Atmospheric Mass Loss from TOI-1259 A b, a Gas Giant Planet With a White Dwarf Companion

The lack of close-in Neptune-mass exoplanets evident in transit surveys has largely been attributed either to photoevaporative mass loss or high-eccentricity migration. To distinguish between these two possibilities, we investigate the origins of TOI-1259 A b, a Saturn-mass (0.4 MJ, 1.0 RJ) exoplanet lying along the upper edge of the Neptune desert. TOI-1259 A b's close-in (P = 3.48 days) orbit and low bulk density make the planet particularly vulnerable to photoevaporation. Using transits observed in the 1083 nm metastable helium line, we probe the upper atmosphere of TOI-1259 A b with the Hale Telescope at Palomar Observatory and the Near-Infrared Spectrograph on Keck II. We report an excess absorption of 0.395±0.072% with Palomar and a blueshifted absorption of 2.4±0.52% with Keck, consistent with an extended escaping atmosphere. Fitting this signal with a Parker wind model, we determine a corresponding atmospheric mass loss rate of log(Ṁ) = 10.33± 0.13 g/s for a thermosphere temperature of 8400⁺¹²⁰⁰₋₁₀₀₀ K based on the Palomar absorption and log(Ṁ) = 10.0± 0.1 g/s for a thermosphere temperature of 8200⁺¹⁰⁰⁰₋₉₀₀ K based on the Keck absorption. This relatively low rate suggests that this planet would not have been significantly altered by mass loss even if it formed in-situ. However, the presence of a white dwarf companion, TOI-1259 B, hints that this planet may not have formed close-in, but rather migrated inward relatively late. Given the estimated parameters of the proto-white dwarf companion, we find that high-eccentricity migration is possible for the system.