Imaging magnetically driven astrospheres: a forward modelling approach

Abstract

An astrosphere is a vast, tailed bubble-like volume around a star, formed through the interaction between the stellar magnetic field, the stellar wind, and the interstellar medium (ISM). Detecting and characterizing astrospheres are essential for constraining stellar wind properties, understanding stellar evolution, and assessing the habitability of surrounding exoplanetary systems. Charge exchanges between ionized stellar wind particles and cold ISM hydrogen atoms populate the astrosphere with neutral hydrogen, which can leave observable signatures in the Lyman-α (Ly α) line absorption profile. Previous studies have inferred stellar mass-loss rates by measuring Ly α absorption in stellar spectra caused by astrospheric neutral hydrogen. However, owing to observational limitations, our knowledge of the global morphology of astrospheres remains limited and largely dependent on sometimes contradictory simulation results. Here we investigate the feasibility of detecting Ly α emission generated by resonant scattering from neutral hydrogen surrounding the star, enabling the construction of a two-dimensional map of the astrosphere. With a three-dimensional magnetohydrodynamic astrosphere model, we perform forward modelling of the Ly α emission and assess the feasibility of this approach by comparing the predicted spectral intensity with the observational limits of the Hubble Space Telescope (HST). Our results indicate that Ly α emission from the hydrogen wall is largely absorbed by the ISM, whereas emission from the near-star astrosphere can remain detectable. The spatially resolved circumstellar Ly α emission could provide important constraints on the astrospheric configuration and stellar wind properties, such as the standing distance of the bow shock, the symmetry of stellar wind mass loss, and the shape of the astro-tail.

Figures (6)

• Figure 1: Illustration of the generation and propagation of the circumstellar Ly $\alpha$ emission.
• Figure 2: The stellar and astrosphere model and observing geometry. (a) The modeled distribution of neutral hydrogen number density in the astrosphere. The directions of upwind and the observer are marked by arrows. (b) The modeled Ly $\alpha$ flux density profile of the central Star. (c) An illustration of the observing geometry.
• Figure 3: Slices of the astrosphere model. (a) Distribution of neutral hydrogen number density. (b) Distribution of the neutral hydrogen temperature. (c) Distribution of the neutral hydrogen speed along the line-of-sight. (d) Distribution of the neutral hydrogen speed along the downwind direction. The 3D astrosphere model is sliced at the $X$-$Z$ plane, where $X$ is the downwind direction, and $Z$ is aligned with the star's rotational axis.
• Figure 4: Modeled Ly $\alpha$ emission from the astrosphere. (a) the distribution of peak intensity, (b) the integrated emission intensity over wavelengths. The red circle represents the bottom boundary of the heliosphere model located at $30~\mathrm{AU}$.
• Figure 5: Modeled Ly $\alpha$ emission from the astrosphere after ISM absorption. (a) Distribution of the peak Ly $\alpha$ intensity. (b) Ly $\alpha$ intensity integrated over wavelength. The red circle indicates the inner boundary of the heliosphere model at 30 AU. The white cross marks the line-of-sight through the hydrogen wall, and the blue cross marks the line-of-sight through the near-star stellar wind. (c) Original emission $E(\nu)$, ISM transmission $T(\nu)$, and resulting throughput profile $E(\nu)\cdot T(\nu)$ along the hydrogen wall line-of-sight. (d) Original emission $E(\nu)$, ISM transmission $T(\nu)$, and resulting throughput profile $E(\nu)\cdot T(\nu)$ along the near-star line-of-sight.