Combining high-dispersion spectroscopy (HDS) with high contrast imaging (HCI): Probing rocky planets around our nearest neighbors (1503.01136v1)

Abstract: Aims: In this work, we discuss a way to combine High Dispersion Spectroscopy and High Contrast Imaging (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 ~1e-5 x 1e-5, although in practice this will be limited by photon noise and/or sky-background. 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 um (d_lambda = 0.07 um) can detect a planet orbiting alpha 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 beta Pictoris b. [abridged]

• The paper demonstrates that combining HDS with HCI can theoretically boost planet-star contrast from 10⁻⁵ to 10⁻¹⁰.
• The method uses advanced simulations and E-ELT instrumentation to detect Earth-sized planets with a signal-to-noise ratio of 5 or higher.
• The study’s breakthrough observation of β Pictoris b confirms the technique's ability to measure exoplanet rotational velocities, advancing exoplanet research.

Analyzing Exoplanets Using Combined High-Dispersion Spectroscopy and High-Contrast Imaging Techniques

The paper presents a method to augment our ability to detect and characterize rocky exoplanets, particularly those in the habitable zones of nearby stars. The authors focus on the integration of high-dispersion spectroscopy (HDS) with high-contrast imaging (HCI), coining this combination as HDS+HCI. This methodological advancement is designed to overcome the respective limitations of HDS and HCI when utilized in isolation, while also maximizing their strengths.

High-dispersion spectroscopy has demonstrated effectiveness in extracting faint planetary signals from the overwhelming starlight by leveraging the Doppler shift of molecular lines in the planet's atmosphere. However, HDS is constrained by stellar noise. In parallel, HCI separates the spatial position of a planet from its star but struggles with speckle noise and quasi-static speckles, achieving planet-star contrast limits of around $10^{-5}$. The paper argues that the combination of HDS with HCI could, in theory, achieve a contrast on the order of $10^{-10}$. However, the practical limit is likely swayed by photon noise and observational limitations.

The implementation of HDS+HCI will be particularly potent with the advent of new astronomical infrastructures like the European Extremely Large Telescope (E-ELT), which is equipped with state-of-the-art adaptive optics systems and high-dispersion integral field spectrographs (IFS) such as METIS. The paper offers simulations illustrating that with the E-ELT, a night of observing with HDS+HCI could detect planetary signals from objects as small as Earth orbiting around $\alpha$ Centauri with a signal-to-noise ratio (S/N) of 5 in the infrared, and potentially even better results in the optical for Earth-sized planets around Proxima Centauri.

A particularly compelling outcome from this paper is the empirical evidence provided by recent HDS+HCI observations of the known exoplanet $\beta$ Pictoris b. These observations managed to discern not only the presence of the planet but its rotational velocity—an important breakthrough for exoplanet science that showcases the potential of HDS+HCI.

These techniques have crucial implications for advancing exoplanetary science, offering significant potential to identify and characterize Earth-like planets in habitable zones of nearby stars. The success of HDS+HCI in practical applications would not only refine our understanding of exoplanetary atmospheres but could herald a new era for spectrographic searches in astronomical research.

This dual-technique approach also suggests a strategic push for telescope and instrument design priorities: namely, prioritizing the development of high-dispersion IFS for the next generation of extremely large telescopes. Such instruments could prove invaluable for dynamically mapping the orbits of extrasolar planets and studying atmospheric compositions with unprecedented precision. The potential for advancing planetary detection efficacies highlights the need for a significant allocation of observational resources towards HDS+HCI techniques for upcoming astrophysical surveys and missions.