Pillars and globules at the edges of H ii regions

Confronting Herschel observations and numerical simulations^⋆

¹ Laboratoire AIM Paris-Saclay (CEA/Irfu – Univ. Paris Diderot – CNRS/INSU), Centre d’études de Saclay, 91191 Gif-sur-Yvette, France
e-mail: pascal.tremblin@cea.fr
² Astrophysics Group, University of Exeter, EX4 4 QL Exeter, UK
³ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁴ CNRS, LAB, UMR 5804, 33270 Floirac, France
⁵ Maison de la Simulation, CEA-CNRS-INRIA-UPS-UVSQ, USR 3441, Centre d’étude de Saclay, 91191 Gif-sur-Yvette, France
⁶ School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
⁷ IAS, CNRS UMR 8617, Université Paris-Sud, Bâtiment 121, 91400 Orsay, France
⁸ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁹ Department of Physics, West Virginia University, Morgantown, WV 26506, USA
¹⁰ Université de Toulouse, UPS, CESR, 9 avenue du Colonel Roche, CNRS, UMR 5187, 31028 Toulouse Cedex 4, France
¹¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹² National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹³ INAF IAPS, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁴ European Space Astronomy Centre, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
¹⁵ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
¹⁶ Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
¹⁷ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
¹⁸ Institut d’Astrophysique de Paris, Université Pierre et Marie Curie (UPMC), CNRS UMR 7095, 75014 Paris, France
¹⁹ APS-INAF, Fosso del Cavaliere 100, 00133 Roma, Italy
²⁰ The Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0NL, UK
²¹ Department of Physics and Astronomy, The Open University, Milton Keynes, UK
²² Institute for Astronomy, University of Hawaii, 96822 Honolulu, Hawaii

Received: 9 July 2013
Accepted: 30 September 2013

Abstract

Context. Herschel far-infrared imaging observations have revealed the density structure of the interface between H ii regions and molecular clouds in great detail. In particular, pillars and globules are present in many high-mass star-forming regions, such as the Eagle nebula (M 16) and the Rosette molecular cloud, and understanding their origin will help characterize triggered star formation.

Aims. The formation mechanisms of these structures are still being debated. The initial morphology of the molecular cloud and its turbulent state are key parameters since they generate deformations and curvatures of the shell during the expansion of the H ii region. Recent numerical simulations have shown how pillars can arise from the collapse of the shell in on itself and how globules can be formed from the interplay of the turbulent molecular cloud and the ionization from massive stars. The goal here is to test this scenario through recent observations of two massive star-forming regions, M 16 and the Rosette molecular cloud.

Methods. First, the column density structure of the interface between molecular clouds and associated H ii regions was characterized using column density maps obtained from far-infrared imaging of the Herschel HOBYS key programme. Then, the DisPerSe algorithm was used on these maps to detect the compressed layers around the ionized gas and pillars in different evolutionary states. Column density profiles were constructed. Finally, their velocity structure was investigated using CO data, and all observational signatures were tested against some distinct diagnostics established from simulations.

Results. The column density profiles have revealed the importance of compression at the edge of the ionized gas. The velocity properties of the structures, i.e. pillars and globules, are very close to what we predict from the numerical simulations. We have identified a good candidate of a nascent pillar in the Rosette molecular cloud that presents the velocity pattern of the shell collapsing on itself, induced by a high local curvature. Globules have a bulk velocity dispersion that indicates the importance of the initial turbulence in their formation, as proposed from numerical simulations. Altogether, this study re-enforces the picture of pillar formation by shell collapse and globule formation by the ionization of highly turbulent clouds.