Does the radial–tangential macroturbulence model adequately describe the spectral line broadening of solar-type stars?

Abstract

Abstract In incorporating the effect of atmospheric turbulence in the broadening of spectral lines, the so-called radial–tangential macroturbulence (RTM) model has been widely used in the field of solar-type stars, which was devised from intuitive appearance of the granular velocity field of the Sun. Since this model assumes that turbulent motions are restricted to only radial and tangential directions, it has a special broadening function with notably narrow width due to the projection effect, the validity of which has not yet been confirmed in practice. With the aim of checking whether this RTM model adequately represents the actual solar photospheric velocity field, we carried out an extensive study on the non-thermal velocity dispersion along the line of sight (Vlos) by analyzing spectral lines at various points of the solar disk based on locally averaged as well as high-spatial-resolution spectra, and found the following results. First, the center-to-limb run of Vlos derived from ground-based low-resolution spectra is simply monotonic with a slightly increasing tendency, which contradicts the specific trend (an appreciable peak at θ ≃ 45°) predicted from RTM. Second, the Vlos values derived from a large number of spectra based on high-resolution space observation were revealed to follow a nearly normal distribution, without any sign of the peculiar distribution expected for the RTM case. These two observational facts indicate that the actual solar velocity field is not simply dichotomous as assumed in RTM, but directionally more chaotic. We thus conclude that RTM is not an adequate model, at least for solar-type stars, as it would significantly overestimate the turbulent velocity dispersion by a factor of ∼2. The classical Gaussian macroturbulence model should be more reasonable in this respect.