Multi-band Spectral and Astrometric Characterization of the HIP 99770 b Planet with SCExAO/CHARIS and Gaia

TL;DR

This work advances the dynamical and atmospheric characterization of the directly imaged planet HIP 99770 b by combining high-resolution $H$ and $K$ band spectra from SCExAO/CHARIS with extensive Hipparcos-Gaia astrometry and a KPIC relative RV measurement. Through orvara-based joint fits, the authors derive refined orbital elements and a dynamical mass in the $13$–$15\,M_{\rm Jup}$ range, with a semi-major axis near $15.7$–$15.8$ AU and eccentricity around $0.22$–$0.29$, depending on the mass prior and data included. Spectroscopic analysis of the CHARIS data, complemented by comparisons to empirical libraries and multiple atmospheric grids, favors an effective temperature around $1300$–$1400$ K and intermediate gravity, placing HIP 99770 b at the L/T transition with cloud properties between younger planets and older brown dwarfs. The results enhance our understanding of substellar atmospheres at the L/T transition, demonstrate the power of joint dynamical constraints with direct imaging, and outline prospects for optical characterization with Roman CGI and JWST.

Abstract

We present and analyze follow-up, higher resolution ($R$ $\sim$ 70) $H$ and $K$ band integral field spectroscopy of the superjovian exoplanet HIP 99770 b with SCExAO/CHARIS. Our new data recover the companion at a high signal-to-noise ratio in both bandpasses and more than double the astrometric baseline for its orbital motion. Jointly modeling HIP 99770 b's position and the star's astrometry from Hipparcos and Gaia yields orbital parameters consistent with those from the discovery paper, albeit with smaller errors, and a slight preference for a smaller semimajor axis ($\sim$15.7--15.8 au)and a larger eccentricity ($\sim$0.28--0.29), disfavoring a circular orbit. We revise its dynamical mass slightly downwards to 15.0$_{-4.4}^{+4.5}$ $M_{\rm Jup}$ for a flat prior and 13.1$_{-5.2}^{+4.8}$ $M_{\rm Jup}$ for a more standard log-uniform mass prior, where the inclusion of its relative radial-velocity measurement is primarily responsible for these changes. We find consistent results for HIP 99770 b's dynamical mass including recent VLTI/GRAVITY astrometry, albeit with a slightly smaller, better constrained eccentricity of $e$ $\sim$ 0.22$^{+0.10}_{-0.13}$. HIP 99770 b is a $\sim$ 1300 K object at the L/T transition with a gravity intermediate between that of the HR 8799 planets and older, more massive field brown dwarfs with similar temperatures but with hints of equilibrium chemistry. HIP 99770 b is particularly well suited for spectroscopic follow up with Roman CGI during the technology demonstration phase at 730 nm to further constrain its metallicity and chemistry; JWST thermal infrared observations could likewise explore the planet's carbon chemistry, metallicity, and clouds.

Figures (21)

• Figure 1: HIP 99770 b SCExAO/CHARIS detections in the H and K bands on 8 July 2023 and 6 June 2023 respectively. The detected companion is circled and the centroid of the star is marked by a yellow star.
• Figure 2: HIP 99770 b H band spectra in red from 8 July 2023 and K band spectra in orange from 6 June 2023 plotted on top of the low resolution spectra from the original discovery in black from 17 October 2021 and published in Currie2023b.
• Figure 3: Corner plot showing MCMC posterior distributions for the flat prior. Fit using absolute astrometry from HGCA, relative astrometry (from both initial discovery and follow-up), and RV measurements from KPIC courtesy of Zhang2024. The contours show the $68\%$$(1\sigma)$, $95\%$$(2\sigma)$, and $99\%$$(3\sigma)$ confidence intervals.
• Figure 4: Orbit fitting with orvara assuming flat (uniform) companion mass priors. (Left) Predicted orbits with most likely orbit in black along with 100 randomly selected orbits (color coded by mass) from the MCMC posterior distribution. Blue circles show our data, while empty circles show predicted location per epoch. (Center) Position angle vs Epoch and (Right) Separation vs Epoch for the selected orbits.
• Figure 5: Corner plot showing MCMC posterior distributions for the log-uniform prior. Fit using absolute astrometry from HGCA, relative astrometry (from both initial discovery and follow-up), and RV measurements from KPIC courtesy of Zhang2024. The contours show the $68\%$$(1\sigma)$, $95\%$$(2\sigma)$, and $99\%$$(3\sigma)$ confidence intervols.