The potential of red supergiants as extragalactic abundance probes at low spectral resolution

Abstract

Red supergiants (RSGs) are among the brightest stars in the local Universe, making them ideal candidates with which to probe the properties of their host galaxies. However, current quantitative spectroscopic techniques require spectral resolutions of R ≳ 17 000, making observations of RSGs at distances greater than 1 Mpc unfeasible. Here we explore the potential of quantitative spectroscopic techniques at much lower resolutions, R ≈ 2-3000. We take archival J-band spectra of a sample of RSGs in the solar neighbourhood. In this spectral region the metallic lines of Fe i, Mg i, Si i and Ti i are prominent, while the molecular absorption features of OH, H2O, CN and CO are weak. We compare these data with synthetic spectra produced from the existing grid of model atmospheres from the MARCS project, with the aim of deriving chemical abundances. We find that all stars studied can be unambiguously fitted by the models, and model parameters of log g, effective temperatures Teff, microturbulence and global metal content may be derived. We find that the abundances derived for the stars are all very close to solar and have low dispersion, with an average of log Z = 0.13 ± 0.14. The values of Teff fitted by the models are ∼150 K cooler than the stars' literature values for earlier spectral types when using the Levesque et al. temperature scale, though we find that this discrepancy may be reduced at spectral resolutions of R = 3000 or higher. In any case, the temperature discrepancy has very little systematic effect on the derived abundances as the equivalent widths (EWs) of the metallic lines are roughly constant across the full temperature range of RSGs. Instead, elemental abundances are the dominating factor in the EWs of the diagnostic lines. Our results suggest that chemical abundance measurements of RSGs are possible at low to medium resolution, meaning that this technique is a viable infrared-based alternative to measuring abundance trends in external galaxies. © 2010 The Authors. Journal compilation © 2010 RAS.