P-L Relation Compared with the P-L relation from Whitelock & Feast (2000) VLBI results (black) show less scatter than the Hipparcos results (red) Source | P-L | Hipparcos | VLBI distance U Her 380 pc 610 pc 277 pc W Hya 90 pc 115 pc 98 pc R Cas 200 pc 106 pc 176 pc S CrB 470 pc 526 pc 433 pc Astrometry of the OH Masers of 4 Mira Stars Wouter Vlemmings 1,2, Huib Jan van Langevelde 3, Phil Diamond 4 1 Center for Radiophysics & Space Research, Cornell University, Ithaca, NY 14853, USA 2 Sterrewacht Leiden, Universiteit Leiden, Niels Bohr Weg 2, 2300 RA Leiden, The Netherlands 3 Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands 4 MERLIN/VLBI National Facility, Jodrell Bank Observatory, Macclesfield, Cheshire, SK11 9DL, UK Astrometry of the OH Masers of 4 Mira Stars Wouter Vlemmings 1,2, Huib Jan van Langevelde 3, Phil Diamond 4 1 Center for Radiophysics & Space Research, Cornell University, Ithaca, NY 14853, USA 2 Sterrewacht Leiden, Universiteit Leiden, Niels Bohr Weg 2, 2300 RA Leiden, The Netherlands 3 Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands 4 MERLIN/VLBI National Facility, Jodrell Bank Observatory, Macclesfield, Cheshire, SK11 9DL, UK Introduction Masers around AGB stars can be monitored with VLBI astrometry We measure the parallaxes of Mira variables through OH OH has limited accuracy but great persistency to allow this to work Early results: van Langevelde et al., 2000 Goals Measure distances of the more enshrouded AGB variables optically bright ones done successfully by Hipparcos Establish occurrence of “amplified stellar image” Special blue spot could be radio-sphere amplified by shell Determine the location of different maser shells with respect to star Galaxy dynamics: 10 km/s at 8 kpc: μ=0.250 mas/yr Stellar astrophysics, binaries, planets Reference frame ties Needs What is the position and motion of the maser wrt the star? amplified stellar image would be beacon Persistent masers in order to connect between epochs. Observations Observed 4 Mira variable stars with the VLBA 8 years of U Her (1994 – 2002) 3 years of W Hya, S CrB and R Cas (1999 – 2002) Phase referencing each target to 2 extra-galactic reference sources continuum calibrators in 2 x 4 MHz bands masers at 1665/67 MHz (1.95 kHz = 0.36 km/s resolution) Estimate remaining errors from calibrator pairs: Dominated by ionospheric errors (uncalibrated) Errors correlate with solar activity Results Fitted proper motions and parallaxes to 4 AGB stars Obtained improved distances to U Her, R Cas and S CrB The maser spot of W Hya shows additional motion, possibly a signature of stellar pulsation The most blue-shifted maser spot position in U Her and W Hya is consistent with being the amplified stellar image Good results from red-shifted spots in R Cas and S CrB OH maser astrometry is limited by intrinsic brightness and ionosphere. The latter can probably be improved, the method is applicable for OH maser stars closer than 1 kpc. Other masers may be more promising/challenging at larger distances (e.g. H 2 O masers, Vemmings et al 2002). References Van Langevelde et al, 2000, A&A, 357, 945 Vlemmings et al, 2002, A&A, 393, L33 Whitelock & Feast, 2000, MNRAS, 319, 759 Status: Astronomy & Astrophysics in press, astroph See also: Vlemmings PhD Thesis 2002, Comparison with Optical Position Astrometric maser positions are compared with Hipparcos Transposing to common epoch with VLBI proper motion and parallax (black) and with the Hipparcos proper motion and parallax (red) For U Her and W Hya the monitored maser spot is on the blue-shifted, front side of the shell. These coincide within the error with the position of the stellar radio-sphere, consistent with being the amplified stellar image. For R Cas and S CrB only red-shifted emission is observed at  80 AU from the stellar position U Her S CrB W Hya R Cas The position of the brightest maser spots for our 4 sources with respect to their extragalactic calibrator. The red line indicates the fitted proper motion and parallax The motion of the most blue-shifted 1667 MHz OH maser spot of U Her for the first 8 epochs of observation Motion fitting results U Her: Π vlbi = 3.61 ± 1.04 mas, μ vlbi = ± 0.38, ± 0.42 mas/yr Π hip = 1.64  1.31 mas, μ hip = ± 0.82, ± 0.92 mas/yr R Cas: Π vlbi = 5.67 ± 1.95 mas, μ vlbi = ± 2.35, ± 1.75 mas/yr Π hip = 9.37  1.10 mas, μ hip = ± 0.95, ± 0.88 mas/yr S CrB: Π vlbi = 2.31 ± 0.33 mas, μ vlbi = ± 0.38, ± 0.33 mas/yr Π hip = 1.90  1.36 mas, μ hip = ± 0.93, ± 0.62 mas/yr W Hya: Π vlbi = ± 2.36 mas, μ vlbi = ± 2.04, ± 2.93 mas/yr Π hip = 8.73  1.09 mas, μ hip = ± 1.18, ± 0.78 mas/yr