A reference framework for extremely metal-poor OB star studies: calibrations for stellar parameters and intrinsic colours

TL;DR

This paper establishes a reference framework for extremely metal-poor OB stars, providing calibrations for stellar parameters and colours, and highlighting differences from Galactic counterparts in temperature and ionizing flux.

Contribution

It introduces the first calibration of stellar parameters and colours for XMP OB stars, accounting for parameter degeneracies and improving candidate selection in low-metallicity environments.

Findings

XMP OB stars are 1-6 kK hotter than Galactic analogues.

XMP stars produce higher ionizing fluxes, with a bimodal HeII-ionizing flux distribution.

Reddening in Sextans A is significant and uneven.

Abstract

We provide the first reference framework for extremely metal-poor (XMP) OB-type stars. We parsed a grid of 0.10 $Z_{\odot}$ FASTWIND models, covering the parameter space of O stars and early-B supergiants, through contemporary spectral classification criteria to deliver a calibration of key stellar properties as a function of spectral type, and tabulated colours for the most common photometric systems. By using an extensive grid of models, we account for the different combinations of stellar parameters that result in the same spectral morphology and provide a range of parameters and colours compatible with each spectral subtype and luminosity class. We supply updated photometric criteria to optimize candidate selection of OB stars in XMP environments. We find 0.10 $Z_{\odot}$ OB stars are 1-6 kK hotter and produce higher ionizing fluxes than their Galactic analogues. In addition, we find a bimodal distribution of the HeII-ionizing flux with spectral type; because of its known dependence on effective temperature and the wind, $\log~q_{HeII}$ for individual XMP late-O type stars could be underestimated by up to 4 orders of magnitude by other calibrations, some of them used by population synthesis codes. Finally, we used our calibrated colours to map the extinction of the 0.10 $Z_{\odot}$ galaxy Sextans A finding that reddening is non-negligible and uneven.