Dense Gas Tracers in Perseus: Relating the N2H+, NH3, and Dust Continuum Properties of Pre- and Proto-Stellar Cores

TL;DR

This study compares N2H+ and NH3 gas tracers in Perseus cores, revealing their tight correlation and similar formation mechanisms, but also highlighting discrepancies with dust continuum observations in proto-stellar cores.

Contribution

It provides new insights into the relationship between nitrogen-bearing gas tracers and dust in dense cores, emphasizing their similar formation processes and the need to understand their differences in proto-stellar stages.

Findings

Strong correlation between N2H+ and NH3 gas properties.

Consistent abundance ratio of N2H+ to para-NH3 across cores.

Discrepancy between gas tracers and dust continuum in proto-stellar cores.

Abstract

We investigate 35 pre-stellar cores and 36 proto-stellar cores in the Perseus molecular cloud. We find a very tight correlation between the physical parameters describing the N2H+ and NH3 gas. Both the velocity centroids and the line widths of N2H+ and NH3 correlate much better than either species correlates with CO, as expected if the nitrogen-bearing species are probing primarily the dense core gas where the CO has been depleted. We also find a tight correlation in the inferred abundance ratio between N2H+ and para-NH3 across all cores, with N(p-NH3)/N(N2H+)= 22 +/- 10. We find a mild correlation between NH3 (and N2H+) column density and the (sub)millimeter dust continuum derived H2 column density for pre-stellar cores, N(p-NH3)/N(H2) ~ 10e-8, but do not find a fixed ratio for proto-stellar cores. The observations suggest that in the Perseus molecular cloud the formation and destruction mechanisms for the two nitrogen-bearing species are similar, regardless of the physical conditions in the dense core gas. While the equivalence of N2H+ and NH3 as powerful tracers of dense gas is validated, the lack of correspondence between these species and the (sub)millimeter dust continuum observations for proto-stellar cores is disconcerting and presently unexplained.