2003ApJ...598.1061H

The 6 cm formaldehyde (H₂CO) maser sources in the compact H II regions NGC 7538 IRS 1 and G29.96-0.02 have been imaged at high resolution (θ_beam<50 mas). Using the Very Long Baseline Array and MERLIN, we find the angular sizes of the NGC 7538 masers to be ∼10 mas (30 AU), corresponding to brightness temperatures ∼10⁸ K. The angular sizes of the G29.96-0.02 masers are ∼20 mas (130 AU), corresponding to brightness temperatures ∼10⁷ K. Using the VLA, we detect 2 cm formaldehyde absorption from the maser regions. We detect no emission in the 2 cm line, indicating the lack of a 2 cm maser and placing limits on the 6 cm excitation process. We find that both NGC 7538 maser components show an increase in intensity on 5-10 yr timescales while the G29.96-0.02 masers show no variability over 2 yr. A search for polarization provides 3 σ upper limits of 1% circularly polarized and 10% linearly polarized emission in NGC 7538 and of 15% circularly polarized emission in G29.96-0.02. A pronounced velocity gradient of 28 km/s/arcsec (1900 km/s/pc) is detected in the NGC 7538 maser gas.