Carbon recombination lines from the Galactic plane at 34.5 & 328 MHz

We present the results of a search for carbon recombination lines in the Ga laxy at 34.5 MHz (C575 alpha) made using the dipole array at Gauribidanur n ear Bangalore. Observations made towards 32 directions resulted in detectio ns of lines in absorption at nine positions. Followup observations at 328 M Hz (C212 alpha) using the Ooty Radio Telescope detected these lines in emis sion. A VLA D-array observation of one of the positions at 330 MHz yielded: no detection implying a lower limit of 10' for the angular size of the line forming region. The longitude-velocity distribution of the observed carbon lines indicate t hat the line forming regions are located mainly between 4 kpc and 7 kpc fro m the Galactic centre. Combining our results with published carbon recombin ation line data near 76 MHz (Erickson, McConnell & Anantharamaiah 1995), we obtain constraintson the physical parameters of the line forming regions. We find thatif the angular size of the line forming regions is greater than or equal to 4 degrees, then the range of parameters that fit the data are: T-e = 20 - 40 K, n(e) similar to 0.1 - 0.3 cm(-3) and pathlengths similar to 0.07 - 0.9 pc which may correspond to thin photodissociated regions arou nd molecular clouds. On the other hand, if the line forming regions are sim ilar to 2 degrees in extent, then warmer gas (T-e similar to 60 - 300 K) Wi th lower electron densities (n(e) similar to 0.03 - 0.05 cm(-3)) extending over several tens of parsecs along the line of sight and possibly associate d with atomic HI gas can fit the data. Based on the range of derived parame ters, we suggest that the carbon line regions are most likely associated wi th photo-dissociation regions.