2020A&A...635A.203R

Context. HD 13724 is a nearby solar-type star at 43.48±0.06pc hosting a long-period low-mass brown dwarf detected with the CORALIE echelle spectrograph as part of the historical CORALIE radial-velocity search for extra-solar planets. The companion has a minimum mass of 26.77_–2.2^+4.4M_Jup and an expected semi-major axis of ∼240mas making it a suitable target for further characterisation with high-contrast imaging, in particular to measure its inclination, mass, and spectrum and thus establish its substellar nature.
Aims. Using high-contrast imaging with the SPHERE instrument on the Very Large Telescope (VLT), we are able to directly image a brown dwarf companion to HD 13724 and obtain a low-resolution spectrum.
Methods. We combine the radial-velocity measurements of CORALIE and HARPS taken over two decades and high-contrast imaging from SPHERE to obtain a dynamical mass estimate. From the SPHERE data we obtain a low-resolution spectrum of the companion from Y to J band, as well as photometric measurements from IRDIS in the J, H, and K bands.
Results. Using high-contrast imaging with the SPHERE instrument at the VLT, we report the first images of a brown dwarf companion orbiting the host star HD 13724. It has an angular separation of 175.6±4.5mas and an H-band contrast of 10.61±0.16mag, and using the age estimate of the star to be ∼1Gyr gives an isochronal mass estimate of ∼44M_Jup. By combining radial-velocity and imaging data we also obtain a dynamical mass of 50.5_–3.5^+3.3M_Jup. Through fitting an atmospheric model, we estimate a surface gravity of logg=5.5 and an effective temperature of 1000K. A comparison of its spectrum with observed T dwarfs estimates a spectral type of T4 or T4.5, with a T4 object providing the best fit.