Ionization compression impact on dense gas distribution and star formation

Probability density functions around H II regions as seen by Herschel^⋆

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 Joint ALMA Observatory, Alonso de Cordova 3107, 7630355 Vitacura, Santiago, Chile
⁶ Department of Physics, West Virginia University, Morgantown WV 26506, USA
⁷ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁸ IAS, CNRS (UMR 8617), Université Paris-Sud, Bâtiment 121, 91400 Orsay, France
⁹ Maison de la Simulation, CEA-CNRS-INRIA-UPS-UVSQ, USR 3441, Centre d’étude de Saclay, 91191 Gif-Sur-Yvette, France
¹⁰ Istituto di Astrofisica e Planetologia Spaziali (INAF-IAPS), via del Fosso del Cavaliere 100, 00133 Roma, Italy
¹¹ Max-Planck Institut für Radioastronomie, Auf dem Hügel, 53121 Bonn, Germany
¹² National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹³ INAF Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
¹⁴ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
¹⁵ European Space Astronomy Centre, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
¹⁶ School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
¹⁷ Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
¹⁸ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
¹⁹ The Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0NL, UK
²⁰ Department of Physics and Astronomy, The Open University, Milton Keynes, UK

Received: 18 September 2013
Accepted: 28 January 2014

Abstract

Aims. Ionization feedback should impact the probability distribution function (PDF) of the column density of cold dust around the ionized gas. We aim to quantify this effect and discuss its potential link to the core and initial mass function (CMF/IMF).

Methods. We used Herschel column density maps of several regions observed within the HOBYS key program in a systematic way: M 16, the Rosette and Vela C molecular clouds, and the RCW 120 H ii region. We computed the PDFs in concentric disks around the main ionizing sources, determined their properties, and discuss the effect of ionization pressure on the distribution of the column density.

Results. We fitted the column density PDFs of all clouds with two lognormal distributions, since they present a “double-peak” or an enlarged shape in the PDF. Our interpretation is that the lowest part of the column density distribution describes the turbulent molecular gas, while the second peak corresponds to a compression zone induced by the expansion of the ionized gas into the turbulent molecular cloud. Such a double peak is not visible for all clouds associated with ionization fronts, but it depends on the relative importance of ionization pressure and turbulent ram pressure. A power-law tail is present for higher column densities, which are generally ascribed to the effect of gravity. The condensations at the edge of the ionized gas have a steep compressed radial profile, sometimes recognizable in the flattening of the power-law tail. This could lead to an unambiguous criterion that is able to disentangle triggered star formation from pre-existing star formation.

Conclusions. In the context of the gravo-turbulent scenario for the origin of the CMF/IMF, the double-peaked or enlarged shape of the PDF may affect the formation of objects at both the low-mass and the high-mass ends of the CMF/IMF. In particular, a broader PDF is required by the gravo-turbulent scenario to fit the IMF properly with a reasonable initial Mach number for the molecular cloud. Since other physical processes (e.g., the equation of state and the variations among the core properties) have already been said to broaden the PDF, the relative importance of the different effects remains an open question.