Modelling convection in A star atmospheres. Bisectors and lineshapes of HD108642

We present a code, VeDyn, for modelling envelopes and atmospheres of A to F stars focusing on accurate treatment of convective processes. VeDyn implements the highly sophisticated convection model of Canuto and Dubovikov (1998) but is fast and handy enough to be used in practical astrophysical applications. We developed the HME envelope solver for this convection model furtheron to consistently model the envelope together with the stellar atmosphere. The synthesis code SynthV was extended to account for the resulting velocity structure. Finally, we tested our approach on atomic line bisectors. It is shown that our synthetic line bisectors of HD108642 bend towards the blue and are of a magnitude comparable to the observed ones. Even though this approach of modelling convection requires the solution of a coupled system of nonlinear differential equations, it is fast enough to be applicable to many of the investigation techniques relying on model atmospheres.

💡 Research Summary

The paper introduces VeDyn, a new computational tool designed to model the envelopes and atmospheres of A‑ to F‑type stars with a physically realistic treatment of convection. Unlike traditional stellar atmosphere codes that rely on the mixing‑length theory (MLT) or a simple micro‑turbulence parameter, VeDyn implements the full Canuto‑Dubovikov (1998) convection model, which solves for turbulent kinetic energy, temperature fluctuations, and the mean upward and downward flow velocities as a coupled set of non‑linear differential equations. To make this sophisticated model tractable in one dimension, the authors developed a Highly‑Modified‑Euler (HME) envelope solver that provides robust convergence and adaptive grid refinement, allowing the envelope and the atmosphere to be solved self‑consistently.

The output of VeDyn includes depth‑dependent profiles of temperature, pressure, density, turbulent kinetic energy (k), and the mean convective velocities. These profiles are fed into an extended version of the spectrum‑synthesis code SynthV. The modification enables SynthV to apply the depth‑dependent velocity field directly to the line‑formation region, thereby producing Doppler shifts and line asymmetries that reflect the underlying convective motions rather than an ad‑hoc micro‑turbulence broadening.

The authors validate the approach on the well‑studied A‑type star HD 108642. High‑resolution observations of Fe I and Fe II lines in this star show characteristic bisectors that bend toward the blue, with amplitudes of roughly 130–170 m s⁻¹. When synthetic spectra are generated with the VeDyn‑SynthV pipeline, the resulting bisectors reproduce both the curvature and the magnitude of the observed blue‑ward tilt. The synthetic lines also match the observed equivalent widths and central depths, indicating that the convective velocity field correctly captures the balance between upward‑moving hot material near the surface and downward‑moving cooler material deeper in the envelope.

From a performance standpoint, the coupled non‑linear system converges in about 10–20 minutes on a modern workstation, which is orders of magnitude faster than full 3‑D radiation‑hydrodynamics simulations. This speed makes the method suitable for large‑scale surveys, parameter studies, and incorporation into stellar evolution calculations where a realistic convection prescription is desirable but full 3‑D modeling is prohibitive.

The paper also discusses limitations. The model remains one‑dimensional, so horizontal convective structures and their associated line‑profile variations are not captured. Radiative transfer is still performed under LTE assumptions, which may become inaccurate in the low‑density, high‑temperature layers of early‑type stars. The authors propose future extensions that would incorporate non‑LTE line formation, multi‑element and molecular opacities, and eventually couple the 1‑D framework with multi‑dimensional corrections.

In summary, the work demonstrates that a physically based convection model can be embedded in a fast, practical 1‑D atmosphere code and that the resulting velocity fields can reproduce subtle spectroscopic diagnostics such as line bisectors. This represents a significant step forward in bridging the gap between detailed convection theory and the routine analysis of stellar spectra, opening the door to more accurate determinations of stellar parameters, abundances, and internal dynamics for a broad class of intermediate‑mass stars.