Herschel-HOBYS study of the earliest phases of high-mass star formation in NGC 6357

D Russeil,M Benedettini,D Elia,T Nony,M Figueira,L D Anderson,J Tigé,Sylvain Bontemps ,L Spinoglio,E Schisano,P Didelon,Ph André ,N Schneider,G J White,A Men’Shchikov,James Di Francesco ,Q Nguyên Luong ,A Zavagno,K L J Rygl,S Pezzuto,V Könyves,F Motte

doi:10.1051/0004-6361/201833870

Abstract

Aims. To constrain models of high-mass star formation it is important to identify the massive dense cores (MDCs) that are able to form high-mass star(s). This is one of the purposes of the Herschel/HOBYS key programme. Here, we carry out the census and characterise of the properties of the MDCs population of the NGC 6357 H II region. Methods. Our study is based on the Herschel/PACS and SPIRE 70−500 μm images of NGC 6357 complemented with (sub-)millimetre and mid-infrared data. We followed the procedure established by the Herschel/HOBYS consortium to extract ~0.1 pc massive dense cores using the getsources software. We estimated their physical parameters (temperatures, masses, luminosities) from spectral energy distribution (SED) fitting. Results. We obtain a complete census of 23 massive dense cores, amongst which one is found to be IR-quiet and twelve are starless, representing very early stages of the star-formation process. Focussing on the starless MDCs, we have considered their evolutionary status, and suggest that only five of them are likely to form a high-mass star. Conclusions. We find that, contrarily to the case in NGC 6334, the NGC 6357 region does not exhibit any ridge or hub features that are believed to be crucial to the massive star formation process. This study adds support for an empirical model in which massive dense cores and protostars simultaneously accrete mass from the surrounding filaments. In addition, the massive star formation in NGC 6357 seems to have stopped and the hottest stars in Pismis 24 have disrupted the filaments.

Highlights

High-mass stars (O-B3 type, >8 M ) are the ionising sources of H II regions
In the framework of the Herschel/HOBYS key programme and in the same way as is done for NGC 6334 by Tigé et al (2017), we performed a study of the massive dense cores in NGC 6357 to better understand how high-mass stars form
We combined the Herschel/HOBYS images to mid-infrared andmillimeter ground-based data to obtain a complete census of 23 massive dense cores (MDCs), among which five are expected to be the most probable progenitors of high-mass stars at 0.1 pc scale

Summary

Introduction

High-mass stars (O-B3 type, >8 M ) are the ionising sources of H II regions. They impact the interstellar medium mainly via their UV radiation, the dynamical expansion of their H II region. Our main goal is to identify and characterise MDCs in order to compare the massive star-formation processes in both regions. Given the Herschel angular resolution (12 at 160 μm) and the homogenous 1–3 kpc distances of HOBYS clouds, the HOBYS key programme is dedicated to identify and characterise 0.1 pc MDCs. Since density is a better criterion than mass to evaluate whether a cloud structure has the ability to form high-mass stars we chose to focus on the densest cloud structures.

Observations

Compact sources physical characterisation

Findings

Comparison with previous massive dense core studies in NGC 6357

Conclusions