2025/08/05 by Shull, J. Michael, Curran, Rachel M., Topping, Michael W.
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #Solar and Stellar Astrophysics (astro-ph.SR)
paper · doi:10.48550/arxiv.2508.03800
Two nearby B-type stars, ε CMa (124±2 pc) and β CMa (151±5 pc), are important contributors to the photoionization of the local interstellar clouds. At spectral type B1 II-III, β CMa is slightly hotter than ε CMa (B2 II-III), but its ionizing flux at Earth is attenuated by a much larger H I column density. At the external surface of the clouds, the two stars produce similar fluxes in the Lyman continuum (LyC). From the β CMa angular diameter, bolometric flux, and position on the Hertzsprung-Russell diagram, we obtain a consistent set of stellar parameters: T\rm eff = 25,180±1120 K, log g = 3.70±0.08, radius R = 8.44±0.56 R\odot, mass M = 13±1 M\odot, and luminosity L = 104.41±0.06 L\odot. The EUVE-observed fluxes and non-LTE model atmospheres are used to determine the ionizing photon production rate Q\rm H = 1046.0 photons s-1 and fluxes incident on the local clouds, Φ\rm HI ≈ 3700 cm-2 s-1 and Φ\rm HeI ≈ 110 cm-2 s-1 in the H I and He I continua. The corresponding photoionization rates are Γ\rm HI ≈ 1.5×10-14 s-1 and Γ\rm HeI ≈ 7.3×10-16 s-1. Within the local cloud, the LyC flux is attenuated by an H I column density N\rm HI = (1.9±0.1)×1018 cm-2, with optical depth τ\rm LL = 12.0± 0.6 at the Lyman limit. The radial velocities and proper motions of β CMa and ε CMa indicate that both stars passed within 10±1 pc of the Sun approximately 4.4 Myr ago, with incident ionizing fluxes 180-200 times larger. Their EUV radiation photoionized and heated the tunnel in the local interstellar gas, associated dynamically with past supernova explosions in the Sco-Cen OB association.