K2 photometry and HERMES spectroscopy of the blue supergiant rho Leo: rotational wind modulation and low-frequency waves

TL;DR

This study combines 80 days of high-cadence K2 photometry and 1800 days of spectroscopy to analyze the rotational modulation, wind variability, and gravity waves in the blue supergiant rho Leo, revealing complex multi-periodic behavior.

Contribution

It provides a detailed analysis of rho Leo's rotational, wind, and gravity wave variability using combined photometric and spectroscopic data, highlighting the star's suitability for studying photosphere-wind interactions.

Findings

Detected a dominant rotation frequency of 0.0373 d$^{-1}$ and its harmonics.

Revealed low-frequency multiperiodic variability consistent across photometry and spectroscopy.

Identified photospheric velocity variations as due to large-scale gravity waves.

Abstract

We present an 80-d long uninterrupted high-cadence K2 light curve of the B1Iab supergiant rho Leo (HD 91316), deduced with the method of halo photometry. This light curve reveals a dominant frequency of $f_{\rmrot}=0.0373$d$^{-1}$ and its harmonics. This dominant frequency corresponds with a rotation period of 26.8d and is subject to amplitude and phase modulation. The K2 photometry additionally reveals multiperiodic low-frequency variability ($<1.5 $d$^{-1}$) and is in full agreement with low-cadence high-resolution spectroscopy assembled during 1800 days. The spectroscopy reveals rotational modulation by a dynamic aspherical wind with an amplitude of about 20km s$^{-1}$ in the H$\alpha$ line, as well as photospheric velocity variations of a few km s$^{-1}$ at frequencies in the range 0.2 to 0.6 d$^{-1}$ in the SiIII 4567\AA\ line. Given the large macroturbulence needed to explain the spectral line broadening of the star, we interpret the detected photospheric velocity as due to travelling super-inertial low-degree large-scale gravity waves with dominant tangential amplitudes and discuss why $\rho$~Leo is an excellent target to study how the observed photospheric variability propagates into the wind.