Time evolution of o-H2D+, N2D+, and N2H+ during the high-mass star formation process
Abstract
Deuterium fractionation is a well-established evolutionary tracer in low-mass star formation, but its applicability to the high-mass regime remains an open question. The abundances and ratios of deuterated species have often been proposed as reliable evolutionary indicators for different stages of the high-mass star formation. We investigate the role of N2H+ and key deuterated molecules as tracers of the different stages of the high-mass star formation, and test whether their abundance ratios can serve as reliable evolutionary indicators. We conducted APEX observations of o-H2D+ (110-111), N2H+ (4-3), and N2d+ (3-2) in 40 high-mass clumps at different evolutionary stages, selected from the ATLASGAL survey. Molecular column densities (N) and abundances (X), were derived through spectral line modelling, both under local thermodynamic equilibrium (LTE) and non-LTE conditions. The N(o-H2D+) show the smallest deviation from LTE results when derived under non-LTE assumptions. In contrast, N2D+ shows the largest discrepancy between the N derived from LTE and non-LTE. In all the cases discussed, we found that X(o-H2D+) decreases more significantly with time than in the case of X(N2D+); whereas X(N2H+) increases slightly. Therefore, the validity of the recently proposed X(o-H2D+)/X(N2D+) ratio as a reliable evolutionary indicator was not observed for this sample. While the deuteration fraction derived from N2D+ and N2H+ clearly decreases with clump evolution, the interpretation of this trend is complex, given the different distribution of the two tracers. Our results suggest that a careful consideration of the observational biases and beam-dilution effects are crucial for an accurate interpretation of the evolution of the deuteration process during the high-mass star formation process.
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