The Spectra of Sgr A and Sgr B2 between 1636 and 1640 MHz

Abstract

The most prominent features in the spectra of Sgr A and Sgr B2 between 1636 and 1640 MHz are absorption lines corresponding to the 2π3/2, J = 3/2 ground state transitions of the isotropic species 18OH, which have rest frequencies of 1637.5 and 1639.4 MHz (Rogers and Barrett 1966). As a result of Doppler shifting, the lines of each transition are spread over a frequency range greater than 1.5 MHz. There are also present emission features which Zuckerman et al. (1971) have identified with the 110 -l11 transition of formic acid (HCOOH), which has a rest frequency of 1638.805 MHz. In view of some disagreement in the literature over the 16OH/18OH abundance ratio (Wilson and Barrett 1970; McGee et al. 1970), and of the baseline uncertainties in the spectra published in support of the HCOOH detection, we have obtained spectra of the two sources over the frequency range under discussion.