Origin of the 6.6-day activity period in TWA 7

Determine the physical origin of the 6.6-day periodic modulation observed in the equivalent widths of the 1083 nm He I infrared triplet and the 1282 nm Paβ line in the young M2 star TWA 7, given that this period is not seen in other activity proxies such as the longitudinal magnetic field Bℓ and the line-by-line differential temperature and yet appears clearly in spectroscopic diagnostics. Ascertain whether the modulation arises from star–planet interactions with a close-in companion, planetary rotation, or intrinsic stellar/magnetospheric variability.

Background

The authors analyzed SPIRou near-infrared spectra of TWA 7 and focused on chromospheric activity tracers, especially the 1083 nm He I infrared triplet and the 1282 nm Paβ line. They detected a coherent periodic modulation at about 6.6 days in the equivalent widths of these lines, which is distinct from the stellar rotation period (5.012 ± 0.007 d) measured from magnetic and temperature proxies.

This 6.6-day signal is clearly present in periodograms and stacked periodograms of the He I line and is also recovered in Paβ with a smaller amplitude, while other activity indicators (longitudinal field Bℓ and differential temperature dT) do not show the same periodicity. The authors note the correlation between He I and Paβ modulations and discuss possible interpretations, including star–planet interaction and planetary rotation, but emphasize that the physical origin of the signal is not yet established.