Very Long Baseline Array Observations of the Zeeman Effect in H2O Masers in W3 IRS 5

We present Very Long Baseline Array (VLBA) observations of the Zeeman effect in H2O masers in W3 IRS 5. These observations have yielded the first ultrahigh spatial resolution maps of the magnetic field in a star-forming region measured via the Zeeman effect—masers for which we can independently measure the magnetic field are separated by ≤25 mas (60 AU). This is the first VLBA detection of the Zeeman effect in H2O masers in a star-forming region. These observations also remove the confusion in observed magnetic fields from closely spaced masers. We shall explicitly demonstrate that observations at lower resolution can yield lower magnetic fields than with the VLBA, even though there are no field reversals within the larger beam.