  Stars in the Galactic disk are born on cold, nearly circular orbits with
small vertical excursions. After their birth, their orbits evolve, driven by
small- or large-scale perturbations in the Galactic disk's gravitational
potential. Here, we study the vertical motions of young stars over their first
few orbital periods, using a sample of OBA stars from \textit{Gaia} E/DR3,
which includes radial velocities and ages $\tau$ from LAMOST. We constructed a
parametric model for the time evolution of the stellar orbits' mean vertical
actions $J_z$ as a function of Galactocentric radius, $R_{\mathrm{GC}}$.
  Accounting for data uncertainties, we use Markov Chain Monte Carlo (MCMC)
analysis in annuli of Galactocentric radius to constrain the model parameters.
Our best-fit model shows a remarkably linear increase of vertical actions with
age across all Galactocentric radii examined. Orbital \textit{heating} by
random scattering could offer a straightforward interpretation for this trend.
However, various other dynamical aspects of the Galactic disk, such as stars
being born in a warped disk, might offer alternative explanations that could be
tested in the future.
