Bolometric correction and spectral energy distribution of cool stars in Galactic clusters

TL;DR

This study investigates the bolometric correction trends for cool red giant stars in Galactic clusters, finding minimal metallicity dependence and providing new fitting functions for stellar parameters.

Contribution

It offers the first extensive empirical analysis of bolometric corrections across a wide metallicity range for red giants, with new fitting functions for BC versus Teff and colors.

Findings

BC(K) is reliable within <0.1 mag across the studied range.

BC(V) is reliable for stars warmer than ~3800K.

No significant dependence of BC(V) and BC(K) on metallicity was observed.

Abstract

We have investigated the relevant trend of the bolometric correction (BC) at the cool-temperature regime of red giant stars and its possible dependence on stellar metallicity. Our analysis relies on a wide sample of optical-infrared spectroscopic observations, along the 3500A-2.5micron wavelength range, for a grid of 92 red giant stars in five (3 globular + 2 open) Galactic clusters, along the -2.2<[Fe/H]<+0.4 metallicity range. Bolometric magnitudes have been found within an internal accuracy of a few percent. Making use of our new database, we provide a set of fitting functions for the V and K BC vs. Teff and broad-band colors, valid over the interval 3300K<Teff<5000K, especially suited for Red Giants. No evident drift of both BC(V) and BC(K) with [Fe/H] is found. Things may be different, however, for the B-band correction, given a clear (B-V) vs. [Fe/H] correlation in place for our data, with metal-poor stars displaying a "bluer" (B-V) with respect to the metal-rich sample, for fixed Teff. Our empirical bolometric scale supports the conclusion that (a) BC(K) from the most recent studies are reliable within <0.1 mag over the whole color/temperature range considered in this paper, and (b) the same conclusion apply to BC(V) only for stars warmer than ~3800K. At cooler temperatures the agreement is less general, and MARCS models are the only ones providing a satisfactory match to observations.