The earliest phases of high-mass star formation, as seen in NGC 6334 by Herschel-HOBYS ^⋆,⋆⋆

¹ Aix-Marseille Univ., CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
e-mail: jeremy.tige@lam.fr
² Université Grenoble Alpes, CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble, 38000 Grenoble, France
³ Laboratoire AIM Paris-Saclay, CEA/IRFU – CNRS/INSU – Université Paris Diderot, Service d’Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
⁴ Max-Planck-Institut für Astronomie, Königsstuhl 17, 69117 Heidelberg, Germany
⁵ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée G. Saint-Hilaire, 33615 Pessac, France
⁶ I. Physik. Institut, University of Cologne, 50937 Cologne, Germany
⁷ Joint ALMA Observatory, 3107 Alonso de Cordova, Vitacura, Santiago, Chile
⁸ Korea Astronomy and Space Science Institute, 776 Daedeok daero, Yuseoung, Daejeon 34055, Korea
⁹ NAOJ Chile Observatory, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁰ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹¹ Department of Physics and Astronomy, University of Victoria, PO Box 355, STN CSC, Victoria, BC V8W 3P6, Canada
¹² Department of Astronomy, Universidad de Chile, Las Condes, Santiago, Chile
¹³ Department of Physics and Astronomy, West Virginia University, Morgantown, WV 26506, USA
¹⁴ National Radio Astronomy Observatory, PO Box 2, Green Bank, WV 24944, USA
¹⁵ INAF-Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133 Rome, Italy
¹⁶ Université de Toulouse, UPS-OMP, CNRS, IRAP, 31028 Toulouse Cedex 4, France
¹⁷ Jeremiah Horrocks Institute, University of Central Lancashire, Preston, Lancashire, PR1 2HE, UK
¹⁸ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
¹⁹ Departamento de Fisica, Universidad de Atacama, Copiapo, Chile
²⁰ Cardiff School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff, Wales, CF24 3AA, UK
²¹ European Space Astronomy Centre ESA/ESAC, 28691 Villanueva de la Cañada, Madrid, Spain
²² Institut d’Astrophysique de Paris, Sorbonne Universités, UPMC Univ. Paris 06, CNRS UMR 7095, 75014 Paris, France
²³ INAF−IRA, via P. Gobetti 101, 40129 Bologna, Italy
²⁴ Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
²⁵ RAL Space, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK

Received: 24 May 2016
Accepted: 14 March 2017

Abstract

Aims. To constrain models of high-mass star formation, the Herschel-HOBYS key program aims at discovering massive dense cores (MDCs) able to host the high-mass analogs of low-mass prestellar cores, which have been searched for over the past decade. We here focus on NGC 6334, one of the best-studied HOBYS molecular cloud complexes.

Methods. We used Herschel/PACS and SPIRE 70−500 μm images of the NGC 6334 complex complemented with (sub)millimeter and mid-infrared data. We built a complete procedure to extract ~0.1 pc dense cores with the getsources software, which simultaneously measures their far-infrared to millimeter fluxes. We carefully estimated the temperatures and masses of these dense cores from their spectral energy distributions (SEDs). We also identified the densest pc-scale cloud structures of NGC 6334, one 2 pc × 1 pc ridge and two 0.8 pc × 0.8 pc hubs, with volume-averaged densities of ~10⁵ cm^-3.

Results. A cross-correlation with high-mass star formation signposts suggests a mass threshold of 75 M_⊙ for MDCs in NGC 6334. MDCs have temperatures of 9.5−40 K, masses of 75−1000 M_⊙, and densities of 1 × 10⁵−7 × 10⁷ cm^-3. Their mid-infrared emission is used to separate 6 IR-bright and 10 IR-quiet protostellar MDCs while their 70 μm emission strength, with respect to fitted SEDs, helps identify 16 starless MDC candidates. The ability of the latter to host high-mass prestellar cores is investigated here and remains questionable. An increase in mass and density from the starless to the IR-quiet and IR-bright phases suggests that the protostars and MDCs simultaneously grow in mass. The statistical lifetimes of the high-mass prestellar and protostellar core phases, estimated to be 1−7 × 10⁴ yr and at most 3 × 10⁵ yr respectively, suggest a dynamical scenario of high-mass star formation.

Conclusions. The present study provides good mass estimates for a statistically significant sample, covering the earliest phases of high-mass star formation. High-mass prestellar cores may not exist in NGC 6334, favoring a scenario presented here, which simultaneously forms clouds, ridges, MDCs, and high-mass protostars.