Physical Properties of Dense Cores: DCO + Observations

We conducted a survey for C¹⁸O, DCO⁺, and H ¹³CO⁺ J = 1 → 0 emission in a sample of low-mass cores previously surveyed in CS, NH₃, and C₃H₂. The DCO⁺ and H¹³CO⁺ observations were used to test deuterium fractionation chemistry models for TMC-1 type clouds. The [DCO⁺/HCO⁺] fractionation ratio (0.045±0.014) was found to be in excellent agreement with current models. In addition, we did a multiple transition study of DCO⁺ emission which revealed the presence of denser gas (n ∝ 10 cm⁻³) than most previous molecular line studies of these cores had found. The observations also suggested that those cores which are associated with young stars have slightly higher densities on average than those cores which have no associated star. We also found that DCO⁺ line-widths for the cores studied are significantly broader than the previously observed NH₃ line widths. The DCO⁺ line width broadening does not appear to be entirely an opacity effect. Instead, the broader line width suggests the presence inside the core of a dense supersonic gas component not traced by NH₃.