The earliest phases of high-mass star formation, as seen in NGC 6334 by \emph{Herschel}

J. Tigé, F. Motte, D. Russeil, A. Zavagno, M. Hennemann, N. Schneider, T. Hill, Q. Nguyen Luong, J. Di Francesco, S. Bontemps, F. Louvet, P. Didelon, V. Konyves, Ph. André, G. Leuleu, J. Bardagi, L. D. Anderson, D. Arzoumanian, M. Benedettini, J. -P. Bernard, D. Elia, M. Figueira, J. Kirk, P. G. Martin, V. Minier, S. Molinari, T. Nony, P. Persi, S. Pezzuto, D. Polychroni, T. Rayner, A. Rivera-Ingraham, H. Roussel, K. Rygl, L. Spinoglio, G. J. White

Published 2017-03-28Version 1

To constrain models of high-mass star formation, the Herschel/HOBYS KP 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 NGC6334, one of the best-studied HOBYS molecular cloud complexes. We used Herschel PACS and SPIRE 70-500mu images of the NGC6334 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 SEDs. A cross-correlation with high-mass star formation signposts suggests a mass threshold of 75Msun for MDCs in NGC6334. MDCs have temperatures of 9.5-40K, masses of 75-1000Msun, and densities of 10^5-10^8cm-3. Their mid-IR emission is used to separate 6 IR-bright and 10 IR-quiet protostellar MDCs while their 70mu 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 1-7x10^4yr and at most 3x10^5yr respectively, suggest a dynamical scenario of high-mass star formation. 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 NGC6334, favoring a scenario presented here, which simultaneously forms clouds and high-mass protostars.