Detection of The..... Transition of 18OH at 1584-MHZ Toward SAGITTARIUS-B2

Observations of the 1584 MHz line of ¹⁸OH (corresponding to the 1612 MHz line of ¹⁶OH) give for the +60 km s^-1 cloud associated with Sgr B2 an abundance ratio [¹⁶OH]:[¹⁸OH] of 220 ± 20 as compared with the terrestrial value of 490.

Departures from LTE for the satellite lines are smaller for ¹⁵OH than for ¹⁶OH, as might be expected if optical depths of the IR transitions connecting the different A-doublets are small at the ¹⁵OH wavelengths.