The low-mass companion of GQ Lup

Abstract

Using NACO on the VLT in the imaging mode we have detected an object at a distance of only 0.7 arcsec from GQ Lup. The object turns out to be co-moving. We have taken two K-band spectra with a resolution of lambda /Delta lambda=700. In here, we analyze the spectra in detail. We show that the shape of spectrum is not spoiled by differences in the Strehl ratio in the blue and in the red part, as well as differential refraction. We reanalyze the spectra and derive the spectral type of the companion using classical methods. We find that the object has a spectral type between M9V and L4V, which corresponds to a Teff between 1600 and 2500 K. Using GAIA-dusty models, we find that the spectral type derivation is robust against different log(g)-values. The Teff derived from the models is again in the range between 1800 and 2400 K. While the models reproduce nicely the general shape of the spectrum, the 12CO-lines in the spectrum have about half the depth as those in the model. We speculate that this difference might be caused by veiling, like in other objects of similar age, and spectral class. We also find that the absolute brightness of the companion matches that of other low-mass free-floating objects of similar age and spectral type. A comparison with the objects in USco observed by Mohanty et al. (2004) shows that the companion of GQ Lup has a lower mass than any of these, as it is of later spectral type, and younger. The same is as true, for the companion of AB Pic. To have a first estimate of the mass of the object we compare the derived Teff and luminosity with those calculated from evolutionary tracks. We also point out that future instruments, like NAHUAL, will finally allow us to derive the masses of such objects more precisely.

arXiv comment6 figures, proceeding of the workshop on Ultralow-mass star formation and evolution held in La Palama June 28 to July 1, 2005

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• Grain growth signatures in the protoplanetary discs of Chamaeleon and Lupus
• Astrometric follow-up observations of directly imaged sub-stellar companions to young stars and brown dwarfs
• Rotation Periods, Inclinations, and Obliquities of Cool Stars Hosting Directly Imaged Substellar Companions: Spin-Orbit Misalignments are Common
• ALMA Measurements of Circumstellar Material in the GQ Lup System
• GQ Lup B Visible & Near-Infrared Photometric Analysis
• First High-Contrast Science with an Integral Field Spectrograph: the Sub-Stellar Companion to GQ Lup
• Near-IR integral-field spectroscopy of the companion to GQ Lup
• The ESO SupJup Survey IV. Unveiling the carbon isotope ratio of GQ Lup B and its host star
• CS Cha B: A disc-obscured M-type star mimicking a polarised planetary companion