TKS III: A Stellar Obliquity Measurement of TOI-1726 c

Kavli Affiliate: Edward Morgan

| First 5 Authors: Fei Dai, Arpita Roy, Benjamin Fulton, Paul Robertson, Lea Hirsch

| Summary:

We report the measurement of a spectroscopic transit of TOI-1726 c, one of
two planets transiting a G-type star with $V$ = 6.9 in the Ursa Major Moving
Group ($sim$400 Myr). With a precise age constraint from cluster membership,
TOI-1726 provides a great opportunity to test various obliquity excitation
scenarios that operate on different timescales. By modeling the
Rossiter-McLaughlin (RM) effect, we derived a sky-projected obliquity of
$-1^{+35}_{-32}~^{circ}$. This result rules out a polar/retrograde orbit; and
is consistent with an aligned orbit for planet c. Considering the previously
reported, similarly prograde RM measurement of planet b and the transiting
nature of both planets, TOI-1726 tentatively conforms to the overall picture
that compact multi-transiting planetary systems tend to have coplanar, likely
aligned orbits. TOI-1726 is also a great atmospheric target for understanding
differential atmospheric loss of sub-Neptune planets (planet b 2.2 $R_oplus$
and c 2.7 $R_oplus$ both likely underwent photoevaporation). The coplanar
geometry points to a dynamically cold history of the system that simplifies any
future modeling of atmospheric escape.

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