The IACOB project

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.

HD 93795: a late-B supergiant star with a square circumstellar nebula

“…This code characterizes the additional broadening in the line profiles combining Fourier transform and goodness-of-fit methodologies. The obtained velocities (see Table 2) are qualitatively in agreement with velocities measured by Simón-Díaz et al (2017) in stars at the similar evolutionary stages (see their fig. 5).…”

Radial abundance gradients in the outer Galactic disk as traced by main-sequence OB stars

“…We note that for spectra with lower S /N ratios, when the line wings are not well defined, the ζ mac -values tend to be less accurate. The ζ mac values also tend to be lower than 20 km s −1 , with only a few stars reaching values up to about 50 km s −1 , which are typical values for massive OB stars (Simón-Díaz, S. et al 2017). We stress that the sample is biased to slow rotators by design, that is, most stars of our sample have v · sin i < 50 km s −1 to allow for a proper detailed elemental abundance study.…”

The spectroscopic Hertzsprung-Russell diagram of Galactic massive stars

“…This wealth of spectroscopic data provides new insights on stellar evolution compared to previous photometric studies, thus providing the required empirical constraints (e.g. Martins & Palacios 2017;Ramírez-Agudelo et al 2017;Sabín-Sanjulián et al 2017;Simón-Díaz et al 2017). In Castro et al (2014), we showed how it is possible to highlight patterns in the spectroscopic Hertzsprung-Russell diagram (sHRD; L ≡ T 4 eff /g, Langer & Kudritzki 2014) and proposed empirical anchors, such as the position of the zero age main-sequence (ZAMS) and of the terminal age main-sequence (TAMS), based on large collections of stellar atmosphere studies in the Milky Way.…”