A search for absorption of MG and CA compounds in molecular clouds towards Galactic continuum sources
Absorption lines of MgH and CaH N=1-0 transitions were searched for in foreground molecular clouds towards the continuum sources associated with Sgr B2 (M) and W49A (N). None of these lines was detected with our sensitivity level of ~20 mK. Millimetric absorption lines of MgO, MgOH, CaO and CaOH were also searched for towards Sgr B2 (M) without success. The fractional abundances relative to molecular hydrogen are <~1.0x10^-11 for MgH, <~7.9x10^-13 for MgO, <~1.6x10^-10 for MgOH, <~1.6x10^-9 for CaH, <~2.0x10^-12 for CaO, and <~2.5x10^-10 for CaOH, respectively. The low abundances measured in absorption indicate that a significant fraction of interstellar magnesium and calcium cannot be tied up in their monohydrides, monoxides and monohydroxides. The low abundance of MgH also implies that grain-surface chemistry involving magnesium is not efficient and that magnesium is depleted on to grains to a factor of >~10^2.5 in well-shielded molecular clouds.