BOWIE-ALIGN: Exploring degeneracies in the muted transmission spectrum of the aligned hot Jupiter NGTS-2b with NIRSpec/G395H

Abstract

We present the first atmospheric observation and characterisation of the aligned, 1468 K hot Jupiter, NGTS-2b, with one JWST NIRSpec/G395H transit. These observations complete the GO 3838 observing campaign of the BOWIE-ALIGN program, which aims to investigate the link between hot Jupiter atmospheric composition and formation history through the atmospheric analysis of planets orbiting F stars that are aligned and misaligned with the host stellar spin axis. The 2.84-5.18 micron spectrum shows weak absorption features attributed to H$_2$O and CO$_2$ absorption, which our free chemistry retrievals fit with posteriors that converge on high mean molecular weight solutions attained through significant H$_2$O mixing ratios. By comparing our results to interior modelling, we show that some of these solutions exceed the 43.5x solar upper limit we obtained from our interior structure models. Such solutions are likely due to cloud-metallicity degeneracies and insufficient wavelength coverage to resolve them. We show that, in the case of our observations, the likelihood distribution of H$_2$O abundances is flat and uninformative, such that our retrievals are biased by the prior. Additionally, our statistically favoured atmospheric solution contains absorption from SO. The chemical abundances retrieved with this model are likely not astrophysically feasible and we demonstrate that the presence of SO is driven by only two data points. Our equilibrium chemistry retrievals hint at a subsolar C/O ratio and supersolar metallicity; however, we find wide posterior distributions that extend to solar values.

Figures (19)

• Figure 1: NGTS-2b broadband transit light curves (left) and residuals (right) from NRS1 and NRS2 using the ExoTiC-JEDI reduction. Data from the different detectors are offset for clarity.
• Figure 2: Binned residual plot for our ExoTiC-JEDI reduction across all spectroscopic light curves (grey) and broad band light curves for NRS1 (green) and NRS2 (blue). The dashed yellow line shows the expected noise properties as the residuals are binned down. This demonstrates that our data aligns with the expectation and is not impacted by unaccounted for red noise in the data.
• Figure 3: NGTS-2b transmission spectra from JWST NIRSpec/G395H at $R$ = 100 (left) and $R$ = 400 (right) using two different reduction pipelines (ExoTiC-JEDI in green and Tiberius in purple). Bottom plots show the residuals between the two pipelines are in very good agreement within the 1$\sigma$ bounds, which are shown by the dashed lines.
• Figure 4: The posterior distribution of our interior structure modelling parameters: mass ($M_\mathrm{J}$), metallicity, log heating efficiency, and age. The parameters used in the model are listed in the figure. We infer a metallicity of $Z_\mathrm{p}=0.15\pm0.04$, below average for this mass but well within the natural dispersion seen in Thorngren2016. Converting this to a number ratio and assuming the limiting fully-mixed case, we obtain an upper-limit on the atmospheric metallicity of 43.5 $\times$Solar.
• Figure 5: Free chemistry retrievals on the ExoTiC-JEDI$R$ = 400 spectrum for the three models tested with POSEIDON (Models I--III) and two models tested with petitRADTRANS (Models A and B). Model I and Model A including SO opacity (purple and pink). Model II and Model B excluding SO opacity (blue and orange). Model II excluding SO opacity and limiting the mean molecular weight to $\upmu\,<\,3$ (light green). While Model I statistically favoured ($\Delta \log Z$ = 5) over Model II ($\upmu\,<\,3$), we discuss the astrophysical likelihood of the retrieved abundance of SO in Section \ref{['sec:SOchem']}, and the role of the mean molecular weight in Section \ref{['sec:mmw']}. We note that due to POSEIDON and petitRADTRANS retrieving offsets for NRS1 and NRS2 respectively, the petitRADTRANS offset has been inverted to directly compare to the POSEIDON retrieved value.