BOWIE-ALIGN: Sub-solar C/O ratio and metallicity atmosphere of the misaligned hot Jupiter HAT-P-30b

TL;DR

Using JWST NIRSpec/G395H transmission spectroscopy, the study characterizes HAT-P-30 b within the BOWIE-ALIGN program to probe formation-linked atmospheric composition of misaligned hot Jupiters. Two independent data reductions and multiple retrieval frameworks (equilibrium, free, and hybrid chemistry, plus BeAR) converge on a sub-solar $C/O$ ratio around $0.29$ and sub-solar metallicity, with detected H2O and CO2 and muted features likely due to clouds or a cool limb. A secondary high-metallicity solution appears in a minority of fits but is not favored by Bayesian evidence, and interior-structure modelling links the atmospheric chemistry to a bulk metallicity $Z\approx 0.28$ with a core mass $\gtrsim 50\,M_\oplus$, constraining formation. The results challenge simple continuous-disc migration models and highlight diversity in formation pathways among misaligned hot Jupiters, with future population-level analyses expected to sharpen these inferences. Overall, the work demonstrates the power of JWST-driven atmospheric demographics to illuminate giant-planet formation histories.

Abstract

We present the JWST NIRSpec/G395H transmission spectrum of the misaligned hot Jupiter HAT-P-30b from 2.8--5.2 $μ$m as part of the BOWIE-ALIGN survey, a comparative survey designed to probe the link between planet formation and atmospheric composition in samples of misaligned and aligned hot Jupiters orbiting F-type stars. Through independent data reductions and retrieval analyses, we find evidence for absorption features of H$_2$O and CO$_2$ in the atmosphere of HAT-P-30b. Our retrieved abundances are consistent with equilibrium chemistry, from which we infer a sub-solar C/O ratio (0.16--0.45), and sub-solar and sub-stellar metallicity (0.2--0.8$\times$solar, compared to a stellar metallicity of 1.1--1.6$\times$solar), with muted spectral features. This composition challenges formation models of continuous migration and accretion within a steady disc of stellar metallicity, and could be the result of low C/O ratio gas accretion within the water ice line, low metallicity accretion due to the trapping of volatiles further out in the disc, or the combined accretion of low metallicity gas and carbon-poor solids.

Figures (12)

• Figure 1: White light curves for HAT-P-30b for each detector, NRS1 (green) and NRS2 (magenta) from the Eureka data reduction (left) and the residuals from the light curve fits (right).
• Figure 2: Transmission spectra of HAT-P-30 b from Tiberius (top) and Eureka! (bottom) data reductions, binned to spectral resolutions of $R=$400 and $R=$100.
• Figure 3: Comparison between transmission spectra obtained by our Tiberius and Eureka! reductions at $R=$100, including the difference between the $R=100$ and $R=400$ spectra in green in the bottom panel. The shaded grey regions are the 1$\times$ and 2$\times$ the median transit depth uncertainty of the Eureka!$R=100$ reduction. We find that the difference between reductions depends linearly on wavelength, with a mean offset of $+$35 ppm and $-$21 ppm in NRS1 and NRS2 respectively.
• Figure 4: 1D isothermal chemical equilibrium forward models of HAT-P-30 b compared to the Eureka!$R=100$ transmission spectrum. The left panel depicts cloud-free models with the isotherm temperature set to 1630 K, the equilibrium temperature of HAT-P-30 b. The right panel depicts a selection of models at different metallicities with comparable fits to the data using different C/O ratios, isotherm temperatures, and opaque grey cloud-top pressures, all noted in the legend. These models demonstrate that the spectral features of HAT-P-30 b are muted compared to those predicted by cloud-free models. The degeneracies between composition, temperature, and clouds highlighted in the right panel demonstrate the need for a full exploration of the parameter space using atmospheric retrievals to interpret the spectrum of HAT-P-30 b.
• Figure 5: Median retrieved spectrum (red) of the Eureka!$R=400$ data (orange) from the petitRADTRANS equilibrium chemistry retrieval with no offset, with the posterior probability distributions of C/O ratio and [M/H].