The HIP 54515 b Superjovian Planet as an Early, Critical Look at the Roman Coronagraph's Performance in the Faint Target Star, Small IWA Limit

The Roman Coronagraph’s capabilities in the faint star, small IWA limit has enormous scientific (programmatic) impacts. Testing its performance in this limit provides a first look at challenges that may be encountered with the Habitable Worlds Observatory in imaging rocky planets around the nearest K and M stars. We propose such a rigorous test with the HLC/575nm targeting a newly-discovered superjovian planet HIP 54515 b, whose predicted contrast is $\sim$4.7 $\times$10$^{-8}$–2.5 $\times$10$^{-7}$. The companion lies close to the coronagraph IWA (well interior to the TTR5 performance region) and orbits a V = 6.8 star, near the limit for which the coronagraph may yield deep contrasts. Multiple reference stars are available that will further test CGI’s performance as a function of $Δ$ pitch angle to assess how the telescope’s thermal environment impacts contrasts.

💡 Research Summary

The paper presents a concrete observing proposal to test the performance limits of the Nancy Grace Roman Space Telescope’s Coronagraph Instrument (CGI) in a regime that pushes both stellar brightness and angular separation constraints. The target is the newly discovered super‑jovian planet HIP 54515 b, which orbits a V = 6.8 star at a projected separation of roughly 0.22–0.24 arcseconds (≈1.5 λ/D at the CGI’s 575 nm band). Atmospheric modeling of the planet’s infrared spectrum predicts a reflected‑light contrast in the 575 nm band of 4.7 × 10⁻⁸ to 2.5 × 10⁻⁷, essentially matching the CGI’s TTR5 benchmark of 10⁻⁷ contrast.

The authors argue that while CGI’s baseline performance has been defined for bright stars (V ≲ 5) and separations of 6–9 λ/D, the technology demonstration must also verify that the system can achieve comparable contrasts on fainter stars (V ≈ 7) and at separations just outside the nominal inner working angle (IWA, ≈3 λ/D). Demonstrating this capability is critical for informing the design and science case of the future Habitable Worlds Observatory, which will need to image Earth‑like planets around nearby K and early‑M dwarfs that are typically fainter and require small‑IWA observations.

The observing strategy follows the “standard typical observing sequence” outlined in the latest CGI documentation. Each visit consists of: (1) dark‑hole digging on a bright nearby PSF reference star, (2) acquisition of PSF reference images, (3) two sets of ±15° telescope rolls on the target star (enabling angular differential imaging, ADI), and (4) a second set of PSF reference observations (enabling reference‑differential imaging, RDI). Two candidate reference stars are identified—HIP 54872 (≈21° pitch‑angle separation) and β Leo (HIP 57632, ≈19° separation). The pitch‑angle difference between target and reference can be varied between 3° and 10°, allowing a systematic study of how thermal environment changes affect wavefront stability and contrast. β Leo offers two ∼70‑day windows per year where the pitch‑angle difference drops below 3°, providing an especially stringent test of thermal‑induced performance degradation.

Exposure time estimates were generated with the Corgi‑ETC tool. For the most optimistic CGI performance scenario, achieving a 5σ contrast of 10⁻⁷ at the predicted planet contrast (4.7 × 10⁻⁸) requires 2.35 hours of on‑source integration; the more conservative scenario requires 5.96 hours. Accounting for the CGI’s overall observing efficiency (24 h of clock time yields ≈14 h of actual integration), each target‑reference pair would consume roughly 4.0 h (optimistic) to 10.2 h (conservative) of clock time. The full program—target plus both reference stars—fits within a total of ~14.3 h of clock time, making it feasible within the CGI technology‑demonstration phase.

If successful, the experiment will deliver three major outcomes: (1) empirical confirmation that CGI can reach TTR5‑level contrasts (10⁻⁷) on stars five times fainter than the nominal benchmark, directly demonstrating the instrument’s photon‑starved performance; (2) quantitative characterization of contrast degradation as a function of pitch‑angle‑induced thermal variations, informing thermal control strategies for future high‑contrast missions; and (3) a high‑signal‑to‑noise detection (SNR > 11) of HIP 54515 b, establishing that the CGI can detect self‑luminous or reflected‑light companions at separations only 1.5 λ/D from the host star.

In summary, the proposed re‑observation of HIP 54515 b provides the most demanding test yet of the Roman CGI’s ability to operate at the faint‑star, small‑IWA frontier. The results will directly impact the feasibility assessments of extended science programs for the Roman mission and will supply critical performance metrics for the design and planning of the Habitable Worlds Observatory, whose primary goal will be to image Earth‑like planets around the nearest K and M dwarfs.