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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

Thayne Currie, Yiting Li, Brianna Lacy, Mona El Morsy, Masayuki Kuzuhara, Naoshi Murakami, Danielle Bovie

Abstract

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.

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

Abstract

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 4.7 10--2.5 10. 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 pitch angle to assess how the telescope's thermal environment impacts contrasts.

Paper Structure

This paper contains 2 figures.

Figures (2)

  • Figure 1: (left) Latest-epoch detection of HIP 54515 b with SCExAO/CHARIS Jovanovic2015Groff2016 at an angular separation of 0$.\!\!^{\prime\prime}$19 from the OASIS survey. The target was selected by the same strategy used to discover and precisely characterize the HIP 99770 b planet and other planets/brown dwarfs by both imaging and astrometry Currie2023aCurrie2023bKuzuhara2022Franson2023Tobin2024ElMorsy2025. Orbital modeling suggests that it will lie at $\rho =$ 0$.\!\!^{\prime\prime}$2--0$.\!\!^{\prime\prime}$24 during the CGI observation phase (e.g., 0$.\!\!^{\prime\prime}$23 in January 2028). (right) HIP 54515 b's predicted contrast in the four main CGI passbands from updated versions of the LacyBurrows2020 models. Modeling its CHARIS spectra and NIRC2 photometry yield a temperature of $\sim$2400 $\pm$ 100 $K$. At 575 nm, it then has a contrast of $\sim$4.7 $\times$10$^{-8}$ to 2.5 $\times$ 10$^{-7}$.
  • 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$^{o}$. (right) Integration time needed to achive a SNR = 13 detection of HIP 54515 b vs. HIP 54515 b contrast at 575 nm for conservative and optimistic CGI performances. A SNR = 11 detection for our worst-cast contrast scenario (4.7$\times$10$^{-8}$) would demonstrate TTR5-level contrasts on a star 5 times fainter than the TTR5 limit and near CGI's inner working angle. We assume a solar system exozodi level, though HIP 54515 lacks evidence for circumstellar dust.