The Nature of Interstellar Gas toward the Pleiades Revealed in Absorption Lines

We present high-resolution, high signal-to-noise ratio absorption-line observations of CN, Ca II, Ca I, CH⁺, and CH along 20 lines of sight toward members of the Pleiades. The acquired data enable the most detailed study to date of the interaction between cluster stars and the surrounding interstellar gas. Total equivalent widths are consistent with previous investigations, except where weaker features are detected owing to our greater sensitivity. Mean b-values for the molecular species indicate that toward most of the Pleiades, CH is associated with the production of CH⁺ rather than CN. An analysis of radial velocities reveals a kinematic distinction between ionized atomic gas and molecular and neutral gas. Molecular components are found with velocities in the local standard of rest of either ~+7 or ~+9.5 km s^-1, with the higher velocity components being associated with the strongest absorption. Atomic gas traced by Ca II shows a strong central component at v_LSR~+7 km s^-1, exhibiting velocity gradients indicative of cloud-cluster interactions. Gas density estimates derived from measured CH/CH⁺ column density ratios show good agreement with those inferred from H₂ rotational populations, yielding typical values of n~50 cm^-3. Our models do not include the important time-dependent effects on CH⁺ formation, which may ultimately be needed to extract physical conditions in these clouds.