A Roman Coronagraph Spectroscopic Mode Demonstration

Abstract

We propose 730 nm high-contrast spectroscopic observations of selected self-luminous directly-imaged planets as a key test of the Roman Coronagraph's planet characterization capabilities. The planet sample draws from ground-based IR discoveries with the NASA headquarters-supported Subaru/OASIS survey -- HIP 99770 b and HIP 54515 b -- and ``emblematic" planets $β$ Pic b and HR 8799 e. All of these planets are likely unsuitable for achieving the coronagraph's core TTR5 goal at 575 nm but are detectable at longer wavelength passbands. Their predicted contrasts at 730 $μm$ cover two orders of magnitude range; all companions reside within the dark hole region enabled by the shaped-pupil coronagraph at 730 nm. These observations will help to fulfill multiple Coronagraph Objectives, providing a first assessment of the wavelength dependence of speckle noise and the ability to extract accurate atmospheric information in the face of this noise. Additionally, they will provide a first experiment at extracting optical planet spectra in the face of signal contamination from a debris disk: prefiguring challenges that the Habitable Worlds Observatory may encounter with imaging Earths in exozodi-contaminated systems.

Figures (2)

• Figure 1: Contrast predictions for $\beta$ Pic b, HIP 54515 b, HR 8799 e, and HIP 99770 b from updates to the LacyBurrows2020 atmosphere models described in Currie2023a. The temperatures and gravities for $\beta$ Pic b, HIP 54515 b, and HIP 99770 b draw from published atmospheric modeling of near-IR spectra and photometry Chilcote2017Currie2025Bovie2025. The assumed temperature of 1200 $K$ for HR 8799 e matches the planet's SED, where the 1300 $K$ model fares slightly worse. $\beta$ Pic b is likely undetectable at 575 nm due to its steep contrast and confusion with the system's bright debris disk. However, it should be detectable at 730 nm, where it is a factor of 30--100 times brighter relative to the star and disk: this prediction is consistent with a recent MagAO-X detection of $\beta$ Pic b at 762nm: see https://nexsci.caltech.edu/workshop/2025/posters/Poster_EdenMcEwen_176.pdf.
• Figure 2: (left) Relative Pitch Angle between HIP 54515 and two potential reference stars during the calendar year of 2027. Combined with keepout maps (not shown), the second PSF reference (HIP 57632) would allow observations with a $\Delta$ Pitch Angle less than 3$^{o}$. (middle, right) SNR vs. integration time for our targets with solar levels of exozodi light and variable ones assuming conservative (dash-dot) or optimistic (solid) coronagraph performances. The latter are either set at 10 times solar system levels or are crudely matched to the system's debris disk brightness.