Medium-resolution integral-field spectroscopy for high-contrast exoplanet imaging

Molecule maps of the β Pictoris system with SINFONI^★

¹ Leiden Observatory, Leiden University, 2333 CA Leiden, The Netherlands
e-mail: hoeijmakers@strw.leidenuniv.nl
² Observatoire de Genève, Chemin des Maillettes 51, 1290 Versoix, Switzerland
³ Universität Bern, Center for space and habitability, Gesellschaftstrasse 6, 3012 Bern, Switzerland
⁴ Department of Astronomy and Astrophysics, University of California, 1156 High St., Santa Cruz, CA 95064, USA
⁵ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁶ Department of Physical Geography, Utrecht University, PO Box 80.115, 3508 TC Utrecht, The Netherlands
⁷ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁸ Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU UMI 3386 and Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 25 February 2018
Accepted: 11 June 2018

Abstract

Context. Angular differential imaging (ADI) and spectral differential imaging (SDI) are well-established high-contrast imaging techniques, but their application is challenging for companions at small angular separations from their host stars.

Aims. The aim of this paper is to investigate to what extent adaptive-optics assisted, medium-resolution (R ~ 5000) integral field spectrographs (IFS) can be used to directly detect the absorption of molecular species in the spectra of planets and substellar companions when these are not present in the spectrum of the star.

Methods. We analysed archival data of the β Pictoris system taken with the SINFONI integral field spectrograph located at ESO’s Very Large Telescope, originally taken to image β Pictoris b using ADI techniques. At each spatial position in the field, a scaled instance of the stellar spectrum is subtracted from the data after which the residuals are cross-correlated with model spectra. The cross-correlation co-adds the individual absorption lines of the planet emission spectrum constructively, while this is not the case for (residual) telluric and stellar features.

Results. Cross-correlation with CO and H₂O models results in significant detections of β Pictoris b with signal-to-noise ratios (S/Ns) of 13.7 and 16.4 respectively. Correlation with a T = 1700 K BT-Settl model provides a detection with an S/N of 22.8. This in contrast to application of ADI, which barely reveals the planet. While the adaptive optics system only achieved modest Strehl ratios of 19–27% leading to a raw contrast of 1:240 at the planet position, cross-correlation achieves a 3σ contrast limit of 2.7 × 10⁻⁵ in this 2.5 hr data set, a factor ~40 below the raw noise level at an angular distance of 0.36′′ from the star.

Conclusions. Adaptive-optics assisted, medium-resolution IFS, such as SINFONI on the VLT and OSIRIS on the Keck Telescope, can be used for high-contrast imaging utilizing cross-correlation techniques for planets that are close to their star and embedded in speckle noise. We refer to this method as molecule mapping, and advocate its application to observations with future medium resolution instruments, in particular ERIS on the VLT, HARMONI on the ELT and NIRSpec, and MIRI on the JWST.