TIC 441725813 spectrum and prewhitened frequencies

The light curve analysis reveals that frequencies are mostly found in the g-mode region, but several p-modes are also detected, indicating that TIC 441725813 is a hybrid sdB pulsator. We identify 25 frequencies that can be associated with l=1 g-modes, 15 frequencies corresponding to l=2 g-modes, and six frequencies characteristic of p-modes. Interestingly, several frequency multiplets interpreted as rotational splittings of deep-probing g-modes indicate a slow rotation period of at least 85.3+/-3.6 day, while splittings of mostly envelope-probing p-modes suggest a significantly shorter rotation period of 17.9+/-0.7 day, which implies the core (mainly the helium mantle with possibly the deeper partially mixed helium-burning core that it surrounds) rotates at least 4.7 times slower than the envelope.

The radial velocity curves indicate that TIC 441725813 is in a close binary system with a low-luminosity companion, possibly a white dwarf. While elusive in the available TESS photometry, a low-frequency signal that would correspond to a period of ~6.7 h is found, albeit at a low S/N. Furthermore, we estimate the inclination angle to be ~60{deg} by two independent means. TIC 441725813 is a particularly interesting sdB star whose envelope rotates faster than the core.

We hypothesise that this might be caused by the effects of a tidal interaction with a companion, although in the present case, the presence of such a companion will have to be further investigated. This analysis paves the way towards a more detailed seismic probing of TIC 441725813 using optimisation techniques, which will be presented in a second paper.