[Paper Review] Wind properties of variable B supergiants

M. Haucke, L. S. Cidale+5 more|arXiv (Cornell University)|Jun 20, 2018

Stellar, planetary, and galactic studies106 references9 citations

TL;DR

This study investigates wind variability in 19 Galactic B-type supergiants using non-LTE synthetic line profile fitting with the FAST-WIND code, revealing that radial pulsations with periods >6 days likely drive mass-loss variations. Key findings include a new empirical link between mass-loss amplitude and photometric/spectroscopic variability on 10–100 day timescales, and evidence that stars on the cool side of the bi-stability jump exhibit reduced V∞/Vesc despite similar or lower mass-loss rates, suggesting enhanced wind clumping or ionization effects.

ABSTRACT

Context. Variable B supergiants (BSGs) constitute a heterogeneous group of stars with complex photometric and spectroscopic behaviours. They exhibit mass-loss variations and experience different types of oscillation modes, and there is growing evidence that variable stellar winds and photospheric pulsations are closely related. Aims. To discuss the wind properties and variability of evolved B-type stars, we derive new stellar and wind parameters for a sample of 19 Galactic BSGs by fitting theoretical line profiles of H, He, and Si to the observed ones and compare them with previous determinations. Methods. The synthetic line profiles are computed with the non-local thermodynamic equilibrium (NLTE) atmosphere code FASTWIND, with a β-law for hydrodynamics. Results. The mass-loss rate of three stars has been obtained for the first time. The global properties of stellar winds of mid/late B supergiants are well represented by a β-law with β > 2. All stars follow the known empirical wind momentum–luminosity relationships, and the late BSGs show the trend of the mid BSGs. HD 75149 and HD 99953 display significant changes in the shape and intensity of the Hα line (from a pure absorption to a P Cygni profile, and vice versa). These stars have mass-loss variations of almost a factor of 2.8. A comparison among mass-loss rates from the literature reveals discrepancies of a factor of 1 to 7. This large variation is a consequence of the uncertainties in the determination of the stellar radius. Therefore, for a reliable comparison of these values we used the invariant parameter Qr. Based on this parameter, we find an empirical relationship that associates the amplitude of mass-loss variations with photometric/spectroscopic variability on timescales of tens of days. We find that stars located on the cool side of the bi-stability jump show a decrease in the ratio V∞∕Vesc, while their corresponding mass-loss rates are similar to or lower than the values found for stars on the hot side. Particularly, for those variable stars a decrease in V∞∕Vesc is accompanied by a decrease in Ṁ. Conclusions. Our results also suggest that radial pulsation modes with periods longer than 6 days might be responsible for the wind variability in the mid/late-type. These radial modes might be identified with strange modes, which are known to facilitate (enhanced) mass loss. On the other hand, we propose that the wind behaviour of stars on the cool side of the bi-stability jump could fit with predictions of the δ−slow hydrodynamics solution for radiation-driven winds with highly variable ionization.

Motivation & Objective

• To derive accurate stellar and wind parameters for a sample of 19 Galactic B supergiants using synthetic line profile fitting.
• To investigate the connection between photospheric pulsations and variable stellar winds in evolved B-type stars.
• To resolve discrepancies in mass-loss rate determinations by introducing the invariant parameter Qr to minimize radius-dependent uncertainties.
• To explore the influence of the bi-stability jump on wind properties, particularly V∞/Vesc and mass-loss rates.
• To identify the pulsation modes responsible for wind variability in mid/late B supergiants.

Proposed method

• Synthetic H, He, and Si line profiles were computed using the non-LTE atmosphere code FAST-WIND with a β-law for wind velocity structure.
• Stellar and wind parameters (Teff, log g, β, Ṁ, V∞, microturbulence, macroturbulence, Vsin i, R⋆) were derived by fitting synthetic profiles to observed spectra.
• The invariant parameter Qr = Ṁ × V∞ × R⋆⁻¹ was used to normalize mass-loss rates and reduce dependence on uncertain stellar radius estimates.
• Wind momentum–luminosity relationship (WLR) was tested across the sample to validate wind model consistency.
• Variability in Hα line profiles (P Cygni vs. absorption) was analyzed to quantify mass-loss rate changes over time.
• Pulsation periods from literature and new fits were correlated with wind variability to identify driving modes.

Experimental results

Research questions

• RQ1 What is the relationship between radial pulsation modes and wind variability in mid/late B supergiants?
• RQ2 How do mass-loss rates derived from different observational tracers compare, and what causes the large discrepancies in the literature?
• RQ3 How do wind properties (especially V∞/Vesc and Ṁ) change across the bi-stability jump, and what physical mechanisms explain the observed trends?
• RQ4 Can the invariant parameter Qr be used to reliably compare mass-loss rates across different studies despite radius uncertainties?
• RQ5 What is the role of strange-mode pulsations in driving time-variable mass loss in evolved B supergiants?

Key findings

• The mass-loss rate was derived for the first time for three B supergiants in the sample, improving the parameter coverage of this class.
• All mid/late B supergiants follow a β-law with β > 2, indicating highly accelerated winds, and all satisfy the empirical wind momentum–luminosity relationship.
• HD 75149 and HD 99953 show Hα profile variations from pure absorption to P Cygni and back, indicating mass-loss rate changes by a factor of ~2.8 over time.
• Using the invariant parameter Qr, a new empirical relationship was found linking the amplitude of mass-loss variation to photometric/spectroscopic variability on timescales of tens of days.
• Stars on the cool side of the bi-stability jump show a significant decrease in V∞/Vesc (from ~2.6 to ~1.3) while maintaining similar or lower mass-loss rates compared to hot-side stars.
• The observed decrease in V∞/Vesc on the cool side is correlated with a decrease in Ṁ, suggesting that enhanced wind clumping or ionization effects (e.g., δ−slow hydrodynamics) may be at play, consistent with predictions of non-stationary radiation-driven wind models.