H2D+ in the high mass star-forming Region Cygnus-X

TL;DR

This study presents the first detailed mapping of H2D+ in a high-mass star-forming region, revealing extensive cold dense gas structures offset from protostars, challenging previous assumptions about core composition and detection methods.

Contribution

It provides the first spatial map of H2D+ in Cygnus-X, uncovering extended cold dense gas structures not detected by previous surveys, and highlights the complexity of core structures in high-mass star formation.

Findings

Discovered five extended H2D+ structures offset from protostars.

H2D+ peak is not associated with dust continuum or N2D+ peaks.

Extended cold dense gas reservoirs are more prevalent than previously detected.

Abstract

H2D+ is a primary ion which dominates the gas-phase chemistry of cold dense gas. Therefore it is hailed as a unique tool in probing the earliest, prestellar phase of star formation. Observationally, its abundance and distribution is however just beginning to be understood in low-mass prestellar and cluster-forming cores. In high mass star forming regions, H2D+ has been detected only in two cores, and its spatial distribution remains unknown. Here we present the first map of the 372 GHz ortho-H2D+ and N2H+ 4-3 transition in the DR21 filament of Cygnus-X with the JCMT, and N2D+ 3--2 and dust continuum with the SMA. We have discovered five very extended (<= 34000 AU diameter) weak structures in H2D+ in the vicinity of, but distinctly offset from embedded protostars. More surprisingly, the H2D+ peak is not associated with either a dust continuum or N2D+ peak. We have therefore uncovered extended massive cold dense gas that was undetected with previous molecular line and dust continuum surveys of the region. This work also shows that our picture of the structure of cores is too simplistic for cluster forming cores and needs to be refined: neither dust continuum with existing capabilities, nor emission in tracers like N2D+ can provide a complete census of the total prestellar gas in such regions. Sensitive H2D+ mapping of the entire DR21 filament is likely to discover more of such cold quiescent gas reservoirs in an otherwise active high mass star-forming region.