Transmission spectroscopy of HAT-P-32b with the LBT: confirmation of clouds/hazes in the planetary atmosphere

Abstract

Spectroscopic observations of a transit event of an extrasolar planet offer the opportunity to study the composition of the planetary atmosphere. We observed a transit of the inflated Hot Jupiter HAT-P-32b with MODS at the LBT to characterize its atmosphere from 3300 to 10000 AA. A time series of target and reference star spectra was binned in two broad-band wavelength channels, from which differential transit light curves were constructed. These broad-band light curves were used to confirm previous transit parameter determinations. To derive the planetary transmission spectrum with a resolution of R ~ 60, we created a chromatic set of 62 narrow-band light curves with an average wavelength width of about 100 AA. The spectrum was corrected for the third-light of a near-by M star, whose spectrum was resolved in the individual exposures. Additionally, we undertook a photometric monitoring campaign of the host star to correct for the influence of starspots. The transmission spectrum of HAT-P-32b shows no pressure-broadened absorption features from Na and K, which is interpreted by the presence of clouds or hazes in the planetary atmosphere. This result is in agreement to previous studies on the same planet. The presence of TiO in gas phase could be ruled out. We find a 2.8 sigma indication of increased absorption in the line core of potassium (KI~7699 AA). No narrow absorption features of Na and Halpha were detected. Furthermore, tentative indications were found for a slope of increasing opacity toward blue wavelengths from the near-IR to the near-UV with an amplitude of two scale heights. If confirmed by follow-up observations, it can be explained by aerosols either causing Mie scattering or causing Rayleigh scattering with an aerosol - gas scale height ratio below unity.

11 pages, 9 figures, Accepted for publication in A&A, revised version including comments from the language editor

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