Epsilon Canis Majoris: The Brightest Extreme-ultraviolet Source with Surprisingly Low Interstellar Absorption
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 measureme...
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| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
IOP Publishing
2025-01-01
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| Series: | The Astrophysical Journal |
| Subjects: | |
| Online Access: | https://doi.org/10.3847/1538-4357/ad9b8b |
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| Summary: | 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) × 10 ^17 cm ^−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. |
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| ISSN: | 1538-4357 |