Characterization of stellar companions from high-contrast long-slit spectroscopy data

The EXtraction Of SPEctrum of COmpanion (EXOSPECO) algorithm

¹ Université de Lyon, Université Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR 5574, 69230 Saint-Genis-Laval, France
e-mail: samuel.the@univ-lyon1.fr, eric.thiebaut@univ-lyon1.fr
² Université Jean-Monnet-Saint-Étienne, CNRS, Institut d’Optique Graduate School, Laboratoire Hubert Curien UMR 5516, 42023 Saint-Étienne, France
e-mail: loic.denis@univ-st-etienne.fr

Received: 28 November 2022
Accepted: 19 July 2023

Abstract

Aims. High-contrast long-slit spectrographs can be used to characterize exoplanets. The resulting spectroscopic data are, however, corrupted by stellar leakages that largely dominate other signals and make the process of extracting the companion spectrum very challenging. This paper presents a complete method to calibrate the spectrograph and extract the signal of interest.

Methods. The proposed method is based on a flexible direct model of the high-contrast long-slit spectroscopic data. This model explicitly accounts for the instrumental response and for the contributions of both the star and the companion. The contributions of these two components and the calibration parameters are jointly estimated by solving a regularized inverse problem. As this problem has no closed-form solution, we propose an alternating minimization strategy to effectively find the solution.

Results. We tested our method on empirical long-slit spectroscopic data and by injecting synthetic companion signals in these data. The proposed initialization and the alternating strategy effectively avoid the self-subtraction bias, even for companions observed very close to the coronagraphic mask. Careful modeling and calibration of the angular and spectral dispersion laws of the instrument clearly reduce the contamination by the stellar leakages. In practice, the outputs of the method are mostly driven by a single hyper-parameter that tunes the level of regularization of the companion’s spectral energy distribution (SED).