Spectroscopic stellar surface gravities from metallic line profiles

Accurate surface gravities are needed for studies of stellar atmospheres and to determine stellar masses. It is suggested that measurements of gravities for late type stars are usually subject to great uncertainties, especially as the results are sensitive to the assumed effective temperature. A method is therefore proposed using the damping wings of metal lines to measure the gravity; and weak lines from the same lower level as the strong line to determine the metal abundance. Low excitation lines of Pel are used to demonstrate the method and new oscillator strengths are given for 51 transitions from the 0.00-0.12 and 0.86-0.99 eV levels. Using the furnace absorption technique the precision obtained is about 0.02 dex. Comparison with other authors' results shows systematic discrepancies between emission and absorption methods. The oscillator strengths are used to interpret the observed line strengths and profiles in the solar spectrum. From 5 model atmospheres a microturbulence of 0.8 km/sec is found, and iron "abundances" of 7.5-7.7. Damping constants are found for strong and medium-strong lines and the values are close to the predictions of classical theories in contrast with the greater damping predicted by more recent calculations. Using the measured oscillator strengths and damping constants the spectrum of Arcturus is used to derive an iron abundance of 6.99 and a microturbulence of 2.1 km/sec. The abundance is sensitive to the effective temperature, and 4400K is proposed from the observed flux distribution. The gravity determined from the profile of the line FeI 5269.5A is however insensitive to the adopted temperature and the measured value is log g = 1.45 ± 0.15 corresponding to a mass of 0.56 M(sun) for a radius of 23 R(sun). The gravity and mass are greater than the results of other methods which assume, and are sensitive to, lower effective temperatures. It is concluded that metallic line profiles in the spectra of cool stars can yield precise gravities which are insensitive to the assumption of LTE or to the temperature in the atmosphere.