CoronaGraph Instrument Reference stars for Exoplanets (CorGI-REx) I. Preliminary Vetting and Implications for the Roman Coronagraph and Habitable Worlds Observatory

The upcoming Roman Coronagraph will be the first high-contrast instrument in space capable of high-order wavefront sensing and control technologies, a critical technology demonstration for the proposed Habitable Worlds Observatory (HWO) that aims to directly image and characterize habitable exoEarths. The nominal Roman Coronagraph observing plan involves alternating observations of a science target and a bright, nearby reference star. High contrast is achieved using wavefront sensing and control, also known as “digging a dark hole”, where performance depends on the properties of the reference star, requiring V<3, a resolved stellar diameter <2 mas, and no stellar multiplicity. The imposed brightness and diameter criteria limit the sample of reference star candidates to high-mass main sequence and post-main sequence objects, where multiplicity rates are high. A future HWO coronagraph may have similarly restrictive criteria in reference star selection. From an exhaustive literature review of 95 stars, we identify an initial list of 40 primary and 18 reserve reference star candidates relevant to both the Roman Coronagraph and HWO. We present results from an initial survey of these candidates with high-resolution adaptive optics imaging and speckle interferometry and identify no new companions. We discuss the need for higher-contrast observations to sufficiently vet these reference star candidates prior to Roman Coronagraph observations along with the implications of reference star criteria on observation planning for Roman and HWO.

💡 Research Summary

The paper presents the CoronaGraph Instrument Reference stars for Exoplanets (CorGI‑Rex) program, a systematic effort to identify and vet suitable reference stars for the upcoming Nancy Grace Roman Space Telescope’s Coronagraph Instrument and for the future Habitable Worlds Observatory (HWO). High‑contrast direct imaging of exoplanets relies on precise wavefront sensing and control (WFSC) and on reference‑differential imaging (RDI). Because many science targets are relatively faint, the Roman Coronagraph will alternate observations between a bright reference star and the science target. The performance of both the high‑order WFSC (HO‑WFSC) “dark‑hole” digging and the subsequent RDI subtraction depends critically on four stringent properties of the reference star: (1) visual magnitude brighter than V = 3 mag to provide sufficient signal‑to‑noise for rapid dark‑hole creation; (2) a uniform‑disk angular diameter smaller than 2 mas to limit low‑order aberration leakage through the coronagraph masks; (3) the absence of visual companions that could introduce incoherent light within the inner working angle or contaminate the PSF wings outside the outer working angle; and (4) a small spacecraft pitch‑angle change (|Δpitch| ≤ 5°) relative to the science target to avoid thermal‑induced contrast degradation.

Using an exhaustive literature and archival search, the authors compiled a list of 95 candidate stars that potentially satisfy these criteria. After applying the four filters, they identified 40 primary and 18 reserve reference stars. Because the criteria preferentially select high‑mass main‑sequence or post‑main‑sequence stars—populations known for high multiplicity rates—the authors performed an initial vetting campaign with high‑resolution adaptive‑optics (AO) imaging and speckle interferometry. The AO and speckle data, limited to contrasts of order 10⁻⁶, revealed no new close companions among the 58 vetted candidates, confirming that the existing catalog information is largely reliable but also highlighting the need for deeper, higher‑contrast observations (e.g., with VLT/SPHERE, Gemini/GPI) to reach the 10⁻⁷–10⁻⁸ regime required for Roman operations.

The paper discusses the operational implications of these constraints. The V < 3 requirement drives the allocation of HO‑WFSC time, while the 2 mas diameter limit is tied to the sensitivity of the Hybrid‑Lyot Coronagraph (HLC) mask to tip‑tilt jitter. Off‑axis companions must be fainter than Δmag ≈ 12–17 depending on separation (0.3″–5″) to keep scattered light below the desired raw contrast of ~10⁻⁷. The pitch‑angle restriction translates into a scheduling requirement that reference–science pairs be within 5° in pitch, although yaw separation can be larger. These constraints reduce the pool of usable reference stars, potentially creating scheduling bottlenecks, especially for HWO where science targets will be even fainter and reference stars may need to be shared across many observations.

Finally, the authors outline ongoing work: continued high‑contrast imaging of the candidate list, incorporation of Gaia astrometry to monitor for unseen companions, and the creation of a publicly accessible database of vetted reference stars. They argue that the CorGI‑Rex effort not only mitigates operational risk for the Roman Coronagraph but also provides a template for reference‑star selection for the next generation of space‑based high‑contrast missions, ensuring that the ambitious goal of directly imaging Earth‑like exoplanets remains achievable.