Deriving Stellar Effective Temperatures of Metal-Poor Stars with the Excitation Potential Method

TL;DR

This paper introduces a new correction method for spectroscopically derived stellar temperatures of metal-poor stars, aligning them more closely with photometric temperatures and improving the consistency of stellar parameter determinations.

Contribution

The authors develop a simple correction scheme for excitation temperatures of metal-poor stars, enhancing their agreement with photometric values and refining related stellar parameters.

Findings

Correction increases temperatures by several hundred degrees for cool red giants.

Adjusted temperatures improve agreement between spectroscopic and photometric measurements.

Method benefits high-resolution stellar surveys by standardizing temperature and abundance determinations.

Abstract

It is well established that stellar effective temperatures determined from photometry and spectroscopy yield systematically different results. We describe a new, simple method to correct spectroscopically derived temperatures ("excitation temperatures") of metal-poor stars based on a literature sample with -3.3<[Fe/H]<-2.5. Excitation temperatures were determined from FeI line abundances in high-resolution optical spectra in the wavelength range of ~3700 to ~7000A, although shorter wavelength ranges, up to 4750 to 6800A, can also be employed, and compared with photometric literature temperatures. Our adjustment scheme increases the temperatures up to several hundred degrees for cool red giants, while leaving the near-main-sequence stars mostly unchanged. Hence, it brings the excitation temperatures in good agreement with photometrically derived values. The modified temperature also influences other stellar parameters, as the FeI-FeII ionization balance is simultaneously used to determine the surface gravity, while also forcing no abundance trend on the absorption line strengths to obtain the microturbulent velocity. As a result of increasing the temperature, the often too low gravities and too high microturbulent velocities in red giants become higher and lower, respectively. Our adjustment scheme thus continues to build on the advantage of deriving temperatures from spectroscopy alone, independent of reddening, while at the same time producing stellar chemical abundances that are more straightforwardly comparable to studies based on photometrically derived temperatures. Hence, our method may prove beneficial for comparing different studies in the literature as well as the many high-resolution stellar spectroscopic surveys that are or will be carried out in the next few years.