Spectral synthesis of 3D unified model atmospheres with winds for O stars

Context. Spectroscopic studies of massive and luminous O-type stellar atmospheres and winds have primarily been done by using one-dimensional (1D), spherically symmetric, and stationary models. However, both observations and modern theoretical models show that such O stars have highly structured and variable atmospheres and winds. Aims. We present a first spectral synthesis based on three-dimensional (3D) time-dependent unified radiation-hydrodynamic (RHD) model atmospheres with winds for O stars. Methods. We first carried out time-dependent, 3D simulations of unified O-star atmospheres with winds. We then used 3D radiative transfer to compute surface brightness maps for the optical continuum as well as integrated flux profiles for select diagnostic lines. To derive occupation numbers and source functions, an approximate non-local thermodynamic equilibrium method was used, as well as scattering source functions. Results. Our continuum intensity maps of a prototypical early O star (⟨Teff⟩ = 40.2 kK, log10 (⟨L⋆⟩/L⊙) = 5.79) in the Galaxy reveal a highly variable and time-dependent surface, characterised by local emergent radiation temperature variations exceeding 10 000 K. Our averaged synthetic line profiles of optical absorption lines have large widths, since the RHD simulations have large velocity dispersions in photospheric layers. Additionally, the absorption line equivalent widths are larger than for comparable 1D models. As such, to reproduce the 3D absorption line results in a corresponding 1D model, we need to apply isotropic Gaussian microturbulence, vmic ~ 10–20 km/s, and macroturbulence, vmac ~ 70 km/s, to the latter. First results using scattering source functions further demonstrate that characteristic features such as the softening of the blue edge of strong ultraviolet wind lines are qualitatively well reproduced by our models. Conclusions. Our 3D simulations clearly predict a highly structured and strongly variable O-star surface, in stark contrast with the smooth surfaces assumed by the 1D models currently used for quantitative spectroscopy of such stars. The first line profile results further suggest that several observed features are naturally reproduced by our models without the need to introduce ad hoc spectral fitting parameters. We also discuss how using 3D rather than 1D simulations as a basis for future studies may affect the derivation of fundamental stellar parameters such as surface gravities and chemical abundances.