Spectral and orbital characterisation of the directly imaged giant planet HIP 65426 b

Abstract

HIP 65426 b is a recently discovered exoplanet imaged during the course of the SPHERE-SHINE survey. Here we present new L ′ and M ′ observations of the planet from the NACO instrument at the VLT from the NACO-ISPY survey, as well as a new Y –H spectrum and K -band photometry from SPHERE-SHINE. Using these data, we confirm the nature of the companion as a warm, dusty planet with a mid-L spectral type. From comparison of its SED with the BT-Settl atmospheric models, we derive a best-fit effective temperature of T eff = 1618 ± 7 K, surface gravity log g = 3.78 −0.03 +0.04 and radius R = 1.17 ± 0.04 R J (statistical uncertainties only). Using the DUSTY and COND isochrones we estimate a mass of 8 ± 1 M J . Combining the astrometric measurements from our new datasets and from the literature, we show the first indications of orbital motion of the companion (2.6 σ significance)and derive preliminary orbital constraints. We find a highly inclined orbit ( i = 1.07 −10 +13 deg) with an orbital period of 800 −400 +1200 yr. We also report SPHERE sparse aperture masking observations that investigate the possibility that HIP 65426 b was scattered onto its current orbit by an additional companion at a smaller orbital separation. From this data we rule out the presence of brown dwarf companions with masses greater than 16 M J at separations larger than 3 AU, significantly narrowing the parameter space for such a companion.