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

Abstract

We present an 80-d long uninterrupted high-cadence K2 light curve of the B1Iab supergiant ρ Leo (HD91316), deduced with the method of halo photometry. This light curve reveals a dominant frequency of frot = 0.0373 d-1 and its harmonics. This dominant frequency corresponds with a rotation period of 26.8 d 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 d. The spectroscopy reveals rotational modulation by a dynamic aspherical wind with an amplitude of about 20 km s-1 in the Ha line, as well as photospheric velocity variations of a few kms-1 at frequencies in the range 0.2-0.6 d-1 in the Si III 4567Å 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 superinertial low-degree large-scale gravity waves with dominant tangential amplitudes and discuss why ρ Leo is an excellent target to study how the observed photospheric variability propagates into the wind.