Spectroscopic Confirmation of Young Planetary-Mass Companions on Wide Orbits

Abstract

We present moderate-resolution ($R$$\sim$4000-5000) near-infrared integral field spectroscopy of the young (1-5 Myr) 6-14 $M_\mathrm{Jup}$ companions ROXs 42B b and FW Tau b obtained with Keck/OSIRIS and Gemini-North/NIFS. The spectrum of ROXs 42B b exhibits clear signs of low surface gravity common to young L dwarfs, confirming its extreme youth, cool temperature, and low mass. Overall, it closely resembles the free-floating 4-7 $M_\mathrm{Jup}$ L-type Taurus member 2MASS J04373705+2331080. The companion to FW Tau AB is more enigmatic. Our optical and near-infrared spectra show strong evidence of outflow activity and disk accretion in the form of line emission from [S II], [O I], H$α$, Ca II, [Fe II], Pa$β$, and H$_2$. The molecular hydrogen emission is spatially resolved as a single lobe that stretches $\approx$0.1" (15 AU). Although the extended emission is not kinematically resolved in our data, its morphology resembles shock-excited H$_2$ jets primarily seen in young Class 0 and Class I sources. The near-infrared continuum of FW Tau b is mostly flat and lacks the deep absorption features expected for a cool, late-type object. This may be a result of accretion-induced veiling, especially in light of its strong and sustained H$α$ emission ($EW$(H$α$)$\gtrsim$290 Å). Alternatively, FW Tau b may be a slightly warmer (M5-M8) accreting low-mass star or brown dwarf (0.03-0.15 $M_{\odot}$) with an edge-on disk. Regardless, its young evolutionary stage is in stark contrast to its Class III host FW Tau AB, indicating a more rapid disk clearing timescale for the host binary system than for its wide companion. Finally, we present near-infrared spectra of the young ($\sim$2-10 Myr) low-mass (12-15 $M_\mathrm{Jup}$) companions GSC 6214-210 B and SR 12 C and find they best resemble low gravity M9.5 and M9 substellar templates.