Dynamical mass of the Ophiuchus intermediate-mass stellar system S1 with DYNAMO-VLBA

We report dynamical mass measurements of the individual stars in the most luminous and massive stellar member of the nearby Ophiuchus star-forming region, the young tight binary system S1. We combine 28 archival data sets with seven recent proprietary Very Long Baseline Array (VLBA) observations obtained as part of the Dynamical Masses of Young Stellar Multiple Systems with the VLBA project (DYNAMO–VLBA), to constrain the astrometric and orbital parameters of the system, and recover high-accuracy dynamical masses. The primary component, S1A, is found to have a mass of 4.11 ± 0.10 M⊙, significantly lower than the typical value ∼6 M⊙ previously reported in the literature. We show that the spectral energy distribution (SED) of S1A can be reproduced by a reddened blackbody with a temperature between roughly 14,000 and 17,000 K. According to evolutionary models, this temperature range corresponds to stellar masses between 4 M⊙ and 6 M⊙, so the SED is not a priori inconsistent with the dynamical mass of S1A. The luminosity of S1 derived from SED fitting, however, is only consistent with models for stellar masses above 5 M⊙. Thus, we cannot reconcile the evolutionary models with the dynamical mass measurement of S1A: The models consistent with the location of S1A in the Hertzsprung-Russel diagram correspond to masses higher by 25% at least than the dynamical mass. For the secondary component, S1B, a mass of 0.831 ± 0.014 M⊙ is determined, consistent with a low-mass young star. While the radio flux of S1A remains roughly constant throughout the orbit, the flux of S1B is found to be higher near apastron.