Kavli Affiliate: Avi Shporer
| First 5 Authors: Mayuko Mori, Akihiko Fukui, Teruyuki Hirano, Norio Narita, John H. Livingston
| Summary:
Spot-crossing transits offer a unique opportunity to probe spot properties
such as temperature, size, and surface distribution. TOI-3884 is a rare system
in which spot-crossing features are persistently observed during every transit.
This is due to its unusual configuration: a nearly poler orbit super-Neptune
transits a pole-on mid-M dwarf, repeatedly crossing a polar spot. However,
previous studies have reported discrepant values in key system parameters, such
as stellar inclination and obliquity. To address this, we conducted multi-band,
multi-epoch transit observations of TOI-3884b using the MuSCAT instrument
series, along with photometric monitoring with the LCO 1m telescopes/Sinistro.
We detected time-dependent variations in the spot-crossing signals, indicating
that the spot is not exactly on the pole. From the monitoring data, we measured
a stellar rotation period of $11.043~_{-0.053}^{+0.054}$ days with a modulation
amplitude of $sim$5% in the r-band, consistent with the time variability in
the spot-crossing features. Our analysis reconciles previous discrepancies and
improves the constraints on the parameters of the system geometry ($i_star =
139.9~^{+1.2}_{-2.0}$ deg and $lambda =41.0~^{+3.7}_{-9.0}$ deg) and those of
the spot properties (spot radius of $0.425~_{-0.011}^{+0.018}~R_star$ and
spot-photosphere temperature difference of $200~_{-9}^{+11}$ K). These results
provide a critical context for interpreting upcoming transmission spectroscopy
of TOI-3884b, as well as yielding new insights into the magnetic activity and
spin-orbit geometry of M dwarfs.
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