Refined Characteristics of L Class Brown Dwarf 2M1821+14

Parallax of the L4.5 dwarf 2M1821+14 from high-precision astrometry with OSIRIS at GTC

Authors:

Sahlmann et al

Abstract:

We used the OSIRIS camera at the 10.4 m Gran Telescopio Canarias (GTC) to monitor the astrometric motion of the L4.5 dwarf 2M1821+14 over 17 months. The astrometric residuals of eleven epochs have a r.m.s. dispersion of 0.4 mas, which is larger than the average precision of 0.23 mas per epoch and hints towards an additional signal or excess noise. Comparison of the point-spread-functions in OSIRIS and FORS2/VLT images reveals no differences critical for high-precision astrometry, despite the GTC's segmented primary mirror. We attribute the excess noise to an unknown effect that may be uncovered with additional data. For 2M1821+14, we measured a relative parallax of 106.15±0.18 mas and determined a correction of 0.50±0.05 mas to absolute parallax, leading to a distance of 9.38±0.03 pc. We excluded at 3-σ confidence the presence of a companion to 2M1821+14 down to a mass ratio of 0.1 (≈5MJupiter) with a period of 50--1000 days and a separation of 0.1--0.7 au. The accurate parallax allowed us to estimate the age and mass of 2M1821+14 of 120--700 Myr and 0.049+0.014−0.024 M⊙, thus confirming its intermediate age and substellar mass. We complement our study with a parallax and proper motion catalogue of 587 stars (i′≃15.5−22) close to 2M1821+14, used as astrometric references. This study demonstrates sub-mas astrometry with the GTC, a capability applicable for a variety of science cases including the search for extrasolar planets and relevant for future astrometric observations with E-ELT and TMT.