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

Abstract

Context. The identification and characterisation of populations of young massive stars in (giant) H ii regions 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 ii regions: M8, G333.6-0.2, and NGC 6357. Methods. We have acquired H-and K-band spectra of around 200 stars using The K-band Multi Object Spectrograph (KMOS) on the ESO Very Large Telescope. The targets for M8 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 and mid-infrared data. For G333.6-0.2, the sample selection is based on the near-infrared 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 M8 and NGC 6357, two populations are identified: (i) OB main-sequence stars (M > 5 M ) 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 M8, G333.6-0.2, and NGC 6357, respectively. We show that MYStIX selected targets have > 90% probability of being members of the H ii region, whereas a selection based on near infrared (NIR) colours leads to a membership probability of only ∼70%. Arias et al. (2007) performed intermediate resolution spectroscopy of PMS stars in M8. They identified 27 classical T Tauri stars, seven weak-lined T Tauri stars and three PMS emission objects with spectral type G. They identified the stars to be younger than 3 Myr and between 0.8 and 2.5 M . Prisinzano et al. (2007, 2019) identified 237 PMS stars as members including 53 binaries. Kumar & Anandarao (2010) identified 64 class 0/I and 168 class II objects using Spitzer near-infrared photometry. Article number, page 2 of 39 M.C. Ramírez-Tannus et al.: The young stellar content of M8, G333.6-0.2, and NGC 6357.

A relation between the radial velocity dispersion of young clusters and their age

“…The sample studied in this Letter consists of the OB stars in M 8 and NGC 6357. The data acquisition and reduction are described in detail in Ramírez-Tannus et al (2020). In short, we obtained around 200 H and K-band intermediate resolution spectra (with spectral resolution power, λ/∆λ, between 6700 and 8500, i.e., 30 < ∆v < 40 km s −1 ) of stars in the abovementioned giant H ii regions with VLT/KMOS (Sharples et al 2013).…”

“…We discarded G333.6−0.2 from our analysis because there are not enough stars with RV measurements to calculate σ 1D . A description of the age and mass range determination can be found in Ramírez-Tannus et al (2020), and Appendix A presents a detailed discussion about the accuracy of the age determination.…”

“…In Ramírez-Tannus et al (2020), VLT/KMOS spectra of around 200 stars in three very young clusters (M 8, NGC 6357, and G333.6−0.2) were obtained. Introducing an automatic method to classify the spectra, the effective temperatures, and luminosities of the observed stars were characterized in order to place them in the Hertzsprung-Russell diagram (HRD).…”

“…Radial-velocity dispersion (σ 1D ) versus age of the clusters. The purple data points show the data fromSana et al (2012), the magenta points show Wd2 and R136(Zeidler et al 2018;Hénault-Brunet et al 2012), and the green data points show the clusters studied inRamírez- Tannus et al (2020) andSana et al (2017). The solid black line represents the linear fit to the data and the gray area shows the 1-σ errors on the fit.…”

A relation between the radial velocity dispersion of young clusters and their age: Evidence for hardening as the formation scenario of massive close binaries

“…In Ramírez-Tannus et al (2020), VLT/KMOS spectra of around 200 stars in three very young clusters (M8, NGC 6357, and G333.6-0.2) were obtained. Introducing an automatic method to classify the spectra, the effective temperatures, and luminosities of the observed stars were characterized in order to place them in the Hertzsprung-Russell diagram (HRD).…”