Stellar halo subtraction alternative for accreting companions' characterization with integral field spectroscopy

Analytical and on-sky demonstration on the PDS70, HTLup, and YSES1 systems

¹ UGA, Institut de Planétologie et d’Astrophysique de Grenoble, CNRS, Saint-Martin-d’Hères, 38400, France
² UGA, Grenoble Images Parole Signal Automatique, Grenoble INP, CNRS, Saint-Martin-d’Hères 38400, France
³ UCBL, Centre de Recherche Astrophysique de Lyon UMR 5574, ENS de Lyon, CNRS, Villeurbanne 69622, France
⁴ Université Côte d’Azur, Nice 06304, France
⁵ Universidad de Chile, Las Condes, 7550000, Chile
⁶ Millennium Nucleus for Planet Formation, Valparaíso 2340000, Chile
⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, Heidelberg 69117, Germany

^★ Corresponding author: remi.julo@univ-grenoble-alpes.fr

Received: 29 November 2024
Accepted: 18 July 2025

Abstract

Context. Medium-resolution integral field spectrographs (IFS), such as the Multi-Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT), are equipped to detect the emission lines (e.g., H_α, H_β) of faint accreting companions when associated with dedicated stellar halo subtraction methods. We recently proposed a new approach based on polynomial modulations of a stellar spectrum estimate across the field of view, with orthogonal polynomials and lines masking. This new technique is designed to better preserve both continuum and emission lines of accreting companions.

Aims. We seek to highlight and quantify analytically and on real data the benefits of this new approach over the one classically used, particularly with regard to distortions of the extracted spectra. We also examine both operating regimes.

Methods. We carried out analytical calculations based on simple toy models of spectra to identify and quantify the main theoretical problems of the state-of-the-art technique, the proposed corrections of our new method, and the remaining limitations of the latter. Simulations of the most extreme situations identified were used to highlight these problems and corrections. Archival VLT/MUSE data of the young PDS70 and HTLup systems were used to vet the detection and characterization capabilities using on-sky observations. New images of the YSES1 planetary system were used to further illustrate the gains.

Results. We show that the state-of-the-art stellar halo subtraction method, based on low-pass filtering, can lead to the self-subtraction of the emission lines and modify the neighboring continua, depending on the line contrast to neighboring continuum contrast ratios. We show that the proposed technique corrects these characterization problems, while maintaining the same detection capabilities. The two protoplanets PDS70 b and c were detected with 5σ significance. The H_α line estimate of the HTLup B stellar companion was improved by ~30% for the integrated flux and by ~8% for the 10%-width. As for YSES1 b, we found it uniquely displays a combination of H_α, H_β, CaII H&K triplet, and HeI lines in emission that can be attributed to accretion and/or chromospheric activity. We derived an accretion rate at H_α of  ~1.45 × 10⁻⁹ M_Jup year with our new method, versus ~1.11 x 10⁻⁹ M_Jup year with the reference method, namely, ~30% less. These new results are compatible with values derived for other companions in this mass range. We note that YSES1 c was not detected in our observations.

Conclusions. The proposed subtraction method better preserves the spectral information, notably the emission line fluxes and profiles, while achieving similar detection performance. Based on a linear and parametric approach, it can be extended and/or combined with additional faint signal search algorithms.