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

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 H 2 O models results in significant detections of β Pictoris b with signal-to-noise ratios of 14.5 and 17.0 respectively. Correlation with a T = 1700K BT-Settl model provides a detection with an SNR of 25.0. 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 2.5 × 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.

Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects

“…Furthermore, the combination of the cross-correlation technique with adaptive optics assisted integral-field spectrographs at medium resolution can also be used for detecting chemical species across the two-dimensional field of the image. Such 'molecular mapping' has been successfully demonstrated for high-significance detections of prominent molecular species such as H2O and CO in directly imaged planets (107,204).…”

Two accreting protoplanets around the young star PDS 70

“…A stellar model could in principle be used to subtract out the star, but this will leave strong residuals either due to intrinsic variability of the host star or due to uncalibrated instrumental effects. We therefore used a data-driven approach to create the reference spectrum, similar to the approach that was used for the characterization of Beta Pictoris b [30]. The changes in the spectrum between spatial pixels can be separated into two parts, low-order effects and high-order effects.…”

Making good use of JWST's coronagraphs: tools and strategies from a user's perspective

“…Combined with mid to high resolution integral field spectroscopy (IFS) HCI can also lead to spectro-astrometric studies 21,22 or molecular mapping. 23,24 Figure 1. Distribution of the instruments' field of view at the focal plane of JWST.…”