A Spectral Line Study of Serpens S68 FIRS1 Region

Abstract

A λ = 1 mm to 3 mm study of the Serpens molecular cloud core on scales of 10' to 10 is presented, concentrating on the northwest condensation and the embedded sources, S68 FIRS1 and S68 N. We adopt temperatures of 25 K for the extended structure (several arcminute scale) and 35 K for the embedded sources. With these values, we use molecular line ratios and LVG statistical equilibrium calculations to derive physical properties in the region. We obtain densities between 0.4-1.2 × 10^6 cm^(-3) and an overall mass of 250-300 M_☉. The majority of the mass is found in extended cloud material with two peaks of 30-45 M_☉ each. Values of the molecular column densities are derived on scales of 60-75 (condensation) and 25-30 (embedded sources). The column densities in the condensation are typically as high as in the embedded sources, despite a factor of 4-5 in beam area, though there is some suggestion of mild depletions on smaller scales based on interferometric observations. Derived abundances for the region show similarities to both warmer cores and colder, dark cloud regions, with some values and ratios falling cleanly between this range of properties. Measurements of several deuterated species indicate enhanced abundances, suggesting the recent evaporation of grain mantles. Overall, the combined properties and measured abundances suggest an intermediate state between the extremes of dark clouds and evolved warm cores like Orion KL. In general, the Serpens NW condensation appears very similar to the IRAS 16293-2422 region, which is itself an example of a cool, low-mass star-forming region with chemical tracers characteristic of hotter, higher mass regions. Our results support the suggestion that the general chemical morphology of a region is determined more by the age of the region than by its mass.