Study of Interstellar Molecular Clouds using Formaldehyde Absorption
  toward Extragalactic Radio Sources
  We present new Very Large Array 6cm H2CO observations toward four
extragalactic radio continuum sources (B0212+735, 3C111, NRAO150, BL Lac) to
explore the structure of foreground Galactic clouds as revealed by absorption
variability. This project adds a new epoch in the monitoring observations of
the sources reported by Marscher and collaborators in the mid 1990's. Our new
observations confirm the monotonic increase in H$_2$CO absorption strength
toward NRAO150. We do not detect significant variability of our 2009 spectra
with respect to the 1994 spectra of 3C111, B0212+735 and BL Lac; however we
find significant variability of the 3C111 2009 spectrum with respect to archive
observations conducted in 1991 and 1992. Our analysis supports that changes in
absorption lines could be caused by chemical and/or geometrical gradients in
the foreground clouds, and not necessarily by small scale (~10 AU) high density
molecular clumps within the clouds.
