Letter to the Editor

Mass, gas, and Gauss around a T Tauri Star with SPIRou

¹ Université de Toulouse, CNRS, IRAP, 14 avenue Belin, 31400 Toulouse, France
² Department of Earth Sciences, University of Hawai’i at Mānoa, Honolulu, Hawai’i 96822, USA
³ Institute for Astrophysics, University of Vienna, 1180 Vienna, Austria
⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden street, Cambridge, MA 02138, USA
⁵ Canada-France-Hawaii Telescope, 65-1238 Mamalahoa Hwy, Kamuela, HI 96743, USA
⁶ Department of Physics and Astronomy, University of North Carolina Chapel Hill, Chapel Hill, NC 27599, USA

^⋆ Corresponding author: jean-francois.donati@irap.omp.eu

Received: 18 March 2025
Accepted: 15 May 2025

Abstract

Studies of young planets help us understand planet evolution and investigate important evolutionary processes such as atmospheric escape. We monitored IRAS 04125+2902, a 3 Myr-old T Tauri star with a transiting planet and a transitional disk, with the SPIRou infrared spectropolarimeter on the Canada-France-Hawaii Telescope. Using these data, we constrained the mass and density of the Jupiter-size companion to < 0.16 M_♃ and < 0.23 g cm⁻³, respectively (90% upper limits). These rule out a Jovian-like object and support the hypothesis that it is an ancestor to the numerous sub-Neptunes found around mature stars. We unambiguously detected magnetic fields at the stellar surface, small-scale fields reaching 1.5 kG and the large-scale field mostly consisting of a 0.80−0.95 kG dipole inclined by 5−15° to the rotation axis. Accretion onto the star is low and/or episodic at a maximum rate of ≃10⁻¹¹ M_⊙ yr⁻¹, indicating that IRAS 04125+2902 is most likely in a magnetic “propeller” regime, presumably explaining the star’s slow rotation (11.3 d). We discovered persistent Doppler-shifted absorption in a metastable He I line, clear evidence for a magnetized wind from a gaseous inner disk. Variability in absorption suggests structure in the disk wind that could reflect disk-planet interactions.