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

TL;DR

This paper provides a comprehensive analysis of the Be binary HD 698, revealing detailed orbital, stellar, and disk properties, and highlighting its evolutionary stage as a post-mass-transfer system with a stripped-envelope companion.

Contribution

It offers the first detailed dynamical and radiative transfer modeling of HD 698, characterizing its binary components, orbit, and circumstellar disk, and identifying it as a rare post-mass-transfer Be binary.

Findings

Component masses: M_Be=7.48 M_sun, M_comp=1.23 M_sun

Dynamical distance: 888 pc

Companion is a luminous, inflated, hydrogen-poor star

Abstract

We present a detailed analysis of the early post-mass-transfer binary HD 698 (V742 Cas) combining high-resolution optical spectroscopy, long-baseline interferometry, and radiative-transfer modeling. Counter-phased radial-velocity curves yield a circular orbit with P=55.927+/-0.001 d and component masses M_Be=7.48+/-0.07 M_sun and M_comp=1.23+/-0.02 M_sun. The Be primary is traced by broad H alpha wings, while narrow metallic absorption lines arise from a slowly rotating companion. The interferometric separation implies a dynamical distance of 888+/-5 pc. The spectral energy distribution is reproduced with E(B-V)=0.321+/-0.016 and a viscous decretion disk of base density rho_0~5x10^-12 g cm^-3 at r=R_eq, declining radially as rho(r)~r^-n with n=3.0. The companion is luminous and inflated, with T_eff=10.0(+0.2,-0.1) kK, R_comp=13.1+/-0.2 R_sun, and log(L/L_sun)=3.19, contributing significantly to the flux (L_comp/L_Be~0.3). Spectral line mismatches further suggest a hydrogen-poor, CNO-processed atmosphere, consistent with a stripped-envelope star. HD 698 thus adds to the emerging class of Be+bloated OB binaries, capturing a brief post-mass-transfer phase when the donor remains spectroscopically detectable prior to the subdwarf stage.