Efficient Spectroscopy of Exoplanets at Small Angular Separations with Vortex Fiber Nulling

Garreth Ruane,Ji Wang,Jacques-Robert Delorme,J. Kent Wallace,Bertrand Mennesson,Dimitri Mawet,Nemanja Jovanovic

doi:10.3847/1538-4357/aae262

Garreth Ruane, Ji Wang + Show 5 more

Open Access

https://doi.org/10.3847/1538-4357/aae262

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Abstract

Abstract Instrumentation designed to characterize potentially habitable planets may combine adaptive optics and high-resolution spectroscopy techniques to achieve the highest possible sensitivity to spectral signs of life. Detecting the weak signal from a planet containing biomarkers will require exquisite control of the optical wavefront to maximize the planet signal and significantly reduce unwanted starlight. We present an optical technique, known as vortex fiber nulling (VFN), that allows polychromatic light from faint planets at extremely small separations from their host stars (≲λ/D) to be efficiently routed to a diffraction-limited spectrograph via a single-mode optical fiber, while light from the star is prevented from entering the spectrograph. VFN takes advantage of the spatial selectivity of a single-mode fiber to isolate the light from close-in companions in a small field of view around the star. We provide theoretical performance predictions of a conceptual design and show that VFN may be utilized to characterize planets detected by radial velocity (RV) instruments in the infrared without knowledge of the azimuthal orientation of their orbits. Using a spectral template-matching technique, we calculate an integration time of ∼400, ∼100, and ∼30 hr for Ross 128 b with Keck, the Thirty Meter Telescope, and the Large Ultraviolet/Optical/Infrared Surveyor, respectively.

Highlights

Perhaps the only practical pathway for detecting biosignatures with ground-based telescopes is to obtain high-resolution spectra of planets orbiting the nearest M-dwarf stars (Riaud & Schneider 2007; Snellen et al 2015; Wang et al 2017)
The discovery of life on these worlds via imaging spectroscopy may need to wait for nextgeneration extreme adaptive optics (AO) on giant segmented mirror groundbased telescopes, such as the Planetary Systems Imager (PSI) on the Thirty Meter Telescope (TMT), or large-aperture space telescopes, such as the Large Ultraviolet/Optical/Infrared Surveyor (LUVOIR; Bolcar et al 2016; Pueyo et al 2017)
We write the signal from the planet and star that enters the spectrograph as Sp = hp FptDlAqT and Ss = hs FstDlAqT, where ηp and ηs are the planet and star throughputs of the vortex fiber nulling (VFN), Φp and Φs are the flux owing to the planet and star, τ is the integration time, Δλ is the full spectral bandwidth, A is the collecting area of the telescope, q is the quantum efficiency of the detector, and T is the transmission of the instrument describing losses that affect the star and planet

Summary

Introduction

Perhaps the only practical pathway for detecting biosignatures with ground-based telescopes is to obtain high-resolution spectra of planets orbiting the nearest M-dwarf stars (Riaud & Schneider 2007; Snellen et al 2015; Wang et al 2017). Using Ross 128 b as an example, we compute the integration time needed to detect potential signs of life in the atmospheres of terrestrial planets orbiting in the habitable zone of nearby M stars with Keck, TMT, and LUVOIR. The feasibility of such observations is dependent on the AO system’s ability to control a select few low-order wavefront error modes, namely tip-tilt and coma, and is relatively insensitive to mid and high spatial frequency aberrations

VFN Concept

Theoretical Performance

Tip-tilt Jitter

Angular Size of the Star

Low-order Aberrations in the Zernike Basis

Aberrations in the Fourier Basis

Other Noise Sources

Estimating Total Integration Time

Requirements for Characterizing Ross 128 b

Keck Telescope

LUVOIR Space Telescope

Detecting New Earth-like Planets Orbiting M Stars