The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not as Cool as We Thought

Abstract

We use moderate-resolution optical spectrophotometry and the new MARCS stellar atmosphere models to determine the effective temperatures of 74 Galactic red supergiants (RSGs). The stars are mostly members of OB associations or clusters with known distances, allowing a critical comparison with modern stellar evolutionary tracks. We find we can achieve excellent matches between the observations and the reddened model fluxes and molecular transitions, although the atomic lines Ca i k4226 and Ca ii H and K are found to be unrealistically strong in the models. Our new effective temperature scale is significantly warmer than those in the literature, with the differences amounting to 400 K for the latest type M supergiants (i.e., M5 I). We show that the newly derived temperatures and bolometric corrections give much better agreement with stellar evolutionary tracks. This agreement provides a completely independent verification of our new temperature scale. The combination of effective temperature and bolometric luminosities allows us to calculate stellar radii; the coolest and most luminous stars (KW Sgr, Case 75, KY Cyg, HD 206936 = Cep) have radii of roughly 1500 R (7 AU), in excellent accordance with the largest stellar radii predicted from current evolutionary theory, although smaller than that found by others for the binary VV Cep and for the peculiar star VY CMa. We find that similar results are obtained for the effective temperatures and bolometric luminosities using only the dereddened V À K colors, providing a powerful demonstration of the self-consistency of the MARCS models.

A comparison of evolutionary tracks for single Galactic massive stars

“…This is a large difference (10%), affecting the interpretation of the properties of red supergiants (e.g. Levesque et al 2005;Davies et al 2013). The tracks from FRANEC and Padova tracks present a blue hook similar to what is observed during core helium burning for lower masses.…”

Open clusters

“…Photometric and spectroscopic studies of this cluster were performed by Ahmed (1962), Schild (1970), Harris (1976), Yilmaz (1976), Mermilliod (1982), Shobbrook (1985), Slettebak (1985), Shobbrook (1987), Moitinho et al (1997), Piatti et al (1998), Tadross (2001), Levesque et al (2005), McSwain & Gies (2005) and McSwain et al (2008). However, the cluster's distance (between 1.5 and 2.2 kpc) and age (from 14.5 to 25 Myr) remain somewhat uncertain.…”

“…On the other hand, spectroscopic investigations of galactic open clusters are quite scarce and are limited to a sample of cluster members that are primarily the Be stars and supergiant populations (Harris 1976;Mermilliod 1982;Slettebak 1985;Levesque et al 2005). Our ignorance of the physical properties of the stellar population (temperature, gravity, luminosity) and the incidence of highly rotating stars and/or binary stars in open clusters leads to high difficulties in determinating the distance modulus, average reddening, and the ages of these systems.…”

Observational evidence for the shrinking of bright maser spots

“…References. A97: Alvarez & Mennessier (1997); C91: Chapman et al (1991); C94: Chapman et al (1994); C07: Cenarro et al (2007); D87 : Diamond et al (1987); H74: Humphreys (1974); K87: Kirrane (1987); L05: Levesque et al (2005) kpc from Earth, without losing significant emission due to missing interferometer spacings. We measured the sizes of individual maser spots by fitting Gaussian components, allowing us to investigate the variations of maser size with intensity, position in the line profiles and other factors.…”