Ionization Sources of the Local Interstellar Clouds: Two B-stars, Three White Dwarfs, and the Local Hot Bubble

Abstract

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, $\epsilon$ CMa ($124\pm2$ pc) and $\beta$ CMa ($151\pm5$ pc), three hot dwarfs, and the local hot bubble (LHB). Line emission (170-912A) from highly ionized metals (Fe, Ne, Mg) in million-degree LHB plasma may be responsible for the elevated ionization fractions of helium ($n_{\rm HeII}/n_{\rm He} \approx 0.4$) compared to hydrogen ($n_{\rm HII} / n_{\rm H} \approx 0.2$) in the local clouds. We update the stellar parameters and ionizing flux for $\beta$ CMa, after correcting the EUV spectra for intervening HI column density, $N_{\rm HI} = 1.9\pm0.1\times10^{18}~{\rm cm}^{-2}$, and its hotter effective temperature, $T_{\rm eff} \approx 25,000$K vs. 21,000K for $\epsilon$ CMa. These two stars produce a combined H-ionizing photon flux $\Phi_{\rm H} \approx 6800\pm1400$ cm$^{-2}$ s$^{-1}$ at the external surface of the local clouds. The hot bubble could produce comparable fluxes, $\Phi_{\rm H} =$ 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 $\beta$ CMa and $\epsilon$ CMa indicate that both stars passed within $10\pm1$ pc of the Sun $4.4\pm0.1$ 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 last 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.