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WFIRST: A New Era in High-Precision Astrometry

A. Bellini (bellini[at]stsci.edu), R. Sanderson (UPenn), and K. Gilbert (STScI)

Ranked as the highest scientific priority for a large space-based mission in the Astro2010 Decadal Survey, the Wide Field Infrared Survey Telescope (WFIRST) will play a pivotal role in astrophysics in the 2020s. With a Hubble Space Telescope (HST)-sized mirror and a field of view 100 times larger, WFIRST's Wide Field Instrument (WFI) will combine true survey-level capabilities with space-based sensitivity and resolution. WFIRST will also include a high-performance coronagraphic instrument. Advances in all areas of astrophysics will be possible, either through targeted General Observer programs, or through archival Guest Investigator analysis of data from core surveys optimized for studies of dark energy or exoplanets.

Astrometry was one of five key "discovery areas" highlighted by the Astro2010 Decadal Survey. Several key characteristics of WFIRST and its WFI make it an excellent astrometric tool. Compared to HST and the James Webb Space Telescope (JWST), WFIRST's much wider field of view, at comparable angular resolution to HST, will provide thousands of astrometric targets in a single pointing anywhere in the sky. In addition, WFIRST will orbit around the Sun-Earth Lagrangian point L2, over a million miles away from Earth. This makes WFIRST's optics extremely stable, especially when compared to low-Earth-orbit telescopes like HST, which suffers from periodic focus changes due to uneven heating from both the Earth and the Sun during each orbit. Furthermore, WFIRST will operate in the infrared regime, so it will be able to see through dust-obscured regions of the sky, like the center of our Galaxy. All in all, WFIRST will enable high-precision astrometric studies at a large variety of size scales, from exoplanetary systems out to the galaxies of the Local Group.

Exoplanets

One of the goals of WFIRST's science mission is to advance our understanding of exoplanets. As demonstrated by the NASA missions Kepler and TESS, wide-field imagers are key to conducting a census of the exoplanets in the Galaxy because of the large sample of objects that can be analyzed in a single pointing. Thanks to its large field of view, WFIRST is set to be a game changer in this research field. For example, a WFIRST exoplanet-microlensing (EML) survey will allow the identification of several hundred exoplanets through the analysis of microlensing light curves. For a subset of them, it will also be possible to measure the astrometric microlensing deflection, which yields additional information on the parameters of the planetary systems (e.g., Bennett et al. 2015). An EML survey will be able to discover planets with masses as small as Mars at separations comparable to Jupiter (Yee et al. 2019; see left panel of Figure 1).

Furthermore, thanks to sophisticated techniques like using the diffraction spikes of saturated stars (Melchior et al. 2018), WFIRST will also be able to detect and characterize Earth-like exoplanets of nearby stars by detecting the wobbling of the host stars due to the gravitational influence of the orbiting planets. The best constraints on planet mass and orbital parameters can be obtained for nearby stars (within about 10 parsecs), which are so bright that their diffraction spikes extend over thousands of high signal-to-noise pixels (right panel of Figure 1). WFIRST will also benefit from the nature of its infrared detectors, which do not show "bleeding" of excess charge from saturated pixels to their neighbors as happens on HST's CCDs. Stellar images with WFIRST will be characterized by 12 diffraction spikes (due to the fact that the secondary mirror is supported by 6 struts), as opposed to the 4 diffraction spikes of HST that extend by at most a few hundred pixels when using its infrared detector. This means a significant gain in term of the signal-to-noise achievable by WFIRST. For competitive measurements of exoplanet masses and orbital parameters, a precision of 10 microarcseconds or better is required, which WFIRST can achieve with integration times of just a few minutes using the diffraction-spike technique.

comparing Kepler and WFIRST exoplanets
Figure 1: (Left) WFIRST will be able to discover and characterize hundreds of exoplanets covering a variety of semimajor axes and masses currently out of reach of present-day exoplanet-hunting missions, as well as many free-floating planets (from Yee et al. 2019). (Right) Bright stars from the Extended Hipparcos Compilation within 10 parsecs. Masses are calculated from the spectral type. Stars are colored according to their J–R color and their size is representative of their apparent optical brightness. The left border of the shaded regions correspond to detections given astrometric signatures of 3, 5, and 10 microarcseconds for a hypothetical three Earth-mass planet with orbital period of 1 year, as well as Neptune-like planets with shorter periods or around more massive or distant stars (from Melchior et al. 2018).

Globular clusters

In the last twenty years, the long-standing view of globular clusters (GCs) as the best example of simple stellar populations (stars formed at the same time from the same molecular cloud) has been demolished by the discovery that essentially all GCs host multiple stellar populations characterized by different abundances of light elements, helium, and in some cases even iron (e.g., Piotto et al. 2015; Milone et al. 2017). Several scenarios have been proposed to explain the formation and evolution of multiple populations in GCs, but all of them have so far failed to account for one or more of the available pieces of observational evidence. The study of the internal kinematics of stars in Milky Way (MW) GCs can help us build a complete picture of the formation of these ancient stellar systems.

In this respect, high-precision proper motions represent a very effective tool. However, because of dynamical evolution, initial kinematical differences between the different stellar populations can be observed only at large radial distances from the center of the clusters, where the relaxation time is comparable to the Hubble time and the stellar density is extremely low (e.g., Bellini et al. 2018). Moreover, due to mass segregation, the outer regions of GCs are mainly populated by low-mass stars that are red and faint. The outskirts of GCs are still uncharted territories observationally, because of the small field of view of current high-resolution space-based telescopes and the poor resolution of current ground-based wide-field telescopes.

