From forced collapse to H ii region expansion in Mon R2: Envelope density structure and age determination with Herschel^⋆,⋆⋆

¹ Laboratoire AIM, CEA/IRFU CNRS/INSU Université Paris Diderot, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
e-mail: pierre.didelon@cea.fr
² Astrophysics Group, University of Exeter, EX4 4QL Exeter, UK
³ Maison de la Simulation, CEA-CNRS-INRIA-UPS-UVSQ, USR 3441, Centre d’étude de Saclay, 91191 Gif-Sur-Yvette, France
⁴ Joint ALMA Observatory, 3107 Alonso de Cordova, Vitacura, Santiago, Chile
⁵ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁶ Department of Physics and Astronomy, West Virginia University, Morgantown, WV 26506, USA
⁷ Also Adjunct Astronomer at the National Radio Astronomy Observatory, PO Box 2, Green Bank, WV 24944, USA
⁸ Université de Bordeaux, OASU, Bordeaux, France
⁹ Cardiff University, Wales Cardiff CF103 XQ, UK
¹⁰ European Space Research and Technology Centre (ESA-ESTEC), Keplerlaan 1, PO Box 299, 2200 AG Noordwijk, The Netherlands
¹¹ Cerro Calan, Observatorio Astronmico Nacional, Camino el Observatorio, 1515 Las Condes, Chile
¹² Laboratoire d’Astrophysique de Marseille, CNRS/INSU–Université de Provence, 13388 Marseille Cedex 13, France
¹³ Queen Mary + Westf. College, Dept. of Physics, London E1 4NS, UK
¹⁴ CESR, 31028 Toulouse, France
¹⁵ INAF–Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133 Rome, Italy
¹⁶ National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Rd., Victoria, BC, V9E 2E7, Canada
¹⁷ University of Victoria, Department of Physics and Astronomy, PO Box 3055, STN CSC, Victoria, BC, V8W 3P6, Canada
¹⁸ National Astronomical Observatory of Japan, Chile Observatory, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
¹⁹ Université de Toulouse, UPS-OMP, IRAP; CNRS, IRAP, 9 avenue colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
²⁰ Jeremiah Horrocks Institute, University of Central Lancashire, Preston, PR1 2 HE Lancashire, UK
²¹ Department of Physical sciences, The Open University, Milton Keynes MK7 6AA, UK
²² RALspace, The Rutherford Appleton Laboratory, Chilton, Didcot OX110 QX, UK

Received: 1 April 2015
Accepted: 13 July 2015

Abstract

Context. The surroundings of H ii regions can have a profound influence on their development, morphology, and evolution. This paper explores the effect of the environment on H ii regions in the MonR2 molecular cloud.

Aims. We aim to investigate the density structure of envelopes surrounding H ii regions and to determine their collapse and ionisation expansion ages. The Mon R2 molecular cloud is an ideal target since it hosts an H ii region association, which has been imaged by the Herschel PACS and SPIRE cameras as part of the HOBYS key programme.

Methods. Column density and temperature images derived from Herschel data were used together to model the structure of H ii bubbles and their surrounding envelopes. The resulting observational constraints were used to follow the development of the Mon R2 ionised regions with analytical calculations and numerical simulations.

Results. The four hot bubbles associated with H ii regions are surrounded by dense, cold, and neutral gas envelopes, which are partly embedded in filaments. The envelope’s radial density profiles are reminiscent of those of low-mass protostellar envelopes. The inner parts of envelopes of all four H ii regions could be free-falling because they display shallow density profiles: ρ(r) ∝ r^{− q} with . As for their outer parts, the two compact H ii regions show a ρ(r) ∝ r^-2 profile, which is typical of the equilibrium structure of a singular isothermal sphere. In contrast, the central UCH ii region shows a steeper outer profile, ρ(r) ∝ r^-2.5, that could be interpreted as material being forced to collapse, where an external agent overwhelms the internal pressure support.

Conclusions. The size of the heated bubbles, the spectral type of the irradiating stars, and the mean initial neutral gas density are used to estimate the ionisation expansion time, t_exp ~ 0.1 Myr, for the dense UCH ii and compact H ii regions and ~ 0.35 Myr for the extended H ii region. Numerical simulations with and without gravity show that the so-called lifetime problem of H ii regions is an artefact of theories that do not take their surrounding neutral envelopes with slowly decreasing density profiles into account. The envelope transition radii between the shallow and steeper density profiles are used to estimate the time elapsed since the formation of the first protostellar embryo, t_inf ~ 1 Myr, for the ultra-compact, 1.5−3 Myr for the compact, and greater than ~6 Myr for the extended H ii regions. These results suggest that the time needed to form a OB-star embryo and to start ionising the cloud, plus the quenching time due to the large gravitational potential amplified by further in-falling material, dominates the ionisation expansion time by a large factor. Accurate determination of the quenching time of H ii regions would require additional small-scale observationnal constraints and numerical simulations including 3D geometry effects.