The young stellar content of the giant H II regions M 8, G333.6−0.2, and NGC 6357 with VLT/KMOS

M C Ramírez-Tannus,M Gennaro,W Brandner,O H Ramírez-Agudelo,H Sana,H Beuther,B Davies,A De Koter,A Bik,D Guo,A Pasquali,T Naylor,H Linz,J Poorta,L Kaper,T Henning,J De Ridder

doi:10.1051/0004-6361/201935941

Abstract

Context.The identification and characterisation of populations of young massive stars in (giant) H IIregions provides important constraints on (i) the formation process of massive stars and their early feedback on the environment, and (ii) the initial conditions for population synthesis models predicting the evolution of ensembles of stars.Aims.We identify and characterise the stellar populations of the following young giant H IIregions: M 8, G333.6−0.2, and NGC 6357.Methods.We have acquiredH- andK-band spectra of around 200 stars using theK-band Multi Object Spectrograph on the ESO Very Large Telescope. The targets for M 8 and NGC 6357 were selected from the Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX), which combines X-ray observations with near-infrared (NIR) and mid-infrared data. For G333.6−0.2, the sample selection is based on the NIR colours combined with X-ray data. We introduce an automatic spectral classification method in order to obtain temperatures and luminosities for the observed stars. We analysed the stellar populations using their photometric, astrometric, and spectroscopic properties and compared the position of the stars in the Hertzprung-Russell diagram with stellar evolution models to constrain their ages and mass ranges.Results.We confirm the presence of candidate ionising sources in the three regions and report new ones, including the first spectroscopically identified O stars in G333.6−0.2. In M 8 and NGC 6357, two populations are identified: (i) OB main-sequence stars (M > 5M⊙) and (ii) pre-main sequence stars (M ≈ 0.5 − 5 M⊙). The ages of the clusters are ∼1−3 Myr, < 3 Myr, and ∼0.5−3 Myr for M 8, G333.6−0.2, and NGC 6357, respectively. We show that MYStIX selected targets have > 90% probability of being members of the H IIregion, whereas a selection based on NIR colours leads to a membership probability of only ∼70%.

Highlights

The assembly of massive stars is a key problem in modern astronomy
We show that MYStIX selected targets have >90% probability of being members of the H ii region, whereas a selection based on NIR colours leads to a membership probability of only ∼70%
In this paper we present H and K band spectroscopic observations performed with VLT/KMOS surveying three giant H ii regions (M 8, G333.6−0.2, and NGC 6357) in order to characterise their stellar populations

Summary

Introduction

From an observational point of view, two lines of research are being pursued to better understand their formation process (Zinnecker & Yorke 2007; Motte et al 2018). One approach is aimed at the very early stages of star formation, observing the cold cores in which massive stars are being formed. A second strategy is to study the final phase and outcome of the formation process, characterising the recently formed stars and looking for remnant signatures of their assembly. The properties of the newly formed massive stars, such as mass, rotational velocity, magnetic field, and multiplicity provide us with information about the formation process. The statistical distributions of these properties – once a large enough sample is scrutinised – define the initial conditions for population synthesis models in which the evolution of entire populations of massive stars is traced

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Conclusion