Zodiacal Exoplanets in Time (ZEIT) XIII: Planet Orbits and Atmospheres
  in the V1298 Tau System, a Keystone in Studies of Early Planetary Evolution
  Studies of planetary systems of stars in star-forming regions and young
clusters open a window on the formative stages of planetary evolution. We
obtained high-cadence high-resolution infrared spectroscopy of the solar-mass
Taurus association-member V1298 Tau during a transit of its 10R$_{\oplus}$-size
"b" planet. We measured the systemic radial velocity and find that the
kinematics of V1298 Tau suggest an affiliation with a $\gtrsim$6 Myr-old
subgroup. A comparison of V1298 Tau and the nearby, co-moving star 2M0405 with
stellar evolution models suggests an age of $\sim$10-25 Myr. We measured the
projected spin-orbit angle of "b" as $\lambda=15_{-16}^{+15}$ and $\lambda =
2_{-4}^{+12}$ degrees using the apparent RV shift and change in line profile,
respectively, induced by the transient occultation of the rotating star by the
planet. These values indicate a prograde orbit like that of the interior "c"
planet of V1298 Tau and point to a co-planar multi-planet system that formed
within a disk. We also measured variation in the strength of the 1083 nm
triplet of neutral orthohelium as a probe of any extended/escaping atmosphere
around "b". We detect a steady decrease in absorption over the transit which
appears to arise from the star or its planetary system. While this variation
could be ascribed to "b" or possibly to the immediately preceding transit of
"d", we cannot rule out that this is due to rapid variation in the stellar
disk-integrated flux in the triplet. The amplitude of variation ($\sim$0.04 nm)
is consistent with moderate estimates of atmospheric escape driven by XUV
radiation from the central star.
