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Enhancing Galactic Astrometry with EVN 2015: Precise Parallaxes and Kinematic Distances of Galactic Sources

This study delves into astrometry of Galactic sources W3(OH) using VLBA observations of CH3OH and H2O masers. By fitting outflow models and calculating proper motions, the research accurately determines distances and constrains Galactic rotation models. The EVN 2015 holds potential for improved image quality and precise observations with higher bandwidth, improved calibration techniques, and flexible scheduling.

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Enhancing Galactic Astrometry with EVN 2015: Precise Parallaxes and Kinematic Distances of Galactic Sources

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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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