First exoplanet transit observation with the Stratospheric Observatory for Infrared Astronomy: confirmation of Rayleigh scattering in HD 189733 b with the High-Speed Imaging Photometer for Occultations

Abstract

Here we report on the first successful exoplanet transit observation with the Stratospheric Observatory for Infrared Astronomy (SOFIA). We observed a single transit of the hot Jupiter HD 189733 b, obtaining two simultaneous primary transit lightcurves in the B and z ′ bands as a demonstration of SOFIA's capability to perform absolute transit photometry. We present a detailed description of our data reduction, in particular the correlation of photometric systematics with various in-flight parameters unique to the airborne observing environment. The derived transit depths at B and z ′ wavelengths confirm a previously reported slope in the optical transmission spectrum of HD 189733 b. Our results give new insights to the current discussion about the source of this Rayleigh scattering in the upper atmosphere and the question of fixed limb darkening coefficients in fitting routines.As an airborne observatory, SOFIA has a number of potential advantages for precise time-domain spectrophotometric observations of transiting exoplanets. Ground-based observations are significantly affected by variations of absorption from telluric gases, in particular H 2 O, in the Earth's atmosphere, and these same gases are also the species of interest in exoplanet atmospheres; SOFIA can observe in important atmospheric windows not observable from the ground (14; 15; 16; 17).These are mostly the water bands but also CO, CH 4 , CO 2 are much better mixed and therefore reduce the temporal variation in these bands, which is a crucial point for time-series observations.

Exoplanet Research with the Stratospheric Observatory for Infrared Astronomy (SOFIA)

“…These two exoplanets presented scientifically compelling cases for preliminary observations, while also providing the opportunity for obtaining high-precision data required to pave the way for future observations of a wide range of exoplanet atmospheres with SOFIA/FLITECAM. Some parts of this program were observed and the results of this observation are published in Angerhausen et al (2015) and are summarized below.…”

“…In Angerhausen et al (2015), the resulting light-curves were then fitted using the Transit Analysis Package (TAP) (Gazak et al 2012) which is built on EXOFAST (Eastman et al 2013). In Angerhausen et al (2017) they used the 'Transit Light Curve Modeller (TCLM)' code (Csizmadia et al 2011(Csizmadia et al , 2015 and the Bayesian model selection, and parameter estimation code EXONEST Placek et al (2014); Placek and Knuth (2015); Placek et al (2015).…”

“…In Angerhausen et al (2015) they summarize this first successful exoplanet transit observation SOFIA as a demonstration of its capability to perform absolute transit photometry, i.e. observing transit light-curves without the need of a comparison star (see Figure 4).…”

“…Our B and z ′ data points reproduce the slope in the optical, most likely caused by Rayleigh scattering, though with an overall smaller transit depth. From Angerhausen et al (2015).…”

“…• bright host stars (like HD 189733b) -for which Angerhausen et al (2015) demonstrated the ability to perform absolute optical photometry • short transit durations in systems like Trappist-1 • science cases that leverage SOFIA's unique capability to observe IR and optical light curves simultaneously (such as starspot characterization) • science cases that complement James Webb Space Telescope (JWST) coverage and/or can be used for JWST target selection and support • transits that are rare and/or time-critical and require a dedicated deployment With the upcoming TESS (Transiting Exoplanet Survey Satellite) (Ricker et al 2014;Sullivan et al 2015) and PLATO (PLAnetary Transits and Oscillations of stars) (Rauer et al 2014; Hippke and Angerhausen 2015) missions we will see a lot more transiting exoplanets that fall into these categories.…”