Combining high-dispersion spectroscopy with high contrast imaging: Probing rocky planets around our nearest neighbors

Abstract

Context. Ground-based high-dispersion (R ∼ 100 000) spectroscopy (HDS) is proving to be a powerful technique with which to characterize extrasolar planets. The planet signal is distilled from the bright starlight, combining ral and time-differential filtering techniques. In parallel, high-contrast imaging (HCI) is developing rapidly, aimed at spatially separating the planet from the star. While HDS is limited by the overwhelming noise from the host star, HCI is limited by residual quasi-static speckles. Both techniques currently reach planet-star contrast limits down to ∼10 −5 , albeit for very different types of planetary systems. Aims. In this work, we discuss a way to combine HDS and HCI (HDS+HCI). For a planet located at a resolvable angular distance from its host star, the starlight can be reduced up to several orders of magnitude using adaptive optics and/or coronography. In addition, the remaining starlight can be filtered out using high-dispersion spectroscopy, utilizing the significantly different (or Doppler shifted) high-dispersion spectra of the planet and star. In this way, HDS+HCI can in principle reach contrast limits of ∼10 −5 ×10 −5 , although in practice this will be limited by photon noise and/or sky-background. In contrast to current direct imaging techniques, such as Angular Differential Imaging and Spectral Differential Imaging, it will work well at small working angles and is much less sensitive to speckle noise. For the discovery of previously unknown planets HDS+HCI requires a high-contrast adaptive optics system combined with a high-dispersion R ∼ 100 000 integral field spectrograph (IFS). This combination currently does not exist, but is planned for the European Extremely Large Telescope. Methods. We present simulations of HDS+HCI observations with the E-ELT, both probing thermal emission from a planet at infrared wavelengths, and starlight reflected off a planet atmosphere at optical wavelengths. For the infrared simulations we use the baseline parameters of the E-ELT and METIS instrument, with the latter combining extreme adaptive optics with an R = 100 000 IFS. We include realistic models of the adaptive optics performance and atmospheric transmission and emission. For the optical simulation we also assume R = 100 000 IFS with adaptive optics capabilities at the E-ELT. Results. One night of HDS+HCI observations with the E-ELT at 4.8 μm (Δλ = 0.07 μm) can detect a planet orbiting α Cen A with a radius of R = 1.5 R earth and a twin-Earth thermal spectrum of T eq = 300 K at a signal-to-noise (S/N) of 5. In the optical, with a Strehl ratio performance of 0.3, reflected light from an Earth-size planet in the habitable zone of Proxima Centauri can be detected at a S/N of 10 in the same time frame. Recently, first HDS+HCI observations have shown the potential of this technique by determining the spin-rotation of the young massive exoplanet β Pictoris b. Conclusions. The exploration of the planetary systems of our neighbor stars is of great scientific and philosophical value. The HD...

Observing Exoplanets with High Dispersion Coronagraphy. I. The Scientific Potential of Current and Next-generation Large Ground and Space Telescopes

“…We suspect that the factor of 2 difference in detection significance may be attributed to the different approach in calculating the CCF S/N. Snellen et al (2015) investigated the detectability of a shortperiod super-Earth around Proxima Cen and concluded that the planet can be detected with a significance of 10 for a wavelength coverage from 0.6 to 0.9 μm with an HDC instrument on the E-ELT. Following the details in their paper, we found a CCF S/N of 4.0 for such a super-Earth.…”

“…At this low level of S/N per pixel, each absorption line is very noisy. Considering the proportionality of CCF S/N to the square root of the number of lines (Snellen et al 2015), we only expect a modest contribution from the line-resolved CCF, as shown in Figure 16, even if the spectral resolution is high enough to resolve individual absorption lines.…”

“…This important topic was later on explored by a few groups (Riaud & Schneider 2007;Kawahara & Hirano 2014;Snellen et al 2015;Lovis et al 2017). The present study builds on previous work in terms of simulation method.…”

“…Star light suppression can be further improved by coupling a high-resolution spectrograph (HRS) with a coronagraphic system (Sparks & Ford 2002;Riaud & Schneider 2007;Kawahara & Hirano 2014;Snellen et al 2015;Lovis et al 2017). In this High Dispersion Coronagraphy (HDC) scheme, the coronagraphic component serves as a spatial filter to separate the light from the star and the planet.…”

“…Here, we develop a framework to simulate the performance of an HDC instrument. Although similar calculations have been performed as part of previous studies (Sparks & Ford 2002;Riaud & Schneider 2007;Kawahara & Hirano 2014;Snellen et al 2015;Lovis et al 2017), a thorough end-to-end simulation that explores the S/N trade space between spectral resolution and star light suppression for ground-based and space-based observations is lacking. In this paper, we simulate a variety of HDC instruments that are either under development or in the conceptual design phase and quantify their potential for detecting new planets as well as particular molecular species in the atmosphere of known planets (e.g., Proxima Cen b, 51 Eri b, and HR 8799 e) and hypothetical Earth-like planets around stars of different spectral types.…”