Evidence for stellar contamination and water absorption in NGTS-5b's transmission spectra with GTC/OSIRIS

TL;DR

This study uses two GTC/OSIRIS optical transits of NGTS-5b to construct chromatic transmission spectra and perform Bayesian atmospheric retrievals. A joint analysis incorporating epoch-dependent stellar contamination reveals strong evidence for unocculted spots (and, to a lesser extent, faculae) shaping the optical spectrum, reducing night-to-night discrepancies. Hybrid retrievals combining atmosphere (equilibrium or free chemistry) with stellar contamination consistently indicate a relatively clear, water-rich atmosphere, with H$_2$O detected at $ m \,log X_{H_2O} = -0.79^{+0.14}_{-0.17}$ in the free-chemistry case, and subsolar metallicity under equilibrium chemistry ($ m log Z \\sim -1.0$; $Z \\sim 0.10^{+0.34}_{-0.05} Z_\\odot$). The results emphasize the impact of TLS on atmospheric inferences and highlight the need for infrared observations (e.g., JWST) to robustly constrain molecular abundances and C/O, aided by simultaneous blue-optical monitoring to mitigate stellar contamination.

Abstract

Transmission spectroscopy serves as a valuable tool for probing atmospheric absorption features in the terminator regions of exoplanets. Stellar surface heterogeneity can introduce wavelength-dependent contamination that complicates the interpretation of planetary spectra. We aim to investigate the atmosphere of the warm sub-Saturn NGTS-5b through optical transmission spectroscopy. Two transits were observed with the low-resolution Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) on the 10.4 m Gran Telescopio Canarias (GTC). Chromatic transit light curves were modeled to derive optical transmission spectra and multiple Bayesian spectral retrievals were performed to characterize the atmospheric properties. Model comparisons provide strong evidence for contamination from unocculted stellar spots. A joint retrieval of the transmission spectra, assuming equilibrium chemistry, indicates a relatively clear atmosphere with a sub-solar C/O ratio of $<$0.22 (90% upper limit) and a low metallicity of $0.10^{+0.34}_{-0.05} \times$ solar. Retrievals assuming free chemistry yield strong evidence for the presence of $\rm H_2O$, with its abundance constrained to $\log X_{\mathrm{H_2O}} = -0.79^{+0.14}_{-0.17}$. However, the abundances of other species remain unconstrained due to the limited optical wavelength coverage. The discrepancies between the two NGTS-5b transit spectra can be attributed to varying levels of stellar contamination. NGTS-5b thus appears to host a relatively clear, water-rich atmosphere, pending confirmation from additional observations of molecular bands in the infrared.

Figures (7)

• Figure 1: Example stellar spectra of NGTS-5 (T; darker line) and its reference star (R; lighter line), observed with the R1000R grism on Night 1 (blue) and the R1000B grism on Night 2 (orange). The vertical lines with gray-shaded regions indicate the adopted spectroscopic passbands.
• Figure 2: White light curves of NGTS-5 observed with GTC/OSIRIS on Night 1 (left) and Night 2 (right). Top panels: observed white light curves (blue circles for Night 1, orange circles for Night 2), best-fit models (black lines), and modeled systematics (green lines). Middle panels: detrended white light curves with the best-fit transit models. Bottom panels: residuals between the observed data and best-fit models.
• Figure 3: Spectroscopic light curves of NGTS-5 observed with GTC/OSIRIS on Night 1 (left) and Night 2 (right). First row: matrix of the raw light curves. Second row: matrix of the detrended light curves. Third row: matrix of the extracted systematics. Fourth row: matrix of the residuals.
• Figure 4: Individual transmission spectra of NGTS-5b derived from Night 1 (blue) and Night 2 (orange) observations.
• Figure 5: Transmission spectra of NGTS-5b with the median, $1\sigma$, and $2\sigma$ confidence intervals of the retrieved hybrid model assuming equilibrium chemistry (Model E, left) and free chemistry (Model F, right). Top row: transmission spectra and retrieved models incorporating both atmospheric and stellar contamination effects, after correcting for the best-fit vertical offset ($\Delta_{\rm offset}$) derived from the joint retrieval. Second row: wavelength-dependent stellar contamination factors inferred from the retrieval. Third row: transmission spectra and corresponding models after correcting for stellar contamination. Bottom row: reference models illustrating the individual contributions of key species.