A search for interstellar CH2 toward HD 154368 and ζ Ophiuchi

We have searched for molecular CH2 in the interstellar gas in the line of sight to HD 154368 and zeta Oph to constrain further the gas-phase chemistry of carbon-bearing molecules in the cool, translucent clouds toward these stars. We report tentative detection of CH2 in the interstellar spectra of both stars. In zeta Oph, slight depressions in the stellar continuum are seen near 1410.1 and 1415.8 Angstrom corresponding to band structure in CH2, while in HD 154368 a single, narrow feature is found at 1410 Angstrom that may be due to CH2. Assuming that these features are produced by CH2, the column densities of CH2 toward HD 154368 and zeta Oph are (5.9 +/- 3.1) x 10(13) cm(-2) (W-lambda 1410.1 = 5.2 m Angstrom) and (3.3 +/- 1.9) x 10(13) cm(-2) (W-lambda 1415.8 = 29 mA), respectively. Although the oscillator strengths for the relevant electronic transitions are somewhat problematic, our deduced column densities are within a factor of 2-5 of those predicted by the model chemistry. These observations may represent the first detection of CH2 interstellar absorption in any astronomical object. Additional theoretical estimates and laboratory measurements of relevant oscillator strengths are needed to provide more accurate estimates of CH2 column densities and stronger constraints on the carbon chemistry in these interstellar molecular clouds.