Globules and pillars in Cygnus X

I. Herschel far-infrared imaging of the Cygnus OB2 environment^⋆

¹ Université Bordeaux, LAB, UMR 5804, 33270 Floirac, France
² CNRS, LAB, UMR 5804, 33270 Floirac, France
³ I. Physik. Institut, University of Cologne, Zülpicher Str. 77, 50937 Koeln, Germany
e-mail: nschneid@ph1.uni-koeln.de
⁴ IRFU/SAp CEA/DSM, Laboratoire AIM CNRS – Université Paris Diderot, 91191 Gif-sur-Yvette, France
⁵ IPAG, University Grenoble Alpes, 38000 Grenoble, France
⁶ Dep. of Physics, West Virginia University, WV 26506, USA
⁷ IAS, CNRS/Université Paris-Sud 11, 91405 Orsay, France
⁸ ESO, Alonso de Cordova 3107, Vitacura, Santiago 19, Chile
⁹ Dep. of Physics and Astronomy, University of Victoria, Victoria, BC V8W 2Y2, Canada
¹⁰ NRCC, Victoria, BC V9E 2E7, Canada
¹¹ Astrophysics Group, University of Exeter, EX4 4 QL Exeter, UK
¹² INAF Osservatorio Astronomico di Palermo, 90134 Palermo, Italy
¹³ Smithsonian Astrophysical Observatory, Cambridge, MA 02138, USA
¹⁴ Joint ALMA Observatory, 3107 Alonso de Cordova, Vitacura, Santiago 19, Chile
¹⁵ Herschel Science Centre, European Space Astronomy Centre (ESAC)/ESA, Villanueva de la Canada, Spain
¹⁶ INAF–ORA, via P.Gobetti 101, 40129 Bologna, Italy
¹⁷ INAF–IAPS, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁸ Maison de la Simulation, CEA-CNRS-INRIA-UPS-UVSQ, USR 3441, CEA Saclay, 91191 Gif-sur-Yvette, France
¹⁹ Department of Physics & Astronomy, The Open University, Milton Keynes MK7 6AA, UK
²⁰ RALSpace, The Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0NL, UK
²¹ Centre for Astrophysics Research, University of Hertfordshire, Hatfield, AL10 9AB, UK

Received: 17 February 2016
Accepted: 5 April 2016

Abstract

The radiative feedback of massive stars on molecular clouds creates pillars, globules and other features at the interface between the H II  region and molecular cloud. Optical and near-infrared observations from the ground as well as with the Hubble or Spitzer satellites have revealed numerous examples of such cloud structures. We present here Herschel far-infrared observations between 70 μm and 500 μm of the immediate environment of the rich Cygnus OB2 association, performed within the Herschel imaging survey of OB Young Stellar objects (HOBYS) program. All of the observed irradiated structures were detected based on their appearance at 70 μm, and have been classified as pillars, globules, evaporating gasous globules (EGGs), proplyd-like objects, and condensations. From the 70 μm and 160 μm flux maps, we derive the local far-ultraviolet (FUV) field on the photon dominated surfaces. In parallel, we use a census of the O-stars to estimate the overall FUV-field, that is 10³–10⁴ G₀ (Habing field) close to the central OB cluster (within 10 pc) and decreases down to a few tens G₀, in a distance of 50 pc. From a spectral energy distribution (SED) fit to the four longest Herschel wavelengths, we determine column density and temperature maps and derive masses, volume densities and surface densities for these structures. We find that the morphological classification corresponds to distinct physical properties. Pillars and globules are massive (~500 M_⊙) and large (equivalent radius r ~ 0.6 pc) structures, corresponding to what is defined as “clumps” for molecular clouds. EGGs and proplyd-likeobjects are smaller (r ~ 0.1 and 0.2 pc) and less massive (~10 and ~30 M_⊙). Cloud condensations are small (~0.1 pc), have an average mass of 35 M_⊙, are dense (~6 × 10⁴ cm^-3), and can thus be described as molecular cloud “cores”. All pillars and globules are oriented toward the Cyg OB2 association center and have the longest estimated photoevaporation lifetimes, a few million years, while all other features should survive less than a million years. These lifetimes are consistent with that found in simulations of turbulent, UV-illuminated clouds. We propose a tentative evolutionary scheme in which pillars can evolve into globules, which in turn then evolve into EGGs, condensations and proplyd-like objects.