GQLup and its common proper motion companion

Abstract

Recently, Neuhaeuser et al. (2005) presented evidence for a sub-stellar, common proper motion companion to GQ Lup. With two theoretical mass estimates, both below the Deuterium burning minimum mass limit, the companion is probably a planet imaged directly. We present here a more detailed astrometric analysis of the GQ Lup system, using all the (different) proper motions published for the primary. The common proper motion is significant in all cases, also when taking into account the error in parallax or distance (140 +- 50 pc). When using the weighted mean, the significance for common proper motion of GQ Lup and its companion is 7 + 4 sigma for no change in separation plus 8 sigma for no change in position angle. We also discuss the question, whether GQ Lup and its common-proper motion companion are not bound, but share the same or similar proper motion as two independent members of the Lupus T association, which is a moving group, where most members should have the same motion anyway. Given our discussion, this hypothesis can be rejected by several sigma: The probability to find by chance an L-dwarf fainter than Ks = 14 mag within 0.7325 arcsec with (almost) the same proper motion of GQ Lup is only < 3 10^{-10}. The orbital motion of the system is not yet detected (1.4 +- 2.2 mas/yr), but is probably smaller than the escape velocity (5.3 +- 2.1 mas/yr), so that the system may well be gravitationally bound and stable. This is different for the 2MASSWJ1207334-393254 system, as we also show.