The Orion Bar is the archetypal edge-on molecular cloud surface illuminated by strong ultraviolet radiation from nearby massive stars. Our relative closeness to the Orion nebula (about 1,350 light years away from Earth) means that we can study the effects of stellar feedback on the parental cloud in detail. Visible-light observations of the Orion Bar1 show that the transition between the hot ionized gas and the warm neutral atomic gas (the ionization front) is spatially well separated from the transition between atomic and molecular gas (the dissociation front), by about 15 arcseconds or 6,200 astronomical units (one astronomical unit is the Earth–Sun distance). Static equilibrium models2,3 used to interpret previous far-infrared and radio observations of the neutral gas in the Orion Bar4,5,6 (typically at 10–20 arcsecond resolution) predict an inhomogeneous cloud structure comprised of dense clumps embedded in a lower-density extended gas component. Here we report one-arcsecond-resolution millimetre-wave images that allow us to resolve the molecular cloud surface. In contrast to stationary model predictions7,8,9, there is no appreciable offset between the peak of the H2 vibrational emission (delineating the H/H2 transition) and the edge of the observed CO and HCO+ emission. This implies that the H/H2 and C+/C/CO transition zones are very close. We find a fragmented ridge of high-density substructures, photoablative gas flows and instabilities at the molecular cloud surface. The results suggest that the cloud edge has been compressed by a high-pressure wave that is moving into the molecular cloud, demonstrating that dynamical and non-equilibrium effects are important for the cloud evolution.
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We thank the ERC for support under grant ERC-2013-Syg-610256-NANOCOSMOS. We also thank MINECO, Spain, for funding support under grants CSD2009-00038 and AYA2012-32032. This work was in part supported by the French CNRS programme ‘Physique et Chimie du Milieu Interstellaire’. We thank P. Schilke and D. Lis for sharing their IRAM-PdBI observations of the H13CN J = 1–0 condensations inside the Orion Bar, and M. Walmsley for sharing his H2 v = 1–0 S(1) and O i 1.3 μm infrared images. ALMA is a partnership of the ESO (representing its member states), the NSF (USA) and NINS (Japan), together with the NRC (Canada), the NSC and ASIAA (Taiwan) and KASI (Republic of Korea) in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by the ESO, the AUI/NRAO and the NAOJ. This Letter makes use of observations obtained with the IRAM 30 m telescope. IRAM is supported by the INSU/CNRS (France), the MPG (Germany), and the IGN (Spain).
We used the ALMA data ADS/JAO.ALMA#2012.1.00352.S available at https://almascience.eso.org/aq/?project_code=2012.1.00352.S.
Reviewer Information Nature thanks R. Plume and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
The incident stellar ultraviolet radiation comes from the left. The velocity of the advancing ionization and dissociation fronts are represented by vIF and vDF respectively. In the Orion Bar, the dissociation front is at about 15″ (about 0.03 pc) from the ionization front. UV, ultraviolet; PAH, polycyclic aromatic hydrocarbons. The snow line refers to the inner cloud layers where molecular gases start to freeze and dust grains become coated by ices.
a, ALMA HCO+ J = 4–3 line integrated intensity. b, ALMA CO J = 3–2 line peak (Orion Bar velocity component). The red contours represent the H13CN J = 1–0 emission (from 0.08 to 0.026 in steps of 0.02 Jy beam−1 km s−1) of dense condensations inside the Orion Bar19. The black contours show the brightest regions of H2 v = 1–0 S(1) emission1 (from 1.5 to 4.5 in steps of 0.5·10−4 erg s−1 cm−2 sr−1). The H2 image is saturated between δx = 19″ and 23″ (that is, no data are shown). Figures have been rotated 127.5° anticlockwise to bring the incident ultraviolet radiation from the left.
a, CO J = 3–2 line peak (for N(CO) = 1018 cm−2). b–d, HCO+ J = 4–3 integrated line intensity at 100 K (b), 200 K (c) and 300 K (d). Each curve represents a different electron abundance model: xe = 0 (blue) and xe = 10−4 (red). Continuous curves are for N(HCO+) = 5 × 1013 cm−2 and dotted lines for N(HCO+) = 2 × 1014 cm−2 (appropriate for deeper inside the Orion Bar, δx > 30″). The horizontal green dashed line represents the average (a) and (b–d) with their standard deviation (grey shaded) towards the dissociation front (at δx ≈ 15″).
a, Vertically averaged cuts perpendicular to the Orion Bar in the HCO+ J = 4–3 line velocity centroid (magenta curve) and FWHM velocity dispersion (grey curve). b, CO and HCO+ spectra at representative positions. The top and middle plots show positions between the ionization and dissociation fronts, the bottom plot is inside the molecular Orion Bar. Offsets are given with respect to the rotated images in Extended Data Fig. 2. The velocity of the background cloud is vLSR ≈ 8.5 km s−1 (black dashed line), whereas the velocity of the Orion Bar is vLSR ≈ 11 km s−1 (green line).
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Goicoechea, J., Pety, J., Cuadrado, S. et al. Compression and ablation of the photo-irradiated molecular cloud the Orion Bar. Nature 537, 207–209 (2016). https://doi.org/10.1038/nature18957