J/A+A/707/A315 Transit light curves of NGTS-5b (Wang+, 2026)
Evidence for stellar contamination and water absorption in NGTS-5b's
transmission spectra with GTC/OSIRIS.
Wang W., Chen G., Jiang C., Palle E., Murgas F., Parviainen H.
<Astron. Astrophys. 707, A315 (2026)>
=2026A&A...707A.315W 2026A&A...707A.315W (SIMBAD/NED BibCode)
ADC_Keywords: Stars, double and multiple ; Stars, K-type ; Exoplanets ;
Spectroscopy
Keywords: methods: data analysis - techniques: spectroscopic -
planets and satellites: atmospheres -
planets and satellites: individual: NGTS-5b
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 subsolar C/O
ratio of <0.22 (90% upper limit) and a low metallicity of
0.10+0.34-0.05solar. Retrievals assuming free chemistry yield
strong evidence for the presence of H2O, with its abundance
constrained to XH2O=-0.79+0.14-0.17. However, the abundances of
other species remain unconstrained due to 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 confi
Description:
We provide here the transit light curves of the warm sub-Saturn
NGTS-5b observed on 10 March 2020 (N1) and 21 March 2021 (N2) using
the OSIRIS spectrograph at the 10.4 m Gran Telescopio CANARIAS (GTC).
The dataset includes one white light curve (wlc) from N1 obtained
with the R1000R grism and one white light curve from N2 obtained with
the R1000B grism, which are also divided into spectroscopic light
curves (slc) in different wavelength bins. The white light curves span
the common passband for joint fitting.
Objects:
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RA (2000) DE Designation(s)
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14 44 13.98 +05 36 19.3 NGTS-5b = 2MASS 14441396+0536195
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
wlcn1.dat 107 174 NGTS-5b GTC/OSIRIS white light curve on N1
wlcn2.dat 107 269 NGTS-5b GTC/OSIRIS white light curve on N2
slcn1.dat 107 12180 NGTS-5b GTC/OSIRIS spectroscopic light curves on N1
slcn2.dat 107 19099 NGTS-5b GTC/OSIRIS spectroscopic light curves on N2
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Byte-by-byte Description of file: slcn1.dat slcn2.dat wlcn1.dat wlcn2.dat
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Bytes Format Units Label Explanations
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1- 15 F15.7 d BJD Barycentric Julian Date (TDB standard)
17- 24 F8.6 --- Flux Relative flux
26- 33 F8.6 --- e_Flux Uncertainty of relative flux
35- 43 F9.6 pix posX Drift of star on CCD in dispersion direction
45- 53 F9.6 pix posY Drift of star on CCD in spatial direction
55- 63 F9.6 pix FWHMX Relative change of full width at half
maximum of the stellar absorption line
profile
65- 73 F9.6 pix FWHMY Full width at half maximum of the stellar
spatial profile
75- 82 F8.6 --- Airmass Airmass during observation
84- 91 I8 --- Night Starting night
93- 97 F5.1 nm wave-left Wavelength at the left border
99-103 F5.1 nm wave-right Wavelength at the right border
105-107 A3 --- Band Band sequence
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Acknowledgements:
Guo Chen, guochen(at)pmo.ac.cn
(End) Patricia Vannier [CDS] 12-Feb-2026