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

TL;DR

This study uses SPIRou infrared spectropolarimetry to characterize a young T Tauri star and its planetary companion, revealing magnetic fields, a non-Jovian planet, and evidence of a magnetized disk wind, advancing understanding of early planetary evolution.

Contribution

First detailed magnetic and planetary characterization of IRAS 04125+2902 using infrared spectropolarimetry, revealing a non-Jovian planet and disk wind interactions in a young star system.

Findings

Planet mass <0.16 M_J, density <0.23 g/cm^3, supporting a sub-Neptune origin.

Detected stellar magnetic fields up to 1.5 kG and a dipole inclined by 5-15 degrees.

Evidence of a magnetized disk wind with variable absorption features.

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_J 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 detect 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 5-15deg to the rotation axis. Accretion onto the star is low and/or episodic at a maximum rate of ~10^{-11} Msun/yr, indicating that IRAS 04125+2902 is most likely in a magnetic 'propeller' regime, possibly maintaining the star's slow rotation (11.3~d). We discover 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.