Convective Line Shifts in the Spectra of Solar-Type Stars

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 convective (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 altitude in the atmosphere in a large range of altitudes from 150 to 700 km were obtained for the first time. All these curves indicate a decrease in blue shifts with altitude, 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 altitudes above 600 km. In cooler K stars, granulation shifts change with altitude on average from –150 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 convective velocity of the star averaged over all analyzed altitudes 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. Large deviations of –21 050 and 1775 m/s were found for the stars HD 102361 and HD 42936, respectively. For the rest of the stars, the deviation does not exceed ±340 m/s, which is probably associated with the use of an average granulation velocity of –300 m/s in the SIMBAD data. Our analysis has shown that the average granulation velocity is not the same for solar-type stars. It is lower in colder stars and higher in hotter stars than the Sun. Therefore, determination of the radial velocities needs to take into account the individual granulation velocities of stars.