2025ApJ...979...21S

The B2 star Epsilon Canis Majoris (ϵ CMa), at parallax distance d = 124 ± 2 pc, dominates the H I photoionization of the Local Interstellar Cloud. At its closer parallax distance compared to previous estimates, ϵ CMa has a 0.9 mag fainter absolute magnitude M_V = -3.97 ± 0.04. We combine measurements of distance with the integrated flux f = (41.5 ± 3.3) × 10^–6 erg cm^–2 s^–1 and angular diameter θ_d = 0.80 ± 0.05 mas to produce a consistent set of stellar parameters: radius R = 10.7 ± 0.7 R_⊙, mass M = 13.1 ± 2.3 M_⊙, gravity $\mathrm{log}g=3.50\pm 0.05$, effective temperature T_eff ≈ 21,000 K, and luminosity L ≈ 20,000 L_⊙. These parameters place ϵ CMa outside the β Cephei instability strip, consistent with its observed lack of pulsations. The observed extreme-ultraviolet spectrum yields a hydrogen photoionization rate Γ_HI ≈ 10^–15 s^–1 (at Earth). The total flux decrement factor at the Lyman limit (Δ_LL = 5000 ± 500) is a combination of attenuation in the stellar atmosphere (Δ_star = 110 ± 10) and interstellar medium (Δ_ISM = 45 ± 5) with optical depth τ_LL = 3.8 ± 0.1. After correcting for interstellar H I column density N_HI = (6 ± 1) × 10¹⁷cm^–2, we find a stellar LyC photon flux Φ_LyC ≈ 3000 cm^–2 s^–1 and ionizing luminosity Q_LyC = 10^45.7±0.3 photons s^–1. The photoionization rate Γ_H ≈ (1-2) × 10^–14 s^–1 at the cloud surface produces an ionization fraction (30%-40%) for total hydrogen density n_H = 0.2 cm^–3. With its 27.3 ± 0.4 km s^–1 heliocentric radial velocity and small proper motion, ϵ CMa passed within 9.3 ± 0.5 pc of the Sun 4.4 Myr ago, with a 180 times higher photoionization rate.