2024/12/09 by J. Michael Shull, Shull, J. Michael, Rachel Curran +3
Physics and Astronomy · #Stellar, planetary, and galactic studies #Astrophysics and Star Formation Studies #Astronomy and Astrophysical Research
paper · pdf · doi:10.48550/arxiv.2412.06919
The B2 star ε CMa, at parallax distance d = 124±2~pc, dominates the H I photoionization of the local interstellar cloud (LIC). At its closer parallax distance compared to previous estimates, ε CMa has a 0.9 mag fainter absolute magnitude MV =-3.97±0.04. We combine measurements of distance with the integrated flux f = (41.5±3.3) × 10-6~\rm erg~cm-2~\rm s-1 and angular diameter θd = 0.80±0.05~mas to produce a consistent set of stellar parameters: radius R = 10.7±0.7~R\odot, mass M = 13.1±2.3~M\odot, gravity log g = 3.50±0.05, effective temperature T\rm eff ≈ 21,000~K, and luminosity L ≈ 20,000~L\odot. These parameters place Epsilon CMa outside the β Cephei instability strip, consistent with its observed lack of pulsations. The observed EUV spectrum yields a hydrogen photoionization rate Γ\rm HI ≈ 10-15 s-1 (at Earth). The total flux decrement factor at the Lyman limit (Δ\rm LL = 5000±500) is a combination of attenuation in the stellar atmosphere (Δ\rm star = 110±10) and interstellar medium (Δ\rm ISM = 45±5) with optical depth τ\rm LL = 3.8±0.1. After correcting for interstellar HI column density N\rm HI = (6±1)×1017~\rm cm-2, we find a stellar LyC photon flux Φ\rm LyC ≈ 3000~\rm cm-2~\rm s-1 and ionizing luminosity Q\rm LyC = 1045.7±0.3 photons s-1. The photoionization rate Γ\rm H ≈ (1-2)× 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±0.4 km/s 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.