The Roasting Marshmallows Program with IGRINS on Gemini South. II. WASP-121 b has Superstellar C/O and Refractory-to-volatile Ratios
Peter Smith, Jorge Abril Sánchez, Michael R. Line, Emily Rauscher, Megan Mansfield, Eliza M.-R. Kempton, Arjun B. Savel, Joost P. Wardenier, Lorenzo Pino, Jacob L. Bean, Hayley Beltz, Vatsal Panwar, Matteo Brogi, Isaac Malsky, Jonathan J. Fortney, Jean-Michel Désert, Stefan Pelletier, Vivien Parmentier, Krishna Kanumalla, Luis Welbanks, Michael R. Meyer, John D. Monnier
Abstract
Abstract A primary goal of exoplanet science is to measure the atmospheric composition of gas giants in order to infer their formation and migration histories. Common diagnostics for planet formation are the atmospheric metallicity ([M/H]) and the carbon-to-oxygen (C/O) ratio as measured through transit or emission spectroscopy. The C/O ratio in particular can be used to approximately place a planet’s initial formation radius from the stellar host, but a given C/O ratio may not be unique to formation location. This degeneracy can be broken by combining measurements of both the C/O ratio and the atmospheric refractory-to-volatile ratio. We report the measurement of both quantities for the atmosphere of the canonical ultrahot Jupiter WASP-121 b using the high-resolution ( R = 45,000) IGRINS instrument on Gemini South. Probing the planet’s direct thermal emission in both pre- and post-secondary eclipse orbital phases, we infer that WASP-121 b has a significantly superstellar C/O ratio of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.70</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.07</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and a moderately superstellar refractory-to-volatile ratio at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>3.83</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.67</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>3.62</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> </mml:math> stellar. This combination is most consistent with formation between the soot line and H 2 O snow line, but we cannot rule out formation between the H 2 O and CO snow lines or beyond the CO snow line. We also measure velocity offsets between H 2 O, CO, and OH, potentially an effect of chemical inhomogeneity on the planet dayside. This study highlights the ability to measure both C/O and refractory-to-volatile ratios via high-resolution spectroscopy in the near-IR H and K bands.