OASIS Survey Direct Imaging and Astrometric Discovery of HIP 71618 B: A Substellar Companion Suitable for the Roman Coronagraph Technology Demonstration

TL;DR

This work presents the discovery and characterization of HIP 71618 B, a substellar companion to the bright A1V star HIP 71618, uncovered by combining Gaia/ Hipparcos astrometry with high-contrast imaging from SCExAO/CHARIS and Keck/NIRC2. Through atmospheric modeling and joint dynamical analysis with orvara, the companion is constrained to a mass near ${60}_{-21}^{+27}$ to ${65}_{-29}^{+54}$ $M_{ m Jup}$, an ~11 au, highly eccentric orbit, and a temperature around $2700 ext{ K}$ with a likely spectral type M6–M8. The system’s brightness, proximity, and orbital geometry place HIP 71618 B within the Roman Coronagraph Dark Hole during its Technology Demonstration phase, offering a concrete path to demonstrate the instrument’s core 575 nm 5σ-contrast requirement of ∼10^-7 (TTR5). The findings highlight the value of combining astrometric and direct-imaging techniques to precisely measure substellar masses and dynamics, while establishing HIP 71618 B as a prime target for early Roman performance validation and atmospheric characterization of young, substellar companions.

Abstract

We present the OASIS survey program discovery of a substellar companion orbiting the young A1V star HIP 71618, detected using precision astrometry from Gaia and Hipparcos and high-contrast imaging with SCExAO/CHARIS and Keck/NIRC2. Atmospheric modeling favors a spectral type of M5--M8 and a temperature of $\sim$2700 $\pm$ 100 $K$. Dynamical modeling constrains HIP 71618 B's mass to be ${60}_{-21}^{+27}$ $M_{\rm Jup}$ or ${65}_{-29}^{+54}$ $M_{\rm Jup}$, depending on the adopted companion mass prior. It has a nearly edge-on, 11 au-orbit with a high eccentricity. HIP 71618 B will be located within Roman Coronagraph's dark hole region during the instrument's technological demonstration phase. A high signal-to-noise ratio detection of HIP 71618 B at 575 nm would demonstrate a 5-$σ$ contrast of 10$^{-7}$ or better. The system is also located within or very close to Roman's Continuous Viewing Zone -- near multiple candidate reference stars for dark-hole digging -- and its primary is bright ($V$ $\approx$ 5). The suitability of HIP 71618 as one potential Roman Coronagraph target for demonstrating the instrument's core requirement (TTR5) should motivate the timely, deep vetting of candidate reference stars.

Figures (6)

• Figure 1: The Gaia color-magnitude diagram for the Pleiades (grey) and Hyades (dark red) compared to PARSEC isochrones for 112 Myr and 750 Myr, young stars with interferometrically measured ages $\lesssim$ 150 Myr, stars in the 400 Myr-old Ursa Majoris Moving group, and other young stars with imaged substellar companions Jones2015Jones2016Jones2016PhDCurrie2023aTobin2024.
• Figure 2: Wavelength-collapsed images of HIP 71618 B with SCExAO/CHARIS in broadband near-infrared light (1.15--2.37 $\mu m$) and with Keck/NIRC2 in the $L_{\rm p}$ filter (3.78 $\mu m$). The white arrow denotes the position of HIP 71618 B in each panel.
• Figure 3: (Top) HIP 71818 b data compared to the best-fit model from the BT-Settl grids in the near-IR spectrum and NIRC2 datapoint. (Bottom) Contour plot showing the confidence intervals for the BT-Settl model grid. The red star indicates the best-fit.
• Figure 4: Corner plot showing posterior distributions of selected orbital parameters from jointly modeling direct imaging and astrometric data assuming a gaussian companion mass prior centered on 65 $M_{\rm Jup}$. The contour lines delineate the regions encompassing 68, 95, and 99% of the posteriors. The corner plot for the simulation adopting a flat companion mass prior is very similar except that its long tail of higher mass solutions results in a slightly larger range of masses within the 68% confidence interval.
• Figure 5: (top) Predicted location of HIP 71618 B in 2027 January from the orvara dynamical modeling results presented in Table \ref{['tab:mcmc_result']}, assuming a gaussian companion mass prior. (bottom) The pitch angle difference between HIP 71618 and two potential reference stars for dark-hole digging in 2027 with the conservative pitch angle threshold displayed as a dashed line.