A search for interstellar C₃ in the translucent cloud toward HD 147889

As part of an effort to search for new interstellar molecules at optical and near-UV wavelengths, we have obtained high S/N (≈ 500) echelle CCD spectra of the well-studied translucent sight line toward HD 147889 in the ρ Oph dark cloud complex. These observations show that HD 147889 is actually a double-lined spectroscopic binary consisting of two B2 IV-V components with rotational (v sin i) velocities of about 30 and 70 km s^-1, an orbital period of about 5 days, and a radial velocity semiamplitude of about 70 km s^-1. The presence and movement of the weak stellar features in the spectrum of HD 147889 may account for some of the discrepancies in previous interstellar absorption-line studies of this object. Despite this potential stellar confusion, we have tentatively identified an interstellar absorption feature at the wavelength (4051.6 Å) expected for an unresolved blend of the strongest Q-branch lines arising from the ground vibrational state of the C₃ A¹Π-X¹Σ electronic transition. Assuming a cloud temperature of 40 K, the strength of this feature would correspond to a C₃ column density of 4 x 10¹² cm^-2 toward HD 147889. This value of N(C₃) is about a factor of 5 smaller than that predicted by the van Dishoeck & Black (1989) model of the HD 147889 cloud.