Photometric and spectroscopic variability of the blue supergiant rho Leo

TL;DR

The paper investigates the photometric and spectroscopic variability of rho Leo by combining long-baseline K2 and TESS photometry with extensive ground-based spectroscopy. Using GLS, GLSp, and WWZ analyses alongside gravity-darkened atmosphere modelling, the authors identify multiple quasi-periodic signals spanning ~0.8–35 days, with persistent components around ~17 days (radial pulsations) and ~12–13 days (stellar rotation) and a plausible inclination of ~$i\approx 22^{\circ}$. The results suggest rho Leo is evolving along a blue loop after the red supergiant phase, with variability driven by a combination of pulsations and rotational modulation, including wind-photosphere interactions reflected in the phase curves. The study demonstrates coherence of multi-method period detections across photometric and spectroscopic data and provides refined stellar parameters and an inclination estimate, contributing to the understanding of blue supergiant evolution and asteroseismic signatures in massive stars.

Abstract

Context. The post-main-sequence evolution of massive stars remains poorly understood, particularly for blue supergiants. These objects play a crucial role in the dynamical and chemical evolution of galaxies and exhibit pronounced photometric and spectroscopic variability, often quasi-periodic rather than strictly periodic. Aims. We investigate the variability of the evolved B-type star rho Leo to determine its physical properties, identify the underlying mechanisms driving its variability, and constrain its evolutionary stage. Methods. We analyse long-term spectroscopic and photometric datasets obtained from multiple sources, including the TESS and Kepler space missions and observations with the 1.5 m telescope in Estonia. Period analysis is performed using the Generalized Lomb-Scargle periodogram, Lomb-Scargle pre-whitening, and the Weighted Wavelet Z-Transform. Fundamental stellar parameters are derived by fitting synthetic line profiles computed with the FastWind code to the HARPS spectrum, while the stellar rotation inclination is estimated using the ZPEKTR code. Results. The He I 6678.151 A line shows significant radial-velocity and line-profile moment variations. We detect a set of periods and harmonics spanning approximately 0.8 to 35 days. Some periods remain stable over time, whereas others vary between observing seasons. A comparison of spectroscopic and photometric variability, together with phase-curve morphology, allows us to constrain the origin of several signals. In particular, the approximately 11 day period is attributed to stellar rotation, while the approximately 17 day period is linked to radial pulsations. Conclusions. Although the variability is quasi-periodic, most detected periods persist across multiple seasons. The wide range of timescales suggests that rho Leo is likely evolving along a blue loop following the red supergiant phase.

Figures (18)

• Figure 1: Comparison between the observed spectrum (black line) and the best-fitting FastWind model (red line).
• Figure 2: Lomb–Scargle power spectrum of the K2 photometry. The first six most powerful frequencies are shown. The dominant frequency (0.0405 d$^{-1}$) refers to a period of 24.7 days.
• Figure 3: Phase diagram of K2 photometry with a period of 24.85 days.
• Figure 4: WWZ scalogram for K2 photometry. The wavelet power is indicated by the colour (see colour bar at left). The lower bright stripe corresponds to the period of $\sim$25 days. The black solid line in the right panel shows the time-averaged wavelet power.
• Figure 5: Lomb–Scargle power spectrum of the TESS photometry. The first seven most powerful frequencies are shown. The dominant frequency of 0.055 d$^{-1}$ refers to a period of 18.2 days.