Equivalent widths of metal-poor stars

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 Fe I line abundances in high-resolution optical spectra in the wavelength range of ~3700-7000{AA}, although shorter wavelength ranges, up to 4750-6800{AA}, 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 Fe I-Fe II 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.