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

Abstract

High‐resolution optical spectra of 57 Galactic B‐type supergiant stars have been analysed 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‐local thermodynamic equilibrium 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 macroturbulence and microturbulent velocity fields is discussed and our results support previous theoretical studies that link the former to subphotospheric 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.

The VLT-FLAMES Tarantula Survey

“…The differences in the two estimates for a given star are generally consistent with these standard deviations. These abundance estimates will also be affected by errors in the atmospheric parameters which has been discussed by, for example, Hunter et al (2007) and Fraser et al (2010). Using a similar methodology, we conservatively estimate a typical uncertainty of 0.2−0.3 dex.…”

“…9 and would lead to the effective temperature estimates as a function of spectral type listed in Table 10. Also listed in this table are the estimates obtained from using a similar procedure on the Galactic samples of Fraser et al (2010) and Crowther et al (2006) and the LMC sample of Trundle et al (2007). Fraser et al (2010), using the same model atmosphere grid as adopted A&A 575, A70 (2015) Table 10.…”

“…Also listed in this table are the estimates obtained from using a similar procedure on the Galactic samples of Fraser et al (2010) and Crowther et al (2006) and the LMC sample of Trundle et al (2007). Fraser et al (2010), using the same model atmosphere grid as adopted A&A 575, A70 (2015) Table 10. Estimates of effective temperatures (in kK) for different spectral types for the current sample (TAR), for supergiants in the LMC (Trundle et al 2007, LMC), a sample of Galactic supergiants (Fraser et al 2010, GAL) and for class Ia supergiants (Crowther et al 2006, CLW here, analysed the spectra of over 50 Galactic supergiants with luminosity classes ranging from Ia + to II.…”

“…The Si  triplet (4552, 4567, and 4574 Å) was adopted as it is observed in effectively all our spectra and as they arise from the same multiplet, the effects of errors in the absolute oscillator strengths and non-LTE effects should be minimised. This method has been used previously by, for example, Dufton et al (2005), Hunter et al (2007), and Fraser et al (2010), who noted that it is sensitive to errors in the equivalent width measurements of the Si  lines. Therefore the microturbulence was also estimated from requiring that the silicon abundance was consistent with that found in the LMC.…”

Critical study of the distribution of rotational velocities of Be stars

“…Howarth et al 1997;Ryans et al 2002;Dufton et al 2006a;Simón-Díaz & Herrero 2007;Fraser et al 2010;Sundqvist et al 2013). The existence of macroturbulence was also noticed in later B-type supergiants (Dufton et al 2006a).…”