Dynamical Masses and Radiative Transfer Modeling of HD 698: A Be Binary in Evolutionary Transition

Gabitova I.A. Carciofi A.C. De Amorim T.H. Suffak M. Miroshnichenko A.S. Zharikov S.V. Rubio A.C. Danford S. Aarnio A.N. Prendergast P. Rudy R.J. Puetter R.C. Perry R.B. Agishev A.T. Vaidman N.L. Khokhlov S.A.
20 December 2025 American Astronomical Society

Astrophysical Journal
2025 #995 Issue 2

We present a detailed analysis of the early post-mass-transfer binary system HD 698 (V742 Cas), combining high-resolution optical spectroscopy, long-baseline interferometry, and radiative transfer modeling. Counter-phased RV curves reveal a circular orbit with a period of 55.927 ± 0.001 d and component masses of M _Be = 7.48 ± 0.07 M _⊙ and M _comp = 1.23 ± 0.02 M _⊙. The Be primary is traced via broad Hα wings, while narrow metallic absorption lines originate from a slowly rotating companion. The angular separation measured via interferometry implies a dynamical distance of 888 ± 5 pc. The spectral energy distribution is best reproduced with a color excess E(B − V) = 0.321 ± 0.016 due to interstellar reddening and a moderately dense viscous decretion disk with base density ρ ₀ ≃ 5 × 10⁻¹² g cm⁻³ at r = R _eq, declining radially as ρ(r) ∝ r ⁻ⁿ with n = 3.0. The companion is found to be a luminous and inflated star with T eff, comp = 10. 0 _{− 0.1}^{+ 0.2} kK, R comp = 13. 1 _{− 0.2}^{+ 0.2} R _⊙, and log L / L _⊙ = 3.19, contributing significantly to the flux (L _comp/L _Be ∼ 0.3). Spectral line mismatches provide further circumstantial evidence that the companion is hydrogen poor, consistent with a stripped-envelope star enriched by CNO processing. HD 698 thus belongs to the emerging class of Be + bloated O/B binaries, representing a short-lived, high-luminosity post-mass-transfer phase, when the stripped donor is still spectroscopically detectable before reaching the subdwarf phase.

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Faculty of Physics and Technology, Al-Farabi Kazakh National University, Al-Farabi Kazakh National University, Al-Farabi Ave., 71, Almaty, 050040, Kazakhstan
Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, São Paulo, 05508-900, Brazil
Department of Physics and Astronomy, Western University, London, N6A 3K7, ON, Canada
Department of Physics and Astronomy, Greensboro, 27402, NC, United States
Fesenkov Astrophysical Institute, Observatory, 23, Almaty, 050020, Kazakhstan
Ensenada, 22860, Mexico
School of Physics and Astronomy, University of Leeds, University of Leeds, Leeds, LS2 9JT, United Kingdom
Kernersville Observatory, Winston-Salem, 27285, NC, United States
Kookoosint Scientific, 1530 Calle Portada, Camarillo, 93010, CA, United States
University of California, San Diego, 9500 Gilman Dr., La Jolla, 92093, CA, United States
Alabaster Scientific, P.O. Box 120, Irvington, 22480, VA, United States

Faculty of Physics and Technology
Instituto de Astronomia
Department of Physics and Astronomy
Department of Physics and Astronomy
Fesenkov Astrophysical Institute
Ensenada
School of Physics and Astronomy
Kernersville Observatory
Kookoosint Scientific
University of California
Alabaster Scientific

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