Direct imaging of the brown dwarf/planet-mass companions to HR 8799 and GJ 758 from 1 to 5 microns: Constraints on atmospheric properties


Thayne Currie (1), Phil Hinz(2), Yoichi Itoh(3), Misato Fukagawa(4), Vanessa Bailey (2), T.J. Rodigas (2), Dan Fabrycky(5), Ruth Murray-Clay(5)


(1) NASA-Goddard; (2) University of Arizona; (3) Kobe University; (4) Nagoya University; (5) Harvard-Smithsonian Center for Astrophysics


I describe new direct imaging data obtained from the MMT and Subaru for substellar-mass companions to GJ 758 and HR 8799 obtained at a variety of broad/narrow-band filters covering 1 to 5 microns (Currie et al. 2010, ApJ, 721, 177L; Currie et al. in prep.). Consistent with the discovery paper of Thalmann et al. (2010), GJ 758B is an ultracool, faint companion with Teff 560 K with a luminosity and H-L’ colors broadly consistent with predictions from the COND evolutionary models. However, our data provide evidence that the putative GJ 758"C" companion is plausibly a background object. The atmospheres of the HR 8799 companions, depart significantly from that of normal L/T dwarfs. Standard atmosphere models (e.g. Burrows et al. 2006) fail to simultaneously match the near and mid-IR portions of their SEDs. Modified Burrows et al models that yield far better agreement provide strong evidence in favor of extensive atmospheric dust absorption/cloud cover and some evidence in favor of low surface gravity and enhanced metallicity compared to the primary star. Best estimated masses for the HR 8799 companions lie below the deuterium-burning limit separating brown dwarfs from planets; however, age estimates for GJ 758 imply that its companion is a brown dwarf, not a planet.

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