Enabling Maximum Technical and Science Return of the Roman-Coronagraph with Precursor Observations and Simulations
PI: Hom, Justin, University Of Arizona
Coronagraph Community Participation Program
The Roman-Coronagraph will be the first visible light high contrast instrument in space capable of high order wavefront sensing and control, critical for maturing techniques for Earth-like exoplanet direct imaging with the Habitable Worlds Observatory (HWO). These technologies require a process known as “digging” a dark hole of high contrast. To both dig and maintain the dark hole contrast, all standard science observing sequences require cyclical observations of the science target and a bright, nearby reference star. Reference star criteria, however, are extremely strict, limited to single bright stars with small resolved diameters. The presence of known binary companions disqualifies most candidates, leaving only 40 options that have yet to be fully vetted for companions. Thorough vetting is therefore critical for the ultimate selection and scheduling of both technical and science targets, as selecting an inadequate reference star will cause dark hole digging to fail and equate to several hours of lost time counted against the guaranteed 90-day Observation Phase. The sky distribution of reference stars directly dictates the scheduling of science targets, meaning adequate sky coverage is needed to ease scheduling flexibility.
Other reference star properties may also limit the maximum achievable contrast without causing dark hole digging to fail. Thresholds on companion brightness and separation have not been thoroughly quantified, and the impact of faint companions outside the dark hole is not well understood. Differences in stellar diameter between science and reference along with the presence of diffuse circumstellar dust may also negatively impact the achievable contrast. Understanding the impact of these factors, along with the complexities of mission scheduling and the depth of a given candidate’s companion vetting campaign, necessitate an informed metric for pairing reference and science targets to optimize the achievable contrast and, therefore, science return of the mission.
We propose to conduct a thorough reference star vetting campaign utilizing archival, approved, and future observing programs. The campaign will utilize datasets from an exhaustive list of ground-based interferometric and high contrast imaging instrumentation operating from the visible to near-infrared. The combination of contrasts across multiple instruments and wavelengths will allow us to identify problematic companions and set deep companion rejection limits at separations greater than 2 mas and contrasts approaching the TTR5 requirement. We will also create high fidelity observing simulations with existing tools to quantify impact on contrast from faint companions, stellar diameter mismatch, and circumstellar dust. In addition to determining the final list of suitable reference stars for the Roman-Coronagraph, we will derive a metric for selecting the optimal reference star for a given science sequence based on the results of our simulations, the depth of rejection limits, and the flexibility of scheduling. After launch, our program will focus on science performance assessment of the delivered reference star list and vetting down to 10-8 contrast with the Roman-Coronagraph itself, allowing us to compare the results of our simulations to on-sky performance while simultaneously vetting candidates for extended mission Coronagraph observations and HWO. The reference-science pairing metric will be amended pending analysis of on-sky data. Other ground-based observing campaigns for a reserve list of reference stars not satisfying initial criteria will also be initiated to both quantify thresholds for dark hole digging and to ease scheduling flexibility for best-effort modes. Our program deliverables will be available to the community and are critical for reducing mission risk and optimizing mission efficiency and science return while also informing planning efforts for HWO.