The IACOB project: III. New observational clues to understand macroturbulent broadening in massive O- and B-type stars

Abstract

Context. The term 'macroturbulence' of O-and B-type stars is commonly used to refer to a source of non-rotational broadening affecting their spectral line-profiles. It has been proposed to be a spectroscopic signature of the presence of stellar oscillations; however, we still lack a definitive confirmation of this hypothesis. Aims. We aim to provide new empirical clues about macroturbulent spectral line broadening in O-and B-type stars to evaluate its physical origin. Methods. We use high-resolution spectra of ≈430 stars with spectral types in the range O4 -B9 (all luminosity classes) compiled in the framework of the IACOB project. We characterize the line-broadening of adequate diagnostic metal lines using a combined Fourier transform and goodness-of-fit technique. We perform a quantitative spectroscopic analysis of the whole sample using automatic tools coupled with a huge grid of fastwind models to determine their effective temperatures and gravities. We also incorporate quantitative information about line asymmetries to our observational description of the characteristics of the line-profiles, and present a comparison of the shape and type of line-profile variability found in a small sample of O stars and B supergiants with still undefined pulsational properties and B main sequence stars with variable line-profiles (due to a well identified type of stellar oscillations or to the presence of spots in the stellar surface). Results. We present a homogeneous and statistically significant overview of the (single snapshot) line-broadening properties of stars in the whole O and B star domain. We find empirical evidence of the existence of various types of non-rotational broadening agents acting in the realm of massive stars. Even though all of them could be quoted and quantified as a macroturbulent broadening from a practical point of view, their physical origin can be different. Contrarily to the early-to late-B dwarfs/giants, which present a mixture of cases in terms of line-profile shape and variability, the whole O-type and B supergiant domain (or, roughly speaking, stars with M ZAMS 15 M ⊙ ) is fully dominated by stars with a remarkable non-rotational broadening component and very similar profiles (including type of variability). We provide some examples illustrating how this observational dataset can be used to evaluate scenarios aimed at explaining the existence of sources of non-rotational broadening in massive stars.

The Effects of Magnetic Fields on the Dynamics of Radiation Pressure–dominated Massive Star Envelopes

“…These fluctuations can impact the spectroscopic measurements of line profiles, potentially affecting both the microturbulence and macroturbulence parameters (Cantiello et al 2009;Jiang et al 2015). Recent spectroscopic observations of a large sample of OB stars revealed a very interesting trend, with macroturbulent velocities increasing with stellar luminosity (Simón-Díaz et al 2016). This has been tentatively attributed to the increasing turbulent pressure in the iron convection zone (Grassitelli et al 2015(Grassitelli et al , 2016, since turbulent pressure fluctuations may trigger high-order high-angular degree oscillations that can collectively mimic the effect of macroturbulence (Aerts et al 2009).…”