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Effect of the stellar absorption line centre-to-limb variation on exoplanet transmission spectrum observations
摘要： Transit spectroscopy is one of the most commonly used techniques for exoplanet atmosphere characterisation. This technique has been used to detect ionised and neutral species in exoplanet atmospheres by comparing the observed stellar lines in and out of transit. The centre-to-limb variation (CLV) of the stellar lines across the stellar disk is an important effect for transmission spectroscopy, since it results in a change of stellar line depth when the planet transits different parts of the stellar disk. We reanalysed the transit data of HD 189733b taken with the HARPS spectrograph to study the CLV effect during transit. The transmission light curve of the Na i D line so obtained shows a clear imprint of the CLV effect. We used a one-dimensional non-LTE stellar spectral model to simulate the CLV effect. After applying the correction, the measurement of the Na i absorption in the atmosphere of HD 189733b becomes better determined. We compared the CLV effect of HD 189733b to that of HD 209458b. The CLV effects are different for these two benchmark planetary systems and this is attributed to their different stellar effective temperatures and transit impact parameters. We then explored the general CLV effect that occurs during exoplanet transits. Normally, a star with a lower effective temperature exhibits a stronger CLV effect and its CLV feature extends over a relatively broad wavelength range. The transit impact parameter (b) describes the transit trajectory on the stellar disk and thus determines the actual manifestation of the CLV effect. We introduced a b-diagram which describes the behaviour of the CLV effect as the function of different impact parameters. With improving observational precision, a careful modelling and correction of the CLV effect is necessary for exoplanet atmosphere characterisation using transit spectroscopy.
关键词： techniques: spectroscopic,stars: atmospheres,planets and satellites: atmospheres
更新于2025-09-04 15:30:14
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NLTE Stellar Population Synthesis of Globular Clusters Using Synthetic Integrated Light Spectra. II. Expanded Photometry and Sensitivity of Near-IR Spectral Features to Cluster Age and Metallicity
摘要： We present an expanded investigation of the library of globular cluster (GC) synthetic integrated light (IL) spectra of Young & Short, focusing on the impact of non-local thermodynamic equilibrium (NLTE) modeling effects on cluster parameters derived from photometric colors and sensitivity of near-IR spectral features to cluster age and metallicity. Johnson–Cousins–Bessel UBVIJK photometric colors are produced for 910 synthetic IL spectra with two degrees of α enhancement, in both NLTE and local thermodynamic equilibrium (LTE). These color values are used to investigate the GC age–metallicity degeneracy and compare NLTE and LTE derived [M/H] values for NGC 104, NGC 5139, and NGC 6205. For a given age, derived [M/H] values are shown to increase by up to 0.05 dex when modeled in NLTE. A total of 86 spectral lines in the range λ?=?12000–22000 ?, representing 14 different atomic species, were identi?ed as sensitive to either cluster age or metallicity, 12 of which were identi?ed as sensitive to both. Equivalent widths of the lines are measured in NLTE and LTE spectra, with NLTE effects changing the widths by up to 0.15–0.25 ? depending on the atomic species.
关键词： stars: fundamental parameters,globular clusters: general,techniques: photometric,stars: atmospheres,techniques: spectroscopic
更新于2025-09-04 15:30:14