Interstellar Bow Shocks around Fast Stars Passing through the Local Interstellar Medium

Abstract

Bow shocks are produced in the local interstellar medium by the passage of fast stars from the Galactic thin-disk and thick-disk populations with velocities V* = 40–80 km s⁻¹. Stellar transits of local H I clouds occur every 3500–7000 yr on average and last between 10⁴ and 10⁵ yr. There could be 10–20 active bow shocks around low-mass stars inside clouds within 15 pc of the Sun. At local cloud distances of 3–10 pc, their turbulent wakes have transverse radial extents R_(wake) ≈ 100–300 au, angular sizes 10''–100'', and Lyα surface brightnesses of 2–8 R in gas with total hydrogen density n_H ≈ 0.1 cm⁻³ and V* = 40–80 km s⁻¹. These transit wakes may cover an area fraction f_A ≈ (R_(wake)/R_(cl)) ≈ 10⁻³ of local H I clouds and be detectable in IR (dust), UV (Lyα, two-photon), or nonthermal radio emission. Turbulent heating in these wakes could produce the observed elevated rotational populations of H₂ (J ≥ 2) and influence the endothermic formation of CH⁺ in diffuse interstellar gas at T > 10³ K.

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