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

TL;DR

The paper defines and vetts a constrained pool of reference stars for the Roman Coronagraph to enable high-contrast wavefront sensing and differential imaging, informing Habitable Worlds Observatory planning. It establishes strict criteria on brightness, angular diameter, and multiplicity, and builds a 40-primary/18-reserve candidate list from literature catalogs. Initial vetting with moderate-contrast AO imaging and speckle interferometry finds no new companions and sets baseline rejection limits, though deeper observations are required for full confidence. The work also analyzes scheduling implications and discusses how relaxing reference-star constraints could broaden target accessibility for Roman and HWO, guiding future instrument design and post-processing strategies.

Abstract

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.

Figures (8)

• Figure 1: One dimensional representations of how an off-axis companion may leak flux into a Roman Coronagraph dark hole. Both panels assume a $V=2$ reference star, and the navy blue line is the unocculted PSF profile from an off-axis companion at a given $\Delta mag$ and separation. The shaded regions represent the extents of the smallest observing mode FOV (light blue; HLC Band 1) and the largest observing mode FOV (gold; SPC-WFOV Band 4). If sufficient flux from a companion is introduced into the dark hole, both dark hole digging and PSF subtraction via RDI can be compromised.
• Figure 2: Primary (circles, $\leq2$ mas) and reserve (triangles, $<5$ mas) candidate reference stars plotted in ecliptic coordinates in epoch J2027 and equinox 2027 with colors indicating rankings. The CVZs are shaded in cyan. Only one rank A star is in any of the CVZs, and the overall number of rank A reference star candidates is very low compared to other ranks. If only Rank A stars are viable reference star candidates, science target scheduling for the Roman Coronagraph will likely be restrictive.
• Figure 3: PHARO $H2/Br\gamma$ image (left, contrast units) and corresponding $5\sigma$ contrast curve measurement (right) of $\beta$ Cas. Assuming bound main sequence stellar companion colors, we also show the $I_C$- and $V$-band corrected contrast curves derived from the $H2/Br\gamma$ contrasts and the system distance. The complete figure set (21 images) is available in the online journal.
• Figure 4: 'Alopeke 562- (left, contrast units) and 832-nm images (center, contrast units), and corresponding $5\sigma$ contrast curve measurements (right) of $\alpha$ Cep. Assuming bound main sequence stellar companion colors, we also show the $I_C$- and $V$-band corrected contrast curves derived from the 562- and 832-nm contrasts, respectively, and the system distance. The complete figure set (43 images) is available in the online journal.
• Figure 5: Outlook for scheduling Roman Coronagraph observations for three potential science targets (columns; $\beta$ Pic, 47 UMa, and HD 206893) assuming observations can commence January 1st, 2027 UT. The first row is the solar angle as a function of days of the year, with the dashed blue line corresponding to the solar angle of the science target and the gray lines corresponding to the solar angles of all considered reference stars. The shaded black regions are the solar keepout zones. A star is not observable any time of the year where a solar angle curve lies within either of these regions. The second row is the absolute value of $\Delta$pitch calculated between the science target and each primary reference star, and the blue curve is the absolute minimum $\Delta$pitch calculated among all primary reference star options on a given day. The third row combines information from the first two rows to determine the actual observation windows where both the science target and any primary reference star where $\Delta$pitch$\leq 5^{\circ}$ are observable. Finally, the fourth row shows the number of primary reference star options with $\Delta$pitch$\leq5^{\circ}$ that are observable with the science target on any given day. Red triangles indicate when $\leq1$ primary reference stars are available. If all primary reference star candidates are viable, scheduling is generally restricted only by science target observability.