The Centre-To-Limb Variations of Solar Fraunhofer Lines Imprinted Upon Lunar Eclipse Spectra Implications for Exoplanet Transit Observations
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A&A 574, A94 (2015) Astronomy DOI: 10.1051/0004-6361/201425220 & c ESO 2015 Astrophysics The centre-to-limb variations of solar Fraunhofer lines imprinted upon lunar eclipse spectra Implications for exoplanet transit observations F. Yan1,2,3,R.A.E.Fosbury3, M. G. Petr-Gotzens3,G.Zhao1, and E. Pallé4,5 1 Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, 100012 Beijing, PR China e-mail: [feiy;gzhao]@nao.cas.cn 2 University of Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, 100049 Beijing, PR China 3 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany 4 Instituto de Astrofísica de Canarias, C/ vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain 5 Dpto. de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain Received 26 October 2014 / Accepted 23 December 2014 ABSTRACT The atmospheres of exoplanets are commonly studied by observing the transit of the planet passing in front of its parent star. The obscuration of part of the stellar disk during a transit will reveal aspects of its surface structure resulting from general centre-to-limb variations (CLVs). These become apparent when forming the ratio between the stellar light in and out of transit. These phenomena can be seen particularly clearly during the progress of a penumbral lunar eclipse, where the Earth transits the solar disk and masks different regions of the solar disk as the eclipse progresses. When inferring the properties of the planetary atmosphere, it is essential that this effect originating at the star is properly accounted for. Using the data observed from the 2014-April-15 lunar eclipse with the ESPaDOnS spectrograph mounted on the Canada France Hawaii Telescope (CFHT), we have obtained for the first time a time sequence of the penumbral spectra. These penumbral spectra enable us to study the centre-to-limb variations of solar Fraunhofer lines when the Earth is transiting Sun. The Na i and Ca ii absorption features reported from previous lunar eclipse observations are demonstrated to be CLV features, which dominate the corresponding line profiles and mask possible planetary signal. Detecting atmospheric species in exoplanets via transit spectroscopy must account for the CLV effect. Key words. planets and satellites: atmospheres – Earth – Moon – eclipses – stars: atmospheres 1. Introduction their atomic and molecular components is very likely to become possible with the next generation of large ground- and space- With the discovery of almost 2000 exoplanets in the last two based telescopes (Hedelt et al. 2013). The Earth itself can be decades, the characterisation of their atmospheres has become used as a benchmark for the future detection of Earth-like exo- a new and rapidly-evolving field. Different atomic, ionic, and planets, for example, using lunar eclipses to obtain the tangential molecular species have already been detected in exoplanet at- long-path transmission spectrum of the Earth’s atmosphere. mospheres. Using Hubble Space Telescope data, Charbonneau Several lunar eclipse observations have been made with the et al. (2002) detected sodium in the atmosphere of HD 209458b aim of obtaining the transmission spectrum of the Earth’s atmo- for the first time by comparing the Na i doublet absorption sphere. Some of these have reported the anomalous behaviour lines in and out of transit. Snellen et al. (2008) then confirmed of certain atomic absorption features. Pallé et al. (2009, here- the Na i absorption in HD 209458b using ground-based tele- after P09) observed the lunar eclipse of 16 August 2008, and scope data. Sodium has also been detected in several other obtained transmission spectra from the umbral lunar eclipse. In giant exoplanets such as HD 189733b (Redfield et al. 2008) P09s spectra, the Ca ii absorption features are detected and weak and WASP-17b (Wood et al. 2011). Potassium was detected in Na i absorptions also appear. Vidal-Madjar et al. (2010, hereafter HD 80606b (Colón et al. 2012) and XO-2b (Sing et al. 2011), V10) observed the same lunar eclipse as P09, but they retrieved while Fossati et al. (2010) reported the detection of Mg ii in the transmission spectrum from the penumbral rather than the WASP-12b. All of these used transmssion spectroscopy during umbral eclipse. In V10, the authors detected relatively strong transits. Molecular features have also been detected in exoplanet Na i D lines absorption but no Ca ii absorption. Arnold et al. atmospheres, such as H2O(Grillmair et al. 2008), CO (Brogi (2014, hereafter A14) observed the penumbral lunar eclipse in et al. 2012), CO2 (Swain et al. 2009), and CH4 (Swain et al. December 2010 and their results are