Wide-Field InfrarRed Survey Telescope-Astrophysics Focused Telescope Assets WFIRST-AFTA 2015 Report (1503.03757v2)

Abstract: This report describes the 2014 study by the Science Definition Team (SDT) of the Wide-Field Infrared Survey Telescope (WFIRST) mission. It is a space observatory that will address the most compelling scientific problems in dark energy, exoplanets and general astrophysics using a 2.4-m telescope with a wide-field infrared instrument and an optical coronagraph. The Astro2010 Decadal Survey recommended a Wide Field Infrared Survey Telescope as its top priority for a new large space mission. As conceived by the decadal survey, WFIRST would carry out a dark energy science program, a microlensing program to determine the demographics of exoplanets, and a general observing program utilizing its ultra wide field. In October 2012, NASA chartered a Science Definition Team (SDT) to produce, in collaboration with the WFIRST Study Office at GSFC and the Program Office at JPL, a Design Reference Mission (DRM) for an implementation of WFIRST using one of the 2.4-m, Hubble-quality telescope assemblies recently made available to NASA. This DRM builds on the work of the earlier WFIRST SDT, reported by Green et al. (2012) and the previous WFIRST-2.4 DRM, reported by Spergel et. (2013). The 2.4-m primary mirror enables a mission with greater sensitivity and higher angular resolution than the 1.3-m and 1.1-m designs considered previously, increasing both the science return of the primary surveys and the capabilities of WFIRST as a Guest Observer facility. The addition of an on-axis coronagraphic instrument to the baseline design enables imaging and spectroscopic studies of planets around nearby stars.

• The paper demonstrates the mission’s primary contributions by advancing dark energy studies through high-latitude IR surveys achieving sub-percent precision on cosmic shear and expansion history.
• The mission employs microlensing in galactic bulge fields to detect a broad spectrum of exoplanets, including approximately 2600 candidates from Super Earths to Earth-mass bodies.
• The report highlights innovative coronagraph technologies that enable direct imaging and spectral characterization of exoplanets with contrast ratios approaching 10⁻⁹.

Overview of the WFIRST-AFTA Mission and Its Scientific Objectives

The "Wide-Field InfraRed Survey Telescope-Astrophysics Focused Telescope Assets" (WFIRST-AFTA) was developed with the primary goal of addressing crucial astronomical inquiries regarding dark energy, cosmic inflation, and exoplanet science. This mission concretizes the high-priority objectives outlined in the National Academy of Sciences' decadal survey. The primary scientific opportunities afforded by WFIRST-AFTA include a high-latitude survey focused on probing dark energy, in-depth galactic bulge fields for microlensing observations, and high-contrast coronagraphy for exoplanet detection in the solar neighborhood.

High-Latitude Survey: Probing Dark Energy and Cosmological Structure

A significant component of the WFIRST-AFTA mission is the high-latitude survey, which is optimized for comprehensive dark energy investigations. Through a meticulously designed combination of wide-field infrared imaging and spectroscopy, WFIRST-AFTA aims to harness three complementary methodologies—weak gravitational lensing, baryon acoustic oscillations, and Type Ia supernovae—to constrain the dark energy equation of state, w(z). Over its scheduled observing run, this survey seeks to achieve sub-percent precision measurements of cosmic shear, galaxy cluster abundance, and cosmic expansion history. Analysis of the high-latitude survey data will provide stringent tests for cosmological models, utilizing parameters such as the density of matter and the fluctuations of cosmic structures (σ8). The mission's sensitivity in the IR spectrum (0.76 to 2.0 µm) and its spatial resolution (enabled by a 2.4m primary telescope aperture) are core strengths, allowing it to survey approximately 2227 deg² of the sky.

Galactic Bulge Field Survey: Exoplanet Microlensing

The mission includes a galactic bulge field survey aimed at completing the census of cold exoplanets through microlensing techniques. This program is distinctive for its capability to detect a wide range of exoplanet types, including those unattainable by existing radial velocity and transit methods. It allows for sensitive detection of Earth-mass and sub-Earth-mass planets beyond the snow line of planetary systems, contributing critically to our understanding of planetary formation and migration. The expected yield for bound exoplanets includes ∼2600 planets, ranging from ∼1030 "Super Earths" to ∼370 Earth-mass planets. These observations will elucidate the mass distribution of planetary bodies in diverse orbital configurations, providing complementary data to the Kepler mission's census of smaller planets.

High-Contrast Coronagraphic Studies: Direct Exoplanet Imaging

The WFIRST-AFTA mission is also set to include a groundbreaking coronagraph instrument optimized for imaging and spectrally characterizing exoplanets and circumstellar disks around nearby stars. This is a substantial technological demonstration, blending cutting-edge coronagraphy techniques with deformable mirror wavefront control, yielding contrast ratios around 10⁻⁹ in certain observational modes, sufficient for direct imaging of gas giants and exo-zodiacal dust. The coronagraph will potentially detect atmospheric features of RV-confirmed planets, offering insights into atmospheric compositions and planet-forming processes. Performance gains in this domain will contribute to the broader goal of imaging Earth-like planets in habitable zones around Sun-like stars in future missions.

Instrumentation Development and Technological Readiness

WFIRST-AFTA relies on advanced optical technology, including a wide-field instrument equipped with a H4RG detector array and powerful integral field spectrograph. The technological readiness of these detectors is crucial, with the paper demonstrating maturity at TRL-6 for the H4RG FPA. The payload’s design reflects optimization for thermal stability, necessary for the stability of photometric measurements required for weak lensing studies. The instrument’s cryocooler configuration and passive radiator design are crucial for thermal management, ensuring optimal operational conditions in geosynchronous orbit.

Synergies and Future Directions

The mission's synergies with concurrent ground-based and upcoming space missions like JWST, Euclid, and LSST represent significant multi-faceted scientific opportunities. Collaborative data analysis with LSST will enhance the photometric redshift accuracy for weak lensing source galaxies. Euclid's complementary surveys will allow cross-validation of dark energy measurements and offer expanded opportunities for scientific exploration in cosmology and galaxy formation. The WFIRST-AFTA mission not only promises transformative scientific returns through its imaging and spectroscopic capabilities but also lays groundwork for the future of exoplanetary science as a pathfinder for imaging technologies. Its considerable contributions will likely influence mission designs and priorities in future astrophysical studies and debates.