The ESPRI project: astrometric exoplanet search with PRIMA

Abstract

Context. The ESPRI project relies on the astrometric capabilities offered by the PRIMA facility of the Very Large Telescope Interferometer for discovering and studying planetary systems. Our survey consists of obtaining high-precision astrometry for a large sample of stars over several years to detect their barycentric motions due to orbiting planets. We present the operation's principle, the instrument's implementation, and the results of a first series of test observations. Aims. We give a comprehensive overview of the instrument infrastructure and present the observation strategy for dual-field relative astrometry in the infrared K-band. We describe the differential delay lines, a key component of the PRIMA facility that was delivered by the ESPRI consortium, and discuss their performance within the facility. This paper serves as reference for future ESPRI publications and for the users of the PRIMA facility. Methods. Observations of bright visual binaries were used to test the observation procedures and to establish the instrument's astrometric precision and accuracy. The data reduction strategy for the astrometry and the necessary corrections to the raw data are presented. Adaptive optics observations with NACO were used as an independent verification of PRIMA astrometric observations. Results. The PRIMA facility was used to carry out tests of astrometric observations. The astrometric performance in terms of precision is limited by the atmospheric turbulence at a level close to the theoretical expectations and a precision of 30 µas was achieved. In contrast, the astrometric accuracy is insufficient for the goals of the ESPRI project and is currently limited by systematic errors that originate in the part of the interferometer beamtrain that is not monitored by the internal metrology system. Conclusions. Our observations led to defining corrective actions required to make the facility ready for carrying out the ESPRI search for extrasolar planets.

Astrometric detection of exoplanets from the ground

“…The ESPRI survey was designed to last several years and to address three main topics: the determination of orbital inclinations for nearby RV planets, the search for long-period giant planets in the solar neighbourhood, and the search for giant planets around young stars. 48…”

“…The goal of the ESPRI project 48 is to discover and characterise extrasolar planets around nearby stars using astrometric observations with the VLT interferometer (VLTI). The dual-feed infrastructure that makes such observations possible at the VLTI is named PRIMA.…”

The ESPRI Project: differential delay lines for PRIMA

“…This will both complement some weakness inherent to the radial velocity method as well as open new discovery spaces. We refer to paper [4] in this same conference proceedings for a detailed the description of our project to contribute in making PRIMA an unique facility for detecting extra-solar planet by the astrometric method. A project overview as well as the description of the method and of our scientific goals is given therein.…”

First results from fringe tracking with the PRIMA fringe sensor unit

“…It consists of star-separator modules at the telescopes (STS 3 ), differential delay lines (DDL 4 ), an internal laser metrology, 5 and the fringe sensor unit (FSU 6 ) in the beam-combination laboratory. The scientific drivers for the development of PRIMA are narrow-angle astrometry 7 and phase referenced imaging in conjunction with the MIDI and AMBER instruments. The general status of the PRIMA facility is presented in Ref.…”