A Wide Planetary-mass Companion to a Young Low-mass Brown Dwarf in Ophiuchus

Abstract

We present the discovery of a planetary-mass companion to CFHTWIR-Oph 98, a low-mass brown dwarf member of the young Ophiuchus star-forming region, with a wide 200 au separation (1"46). The companion was identified using Hubble Space Telescope images, and confirmed to share common proper motion with the primary using archival and new ground-based observations. Based on the very low probability of the components being unrelated Ophiuchus members, we conclude that Oph 98 AB forms a binary system. From our multiband photometry, we constrain the primary to be an M9-L1 dwarf, and the faint companion to have an L2-L6 spectral type. For a median age of 3 Myr for Ophiuchus, fits of evolutionary models to measured luminosities yield masses of 15.4 ± 0.8 M_Jup for Oph 98 A and 7.8 ± 0.8 M_Jup for Oph 98 B, with respective effective temperatures of 2320 ± 40 K and 1800 ± 40 K. For possible system ages of 1-7 Myr, masses could range from 9.6-18.4 M_Jup for the primary, and from 4.1-11.6 M_Jup for the secondary. The low component masses and very large separation make this binary the lowest binding energy system imaged to date, indicating that the outcome of low-mass star formation can result in such extreme, weakly bound systems. With such a young age, Oph 98 AB extends the growing population of young free-floating planetary-mass objects, offering a new benchmark to refine formation theories at the lowest masses.