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Variability of Southern Blue Supergiants with TESS

Published online by Cambridge University Press:  09 November 2026

Michalis Kourniotis Open the ORCID record for Michalis Kourniotis [Opens in a new window] ,
Lydia S. Cidale ,
Michaela Kraus Open the ORCID record for Michaela Kraus [Opens in a new window] ,
Matias A. Ruiz Diaz  and
Aldana Alberici Adam
Michalis Kourniotis*
Affiliation:
Astronomical Institute, Czech Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic
Lydia S. Cidale
Affiliation:
Instituto de Astrofísica La Plata, CCT La Plata, CONICET-UNLP, Paseo del Bosque S/N, B1900FWA, La Plata, Argentina Departamento de Espectroscopía, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata (UNLP), Paseo del Bosque S/N, B1900FWA, La Plata, Argentina
Michaela Kraus
Affiliation:
Astronomical Institute, Czech Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic
Matias A. Ruiz Diaz
Affiliation:
Instituto de Astrofísica La Plata, CCT La Plata, CONICET-UNLP, Paseo del Bosque S/N, B1900FWA, La Plata, Argentina Departamento de Espectroscopía, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata (UNLP), Paseo del Bosque S/N, B1900FWA, La Plata, Argentina
Aldana Alberici Adam
Affiliation:
Instituto de Astrofísica La Plata, CCT La Plata, CONICET-UNLP, Paseo del Bosque S/N, B1900FWA, La Plata, Argentina Departamento de Espectroscopía, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata (UNLP), Paseo del Bosque S/N, B1900FWA, La Plata, Argentina
*
email: kourniotis@asu.cas.cz
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Abstract

The photometric variability of massive stars provides the opportunity to study the dynamics in their interior and atmosphere, in turn assisting in characterizing their evolutionary stage. We explore correlations between the stellar parameters of blue supergiants (BSGs) in the Galaxy and their variability properties assessed with the Transiting Exoplanet Survey Satellite (TESS). We explored the time and frequency domain of 41 stars, including also modeling of their stochastic low-frequency (SLF) variability. We report a positive trend between the light curve amplitude and log (L/L$_{\odot})$, with the less luminous stars displaying frequencies that can be interpreted as rotationally induced. The more luminous BSGs are mixed with pulsators of the $\alpha$ Cygni class, which display diverse and/or time-variant photometric properties. A significant positive trend is also found between the SLF variability amplitude and log $T_{\rm eff}$, highlighting an influential role of the stellar age on the emergence of the ambiguous signal beyond the main sequence. Notably, the $\alpha$ Cygni variables display a suppressed SLF variability, which may mirror their rather advanced stage as post-red supergiants.

Keywords

stars: massivestars: variables: generalstars: rotationstars: oscillationsstars: evolution

Information

Type
Poster Paper
Information
Proceedings of the International Astronomical Union , Volume 21 , Symposium S402: Massive Stars Across Redshifts in the Era of JWST and Large Surveys , January 2025 , pp. 293 - 296
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Copyright
© The Author(s), 2026. Published by Cambridge University Press on behalf of The International Astronomical Union

1. Introduction

The class of blue supergiants (BSGs) consists of objects that span an appreciable fraction of the massive star lifetime between the main sequence and the pre-supernova stages. The observed population statistics of BSGs (e.g. Castro et al. Reference Castro2014), however, contradict the expectations of the standard evolutionary theory (Bellinger et al. Reference Bellinger2024), highlighting the need to revisit the origin and evolutionary state of these objects. In support of the latter aim, the study of BSG variability provides robust diagnostics of the stellar structure and wind, setting valuable constraints on the physics of the post-main-sequence evolution and facilitating the classification of the respective counterparts. Motivated by this potential, we conducted a variability study of Galactic B-type BSGs and explored associations between their well-determined physical parameters and the features of their time and frequency domain.

2. Methodology

We undertook a study of 41 BSGs in the Galaxy with determined spectroscopic parameters (Fraser et al. Reference Fraser2010) using TESS data from sectors 5 minus 66 $5-66$ (Ricker et al. Reference Ricker2015). Both pre-processed (PDCSAP) and our own extracted/corrected fluxes were explored, taking into account possible contamination effects in the TESS aperture. The light curves were de-trended and normalized, and consecutive sectors were joined into larger observing windows. The entirety of the studied data are available on the Zenodo repository.Footnote †

We described the time domain of the stars by means of three statistical measures: the standard deviation sigma $\sigma$ , the coherency parameter ( psi squared $\psi^2$ ), and the skewness, to assess the degree of scatter, stochasticity, and asymmetry of the light curves. In the frequency domain, we extracted prominent frequencies via an iterative pre-whitening process and modeled the background signal –so called stochastic low-frequency (SLF) variability– that manifests ubiquitously in all amplitude spectra. The luminosities of the stars were calculated from the modeling of their spectral energy distribution. We assessed correlations between the variability and stellar parameters by calculating the Pearson’s correlation coefficient r, with a p minus $p-$ value significance.

