Roman Observations Time Allocation Committee: Final Report and Recommendations

TL;DR

This paper presents ROTAC's final allocations and rationale for Roman WFI surveys, balancing Core Community Surveys (HLWAS, HLTDS, GBTDS) with General Astrophysics Surveys (GAS) across a 5-year prime mission. It details nominal, underguide, and overguide options evaluated by the CCS Definition Committees, selects HLWAS (520 days) and HLTDS (180 days) as nominal, and endorses an overguide GBTDS (438 days) with targeted modifications, plus 389 days for GAS (25.5% of science time). The document also describes the observational designs, mission scheduling considerations, external data dependencies, and the need for flexibility to adapt to changing science in exoplanets, dark energy/matter, and general astrophysics, all while keeping data publicly accessible via the Science Operations Center. By outlining process, timeline, and data strategy, the paper underlines a community-driven approach that maximizes scientific return and enables broad GAS opportunities within Roman’s survey ecosystem, with explicit attention to future mission extensions and mid-mission adjustments.

Abstract

The Nancy Grace Roman Space Telescope is poised to revolutionize our scientific understanding of exoplanets, dark matter, dark energy, and general astrophysics, including through an innovative community approach to defining and executing sky surveys. The Roman Observations Time Allocation Committee (ROTAC) was convened to recommend time allocations for the three Core Community Surveys (CCS) using the Wide Field Instrument (WFI): the High Latitude Wide Area Survey, the High Latitude Time Domain Survey, and the Galactic Bulge Time Domain Survey, as well as balance the time allocation for the General Astrophysics Surveys. Each CCS had a corresponding Definition Committee that collected community input and designed proposals for a nominal (in-guide) survey, as well as underguide and overguide options with smaller and larger time allocations, respectively. These options explored different ways of fulfilling the mission science requirements while maximizing general astrophysics science goals enabled by the surveys. In this report, the ROTAC lays out its recommendations for the three CCS observing designs and the WFI time allotment for CCS (74.5%) and the General Astrophysics Surveys (25.5%).

Figures (6)

• Figure 2: Equatorial projection of the current footprint of the recommended CCS design options: HLWAS tiers in light green (wide tier), medium green (medium tier), and dark green (deep tiers); HLTDS in blue; and GBTDS in yellow (see \ref{['fig:gbtds_field_layout']} for a more detailed GBTDS field layout). Also shown in red is the early-definition Galactic Plane Survey (GPS). The total footprint is here overlaid on a background stellar density from Gaia. Figure code credit: Javier Sanchez and Saurabh W. Jha, with assistance from Eli Rykoff and Alex Drlica-Wagner; data credit: ESA/Gaia/DPAC.
• Figure 3: Illustrative simulation of the Roman observing timeline. The GBTDS observes nearly continuously in the first three Galactic bulge seasons and then again late in the mission. The HLTDS main survey observes at high cadence during the middle years of the prime mission. The HLWAS and GPS are scheduled around the time-domain surveys, but all of the programs obtain data early on in the mission. This timeline will be updated to include observations with the Coronagraph Instrument and Calibrations, and it will shift depending on the actual start date of survey operations (a late 2026 launch was assumed here). The brief periods when resource demand exceeds availability ($>$1.0) are largely artifacts here and will be rectified. The remaining available observing time will be dedicated to General Astrophysics Surveys. Figure credit: Roman Science Operations Center at STScI.
• Figure 4: Top: nominal timeline of HLTDS components. Bottom: Relative area covered by the different imaging and spectroscopy tiers. Graphic from the HLTDS DC report (\ref{['app:hltds']}).
• Figure 5: Field layout for the overguide GBTDS survey as recommended by the ROTAC. Background contours show the expected yield for 1 $M_{\rm Earth}$ free-floating planets. Graphic from the GBTDS DC report (\ref{['app:gbtds']}).
• Figure 6: Yield simulations for transiting planets and asteroseismology using the recommended survey designs. Left: Transiting exoplanet yield relative to the nominal survey as a function of planet radius (x-axis) and host star spectral types (y-axis). Blue bars denote the underguide, and orange bars the overguide survey. Numbers in each panel denote the number of planets expected to be detected in the nominal survey design. Figure credit Robby Wilson (GSFC). Right: Signal-to-noise ratio histogram for asteroseismic detections in red clump stars for the nominal and overguide recommendations. Figure credit: Trevor Weiss (CSULB) and Noah Downing (OSU).