Epsilon Canis Majoris: The Brightest EUV Source with Surprisingly Low Interstellar Absorption

TL;DR

This paper characterizes the B2 star epsilon CMa as the brightest EUV source with unexpectedly low interstellar absorption, providing detailed stellar parameters and implications for local interstellar ionization.

Contribution

It offers a comprehensive analysis of epsilon CMa's stellar properties and EUV spectrum, revealing its significant role in local interstellar ionization with precise measurements and modeling.

Findings

Epsilon CMa is the brightest EUV source with low interstellar absorption.

Derived stellar parameters place epsilon CMa outside the beta Cephei instability strip.

The star's ionizing photon flux significantly impacts local interstellar hydrogen ionization.

Abstract

The B2 star $\epsilon$ CMa, at parallax distance $d = 124\pm2$~pc, dominates the H I photoionization of the local interstellar cloud (LIC). At its closer parallax distance compared to previous estimates, $\epsilon$ CMa has a 0.9 mag fainter absolute magnitude $M_V =-3.97\pm0.04$. We combine measurements of distance with the integrated flux $f = (41.5\pm3.3) \times 10^{-6}~{\rm erg~cm}^{-2}~{\rm s}^{-1}$ and angular diameter $\theta_d = 0.80\pm0.05$~mas to produce a consistent set of stellar parameters: radius $R = 10.7\pm0.7~R_{\odot}$, mass $M = 13.1\pm2.3~M_{\odot}$, gravity $\log g = 3.50\pm0.05$, effective temperature $T_{\rm eff} \approx 21,000$~K, and luminosity $L \approx 20,000~L_{\odot}$. These parameters place Epsilon CMa outside the $\beta$ Cephei instability strip, consistent with its observed lack of pulsations. The observed EUV spectrum yields a hydrogen photoionization rate $\Gamma_{\rm HI} \approx 10^{-15}$ s$^{-1}$ (at Earth). The total flux decrement factor at the Lyman limit ($\Delta_{\rm LL} = 5000\pm500$) is a combination of attenuation in the stellar atmosphere ($\Delta_{\rm star} = 110\pm10$) and interstellar medium ($\Delta_{\rm ISM} = 45\pm5$) with optical depth $\tau_{\rm LL} = 3.8\pm0.1$. After correcting for interstellar HI column density $N_{\rm HI} = (6\pm1)\times10^{17}~{\rm cm}^{-2}$, we find a stellar LyC photon flux $\Phi_{\rm LyC} \approx 3000~{\rm cm}^{-2}~{\rm s}^{-1}$ and ionizing luminosity $Q_{\rm LyC} = 10^{45.7\pm0.3}$ photons s$^{-1}$. The photoionization rate $\Gamma_{\rm H} \approx$ (1-2)$\times 10^{-14}~{\rm s}^{-1}$ at the cloud surface produces an ionization fraction (30-40\%) for total hydrogen density $n_{\rm H} = 0.2$ cm$^{-3}$. With its $27.3\pm0.4$ km/s heliocentric radial velocity and small proper motion, $\epsilon$ CMa passed within $9.3\pm0.5$ pc of the Sun 4.4 Myr ago, with a 180 times higher photoionization rate.