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Spi-OPS: Spitzer and CHEOPS confirm the near-polar orbit of MASCARA-1 b and reveal a hint of dayside reflection


Spi-OPS: Spitzer and CHEOPS confirm the near-polar orbit of MASCARA-1 b and reveal a hint of dayside reflection Thumbnail



<jats:p><jats:italic>Context.</jats:italic> The light curves of tidally locked hot Jupiters transiting fast-rotating, early-type stars are a rich source of information about both the planet and star, with full-phase coverage enabling a detailed atmospheric characterisation of the planet. Although it is possible to determine the true spin–orbit angle ? – a notoriously difficult parameter to measure – from any transit asymmetry resulting from gravity darkening induced by the stellar rotation, the correlations that exist between the transit parameters have led to large disagreements in published values of ? for some systems.</jats:p> <jats:p><jats:italic>Aims.</jats:italic> We aimed to study these phenomena in the light curves of the ultra-hot Jupiter MASCARA-1 b, which is characteristically similar to well-studied contemporaries such as KELT-9 b and WASP-33 b.</jats:p> <jats:p><jats:italic>Methods.</jats:italic> We obtained optical CHaracterising ExOPlanet Satellite (CHEOPS) transit and occultation light curves of MASCARA-1 b, and analysed them jointly with a <jats:italic>Spitzer</jats:italic>/IRAC 4.5 µm full-phase curve to model the asymmetric transits, occultations, and phase-dependent flux modulation. For the latter, we employed a novel physics-driven approach to jointly fit the phase modulation by generating a single 2D temperature map and integrating it over the two bandpasses as a function of phase to account for the differing planet–star flux contrasts. The reflected light component was modelled using the general ab initio solution for a semi-infinite atmosphere.</jats:p> <jats:p><jats:italic>Results.</jats:italic> When fitting the CHEOPS and <jats:italic>Spitzer</jats:italic> transits together, the degeneracies are greatly diminished and return results consistent with previously published Doppler tomography. Placing priors informed by the tomography achieves even better precision, allowing a determination of ? = 72.1<jats:sub>-2.4</jats:sub><jats:sup>+2.5</jats:sup> deg. From the occultations and phase variations, we derived dayside and nightside temperatures of 3062<jats:sub>-68</jats:sub><jats:sup>+66</jats:sup> K and 1720 ± 330 K, respectively.Our retrieval suggests that the dayside emission spectrum closely follows that of a blackbody. As the CHEOPS occultation is too deep to be attributed to blackbody flux alone, we could separately derive geometric albedo A<jats:sub>g</jats:sub> = 0.171<jats:sub>-0.068</jats:sub><jats:sup>+0.066</jats:sup> and spherical albedo A<jats:sub>s</jats:sub> = 0.266<jats:sub>-0.100</jats:sub><jats:sup>+0.097</jats:sup> from the CHEOPS data, and Bond albedoA<jats:sub>B</jats:sub> = 0.057<jats:sub>-0.101</jats:sub><jats:sup>+0.083</jats:sup> from the <jats:italic>Spitzer</jats:italic> phase curve.Although small, the <jats:italic>A</jats:italic><jats:sub>g</jats:sub> and <jats:italic>A</jats:italic><jats:sub>s</jats:sub> indicate that MASCARA-1 b is more reflective than most other ultra-hot Jupiters, where H<jats:sup>-</jats:sup> absorption is expected to dominate.</jats:p> <jats:p><jats:italic>Conclusions.</jats:italic> Where possible, priors informed by Doppler tomography should be used when fitting transits of fast-rotating stars, though multi-colour photometry may also unlock an accurate measurement of ?. Our approach to modelling the phase variations at different wavelengths provides a template for how to separate thermal emission from reflected light in spectrally resolved <jats:italic>James Webb</jats:italic> Space Telescope phase curve data.</jats:p>


Maxted. (2022). Spi-OPS: Spitzer and CHEOPS confirm the near-polar orbit of MASCARA-1 b and reveal a hint of dayside reflection. Astronomy & Astrophysics,

Acceptance Date Sep 10, 2021
Publication Date Feb 1, 2022
Journal Astronomy & Astrophysics
Print ISSN 0004-6361
Publisher EDP Sciences
Publisher URL


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