3. Results

We found a strong positive correlation between sigma $\sigma$ and log ( upper L divided by upper L Subscript circled dot $L/L_{\odot}$ ) ( r equals 0.74 $r=0.74$ , p less than 0.0001 $p\lt0.0001$ ); the more luminous BSGs display greater variability amplitudes (Fig. 1). The frequencies of the less massive stars comply with their rotational periods (red markers; Fig. 1), attributing their variability to surface spots or clumped winds. A positive correlation is also seen between psi squared $\psi^2$ and log (L/L Subscript circled dot Baseline right parenthesis $_{\odot})$ ( r equals 0.52 $r=0.52$ , p less than 0.001 $p\lt0.001$ ), indicating that the more luminous variables possess smooth light curves with a higher degree of coherency (yet with larger fluctuations across the different sectors) compared to less luminous objects. The positive trend between the Eddington factor normal upper Gamma Subscript e $\Gamma_{e}$ and the skewness ( r equals 0.33 $r=0.33$ , p equals 0.04 $p=0.04$ ) implies that younger and less massive BSGs display short-term brightening, being indicative of wind, line-driven, instabilities (Krtička & Feldmeier Reference Krtička and Feldmeier2021). Less bound to the above trends are several of the alpha $\alpha$ Cygni variables (outer circles; Fig. 1), stars widely claimed to be post-red supergiants (post-RSGs). The irregularity in their light curves follows the inhomogeneity in the excitation mechanisms of their pulsations, these including oscillatory convection modes and radial strange modes (Saio et al. Reference Saio2013).

Figure 1.

Stellar parameters of the sample BSGs against their time-domain statistics. Red points indicate the candidate rotating variables; the outer circles the alpha $\alpha$ Cygni ones. Taken from Kourniotis et al. (Reference Kourniotis2025).

Multiple scatter plots showing relationships between stellar parameters and variability properties of blue supergiants.
Figure 1 Long description

Panel A: Scatter plot of log(T_eff [K]) versus sigma [mag]. Panel B: Scatter plot of log(T_eff [K]) versus psi squared. Panel C: Scatter plot of log(T_eff [K]) versus skw. Panel D: Scatter plot of log(L/L_sun) versus sigma [mag]. Panel E: Scatter plot of log(L/L_sun) versus psi squared. Panel F: Scatter plot of log(L/L_sun) versus skw. Panel G: Scatter plot of F_e versus sigma [mag]. Panel H: Scatter plot of F_e versus psi squared. Panel I: Scatter plot of F_e versus skw. Each panel includes red points indicating candidate rotating variables and blue circles indicating alpha Cygni variables.

We locate our studied objects in the Hertzsprung-Russell diagram (Fig. 2) and superimpose them on models of stellar evolution at solar metallicity (Ekström et al. Reference Ekström2012). The alpha $\alpha$ Cygni variables are in good agreement with their definition of post-RSGs, meaning they span tracks that suggest blueward evolution ( 5.0 less than or equivalent to normal l normal o normal g left parenthesis upper L divided by upper L Subscript circled dot Baseline right parenthesis less than or equivalent to 5.8 $5.0\lesssim{\rm log}(L/L_{\odot})\lesssim5.8$ ). We report a significant trend between the SLF variability amplitude alpha 0 $\alpha_{0}$ and log upper T Subscript normal e normal f normal f $T_{\rm eff}$ ( r equals 0.59 $r=0.59$ , p less than 0.0001 right parenthesis $p\lt0.0001)$ , showing that the background signal is pronounced at the earlier stages of evolution and drops as the stars age.

Figure 2.

Hertzsprung-Russell diagram for rotating massive stars at solar metallicity (Ekström et al. Reference Ekström2012). The marker size of our sample stars is proportional to the SLF variability amplitude. Taken from Kourniotis et al. (Reference Kourniotis2025).

Scatter plot showing the relationship between luminosity and effective temperature of rotating massive stars.

4. Discussion and Concluding Remarks

Our study of post-main-sequence BSGs (Kourniotis et al. Reference Kourniotis2025) complements studies on earlier-type objects (Bowman et al. Reference Bowman2020; Shen et al. Reference Shen2024) showing that the SLF variability is amplified at or shortly after the end of the main-sequence phase. Less plausible is the scenario that the signal is generated by subsurface convection zones driven by the iron opacity peak, as it would raise expectations for amplification towards the upper cool evolutionary diagram (Cantiello et al. Reference Cantiello2021) which, however, does not comply with the current picture (Fig. 2). On the other hand, when confining ourselves to lower luminosities that exclude the alpha $\alpha$ Cygni variables, we found a weak positive trend between alpha 0 $\alpha_{0}$ and the stellar luminosity/mass (see Fig. 10 in Kourniotis et al. Reference Kourniotis2025). The latter trend is similar to that of BSGs in the Large Magellanic Cloud (Bowman et al. Reference Bowman2019), attributing the debated signal possibly to a metallicity-independent mechanism.

To improve mapping of the BSG variability across the evolutionary diagram, an exploration of a large statistical sample using machine learning is ongoing (Kourniotis et al. in prep.).

Acknowledgements

Funded by the European Union (Project 101183150 – “OCEANS”) and the Czech Science Foundation (GAČR grant number 25-17532S). LC acknowledges financial support from CONICET (PIP 1337) and the University of La Plata (Programa de Incentivos 11/G160). The Astronomical Institute Ondřejov is supported by RVO:67985815. MRD and AAA acknowledge support from a CONICET fellowship.

References

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Figure 0

Figure 1. Figure 1 long description.Stellar parameters of the sample BSGs against their time-domain statistics. Red points indicate the candidate rotating variables; the outer circles the α$\alpha$ Cygni ones. Taken from Kourniotis et al. (2025).

Figure 1

Figure 2. Hertzsprung-Russell diagram for rotating massive stars at solar metallicity (Ekström et al. 2012). The marker size of our sample stars is proportional to the SLF variability amplitude. Taken from Kourniotis et al. (2025).