[Paper Review] Bright OB stars in the Galaxy IV. Stellar and wind parameters of early to late B supergiants

N. Markova, J. Puls|ArXiv.org|Nov 7, 2007

Stellar, planetary, and galactic studies69 references100 citations

TL;DR

This study uses the NLTE atmosphere code FASTWIND to determine stellar and wind parameters for 11 Galactic B-supergiants (B0 to B9), revealing that line- and wind-blanketing require a 10–20% downward revision of the effective temperature scale. The key finding is that mass-loss rates ($\dot{M}$) change by only a factor of 0.4–2.5 across the bi-stability region, contradicting theoretical predictions that $v_\infty$ decreases while $\dot{M}$ increases to compensate.

ABSTRACT

We apply the NLTE atmosphere code FASTWIND to perform a spectroscopic study of a small sample of Galactic B-supergiants from B0 to B9. By means of the resulting data and incorporating additional datasets from alternative studies, we investigate the properties of OB-supergiants and compare our findings with theoretical predictions. As a result we find that due to the combined effects of line- and wind-blanketing, the temperature scale of Galactic B-supergiants needs to be revised downwards, by 10 to 20 percent, the latter value being appropriate for stronger winds. In fair accordance with recent results, our sample furthermore indicates a gradual decrease in wind terminal velocities over the bi-stability region, where the limits of this region are located at lower temperatures than the predicted ones. Introducing a distance-independent quantity Q' related to wind-strength, we also show that this quantity is a well defined, monotonically increasing function of Teff outside this region. Inside and from hot to cool, the mass loss rate changes by a factor (in between 0.4 and 2.5) which is (much) smaller than the predicted factor of 5. All this indicates that the decrease in wind terminal velocity over the bi-stability region is not over-compensated by an increase of mass loss rate, as frequently argued (provided the wind-clumping properties on both sides of this region do not differ substantially).

Motivation & Objective

• To determine reliable stellar and wind parameters for a sample of Galactic B-supergiants from early to late spectral subtypes (B0 to B9).
• To investigate the effects of line- and wind-blanketing on the effective temperature scale of B-supergiants.
• To test theoretical predictions of wind properties (e.g., $v_\infty$, $\dot{M}$) against observed spectroscopic data.
• To examine the behavior of a new distance-independent wind-strength parameter $Q'$ across the bi-stability region.
• To assess the role of wind-clumping and macro-turbulence in interpreting observed line profiles and wind diagnostics.

Proposed method

• Employed the NLTE atmosphere code FASTWIND to synthesize spectral line profiles for observed B-supergiant stars.
• Used line profile fitting with synthetic profiles to derive stellar and wind parameters, including $T_{\rm eff}$, $\dot{M}$, $v_\infty$, and micro-turbulent velocity.
• Applied a Fourier technique to disentangle contributions from stellar rotation and macro-turbulence to line broadening.
• Incorporated Si abundance determinations in parallel with micro-turbulent velocity estimates to improve spectral fitting accuracy.
• Introduced a new distance-independent wind-strength parameter $Q' = \dot{M} / (R_\star^{1.5} \, g_{\rm eff} / v_\infty)$ to analyze wind behavior across $T_{\rm eff}$.
• Compared observed $Q'$ and wind properties with theoretical predictions from Vink et al. (2000) and recent observational datasets.

Experimental results

Research questions

• RQ1 How do line- and wind-blanketing effects influence the effective temperature scale of B-supergiants, particularly in mid- and late-type stars?
• RQ2 To what extent do observed wind parameters ($v_\infty$, $\dot{M}$) deviate from theoretical predictions in the bi-stability region?
• RQ3 Is the wind-strength parameter $Q'$ a well-behaved function of $T_{\rm eff}$ outside the bi-stability region, and how does it behave inside?
• RQ4 Does the observed change in $\dot{M}$ across the bi-stability region overcompensate for the decrease in $v_\infty$, as predicted by theory?
• RQ5 How do clumping effects and macro-turbulence influence the interpretation of wind diagnostics and the derived parameters?

Key findings

• The effective temperature scale for Galactic B-supergiants must be revised downward by 10–20%, with the larger correction applying to stars with strong winds, due to line- and wind-blanketing effects.
• The bi-stability region is located at lower $T_{\rm eff}$ (18–23 kK) than predicted (22.5–27 kK), with $v_\infty$ showing a gradual decrease across this region.
• The new wind-strength parameter $Q'$ is a well-defined, monotonically increasing function of $T_{\rm eff}$ outside the bi-stability region, indicating consistent wind behavior.
• Inside the bi-stability region, $\dot{M}$ changes by a factor between 0.4 and 2.5 when moving from hot to cool stars, much smaller than the predicted factor of 5.
• The decrease in $v_\infty$ is not over-compensated by an increase in $\dot{M}$, contradicting theoretical predictions that wind-momentum should increase across the jump.
• Theoretical mass-loss rates are likely overestimated, especially in the cool end of the bi-stability region, suggesting a need for revised wind models including clumping and radiation field effects.