Metastable Helium Absorptions with 3D Hydrodynamics and Self-consistent Photochemistry. II. WASP-107b, Stellar Wind, Radiation Pressure, and Shear Instability
Lile Wang, Fei Dai
Abstract
Abstract This paper presents simulations of the metastable helium (He*) observations of WASP-107b, so far the highest signal-to-noise ratio detection that is confirmed by three different instruments. We employ full 3D hydrodynamics coupled with coevolving nonequilibrium thermochemistry and ray-tracing radiation, predicting mass-loss rates, temperature profiles, and synthetic He* line profiles and light curves from first principles. We find that a stellar wind stronger than solar is demanded by the observed heavily blueshifted line profile and asymmetric transit light curve. Radiation pressure can be important for Ly α observations, but not He*. Our model finds that WASP-107b is losing mass at a rate of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>≃</mml:mo> <mml:mn>1.0</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>9</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊕</mml:mo> </mml:mrow> </mml:msub> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi>yr</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> . Although <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:math> varies by <1% given constant wind and irradiation from the host, shear instabilities still emerge from wind impacts, producing ∼10% fluctuations of He* transit depths over hour-long timescales. The common assumption that He* transit depth indicates the fluctuation of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:math> is problematic. The trailing tail is more susceptible than planet adjacency to the shear instabilities; thus, the line profile is more variable in the blueshifted wing, while the transit light curve is more variable after midtransit. We stress that the synergy between Ly α (higher altitudes, lower density) and He* (lower altitudes, higher density) transit observations, particularly simultaneous ones, yields better understanding of planetary outflows and stellar wind properties.