Observations of interstellar C2 toward KHI Oph, HD 154368, 147889 and149404.

Interstellar absorption lines of the C2 (2-0) Phillips band at 8750 A have been searched for in the spectra of southern stars. Seventeen lines originating from the lowest eight rotational levels have been detected toward Chi Oph, and eleven lines originating from the lowest five rotational levels toward HD 154368 and 147889. No C2 lines were seen toward HD 149404. A recently developed theory has been used to extract information about the density, temperature or strength of the radiation field in the line-forming interstellar regions from the observed rotational populations. The results are compared with those obtained from other molecular observations. Toward Chi Oph, the interstellar radiation field appears enhanced in the ultraviolet part of the spectrum relative to the infrared part. The C2 data suggest a higher kinetic temperature for the material in front of HD 147889 than is inferred from radio observations.