similar to those of V10, i.e. 2008). Na i absorption is detected while Ca ii is not. Yan et al. (2014) In recent years, nearly 100 Earth-sized or smaller exoplan- observed the lunar eclipse of December 2011 and neither the ets have been discovered, one of them being in the habitable Na i nor the Ca ii absorption features are detected in the trans- zone of its host star (Quintana et al. 2014). Although characteris- mission spectrum obtained from the umbral eclipse. ing the atmospheres of these terrestrial exoplanets is not within The interpretation of these discrepancies is not straight- the reach of current instrumentation, the detection of some of forward since the Earth’s transmission spectra from these Article published by EDP Sciences A94, page 1 of 9 A&A 574, A94 (2015) observations are retrieved using different methods. Although there are Na i,Cai,andCaii layers in the Earth’s ionosphere, according to our research, the Na i or Ca ii absorption features in the observed Earth’s transmission spectra are probably due to the centre-to-limb variation (CLV) of the solar lines rather than the absorptions in the Earth’s atmosphere. The strong solar Fraunhofer lines have prominent variations in both line intensity and profile from centre to limb cross the solar disk. Athay et al. (1972)usedtheFei line’s CLV spec- trum to generally interpret the CLV effect. For most strong so- lar Fraunhofer lines, the normalised spectral line from the cen- tre disk is deeper than that from the limb. The observed CLV Fig. 1. A schematic of a penumbral eclipse as seen from the Moon. Here features are used to understand detailed solar physics and to Penumbra A and Penumbra B represent the views from two different guide the modelling work. For example, Allende Prieto et al. penumbral locations (indicated in Fig. 2). (2004) observed the CLV of solar lines and used this to test non- Local Thermodynamic Equilibrium line formation calculations and Koesterke et al. (2008) used the CLV data to test 3D solar hydrodynamic simulations. refracted. However, the line shape difference between the um- Since a lunar eclipse shares similarities with an exoplanet bral spectrum and the bright Moon spectrum is relatively small, transit, this CLV effect can also be present in transit spectra. and the penumbral spectrum exhibits significant differences (see The corresponding features will be considerably weaker since Fig. 3 for details). the planet only blocks a small fraction of the stellar surface dur- ing transit. However, the CLV effect can still have an influence 2.1. The April 2014 lunar eclipse observations on the interpretation of atomic line detections in exoplanet at- mospheres and should be properly treated. We used the fiber-fed ESPaDOnS spectrograph mounted on the For the detection of sodium in HD 209458b, Charbonneau i Canada France Hawaii Telescope (CFHT) for our observations. et al. (2002) considered the CLV in the Na D lines and con- The spectrograph covers a wavelength range of 370−1050 nm in cluded that the contribution was small for their observation. a single exposure. The “object only” mode was used to achieve ff i Redfield et al. (2008) modelled the CLV e ect on the Na D lines high spectral resolving power (average resolution: λ/Δλ ∼ ff in HD 189733b (the CLV is referred to as di erential limb dark- 81 000). The observation began when the Moon was in the ening in their paper), and the model shows that this contribution i Earth’s umbra and lasted until the Moon was fully out of the is much smaller than the observed Na absorption. However, for Earth’s shadow. We used non-sidereal tracking to locate the fiber ff stellar lines that have significant CLV e ects or if the observa- at the Eudoxus crater during the entire observation. The Eudoxus ff tion is performed at high spectral resolution, this e ect could crater is chosen because it enables us to access different phases become important. of the umbral eclipse as its trace is close to the umbral center. We observed the lunar eclipse in April 2014 and obtained a Figure 2 shows the trace of Eudoxus with respect to the Earth’s ff set of spectra at di erent stages of the eclipse. The changes in the shadow. We obtained a sequence of spectra from the umbra, solar line profiles caused by the CLV can clearly be seen in our penumbra, and bright Moon, and we performed the standard data data. In Sect. 2, we use our lunar eclipse data to demonstrate this reduction procedure using the CFHT Upena pipeline1. effect and compare it with the solar spectral Atlas. In Sect. 3, the i ii The penumbral spectral sequence consists of about 60 spec- Na and Ca features observed in previous lunar eclipse obser- tra from different penumbral locations, enabling us to study the vations are compared with the spectral features caused by CLV. ff line shape change.