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

TL;DR

This work demonstrates, for the first time, a 3D time-dependent unified atmosphere with winds for an O-type star and performs spectral synthesis with 3D radiative transfer and an approximate NLTE treatment. The approach reveals a highly structured and variable stellar surface, with large velocity dispersions producing broad photospheric lines and larger EWs than comparable 1D models, reducing the need for ad hoc turbulence parameters. Scattering effects reproduce qualitative features of UV wind lines, and the study discusses how 3D physics may alter determinations of fundamental parameters such as surface gravity and chemical abundances. The results underscore the importance of 3D modelling for accurate spectroscopic analyses of massive stars and outline steps to extend the framework toward full NLTE, line damping, and doublet treatment for direct comparison with observations.

Abstract

Spectroscopic studies of massive and luminous O-type stellar atmospheres and winds have primarily been done by using 1D, spherically symmetric and stationary models. Both observations and modern theoretical models show that such stars have highly structured and variable atmospheres and winds. We present first spectral synthesis based on 3D time-dependent unified RHD model atmospheres with winds for O stars. We first carried out time-dependent, 3D simulations of unified 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 aNLTE method was used, as well as scattering source functions. Our continuum intensity maps of a prototypical early O star reveal a highly variable and time-dependent surface, characterised by local emergent radiation temperature variations. Our averaged synthetic line profiles of optical absorption lines have large widths, without applying any macro- or microturbulence. From the simulations we find correspondingly large velocity dispersions in the photospheric layers. Additionally, the absorption line EWs are larger than for comparable 1D models. First results using scattering source functions further demonstrate that characteristic features like the softening of the blue edge of strong ultra-violet wind lines are qualitatively well-reproduced by our models. Our 3D simulations clearly predict a highly structured and strongly variable O star surface. 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.

Figures (13)

• Figure 1: Volume rendering of relative density (displayed colours are $\log_{10} \rho/\langle \rho \rangle$) for the 3D model atmosphere and wind. Angle brackets denote lateral averaging at each radial position.
• Figure 2: Maps of logarithm of density, radial velocity, and radiation temperature, in three selected vertical planes with laterally averaged continuum optical depth according to the legends. The lateral X and Y axes are labelled in units of $R_0$.
• Figure 3: Emergent optical continuum intensities of local patches on the 3D O star model, for six different snapshots. Each panel here corresponds to one $\mu$=1 (local surface normal and line of sight are aligned) 3D patch simulation, and is showing the complete patch. Colour bars display local emergent radiation temperature defined through $I_{\nu} \equiv B_\nu(T_{\rm rad})$. The spatial extent of each X and Y axis is $0.2~R_0$.
• Figure 4: Figurative illustrations of (a) the snapshots (3D unified atmosphere and wind simulations of O-type stars), (b) the stacked snapshots (stacked snapshot array), (c) one of the six sectors that is constructed from our stacked snapshot array and (d) the sphere that is constructed. For this illustration only one snapshot was used; hence a repetitive pattern is visible. The colour represents an arbitrary field in the simulation.
• Figure 5: Surface brightness plots of the O-star simulations described above, at the wavelength of the O III 5594 line: (a) for a snapshot-sphere model, (b) for a mixed-sphere model.