The GTC exoplanet transit spectroscopy survey

Abstract

We observed the hot Jupiter HAT-P-32b (also known as HAT-P-32Ab) to determine its optical transmission spectrum by measuring the wavelength-dependent planet-to-star radius ratios in the region between 518 -918 nm. We used the OSIRIS instrument at the GTC in long slit spectroscopy mode, placing HAT-P-32 and a reference star in the same slit and obtaining a time series of spectra covering two transit events. Using the best quality data set, we were able to yield 20 narrow-band transit light curves, with each passband spanning a 20 nm wide interval. After removal of all systematic noise signals and light curve modeling the uncertainties for the resulting radius ratios lie between 337 and 972 ppm. The radius ratios show little variation with wavelength suggesting a high altitude cloud layer masking any atmospheric features. Alternatively, a strong depletion in alkali metals or a much smaller than expected planetary atmospheric scale height could be responsible for the lack of atmospheric features. Our result of a flat transmission spectrum is consistent with a previous ground-based study of the optical spectrum of this planet. This agreement between independent results demonstrates that ground-based measurements of exoplanet atmospheres can give reliable and reproducible results despite the fact that the data often is heavily affected by systematic noise, as long as the noise source is well understood and properly corrected. We also extract an optical spectrum of the M-dwarf companion HAT-P-32B. Using PHOENIX stellar atmosphere models we determine an effective temperature of T eff = 3187 +60 −71 K, slightly colder than previous studies relying only on broadband infra-red data.

Near-IR Transmission Spectrum of HAT-P-32b using HST/WFC3

“…Previous ground-based observations of the transit of HAT-P-32b in the optical wavelengths (Gibson et al 2013;Zhao et al 2014;Mallonn & Strassmeier 2016;Nortmann et al 2016) did not find evidence of spectral modulations due to molecules. Our cloud top pressure is consistent with their measurements within 1σ, hence the water detection in the infrared is not controversial.…”

“…HAT-P-32b is one of the most inflated exoplanets discovered, being less massive than Jupiter (M M 0.79 p J u p = ) but having almost twice its radius (R R 1.789 p J u p = ). The atmosphere of HAT-P-32b has been observed with ground-based instruments in the optical wavelengths, revealing a featureless transmission spectrum (Gibson et al 2013;Zhao et al 2014;Mallonn & Strassmeier 2016;Nortmann et al 2016). In addition, Zhao et al (2014) suggested the presence of a thermal inversion in the atmosphere of HAT-P-32bto interpret eclipse observations.…”

The GTC exoplanet transit spectroscopy survey

“…As in Nortmann et al (2016), the Bayesian Information Criterion (BIC) was used to select the best light curve model. The BIC penalizes a high χ 2 value and/or the number of free parameters used in the fitting procedure; the model that presents the lowest BIC is the one selected.…”

“…All the tested models present a synthetic transit model with a time dependency to reproduce a second-order color effect not corrected by the differential photometry, and a FWHM dependent polynomial to model the systematic effects produced by seeing variations. The FWHM correction was introduced because seeing variations affected the number of pixels in which the major part of the stellar flux is contained, and this pixel-dependent effect will act as a systematic correlated with FWHM (see Nortmann et al 2016;Chen et al 2017). With a slit 40 arcsec in width, the flux losses produced by not being able to measure the flux contained in the wings of the spectral profile are negligible.…”

“…The principle of transmission spectroscopy is to measure the change in the planetary radius at different wavelengths during a transit event, and to use this change to infer the presence of particular components of the exoplanet atmosphere. Because of their relatively big atmospheric scale heights, hot Jupiters are excellent targets for atmospheric detection from the ground and they are the main targets of our survey (e.g., Murgas et al 2014;Parviainen et al 2016;Pallé et al 2016;Nortmann et al 2016;Chen et al 2017).…”