Abstract
Studies of atmospheres of directly imaged extrasolar planets with high-resolution spectrographs have shown that their characterization is predominantly limited by noise on the stellar halo at the location of the studied exoplanet. An instrumental combination of high-contrast imaging and high spectral resolution that suppresses this noise and resolves the spectral lines can therefore yield higher quality spectra. We study the performance of the proposed HiRISE fiber coupling between the direct imager SPHERE and the spectrograph CRIRES+ at the Very Large Telescope for spectral characterization of directly imaged planets. Using end-to-end simulations of HiRISE we determine the signal-to-noise ratio (S/N) of the detection of molecular species for known extrasolar planets inHandKbands, and compare them to CRIRES+. We investigate the ultimate detection limits of HiRISE as a function of stellar magnitude, and we quantify the impact of different coronagraphs and of the system transmission. We find that HiRISE largely outperforms CRIRES+ for companions around bright hosts likeβPictoris or 51 Eridani. For anH = 3.5 host, we observe a gain of a factor of up to 16 in observing time with HiRISE to reach the same S/N on a companion at 200 mas. More generally, HiRISE provides better performance than CRIRES+ in 2 h integration times between 50 and 350 mas for hosts withH < 8.5 and between 50 and 700 mas forH < 7. For fainter hosts like PDS 70 and HIP 65426, no significant improvements are observed. We find that using no coronagraph yields the best S/N when characterizing known exoplanets due to higher transmission and fiber-based starlight suppression. We demonstrate that the overall transmission of the system is in fact the main driver of performance. Finally, we show that HiRISE outperforms the best detection limits of SPHERE for bright stars, opening major possibilities for the characterization of future planetary companions detected by other techniques.
Highlights
In contrast to indirect methods, direct imaging permits us to spatially separate and directly measure radiation from an exoplanet, which allows us to spectrally analyze its atmosphere with minimized impact from the host star
We study the performance of the proposed High-Resolution Imaging and Spectroscopy of Exoplanets project (HiRISE) fiber coupling between the direct imager SPHERE and the spectrograph CRIRES+ at the Very Large Telescope for spectral characterization of directly imaged planets
Their characterization is still in large part limited by the accessible spectral resolution (e.g., Zurlo et al 2016; De Rosa et al 2016; Greenbaum et al 2018; Samland et al 2017; Rajan et al 2017; Cheetham et al 2019), recent progress has been made with VLTI/GRAVITY (GRAVITY Collaboration 2019, 2020) at R = 4000
Summary
In contrast to indirect methods, direct imaging permits us to spatially separate and directly measure radiation from an exoplanet, which allows us to spectrally analyze its atmosphere with minimized impact from the host star. This combination of the spatial separation of planet and star and mid- to high-resolution spectroscopy has been given a clear demonstration through the detection of the atmosphere of the directly imaged planet β Pictoris b (Snellen et al 2014) using the CRIRES instrument (Kaeufl et al 2004) with its MACAO adaptive optics system (Arsenault et al 2003) on the ESO Very Large Telescope (VLT) This measurement allowed us to determine the planet’s orbital velocity, which was used to better constrain its orbital parameters, and to determine the planet’s rotational period and probable length of day, which were derived through the broadening of the CO and H2O lines.
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