Center--limb variation of solar photospheric microturbulence

TL;DR

This study investigates how solar photospheric microturbulence varies from the disk center to the limb, revealing a gradual increase that aligns with predictions from 3D hydrodynamical models.

Contribution

It provides the first detailed measurement of microturbulence variation across the solar disk, filling a gap in understanding stellar atmospheric dynamics.

Findings

Microturbulence increases from ~1.0 km/s at disk center to ~2 km/s at the limb.

The disk-integrated microturbulence is about 20% larger than at the disk center.

Results are consistent with 3D hydrodynamical model predictions.

Abstract

Microturbulence (\xi) is a key parameter introduced in stellar spectroscopy to explain the strength of saturated lines by formally incorporating an additional thermal broadening term in the line opacity profile. Although our Sun can serve as an important testing bench to check the usual assumption of constant \xi, the detailed behavior of how \xi varies from the disk center through the limb seems to have never been investigated so far. In order to fill this gap, local \xi values on the solar disk were determined from the equivalent widths of 46 Fe I lines at 32 points from the center to the limb by requiring the consistency between the abundances derived from lines of various strengths. The run of \xi with \theta (angle between line of sight and the surface normal) was found to be only gradual from ~1.0km/s (at sin\theta = 0: disk center) to ~1.3km/s (at sin\theta ~ 0.7: two-thirds of radial distance); but thereafter increasing more steeply up to ~2km/s (at sin\theta = 0.97: limb). This result further suggests that the microturbulence derived from the flux spectrum of the disk-integrated Sun is by ~20% larger than that of the disk-center value, which is almost consistent with the prediction from 3D hydrodynamical model atmospheres.