Detection of carbon recombination lines below 100 MHz towards the Galactic centre and M16.

Abstract

The detection of highly excited recombination lines of carbon in absorption toward the Galactic center at 75 MHz and toward M16 at 68 and 80 MHz is reported. Toward M16 the line was also detected in emission at 325 MHz. The central velocity of the line toward the Galactic center is near 0 km/s and corresponds to any cloud along the line of sight. The central velocity toward M16 is + 19 km/s and agrees with a known H-I absorption feature in this direction. The optical depths range from 0.0006 to 0.002 and the linewidths are about 15 km/s. These two detections were the only positive ones in observations at several frequencies below 100 MHz toward 19 directions in the galactic plane.