A spectral survey of an ultra-hot Jupiter

Abstract

Context. KELT-9 b exemplifies a newly emerging class of short-period gaseous exoplanets that tend to orbit hot, early type stars – termed ultra-hot Jupiters. The severe stellar irradiation heats their atmospheres to temperatures of ~4000 K, similar to temperatures of photospheres of dwarf stars. Due to the absence of aerosols and complex molecular chemistry at such temperatures, these planets offer the potential of detailed chemical characterization through transit and day-side spectroscopy. Detailed studies of their chemical inventories may provide crucial constraints on their formation process(es) and evolution history. Aims. We aim to search the optical transmission spectrum of KELT-9 b for absorption lines by metals using the cross-correlation technique. Methods. We analysed two transit observations obtained with the HARPS-N spectrograph. We used an isothermal equilibrium chemistry model to predict the transmission spectrum for each of the neutral and singly ionized atoms with atomic numbers between three and 78. Of these, we identified the elements that are expected to have spectral lines in the visible wavelength range and used those as cross-correlation templates. Results. We detect (>5σ) absorption by Na I, Cr II, Sc II and Y II, and confirm previous detections of Mg I, Fe I, Fe II, and Ti II. In addition, we find evidence of Ca I, Cr I, Co I, and Sr II that will require further observations to verify. The detected absorption lines are significantly deeper than predicted by our model, suggesting that the material is transported to higher altitudes where the density is enhanced compared to a hydrostatic profile, and that the material is part of an extended or outflowing envelope. There appears to be no significant blue-shift of the absorption spectrum due to a net day-to-night side wind. In particular, the strong Fe II feature is shifted by 0.18 ± 0.27 km s−1, consistent with zero. Using the orbital velocity of the planet we derive revised masses and radii of the star and the planet: M* = 1.978 ± 0.023 M⊙, R* = 2.178 ± 0.011 R⊙, mp = 2.44 ± 0.70 MJ and Rp = 1.783 ± 0.009 RJ.

Detection of Fe I and Fe II in the atmosphere of MASCARA-2b using a cross-correlation method

“…Fe I and Fe II have been previously detected in the atmosphere of KELT-9b by Hoeijmakers et al (2018Hoeijmakers et al ( , 2019 using the cross-correlation method and by Cauley et al (2019) using transmission spectroscopy. KELT-9b, however, has an extremely high day-side equilibrium temperature (T eq = 3920 K), while MASCARA-2b's is much lower (T eq = 2260 K).…”

“…Chemical composition studies of the hottest planet known to date, KELT-9b (Gaudi et al 2017), reveal an extended hydrogen atmosphere (Balmer Hα line, Yan & Henning (2018)). Moreover, Hoeijmakers et al (2018Hoeijmakers et al ( , 2019 reported the detection of Cr ii, Fe i, Fe ii, Mg ii, Na i, Sc ii, Ti ii and Y ii, as well as evidence of Ca i, Cr i, Co i and Sr ii in its atmosphere. Additionally Cauley et al (2019) detected in this planet H β and Mg i triplet.…”

Detection of neutral atomic species in the ultra-hot Jupiter WASP-121b

“…We can search for these species by cross-correlating with a model transmission spectrum, which stacks the signal from all of the absorption lines. This approach has been used successfully in the optical regime (Nugroho et al 2017;Hoeijmakers et al 2018Hoeijmakers et al , 2019, as well as the near-infrared (Snellen et al 2010;Brogi et al 2012;Rodler et al 2013;Lockwood et al 2014;Birkby et al 2013;Piskorz et al 2016;Birkby et al 2017;Brogi et al 2018;Hawker et al 2018). We use the X-COR pipeline, which was previously used to detect CO, H 2 O, and HCN in the dayside atmospheres of hot Jupiters (Hawker et al 2018;Cabot et al 2019).…”

“…The cross-correlation approach (Snellen et al 2010) involves comparing observed spectra with a model template of a species in order to detect forests of weak absorption lines. Molecular, atomic and ionized species have been detected this way (Snellen et al 2010;Birkby 2018;Alonso-Floriano et al 2019;Hoeijmakers et al 2019). Strong features, such as the Na doublet and Balmer lines can be directly recovered by co-adding multiple in-transit spectra (Wyttenbach et al 2015;Casasayas-Barris et al 2019).…”

HST/STIS transmission spectrum of the ultra-hot Jupiter WASP-76 b confirms the presence of sodium in its atmosphere

“…For example, WASP-12 b shows a Rayleigh slope with an amplitude of ∼2 scale heights (Stevenson et al 2014). Hoeijmakers et al (2019) found heavy metals in one of the hottest exoplanets known to date, KELT-9 b. The controversial WASP-19 b shows either titanium oxide (Sedaghati et al 2017) or a flat spectrum (Espinoza et al 2019).…”