Skymaps of observables of three-dimensional magnetohydrodynamic astrosphere models

Abstract

Context. Three-dimensional models of astrospheres have recently become of interest. However, comparisons between these models and observations are non-trivial because of the two-dimensional nature of observations. Aims. By projecting selected physical values of three-dimensional models of astrospheres onto the surface of a sphere that is centred on a virtual all-sky observer, these models can be compared to observational data in different observables: the column density, bremsstrahlung flux, rotation measure, Hα flux, and synchrotron or cyclotron flux. Methods. Projections were calculated by rotating and moving the astrosphere model to the desired position and orientation and by then computing the value of a given patch on the sphere by a modified line-of-sight integration. Contributions to the selected observable made by all model cells that are connected to the patch by the line of sight in question were taken into account.Results. When the model produces a bow shock, a distinct parabolic structure produced by the outer astrosheath can be seen in every observable of the projection, the exact shape depending on the orientations of the line of sight and the stellar motion. Of all four examined astrosphere models, only that of λ Cephei shows fluxes that are higher than current observational thresholds. This is due to the strong stellar wind and interstellar inflow of the λ Cephei model.

Modelling O-star astrospheres with different relative speeds between the ISM and the star: 2D and 3D MHD model comparison

“…In order to generate synthetic observational images, the models were projected onto a virtual sky as described by Baalmann et al (2020). To this effect the model was shifted to a desired position, (d 0 , b 0 , l 0 ), in Galactic coordinates, where d 0 is the distance modulus of the star, b its Galactic latitude, and l its Galactic longitude.…”

“…Within this study the astrosphere around the runaway Ostar λ Cephei has been used as a basis for the modelling procedure (cf. Scherer et al 2020;Baalmann et al 2020. Following the classification scheme proposed by Henney & Arthur (2019a,b,c), this astrosphere features a wind-supported BS, that is, the dynamics are determined by the MHD flows instead of the stellar radiation, and the interstellar dust is highly coupled to the plasma.…”

Simulating observable structures due to a perturbed interstellar medium in front of astrospheric bow shocks in 3D MHD

“…with α i (T ) the temperature-dependent recombination rate coefficient of state i, and P i,k the branching ratio from state i through state k. The α i (T ) were taken from Mao & Kaastra (2016) and the P i,k computed as in Baalmann et al (2020) with data from Wiese and decreases strictly monotonously from α eff,3→2 (10 1 K) ≈ 3.6 × 10 −12 cm 3 /s to α eff,3→2 (10 8 K) ≈ 1.1 × 10 −18 cm 3 /s. All models were also analysed in the spectral emission due to dust at a wavelength of 70 µm.…”

“…The Gaunt factor was calculated following van Hoof et al (2014) and resulted in values g ff (T ) ∈ [1.0, 1.5[, so the usual approximation ḡ = 1.2 holds (cf. Baalmann et al 2020). The j brs is the flux over the entire electromagnetic spectrum without any modulation by, for example, (self-)absorption.…”

“…Three-dimensional (3D) magnetohydrodynamics (MHD) have recently come into focus as a viable tool for further examining and understanding the structures of astrospheres (e.g. Katushkina et al 2018;Gvaramadze et al 2018;Scherer et al 2020). By changing these models' orientations with respect to an observer's line of sight (LOS), the generation of synthetic observations produces images similar to genuine data of actual bow shocks (BSs; e.g.…”