The 6 cm formaldehyde ( H _ { 2 } CO ) maser sources in the compact H ii regions NGC 7538-IRS1 and G29.96-0.02 have been imaged at high resolution ( \theta _ { beam } < 50 mas ) .
Using the VLBA and MERLIN , we find the angular sizes of the NGC 7538 masers to be \sim 10 mas ( 30 AU ) corresponding to brightness temperatures \sim 10 ^ { 8 } K. The angular sizes of the G29.96-0.02 masers are \sim 20 mas ( 130 AU ) corresponding to brightness temperatures \sim 10 ^ { 7 } 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 year timescales while the G29.96-0.02 masers show no variability over 2 years .
A search for polarization provides 3- \sigma 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 } ^ { -1 } { arcsecond } ^ { -1 } ( 1900 { km } { s } ^ { -1 } { pc } ^ { -1 } ) is detected in the NGC 7538 maser gas .