Convective Line Shifts in the Spectra of Solar-Type Stars

TL;DR

This study measures and analyzes convective line shifts in solar-type stars, revealing how granulation velocities vary with height, temperature, and stellar properties, and emphasizing their importance in radial velocity measurements.

Contribution

First detailed curves of convection velocities as a function of atmospheric height for FGK stars, linking granulation dynamics with stellar parameters.

Findings

Granulation velocities decrease with height, reaching zero in the photosphere.

Red shifts in the chromosphere indicate reversal of granulation velocities above 600 km.

Velocity gradients increase with stellar temperature and decrease with gravity, metallicity, and age.

Abstract

The Doppler line shifts in the spectra of the Sun and stars with effective temperatures from 4800 to 6200 K were measured and the average connective (granulation) velocities were estimated. The absolute scale of the line shifts for the stars was established on the basis of the derived dependence of the shifts of solar lines on optical depth. For FGK solar-type stars, curves of convection velocities as a function of the height in the atmosphere in a large range of heights from 150 to 700 km were obtained for the first time. All these curves indicate a decrease in blue shifts with height, which means that the granulation velocities through the photosphere slow down to zero. In the lower chromosphere, red shifts of strong Mg I lines are observed, which indicate a change in the direction of granulation velocities to the opposite and confirm the effects of reversal of granulation at heights above 600 km. In cooler K stars, granulation shifts change with height on average from -50 to 100 m/s, while they change more sharply in hotter FG stars from -700 to 300 m/s. The gradient of the line shift curves increases with an increase in the effective temperature and a decrease in gravity, metallicity, and age of the star. The connective velocity of the star averaged over all analyzed heights increases from -90 to -560 m/s from colder to hotter stars. It correlates with macroturbulence, asymmetry of spectral lines, and the rotation velocity of the star. We also obtained the radial velocities of the stars and compared them with the SIMBAD data. Our analysis has shown that the individual granular velocities of the stars must be taken into account when determining the radial velocities.