Astrometric planet search around southern ultracool dwarfs II: Astrometric reduction methods and a deep astrometric catalogue

摘要

We describe the astrometric reduction of images obtained with the FORS2/VLTcamera in the framework of an astrometric planet search around 20M/L-transition dwarfs. We present the correction of systematic errors, theachieved astrometric performance, and a new astrometric catalogue containingthe faint reference stars in 20 fields located close to the Galactic plane. Wedetected three types of systematic errors in the FORS2 astrometry: the relativemotion of the camera's two CCD chips, errors that are correlated in space, andan error contribution of yet unexplained origin. The relative CCD motion hasprobably a thermal origin and usually is 0.001-0.010 px (~0.1-1 mas), butsometimes amounts to 0.02-0.05 px (3-6 mas). This instability andspace-correlated errors are detected and mitigated using reference stars. Thethird component of unknown origin has an amplitude of 0.03-0.14 mas and isindependent of the observing conditions. We find that a consecutive sequence of32 images of a well-exposed star over 40 min at 0.6" seeing results in a medianr.m.s. of the epoch residuals of 0.126 mas. Overall, the epoch residuals aredistributed according to a normal law with a chi2~1. We compiled a catalogue of12000 stars with I-band magnitudes of 16-22 located in 20 fields, each covering~2x2'. It contains I-band magnitudes, ICRF positions with 40-70 mas precision,and relative proper motions and absolute trigonometric parallaxes with aprecision of 0.1 mas/yr and 0.1 mas at the bright end, respectively.