A Very High Spectral Resolution Study of Ground‐State OH Masers in W3(OH)

We present VLBA observations of the ground-state hydroxyl masers in W3(OH) at0.02 km s-1 spectral resolution. Over 250 masers are detected, including 56Zeeman pairs. Lineshapes are predominantly Gaussian or combinations of severalGaussians, with normalized deviations typically of the same magnitude as inmasers in other species. Typical FWHM maser linewidths are 0.15 to 0.38 km s-1and are larger in the 1665 MHz transition than in the other three ground-statetransitions. The satellite-line 1612 and 1720 MHz masers show no evidence ofsigma^+/-2,3 components. The spatial positions of most masers are seen to varyacross the line profile, with many spots showing clear, organized positionalgradients. Equivalent line-of-sight velocity gradients in the plane of the skytypically range from 0.01 to 1 km s-1 AU-1 (i.e., positional gradients of 1 to100 AU (km s-1)-1). Small velocity gradients in the 1667 MHz transition supporttheoretical predictions that 1667 MHz masers appear in regions with smallvelocity shifts along the amplification length. Deconvolved maser spot sizesappear to be larger in the line wings but do not support a spherical masergeometry.Comment: 17 pages, including 13 figures and 2 tables, uses emulateapj.cls, accepted to Ap