Strong accretion on a deuterium-burning brown dwarf^⋆

¹ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
e-mail: fcomeron@eso.org
² Osservatorio Astrofisico di Arcetri, INAF, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 8 November 2009
Accepted: 1 July 2010

Abstract

Context. The accretion processes that accompany the earliest stages of star formation have been shown in recent years to extend to masses well below the substellar limit, and even to masses close to the deuterium-burning limit, suggesting that the features characteristic of the T Tauri phase are also common to brown dwarfs.

Aims. We discuss new observations of GY 11, a young brown dwarf in the embedded ρ Ophiuchi cluster.

Methods. We have obtained for the first time low-resolution, long-slit spectroscopy of GY 11 in the red visible region, using the FORS1 instrument at the VLT. The spectral region includes accretion diagnostic lines such as Hα and the CaII infrared triplet.

Results. The visible spectrum allows us to confirm that GY 11 lies well below the hydrogen-burning limit, in agreement with earlier findings based on the near-infrared spectral energy distribution. We obtain an improved derivation of its physical parameters, which suggest that GY 11 is on or near the deuterium-burning phase. We estimate a mass of 30 M_Jup, a luminosity of 6 × 10^-3 L_⊙, and a temperature of 2700 K. We detect strong Hα and CaII triplet emission, and estimate from the latter an accretion rate Ṁ_acc = 9.5 × 10^-10   M_⊙ yr^-1, which places GY 11 among the objects with the highest Ṁ_acc/M^∗ ratios measured thus far in their mass range. This might indicate that accretion in GY 11 is driven by the gravitational instability of its circum(sub-)stellar disk. The intense Hα emission contrasts with the previously reported non-detection of Paβ and Brγ emission, and we discuss possible implications of this on the physical characteristics of the region where hydrogen emission is produced. Using archival near-infrared imaging obtained at different epochs, we prove that the H₂ emission previously reported in infrared spectra of GY 11 comes from a chance coincidence with Herbig-Haro knots from the nearby source VLA1623 and not from a molecular outflow driven by GY 11. As a byproduct of our observations, we also obtained a spectrum of the neighboring, embedded low-mass star GY 10, which we classify as M 5.5