Detection of Dust Condensations in the Orion Bar Photon-dominated Region

TL;DR

This study uses submillimeter observations to identify and analyze dust condensations in the Orion Bar PDR, revealing their properties, dynamics, and likely formation mechanism due to HII region expansion.

Contribution

First detection of compact dust condensations in the Orion Bar PDR and detailed analysis of their physical and kinematic properties.

Findings

Nine dust condensations identified within 0.03 pc of the dissociation front.

Condensations are not gravitationally bound and unlikely to form stars.

Evidence suggests condensations formed by high-pressure wave from HII region expansion.

Abstract

We report Submillimeter Array dust continuum and molecular spectral line observations toward the Orion Bar photon-dominated region (PDR). The 1.2 mm continuum map reveals, for the first time, a total of 9 compact (r < 0.01pc) dust condensations located within a distance of ~0.03 pc from the dissociation front of the PDR. Part of the dust condensations are seen in spectral line emissions of CS (5-4) and H$_2$CS ($7_{1,7}$-$6_{1,6}$), though the CS map also reveals dense gas further away from the dissociation front. We detect compact emissions in H$_2$CS ($6_{0,6}$-$5_{0,5}$), ($6_{2,4}$-$5_{2,3}$) and C$^{34}$S, C$^{33}$S (4-3) toward bright dust condensations. The line ratio of H$_2$CS ($6_{0,6}$-$5_{0,5}$)/($6_{2,4}$-$5_{2,3}$) suggests a temperature of $73\pm58$ K. A non-thermal velocity dispersion of ~0.25 - 0.50 km s$^{-1}$ is derived from the high spectral resolution C$^{34}$S data, and indicates a subsonic to transonic turbulence in the condensations. The masses of the condensations are estimated from the dust emission, and range from 0.03 to 0.3 $M_{\odot}$, all significantly lower than any critical mass that is required for self-gravity to play a crucial role. Thus the condensations are not gravitationally bound, and could not collapse to form stars. In cooperating with recent high resolution observations of the surface layers of the molecular cloud in the Bar, we speculate that the condensations are produced as a high-pressure wave induced by the expansion of the HII region compresses and enters the cloud. A velocity gradient along a direction perpendicular to the major axis of the Bar is seen in H$_2$CS ($7_{1,7}$-$6_{1,6}$), and is consistent with the scenario that the molecular gas behind the dissociation front is being compressed.