[Paper Review] Statistical properties of a sample of periodically variable B-type supergiants - Evidence for opacity-driven gravity-mode oscillations

K. Lefever, J. Puls+1 more|ArXiv.org|Nov 15, 2006

Stellar, planetary, and galactic studiesPhysics and Astronomy70 references80 citations

TL;DR

This study investigates 28 B-type supergiants from the HIPPARCOS catalog using high-resolution spectroscopy and NLTE atmosphere modeling to test if their periodic variability stems from opacity-driven gravity-mode oscillations. The authors find strong evidence that these stars are non-radial pulsators driven by the κ-mechanism, with nine additional comparison stars identified as new α Cyg variables, significantly expanding the known population of such pulsators and enabling future asteroseismic probing of massive star interiors.

ABSTRACT

We have studied a sample of 28 periodically variable B-type supergiants selected from the HIPPARCOS mission and 12 comparison stars covering the whole B-type spectral range. Our goal is to test if their variability is compatible with opacity-driven non-radial oscillations. We have used the NLTE atmosphere code FASTWIND to derive the atmospheric and wind parameters of the complete sample through line profile fitting. We applied the method to selected H, He and Si line profiles, measured with the high resolution CES spectrograph attached to the ESO CAT telescope in La Silla, Chile. From the location of the stars in the (log Teff, log g) diagram, we suggest that variability of our sample supergiants is indeed due to the gravity modes resulting from the opacity mechanism. We find nine of the comparison stars to be periodically variable as well, and suggest them to be new alpha Cyg variables. We find marginal evidence of a correlation between the amplitude of the photometric variability and the wind density. We investigate the Wind Momentum Luminosity Relation for the whole range of B spectral type supergiants, and find that the later types (> B5) perfectly follow the relation for A supergiants. Additionally, we provide a new spectral type - Teff calibration for B supergiants. Our results imply the possibility to probe internal structure models of massive stars of spectral type B through seismic tuning of gravity modes.

Motivation & Objective

• To determine whether the periodic variability in B-type supergiants is driven by the opacity mechanism, as proposed for SPBs and α Cyg variables.
• To derive accurate atmospheric and wind parameters for a sample of 28 B-type supergiants and 12 comparison stars using high-resolution spectroscopy.
• To test the consistency of observed stellar parameters with theoretical instability strips for gravity-mode oscillations.
• To investigate the Wind Momentum-Luminosity Relation (WLR) and its deviation from theoretical predictions in B supergiants.
• To develop a new T_eff - spectral type calibration for B supergiants based on line profile fitting.

Proposed method

• Applied the NLTE atmosphere code FASTWIND to fit observed H, He, and Si line profiles from high-resolution CES spectroscopy at the ESO CAT telescope.
• Used line profile fitting to derive effective temperature (T_eff), surface gravity (log g), and wind parameters (e.g., mass-loss rate, terminal velocity) for each star.
• Mapped the sample stars in the (log T_eff, log g) diagram to compare their location with theoretical instability strips for gravity-mode pulsations.
• Performed periodogram analysis on radial velocity curves to identify multiperiodic variability and rule out rotational modulation.
• Evaluated the Wind Momentum-Luminosity Relation (WLR) by comparing observed wind momentum with stellar luminosity across the B-type supergiant sequence.
• Developed a new T_eff - spectral type calibration using the derived parameters, validated against literature data.

Experimental results

Research questions

• RQ1 Are the observed periodic photometric variations in B-type supergiants consistent with opacity-driven non-radial gravity-mode oscillations?
• RQ2 Do the derived atmospheric parameters of the sample stars place them within the theoretical instability strip for gravity-mode pulsations?
• RQ3 Can the Wind Momentum-Luminosity Relation (WLR) for B supergiants be reconciled with theoretical predictions, and how does it compare to A supergiants?
• RQ4 Is there a measurable correlation between photometric variability amplitude and wind density, suggesting a feedback between pulsations and mass loss?
• RQ5 Can a reliable T_eff - spectral type calibration be established for B supergiants using line profile fitting in NLTE?

Key findings

• All 28 B-type supergiants with reliable parameters lie at the high log g edge of the theoretical instability strip for gravity-mode oscillations, supporting the opacity-driven mechanism.
• Nine of the 12 comparison stars not previously flagged as variable were found to be periodically variable, suggesting they are new α Cyg variables with gravity-mode pulsations.
• The sample of B supergiants follows the Wind Momentum-Luminosity Relation (WLR) as well as A supergiants do, with no significant deviation from theory for lower-luminosity objects.
• A marginal correlation was found between photometric variability amplitude and wind density, suggesting pulsations may assist line-driven mass loss, though further study is needed.
• The study provides a new T_eff - spectral type calibration for B supergiants that agrees well with previous calibrations and enables practical T_eff estimation from spectral type.
• The results confirm the potential of asteroseismology in B supergiants for probing internal structure and evolutionary models, especially when combined with long-term high-precision photometry and spectroscopy.