A. J. Beasley — Page 2

arXiv1999

Sub-Milliarcsecond Precision of Pulsar Motions: Using In-Beam Calibrators with the VLBA

We present Very Long Baseline Array phase-referenced measurements of the parallax and proper motion of two pulsars, B0919+06 and B1857-26. Sub-milliarcsecond positional accuracy was obtained by simultaneously observing a weak calibrator source within the 40' field of view of the VLBA at 1.5 GHz. We discuss the merits of using weak close calibrator sources for VLBI observations at low frequencies, and outline a method of observation and data reduction for these type of measurements. For the pulsar B1919+06 we measure a parallax of 0.31 +/- 0.14 mas. The accuracy of the proper motions is approximately 0.5 mas, an order of magnitude improvement over most previous determinations.

astro-ph↗

arXiv1996

VLBI observations of the 6.7 and 12.2 GHz methanol masers associated with NGC 6334F

We present milliarcsecond resolution images of the 6.7 and 12.2 GHz methanol maser emission associated with the well-known star formation region NGC 6334F. The images agree well with previous lower resolution observations, but detect approximately double the number of spots seen in the earlier work. Comparison of the relative positions of the 6.7 and 12.2 GHz maser spots shows that five of them are coincident to within the positional accuracy of these observations (approximately 4 milli-arcsec). Menten et al. (1992) observed similar positional coincidence for W3(OH) and in each case the flux density of the 6.7 GHz maser spot was greater than that of the 12.2 GHz methanol maser spot. However, for NGC 6334F several of the coincident maser spots have a larger flux density at 12.2 GHz than at 6.7 GHz. We also detected several 12.2 GHz methanol maser spots with no coincident 6.7 GHz emission. This implies that, although the 6.7 GHz methanol masers usually have a greater flux density than their 12.2 GHz counterparts, regions within the gas cloud exist where the conditions are more favourable for 12.2 than for 6.7 GHz methanol maser emission. If the conditions which give rise to the various observed ratios can be determined, then VLBI images of class II methanol masers will allow the physical conditions of the star formation region to be probed with unprecedented resolution.

astro-ph↗