ExoplaNeT accRetion mOnitoring sPectroscopic surveY (ENTROPY). I. Evidence for magnetospheric accretion in the young isolated planetary-mass object 2MASS J11151597+1937266

Context . Accretion among planetary mass companions is a poorly understood phenomenon, due to the lack of both observational and theoretical studies. The detection of emission lines from accreting gas giants facilitates detailed investigations into this process. Aims . This work presents a detailed analysis of Balmer lines from one of the few known young, planetary-mass objects with observed emission, the isolated L2 γ dwarf 2MASS J11151597+1937266 with a mass between 7 and 21 M Jup and an age of 5–45 Myr, located at 45 ± 2 pc. Methods . We obtained the first high-resolution ( R ~ 50 000) spectrum of the target with VLT/UVES, an echelle spectrograph operating in the near-ultraviolet to visible wavelengths (3200–6800 Å). Results . We report several resolved hydrogen (H I ; H3–H6) and helium (He I ; λ5875.6) emission lines in the spectrum. Based on the asymmetric line profiles of H α and H β , the 10% width of H α (199 ± 1 km s −1 ), tentative He I λ 6678 emission, and indications of a disk from mid-infrared excess, we confirm ongoing accretion at this object. Using the Gaia update of the parallax, we revise its temperature to 1816 ± 63 K and radius to 1.5 ± 0.1 R Jup . Analysis of observed H I profiles using a 1D planet-surface shock model implies a pre-shock gas velocity, v 0 = 120 −40 + 80 km s −1 , and a pre-shock density, log( n 0 /cm −3 ) = 14 −5 + 0 . The pre-shock velocity points to a mass, M p = 6 −4 + 8 M Jup , for the target. Combining H I line luminosities ( L line ) and planetary L line −L acc (accretion luminosity) scaling relations, we derived a mass accretion rate, Ṁ acc = 1.4 −0.9 + 2.8 × 10 −8 M Jup yr −1 . Conclusions . The line-emitting area predicted from the planet-surface shock model is very small (~0.03%), and points to a shock at the base of a magnetospherically induced funnel. The H α profile exhibits a much stronger flux than predicted by the model that best fits the rest of the H I profiles, indicating that another mechanism than shock emission contributes to the H α emission. Comparison of line fluxes and Ṁ acc from archival moderate-resolution SDSS spectra indicate variable accretion at 2MASS J11151597+1937266.