Lack of high-mass pre-stellar cores in the starless MDCs of NGC 6334^★

¹ Universidad de Chile, Camino el Observatorio 1515, Las Condes, Santiago de Chile, Chile
e-mail: flouvet@das.uchile.cl
² CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Aix-Marseille Université, Marseille, France
³ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁴ IPAG, Université Grenoble Alpes, 38000 Grenoble, France

Received: 12 November 2017
Accepted: 31 October 2018

Abstract

Context. The formation of high-mass stars remains unknown in many aspects. There are two competing families of models to explain the formation of high-mass stars. On the one hand, quasi-static models predict the existence of high-mass pre-stellar cores sustained by a high degree of turbulence. On the other hand, competitive accretion models predict that high-mass proto-stellar cores evolve from low or intermediate mass proto-stellar cores in dynamic environments.

Aims. The aim of the present work is to bring observational constraints at the scale of high-mass cores (~0.03 pc).

Methods. We targeted with ALMA and MOPRA a sample of nine starless massive dense cores (MDCs) discovered in a recent Herschel/HOBYS study. Their mass and size (~110 M_⊙ and r = 0.1 pc, respectively) are similar to the initial conditions used in the quasi-static family of models explaining for the formation of high-mass stars. We present ALMA 1.4 mm continuum observations that resolve the Jeans length (λ_Jeans ~ 0.03 pc) and that are sensitive to the Jeans mass (M_Jeans ~ 0.65 M_⊙) in the nine starless MDCs, together with ALMA-¹²CO(2–1) emission line observations. We also present HCO⁺(1–0), H¹³CO⁺(1–0) and N₂H⁺(1–0) molecular lines from the MOPRA telescope for eight of the nine MDCs.

Results. The nine starless MDCs have the mass reservoir to form high-mass stars according to the criteria by Baldeschi et al. (2017). Three of the starless MDCs are subvirialized with α_vir ~ 0.35, and four MDCs show sign of collapse from their molecular emission lines. ALMA observations show very little fragmentation within the MDCs. Only two of the starless MDCs host compact continuum sources, whose fluxes correspond to <3 M_⊙ fragments. Therefore, the mass reservoir of the MDCs has not yet been accreted onto compact objects, and most of the emission is filtered out by the interferometer.

Conclusions. These observations do not support the quasi-static models for high-mass star formation since no high-mass pre-stellar core is found in NGC 6334. The competitive accretion models, on the other hand, predict a level of fragmentation much higher than what we observe.