The Sizes of OH (1720 MHz) Supernova Remnant Masers: MERLIN and Very Long Baseline Array Observations of IC 443

MERLIN and Very Long Baseline Array (VLBA) observations of the 1720 MHz maser emission from the OH molecule in the supernova remnant IC 443 are presented. Based on MERLIN data with a resolution of 160 mas, the deconvolved sizes of the maser sources are in the range 90-180 mas (135-270 AU). The 12 mas resolution VLBA images show compact cores with sizes in the range of 15-55 mas. The maser brightness temperatures are (2-34) × 106 K for the MERLIN sources and (5-19) × 108 K for the VLBA cores, in agreement with theory. Unlike the Zeeman Stokes V profiles observed in other OH (1720 MHz) SNR masers, single-handed circular polarization line profiles are observed in IC 443 on all angular scales from 1000 to 10 mas resolution. For one line component, the observed line width is 0.24 ± 0.07 km s-1, compared to an estimated Doppler width of 0.49 km s-1. This discrepancy in line widths can be accounted for if the maser emission arises from an elongated ellipsoidal region of masing gas.