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 $V_{\ast }=$ 40-80 km/s. Stellar transits of local H I clouds occur every 3500-7000 yr on average and last between $10^4$ and $10^5$ yr. There could be 10-20 active bow shocks around low-mass stars inside clouds within 10-15 pc of the Sun. At local cloud distances of 3-10 pc, their turbulent wakes have transverse radial extents $R_{\mathrm{wake}}\approx$ 10-300 AU, angular sizes 10-100 arcsec, and Lyman-alpha surface brightnesses of 2-8 Rayleighs in gas with total hydrogen density $n_H\approx 0.1{\mathrm{cm}}^{-3}$ and $V_{\ast }=$ 40-80 km/s. These transit wakes may cover an area fraction $f_A\approx (R_{\mathrm{wake}}/R_{\mathrm{cl}})\approx 10^{-3}$ of local H I clouds and be detectable in IR (dust), UV (Lya, two-photon), or non-thermal radio emission. Turbulent heating in these wakes could produce the observed elevated rotational populations of H${}_2$ ($J\ge 2$) and influence the endothermic formation of CH${}^{+}$ in diffuse interstellar gas at $T>10^3$ K.

Accepted to Astrophysical Journal, 12 pages with one table