Atmospheric parameters and rotational velocities for a sample of Galactic B-type supergiants

TL;DR

This study analyzes 57 Galactic B-type supergiants to determine their atmospheric and rotational properties, compares observations with stellar evolution models, and discusses turbulence origins and differential rotation evidence.

Contribution

It provides detailed measurements of velocities and atmospheric parameters for B-type supergiants, testing stellar evolution predictions and exploring turbulence and rotation phenomena.

Findings

Good agreement between observed and predicted rotational velocities.

Some stars show higher rotational velocities possibly due to binarity.

No correlation between nitrogen abundance and current rotational velocity.

Abstract

High resolution optical spectra of 57 Galactic B-type supergiant stars have been analyzed to determine their rotational and macroturbulent velocities. In addition, their atmospheric parameters (effective temperature, surface gravity and microturbulent velocity) and surface nitrogen abundances have been estimated using a non-LTE grid of model atmospheres. Comparisons of the projected rotational velocities have been made with the predictions of stellar evolutionary models and in general good agreement was found. However for a small number of targets, their observed rotational velocities were significantly larger than predicted, although their nitrogen abundances were consistent with the rest of the sample. We conclude that binarity may have played a role in generating their large rotational velocities. No correlation was found between nitrogen abundances and the current projected rotational velocities. However a correlation was found with the inferred projected rotational velocities of the main sequence precursors of our supergiant sample. This correlation is again in agreement with the predictions of single star evolutionary models that incorporate rotational mixing. The origin of the macroturbulent and microturbulent velocity fields is discussed and our results support previous theoretical studies that link the former to sub-photospheric convection and the latter to non-radial gravity mode oscillations. In addition, we have attempted to identify differential rotation in our most rapidly rotating targets.