Deep high-resolution L band spectroscopy in the β Pictoris planetary system^★

¹ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
² École Polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands

^★★ Corresponding author; markus.janson@astro.su.se

Received: 29 September 2024
Accepted: 7 January 2025

Abstract

The β Pictoris system, with its two directly imaged planets β Pic b and β Pic c and its well characterised debris disk, is a prime target for detailed characterisation of young planetary systems. Here, we present high-resolution and high-contrast LM band spectroscopy with CRIRES+ of the system, primarily for the purpose of atmospheric characterisation of β Pic b. We developed methods for determining slit geometry and wavelength calibration based on telluric absorption and emission lines, as well as methods for point spread function (PSF) modelling and subtraction, and artificial planet injection, in order to extract and characterise planet spectra at a high signal-to-noise ratio (S/N) and spectral fidelity. Through cross-correlation with model spectra, we detected H₂O absorption for planet b in each of the 13 individual observations spanning four different spectral settings. This provides a clear confirmation of previously detected water absorption, and allowed us to derive an exquisite precision on the rotational velocity of β Pic b, υ_rot = 20.36 ± 0.31 km/s, which is consistent within error bars with previous determinations. We also observed a tentative H₂O cross-correlation peak at the expected position and velocity of planet c; the feature is however not at a statistically significant level. Despite a higher sensitivity to SiO than earlier studies, we do not confirm a tentative SiO feature previously reported for planet b. When combining data from different epochs and different observing modes for the strong H₂O feature of planet b, we find that the S/N grows considerably faster when sets of different spectral settings are combined, compared to when multiple data sets of the same spectral setting are combined. This implies that maximising spectral coverage is often more important than maximising integration depth when investigating exoplanetary atmospheres using cross-correlation techniques.