Beta Canis Majoris: The Other Major Ionization Source of the Local Interstellar Clouds

TL;DR

This paper investigates the ionizing influence of the nearby star Beta Canis Majoris on local interstellar clouds, providing detailed stellar parameters, ionizing flux calculations, and historical context of their proximity to the Sun.

Contribution

It offers a comprehensive analysis of Beta Canis Majoris's ionizing flux and its impact on local interstellar clouds, including new stellar parameters and historical trajectory insights.

Findings

Beta CMa's ionizing photon rate is 10^46 photons/sec.

The star's EUV flux significantly ionized local interstellar gas.

Both stars passed within 10 pc of the Sun 4.4 million years ago.

Abstract

Two nearby B-type stars, $\epsilon$ CMa ($124\pm2$ pc) and $\beta$ CMa ($151\pm5$ pc), are important contributors to the photoionization of the local interstellar clouds. At spectral type B1 II-III, $\beta$ CMa is slightly hotter than $\epsilon$ 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 $\beta$ CMa angular diameter, bolometric flux, and position on the Hertzsprung-Russell diagram, we obtain a consistent set of stellar parameters: $T_{\rm eff} = 25,180\pm1120$ K, $\log g = 3.70\pm0.08$, radius $R = 8.44\pm0.56\,R_{\odot}$, mass $M = 13\pm1\,M_{\odot}$, and luminosity $L = 10^{4.41\pm0.06}\, L_{\odot}$. The EUVE-observed fluxes and non-LTE model atmospheres are used to determine the ionizing photon production rate $Q_{\rm H} = 10^{46.0}$ photons s$^{-1}$ and fluxes incident on the local clouds, $\Phi_{\rm HI} \approx 3700$ cm$^{-2}$ s$^{-1}$ and $\Phi_{\rm HeI} \approx 110$ cm$^{-2}$ s$^{-1}$ in the H I and He I continua. The corresponding photoionization rates are $\Gamma_{\rm HI} \approx 1.5\times10^{-14}$ s$^{-1}$ and $\Gamma_{\rm HeI} \approx 7.3\times10^{-16}$ s$^{-1}$. Within the local cloud, the LyC flux is attenuated by an H I column density $N_{\rm HI} = (1.9\pm0.1)\times10^{18}$ cm$^{-2}$, with optical depth $\tau_{\rm LL} = 12.0\pm 0.6$ at the Lyman limit. The radial velocities and proper motions of $\beta$ CMa and $\epsilon$ CMa indicate that both stars passed within $10\pm1$ 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.