WFIRST's WFI combines the best of these two worlds and thus represents the perfect tool for observing GC outskirts. Its large field of view minimizes the impact of low-number statistics, while its infrared filters, combined with HST-like spatial resolution, can measure astrometry and photometry of stars very efficiently. The efficiency gains will be most significant for Bulge GCs that are particularly expensive to study in terms of HST time, due to the high reddening towards the Bulge. Typical MW GCs can entirely fit within the field of view of the WFI. Even the largest MW GCs, like NGC 104, would require just a few WFIRST pointings to observe their full extent, while several hundreds of HST pointings would be required to map the same region of the sky (Figure 2). Typical internal velocity dispersions in the outskirts of GCs are of the order of a few (1–2) km/s. At a typical MW GC distance of 10 kpc, this translates into 20–40 microarcseconds per year, a level of precision that WFIRST can easily achieve with two epochs separated by 3–5 years of temporal baseline, and with just a few tens of images per epoch.

WFIRST's field of view
Figure 2: Both a typical MW GC (left) and one of the largest MW GCs (right) can easily be observed with just one to a few WFIRST pointings. The HST and JWST field of views are also shown on the same scale, for comparison. In both panels, the red circle shows the nominal tidal radius of the cluster (adapted from Bellini et al. 2019).

Star formation and structure of the Milky Way

The study of the MW provides a close-up view of the interplay between cosmology, dark matter, and galaxy formation. In fact, the MW represents a unique laboratory to tackle fundamental questions in astrophysics such as "How do galaxies of different masses form and evolve over cosmic time?" and "What really is dark matter?" The MW is the only galaxy for which we can obtain both complete 3D positions and velocities (6D phase-space positions) and multiple elemental abundance measurements for individual stars across the Hertzsprung-Russell diagram and throughout the Galaxy to its virial radius. This multidimensional view is key to answering these fundamental questions.

The ESA mission Gaia is currently revolutionizing our understanding of the MW's structure in its outer parts, including its halo. However, Gaia's optical filters make it nearly blind when it looks towards the inner MW, due to the presence of a large amount of extinction in the Galactic plane at optical wavelengths (see Figure 3). WFIRST will be able to probe significantly deeper into the MW thanks to its infrared filters that minimize the obscuring effects of interstellar extinction. This will allow us to potentially map the structure and the kinematics of the entire MW Bulge. The astrometric study of the Bulge requires precisions of the order of 10 microarcseconds or less (to measure the distance of Bulge stars via parallax determinations with an error of 9% or better) over a very large field of view, and can directly exploit the data of a WFIRST EML survey. In particular, the end-of-mission EML proper-motion accuracy of Bulge stars translates into a velocity precision of about 1 km/s, which allows a clean determination of the kinematics of the Bulge and of Disk stars in the foreground.

Moreover, the inner MW hosts many massive young stellar clusters, which are ideal laboratories for studies of cluster formation, cluster dynamics and stellar evolution. Historically, the detailed analyses of these objects have been largely neglected, due to high and spatially variable extinction, high stellar densities and confusion with foreground sources. Many of these limitations can be overcome with WFIRST thanks to its ability to measure high-precision proper motions with which we can isolate cluster members from field stars and study their internal kinematics, and the use of infrared filters to minimize the impact of interstellar extinction. A typical proper-motion precision of 50 microarcseconds per year is needed for these studies, which is a precision easily within WFIRST's reach.

WFIRST sees red stars
Figure 3: Simulated completeness of the distribution of red-clump stars within 500 parsecs of the Galactic plane. The MW center is at (0,0), as seen face-on from the North Galactic Pole. The Sun's position is marked by a "+". The left panel shows what Gaia would detect in the optical, which is nearly complete around the solar neighborhood, but the completeness quickly drops for most of the MW near and beyond the Bulge. The right panel shows that WFIRST is expected to detect a complete sample of red-clump stars throughout the MW (from Sanderson et al. 2019).

Summary

We have highlighted just a few of the many examples of science enabled by the astrometric precision and wide field of view possible with WFIRST. WFIRST's astrometric capabilities will also expand our knowledge of the dynamics of the Local Group through measuring the absolute proper motions of its galaxies, help reconstruct the dynamical evolution of the Milky Way by measuring the motion of tidally disrupted remains of previously accreted galaxies in the distant halo, discover isolated black holes and neutron stars through astrometric microlensing, and characterize thousands of asteroids and Kuiper-belt objects (the WFIRST Astrometry Working Group et al. 2019). Astrometry is just one of the many research fields in which WFIRST is going to excel: the abundance of WFIRST data, which will be nonproprietary and available to the entire community through the WFIRST archive from day one, will enable researchers to pursue their areas of expertise with ease.

The WFIRST mission is managed by NASA’s Goddard Space Flight Center with participation by NASA’s Jet Propulsion Laboratory (JPL), the Space Telescope Science Institute (STScI), the Infrared Processing and Analysis Center (IPAC), industrial and foreign partners, and a science team comprised of members from U.S. research institutions across the country. Additional information on WFIRST and its scientific capabilities can be found at the following partner websites:

https://wfirst.gsfc.nasa.gov

https://science.jpl.nasa.gov/projects/WFIRST/

WFIRST at STScI

https://wfirst.ipac.caltech.edu

References

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