Determine the origin of filamentary HCN structures

Determine the origin of the highly structured, filamentary HCN $J=4-3$ emission associated with the redshifted $v_{\rm LSR}\simeq11.5$ km s$^{-1}$ component in the Orion Bar, including whether these structures are compressed substructures produced by a UV-driven shock wave.

Background

The ALMA observations resolve two HCN velocity components in the Orion Bar: one near the systemic Bar velocity of 10.5 km s−1^{-1} and another near 11.5 km s−1^{-1}. The latter component exhibits narrow, filamentary structures near the dissociation fronts, whereas the systemic component is smoother.

The paper argues that the morphology and velocity shift may be consistent with compression by a UV-driven shock associated with H II-region expansion or PDR photoevaporation. However, the physical origin of these structures is not established observationally, leaving open whether they are shock-compressed filaments or reflect another component of the cloud’s complex kinematics.

References

The question is what is the origin of these structures. These filamentary structures appear perpendicular to the DFs and have width around 1", hence very close to what is observed for the H$_2$ filaments (see Fig. \ref{fig:MO_maps_ALMA} and Zannese et al. submitted). Thus, this seems compatible with compressed substructures produced by the propagation of a UV-driven shock wave due to the expansion of the HII region and/or the photoevaporation of the PDR.

— Sub-arcsecond imaging of HCN and CN in the Orion Bar. UV-driven warm Nitrogen chemistry in dense environments  (2609.19361 - Zannese et al., 16 Sep 2026) in Section 3.4, “High density clumps or compressed filaments?”