Kavli Affiliate: Sara Seager
| First 5 Authors: Rosa E. Keers, Alexander I. Shapiro, Nadiia M. Kostogryz, Ana Glidden, Prajwal Niraula
| Summary:
Stellar limb darkening must be properly accounted for to accurately determine
the radii of exoplanets at various wavelengths. The standard approach to
address limb darkening involves either using laws with coefficients from
modelled stellar spectra or determining the coefficients empirically during
light curve fitting of the data. Here, we test how accurately three common laws
— quadratic, power, and a three-parameter law — can reproduce stellar limb
darkening at different wavelengths and across a broad range of stars. We show
that using a quadratic limb darkening law, which is most frequently employed by
the community, leads to wavelength-dependent offsets in retrieved transmission
spectra. For planets with high impact parameters ($b$ larger than about 0.5)
the amplitude of these offsets can reach 1% of the transit depth which is some
cases is comparable to and can even exceed the expected signals from the
planetary atmosphere. Furthermore, the quadratic law causes an offset in the
value of the impact parameter when it is determined by fitting the broadband
transit light curves. In contrast, using the Kipping–Sing three-parameter law
leads to robust retrievals. We advocate the use of this law in retrievals,
especially for transits with large impact parameters.
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