Astrometric and photometric characterization of eta Tel B combining two decades of observations

Abstract

eta Tel is an 18 Myr system composed of a 2.09 M$_ odot $ A-type star with an M7-M8 brown dwarf companion, eta Tel B. The two objects have a projected separation of 4farcs 2 (sim 208 au). This system has been targeted by high-contrast imaging campaigns for over 20 years, facilitating its orbital and photometric characterization. The companion, eta Tel B, both bright and on a wide orbit, is an ideal candidate for a detailed examination of its position and the characterization of its atmosphere. To explore the orbital parameters of eta Tel B, measure its contrast, and investigate its close surroundings, we analyzed three new SPHERE/IRDIS coronagraphic observations. Our objectives are to investigate the possibility of a circumplanetary disk or a close companion around eta Tel B, and characterize its orbit by combining this new data set with archival data acquired in the past two decades. The IRDIS data are reduced with state-of-the-art algorithms to achieve a contrast with respect to the star of 1.0$ $ at the location of the companion. Using the NEGative Fake Companion technique (NEGFC), we measure the astrometric positions and flux of eta Tel B for the three IRDIS epochs. Together with the measurements presented in the literature, the baseline of the astrometric follow-up is 19 years. We calculate a contrast for the companion of 6.8 magnitudes in the H band. The separation and position angle measured are 4farcs 218 and 167.3 degrees, respectively. The astrometric positions of the companions are calculated with an uncertainty of 4 milliarcseconds (mas) in separation and 0.2 degrees in position angle. These are the smallest astrometrical uncertainties of eta Tel B obtained so far. The orbital parameters are estimated using the Orvara code, including all available epochs. The orbital analysis is performed taking into account the Gaia-Hipparcos acceleration of the system. Suppressing its point spread function (PSF), we have produced contrast curves centered on the brown dwarf in order to constrain our detection capabilities for a disk or companions around it. After considering only orbits that could not disrupt the outer debris disk around eta Tel A, our orbital analysis reveals a low eccentric orbit (e sim 0.34) with an inclination of 81.9 degrees (nearly edge-on) and a semi-major axis of 218 au. Furthermore, we determine the mass of eta Tel B to be 48 consistent with previous calculations from the literature based on evolutionary models. Finally, we do not detect any significant residual pointing to the presence of a satellite or a disk around the brown dwarf. The retrieved detection limits allow us to discard massive objects around eta Tel B with masses down to 1.6 at a separation of 33 au.