Molecules toward HD 62542 - A high-density, peculiar extinction sight line in the GUM nebula complex

We present high signal-to-noise (S/N ≍ 200-500) absorption-line data covering the violet transitions of CN, CH, CH ⁺, Ca II, Ca I, and Fe I toward HD 62542, an extremely peculiar ultraviolet extinction sight line passing through a dense wind-modified ridge of gas and dust located inside the Gum nebula. The column densities of CN and CH, 1013.41 and 1013.55 cm⁻², respectively, are quite large, especially for a sight line with only 1 mag of visual extinction, A(V). In fact, per unit A(V), this is one of the richest molecular environments for which optical spectroscopy exists. However, no CH⁺ is detected yielding a 2 upper limit to the column density of ≤ 1011.83 cm⁻². A detailed quantitative analysis of the CN rotational lines yields a N = 0 → 1 excitation temperature, T₀₁ = 3.7±0.9 K (2 σ limits). T₀₁ is larger than the nominal value of 2.76 K associated with excitation by the cosmic microwave background, suggesting significant excitation from collisional processes. This result, along with a comparison of the column densities of CN and CH to recent models by van Dishoeck and Black, suggests a large spatial density (n ≍10⁴ cm⁻³). For A(V) = 1 mag, a density as large as 10⁴ cm⁻³ is generally only observable in small, low-mass clouds (≤ few Msun). Close examination of the environment indicates that clouds of this type are prevalent. We conclude that the matter through which the sight line passes is probably the dense, remnant core of a cloud that has been stripped of its outer envelope by strong wind and radiation pressure from the nearby stars ζ Pup (04 Iaf) and γ² Vel (WC8 + O9 I). Given the likelihood of strong shocks in the environment, the absence of CH⁺ in this sight line provides an interesting test of the theories of shock production of CH ⁺.