title: Compression and ablation of the photo-irradiated molecular cloud the Orion Bar.
abstract: 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 Bar 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 models used to interpret previous far-infrared and radio observations of the neutral gas in the Orion Bar (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 predictions, there is no appreciable offset between the peak of the H2vibrational emission (delineating the H/H2transition) and the edge of the observed CO and HCO+emission. This implies that the H/H2and 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.
authors: Grupo de Astrofísica Molecular, Instituto de Ciencia de Materiales de Madrid (CSIC), Calle Sor Juana Ines de la Cruz 3, E-28049 Cantoblanco, Madrid, Spain.
authors: Javier R
authors: JR
authors: Goicoechea
authors: Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France.|Laboratoire d'Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères (LERMA), Observatoire de Paris, Centre National de la Recherche Scientifique (CNRS), Unité Mixte de Rechersche (UMR) 8112, École Normale Supérieure, PSL Research University, 24 rue Lhomond, 75231, Paris Cedex 05, France.
authors: Jérôme
authors: J
authors: Pety
authors: Grupo de Astrofísica Molecular, Instituto de Ciencia de Materiales de Madrid (CSIC), Calle Sor Juana Ines de la Cruz 3, E-28049 Cantoblanco, Madrid, Spain.
authors: Sara
authors: S
authors: Cuadrado
authors: Grupo de Astrofísica Molecular, Instituto de Ciencia de Materiales de Madrid (CSIC), Calle Sor Juana Ines de la Cruz 3, E-28049 Cantoblanco, Madrid, Spain.
authors: José
authors: J
authors: Cernicharo
authors: Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, F-38406 Saint Martin d'Hères, France.|Laboratoire d'Astrophysique de Bordeaux (LAB), Université de Bordeaux, UMR 5804, F-33270 Floirac, France.|CNRS, LAB, UMR 5804, F-33270 Floirac, France.
authors: Edwige
authors: E
authors: Chapillon
authors: Observatorio Astronómico Nacional (OAN-IGN). Apartado 112, 28803 Alcalá de Henares, Spain.
authors: Asunción
authors: A
authors: Fuente
authors: Laboratoire d'Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères (LERMA), Observatoire de Paris, Centre National de la Recherche Scientifique (CNRS), Unité Mixte de Rechersche (UMR) 8112, École Normale Supérieure, PSL Research University, 24 rue Lhomond, 75231, Paris Cedex 05, France.|Sorbonne Universités, Université Pierre et Marie Curie (UPMC), Université Paris 06, 75000, France.
authors: Maryvonne
authors: M
authors: Gerin
authors: Université de Toulouse, Université Paul-Sabatier-Observatoire Midi-Pyrénées (UPS-OMP), Institut de Recherche en Astrophysique et Planétologie (IRAP), 31028, Toulouse, France.|CNRS, IRAP, 9 Avenue du Colonel Roche, BP 44346, 31028 Toulouse, France.
authors: Christine
authors: C
authors: Joblin
authors: Grupo de Astrofísica Molecular, Instituto de Ciencia de Materiales de Madrid (CSIC), Calle Sor Juana Ines de la Cruz 3, E-28049 Cantoblanco, Madrid, Spain.
authors: Nuria
authors: N
authors: Marcelino
authors: Université de Toulouse, Université Paul-Sabatier-Observatoire Midi-Pyrénées (UPS-OMP), Institut de Recherche en Astrophysique et Planétologie (IRAP), 31028, Toulouse, France.|CNRS, IRAP, 9 Avenue du Colonel Roche, BP 44346, 31028 Toulouse, France.
authors: Paolo
authors: P
authors: Pilleri