M. D'Onofrio, T. Stolker, S. Y. Haffert, A. Y. Kesseli, R. G. van Holstein, Y. Aoyama, J. Brinchmann, G. Cugno, J. H. Girard, G. Marleau, M. R. Meyer, J. Milli, S. P. Quanz, I. A. G. Snellen, K. O. Todorov

Abstract
GQ Lup B is a young and accreting, substellar companion that appears to drive a spiral arm in the circumstellar disk of its host star. We report high-contrast imaging observations of GQ Lup B with VLT/NACO at 4–5 μm and medium-resolution integral field spectroscopy with VLT/MUSE. The optical spectrum is consistent with an M9 spectral type, shows characteristics of a low-gravity atmosphere, and exhibits strong Hα emission. The H − M' color is ≳1 mag redder than field dwarfs with similar spectral types, and a detailed analysis of the spectral energy distribution (SED) from optical to mid-infrared wavelengths reveals excess emission in the L', NB4.05, and M' bands. The excess flux is well described by a blackbody component with T_disk ≈ 460 K and R_disk ≈ 65 R_J and is expected to trace continuum emission from small grains in a protolunar disk. We derive an extinction of AV ≈ 2.3 mag from the broadband SED with a suspected origin in the vicinity of the companion. We also combine 15 yr of astrometric measurements and constrain the mutual inclination with the circumstellar disk to 84 ± 9 deg, indicating a tumultuous dynamical evolution or a stellar-like formation pathway. From the measured Hα flux and the estimated companion mass, Mp ≈ 30 M_J, we derive an accretion rate of M˙≈ 10^−6.5 M_J yr⁻¹ . We speculate that the disk is in a transitional stage in which the assembly of satellites from a pebble reservoir has opened a central cavity while GQ Lup B is in the final stages of its formation.

Keywords
Accretion; Brown dwarfs; Direct imaging; Planet formation; Natural satellite formation; High angular resolution

Notes
Based on observations collected under ESO programmes 0101.C-0502(B), 0102.C-0649(A), and 0103.C-0524(A).

The Astronomical Journal
Volume 162, Number 6
2021 December

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