Further deep imaging of HR7329 A (η Tel A) and its brown dwarf companion B

Abstract

About 4 arcsec south of the young A0-type star HR7329, a faint companion candidate was found by Lowrance et al. Its spectral type of M7-8 is consistent with a young brown dwarf companion. Here we report 10 new astrometric imaging observations of the pair HR7329 A and B, obtained with the Hubble Space Telescope and the Very Large Telescope, aimed at showing a common proper motion with high significance and possible orbital motion of B around A. With 11 yr of epoch difference between the first and our last image, we can reject the possibility by more than 21σ that B would be a non-moving background object unrelated to A. We detect no change in position angle and small or no change in separation (2.91 ± 2.41 masyr-1), so that the orbit of HR7329 B around A is inclined and/or eccentric and/or the orbital motion is currently only in the radial direction. If HR7329 B is responsible for the outer radius of the debris disc around HR7329 A being 24 au, and if HR7329 B currently is at its apastron at 200 au (4.2arcsec at 47.7pc), we determine its pericentre distance to be 71 au, its semimajor axis to be 136 au, and its eccentricity to be e=0.47. From the magnitude differences between HR7329 A and B and the 2MASS magnitudes for the HR7329 A+B system, we can estimate the magnitudes of HR7329 B (J= 12.06 ± 0.19,H=11.75±0.10,Ks=11.6 ± 0.1,L=11.1 ± 0.2mag) and then, with a few otherwise known parameters, its luminosity and mass (20-50 Jupiter masses). In the deepest images available, we did not detect any additional companion candidates up to ≤9 arcsec, but we determined the upper limits in the planetary mass regime. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.