2025ApJ...994..157S

The dominant sources of photoionizing radiation in the extreme-ultraviolet (EUV) incident on the exterior of the local interstellar clouds include two nearby early B-type stars, ϵ CMa (124 ± 2 pc) and β CMa (151 ± 5 pc), three hot dwarfs, and the Local Hot Bubble (LHB). Line emission (170-912 Å) from highly ionized metals (Fe, Ne, Mg) in million-degree LHB plasma may be responsible for the elevated ionization fractions of helium (n_He II_/n_He ≈ 0.4) compared to hydrogen (n_H II_/n_H ≈ 0.2) in the local clouds. We update the stellar parameters and ionizing flux for β CMa, after correcting the EUV spectra for intervening H I column density, N_H I_ = (1.9 ± 0.1) × 10¹⁸ cm^–2, and its hotter effective temperature, T_eff ≈ 25,000 K versus 21,000 K for ϵ CMa. These two stars produce a combined H-ionizing photon flux Φ_H ≈ 6800 ± 1400 cm^–2 s^–1 at the external surface of the local clouds. The hot bubble could produce comparable fluxes, Φ_H = 2000-9000 cm^–2 s^–1, depending on the amount of metal depletion into dust grains that survive sputtering. The radial velocities and proper motions of β CMa and ϵ CMa indicate that both stars passed within 10 ± 1 pc of the Sun 4.4 ± 0.1 Myr ago, with 100-200 times higher local ionizing fluxes. At that time, the local clouds were likely farther from the Sun, owing to their transverse motion. Over the past few Myr, EUV radiation from these two stars left a wake of highly ionized gas in a hot, low-density cavity produced by past supernova explosions in the Sco-Cen OB association and connected with the LHB.