Predictions of the WFIRST Microlensing Survey. I. Bound Planet Detection Rates

Abstract

The Wide Field InfraRed Survey Telescope (WFIRST) is the next NASA astrophysics flagship mission, to follow the James Webb Space Telescope (JWST). The WFIRST mission was chosen as the top-priority large space mission of the 2010 astronomy and astrophysics decadal survey in order to achieve three primary goals: to study dark energy via a wide-field imaging survey, to study exoplanets via a microlensing survey, and to enable a guest observer program. Here we assess the ability of the several WFIRST designs to achieve the goal of the microlensing survey to discover a large sample of cold, low-mass exoplanets with semimajor axes beyond roughly one AU, which are largely impossible to detect with any other technique. We present the results of a suite of simulations that span the full range of the proposed WFIRST architectures, from the original design envisioned by the decadal survey, to the current design, which utilizes a 2.4-m telescope donated to NASA. By studying such a broad range of architectures, we are able to determine the impact of design trades on the expected yields of detected exoplanets. In estimating the yields we take particular care to ensure that our assumed Galactic model predicts microlensing event rates that match observations, consider the impact that inaccuracies in the Galactic model might have on the yields, and ensure that numerical errors in lightcurve computations do not bias the yields for the smallest mass exoplanets. For the nominal baseline WFIRST design and a fiducial planet mass function, we predict that a total of ∼1400 bound exoplanets with mass greater than ∼0.1 M ⊕ should be detected, including ∼200 with mass 3 M ⊕ . WFIRST should have sensitivity to planets with mass down to ∼0.02 M ⊕ , or roughly the mass of Ganymede.

Joint Radial Velocity and Direct Imaging Planet Yield Calculations. I. Self-consistent Planet Populations

“…ied until WFIRST commences its microlensing survey (Penny et al 2019). RV technologies will continue to improve in the interim (National Academies of Sciences & Medicine 2018).…”

PopSyCLE: A New Population Synthesis Code for Compact Object Microlensing Events

“…Kerins et al (2009) generated synthetic maps of the microlensing optical depth, event rate, and average Einstein crossing time of the Galactic bulge, incorporating a 3-D extinction map. The GULLS code of Penny et al (2013Penny et al ( , 2019 improved upon this work by taking into account the effects of blending, while the MaBµLS code of Awiphan et al (2016) also included low-mass stars and brown dwarfs. All three of these simulations follow a similar method, as summarized in P19, of drawing sources and lenses from a distribution described by the Besançon model, then assigning weights proportional that sourcelens pair's contribution to the total event rate along that sight line.…”

“…Currently, microlensing is being used to search for isolated BHs by combining information from the photometric brightening and parallax signal and the astrometric shift of the source image to constrain the mass of the lens (Lu et al 2016;Rybicki et al 2018). As all current microlensing surveys are ground-based photometric surveys, a substantial development is space-based survey telescopes such as Gaia and WFIRST, which will provide astrometric measurements for microlensing events (Gaia Collaboration et al 2016;Rybicki et al 2018;Spergel et al 2015;Penny et al 2019).…”

“…This issue of too short microlensing events is also a problem described in §6.2.2 ofPenny et al (2019), who implement the Besançon model to study microlensing, although it should be noted that the versions of the Besançon model inPenny et al (2019) and Galaxia are slightly different.…”

The Feasibility of Directly Imaging Nearby Cold Jovian Planets with MIRI/JWST

“…WFIRST's microlensing survey will be extremely sensitive to Jovianmass and smaller planets at intermediate orbital distances of 1-10 AU (see Fig. 9 in Penny et al (2019)), and WFIRST will also be capable of high-contrast coronagraphic imaging from 575 -825 nm at IWAs down to 0.15 (Mennesson et al 2018). Jovian planet statistics from the microlensing sample could inform future JWST imaging campaigns, and WFIRST direct imaging will usher in a new era of exoplanet characterization through reflected light spectroscopy.…”