Asymmetry of Lines in the Spectra of the Sun and Solar-Type Stars

TL;DR

This study analyzes line asymmetries in solar and stellar spectra to understand convective motions, revealing how rotation, temperature, gravity, and metallicity influence line bisectors and convective velocities.

Contribution

It provides new measurements of average convective velocities and bisector shapes across different stars, linking these to stellar parameters and improving understanding of stellar surface convection.

Findings

Convective velocity in the Sun is about -420 m/s.

Bisector shapes vary with stellar rotation and temperature.

Wavelength scale shifts in spectral atlases can reach -330 m/s.

Abstract

We have analysed the asymmetry of lines Fe I and Fe II in spectra of a solar flux using three FTS atlases and the HARPS atlas and also in spectra of 13 stars using observation data on the HARPS spectrograph. To reduce observation noise individual line bisectors of each star have been averaged. The obtained average bisectors in the stellar spectra are more or less similar to the shape C well known to the Sun. In stars with rotation speeds greater than 5 km/s the shape of the bisectors is more like /. The curvature and span of the bisectors increase with the temperature of the star. Our results confirm the known facts about strong influence of rotation velocity on the span and shape of bisectors. The average convective speed was determined based on the span of the average bisector, which shows the largest difference between the velocity of cold falling and hot rising convective flows of the matter. It's equal to -420 m/s for the Sun as a star. In solar-type stars, it grows from -150 to -700 m/s with an effective temperature of 4800 to 6200 K, respectively. For stars with greater surface gravity and greater metallicity, the average convective velocity decreases. It also decreases with star age and correlates with the speed of micro and macroturbulent movements. The results of solar flux analysis showed that absolute wavelength scales in the FTS atlases used coincide with an accuracy of about -10 m/s, except for the atlas of Hinkle, etc., whose scale is shifted and depends on the wavelength. In the range from 450 to 650 nm, the scale shift of this atlas varies from -100 to -330 m/s, respectively, and it equals on average of -240 m/s. The resulting average star bisectors contain information about the fields of convective velocities and may be useful for hydrodynamic modeling of stellar atmospheres in order to study the characteristic features of surface convection.