First High‐Contrast Science with an Integral Field Spectrograph: The Substellar Companion to GQ Lupi

We present commissioning data from the OSIRIS integral field spectrograph(IFS) on the Keck II 10 m telescope that demonstrate the utility of adaptiveoptics IFS spectroscopy in studying faint close-in sub-stellar companions inthe haloes of bright stars. Our R~2000 J- and H-band spectra of the sub-stellarcompanion to the 1-10 Myr-old GQ Lup complement existing K-band spectra andphotometry, and improve on the original estimate of its spectral type. We findthat GQ Lup B is somewhat hotter (M6-L0) than reported in the discovery paperby Neuhauser and collaborators (M9-L4), mainly due to the surface-gravitysensitivity of the K-band spectral classification indices used by thediscoverers. Spectroscopic features characteristic of low surface gravityobjects, such as lack of alkali absorption and a triangular H-band continuum,are indeed prominent in our spectrum of GQ Lup B. The peculiar shape of theH-band continuum and the difference between the two spectral type estimates iswell explained in the context of the diminishing strength of H2 collisioninduced absorption with decreasing surface gravity, as recently proposed foryoung ultra-cool dwarfs by Kirkpatrick and collaborators. Using our updatedspectroscopic classification of GQ Lup B and a re-evaluation of the age andheliocentric distance of the primary, we perform a comparative analysis of theavailable sub-stellar evolutionary models to estimate the mass of thecompanion. We find that the mass of GQ Lup B is 0.010-0.040 Msun. Hence, it isunlikely to be a wide-orbit counterpart to the known radial-velocity extrasolarplanets, whose masses are < 0.015 Msun. Instead, GQ Lup A/B is probably amember of a growing family of very low mass ratio widely separated binariesdiscovered through high-contrast imaging.Comment: 27 pages, 6 figures; accepted for publication to Ap