JWST COMPASS: A NIRSpec G395H Transmission Spectrum of Radius Valley-Dweller TOI-260 b

Abstract

We present a JWST/NIRSpec G395H transmission spectrum of TOI-260 b, a $T_\mathrm{eq}\sim 490$ K, $R_\mathrm{p} = 1.76\,R_\oplus$ planet. The transmission spectrum is derived by combining two transit observations, collected as part of the JWST COMPASS program. We achieved the same median transit depth precision of 37 ppm in both visits, and a median precision of 26 ppm when combining the spectroscopic light curves from the two visits. Implementing a 30-pixel-wide ($R\sim 200$) spectroscopic binning scheme, we find that the transmission spectrum is mostly featureless, with a possible feature around 3.17 $μ$m. We assess the significance of any features in the transmission spectrum with a suite of non-parametric models, which confirm the presence of a potential feature in the NRS1 bandpass and an offset between the NRS1 and NRS2 detectors. To investigate the atmospheric composition of TOI-260 b, we run a series of PLATON retrievals. We do not detect any clear molecular signatures, but the combined data from the two visits are sufficient to constrain the atmospheric metallicity to greater than $200\times$ solar, assuming no opaque deck $\lesssim2.5$ mbar. We also investigate causes of the potential feature near 3.17 $μ$m; while we find some compatible gaseous species and cannot fully discard an astrophysical origin, we suspect a systematics origin due to the variance in strength and position of the feature. Overall, this look at TOI-260 b adds to the small sample of radius-valley planets, which already seem to show a diversity in their atmospheric compositions. Determining the true nature of these enigmatic planets will require a larger telescope time investment.

Figures (21)

• Figure 1: Left: The normalised TOI-260 b white light curves from visit 1 as extracted and detrended with (top row)Tiberius and (bottom row)Eureka!, centered on the fitted mid-transit time. The NRS1 light curve is shown in the darker shade (and is offset from unity for clarity) above the NRS2 light curve in the lighter shade. The best-fit systematics + transit models are overplotted. The binned light curves are shown in alternate colors, and data points that were clipped prior to fitting are shown in grey, for reference only. Right: The associated residuals for each visit and detector. We attach the histograms of the residuals to the right.
• Figure 2: The TOI-260 b white light curves from visit 2; same format as Fig. \ref{['fig:WLC_v1']}.
• Figure 3: The RMS of the binned white light residuals for (top row) visit 1 and (bottom row) visit 2. Tiberius and Eureka! are shown on the left and right respectively. The expectation from pure Gaussian noise is indicated for reference in grey.
• Figure 4: Top row: The transmission spectra of TOI-260 b, extracted with (left)Tiberius and (right)Eureka!. The spectra from the two visits are distinguished in different shades, and their difference plotted below in black (ppm). Bottom row: The weighted average transmission spectra of TOI-260 b, from Tiberius (blue) and the transit depths from the jointly fit SLCs from Eureka! (pink). The difference between the two joint spectra is plotted below in black. The median difference of 3.2 ppm is marked with the grey dashed line, with $1\sigma$ and $2\sigma$ regions shaded.
• Figure 5: The transit depth precisions measured by (blue) and (pink) compared to the predicted uncertainties from our simulations (black).