurate zenith delay τ 0 at each antenna in correlator-model - different phase-errors for different sources Δφ 1 = 2πν·Δτ 0 sec(Z1) Δφ 2 = 2πν·Δτ 0 sec(Z2) } τ0τ0 τ 0 sec(Z 1 ) τ 0 sec(Z 2 ) Z Δτ 0  degrade image quality

‘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

‘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

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

high correlation between parallax and proper motion