Unveiling very young O stars. Two Galactic O2V((f*))z in Westerlund 2

TL;DR

This study reclassified several Galactic O-type stars in Westerlund 2, identifying two very young O2V stars, and provided new temperature calibrations, distance estimates, and insights into their evolutionary status.

Contribution

The paper presents the first firm identification of two O2V stars in Westerlund 2 and introduces new spectral type and temperature calibrations for early O-type stars.

Findings

Identification of two O2V stars in Westerlund 2

Derived new temperature calibrations for O2-O4 stars

Estimated Westerlund 2 distance as approximately 5 kpc

Abstract

O-type stars are known to significantly contribute to both the dynamics and evolution of galaxies. Massive and luminous, they probably control and regulate the galaxies star formation rates. For this work I performed a redetermination of the spectral types and effective temperatures of the Galactic O-type stars MSP182, MSP183, MSP199, VPHAS-01338, and VPHAS-01273. From a careful examination of the spectral features present in the blue optical spectral region, it was possible to identify several nitrogen lines usually only seen in the blue optical spectra of O2-O3 stars. From the nitrogen ionic equivalent width ratios measured in the spectra of MSP182, MSP183, MSP199, VPHAS-01338, and VPHAS-01273, and in those of standard stars of the O2-O4 spectral types, earlier spectral types and hotter effective temperature values were derived. Two O2V, together with three new O3V stars, are now firmly identified in the Westerlund 2 region. Besides RFS1 in NGC3603, the O2 stars found in Westerlund 2 are the only other exemplars known to date in the Milky Way. From the nitrogen equivalent width line ratios measured in the spectra of standard stars of the O2-O4 spectral types, linear relations between the NIV4058-NIII4640 ratio and the effective temperature in the 47000K-51000K range were derived. Based on my spectroscopic analysis of the science targets and the use of a HRD, a mean heliocentric distance of 5kpc to Westerlund 2 was computed, a result that is in line with the mean heliocentric distance of 5.3(1.5) kpc obtained from the associated Gaia DR3 parallaxes and distances. The Westerlund 2 massive stars studied in this work probably share a common evolutionary process that might be representative of the evolutionary ages of a large fraction of the cluster's O-type stellar population, which seems to be much younger than 1 Myrs.