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Session 44 - Circumstellar Material.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
We present results of a snapshot polarization survey of OH masers in circumstellar envelopes conducted with the VLBA. Early observations of 1612 MHz OH maser emission towards OH/IR stars revealed little polarization in contrast to the interstellar masers where high polarization is frequently found. Single-dish studies have shown that the degree of polarization is strongly dependent on the velocity resolution of the observations. More recent VLBI observations have also shown that high spatial resolution is also essential to determining individual component polarization. We observed the OH maser emission towards 70 OH/IR stars characterized by their OH and IR radiation. In our sample, we have selected sources with fluxes greater than 2 Jy and declinations greater than -40 deg. Using the VLBA with 2 baseband channels each with a bandwidth of 125 kHz, we were able to observe the 1612 and 1667 MHz transitions simultaneously in both right and left circular polarization. The VLBA correlator can produce 128 spectral channels per baseband channel providing us with a velocity resolution of better than 0.2 km s^-1. We are thus able to accurately determine polarization characteristics of a large sample of OH/IR stars.
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