Dynamics of star-disk interaction processes in young stars revealed by Kepler/K2

High-precision time series photometry provides a unique window into the dynamics of the inner disk regions (<1 AU) around young stars (<5-10 Myr). Exquisite surveys carried out with CoRoT and Kepler have revealed a huge variety of photometric behaviors for young stars with disks, driven by variable mass accretion, stellar magnetic activity, or rapidly evolving inner disk structures. Over the past few years, such surveys have allowed us to achieve a detailed picture of variability for 3-10 Myr old stars of K/M spectral type. Multiple classes of young star-disk variables were identified, each characterized by different degrees of (a)periodicity and (a)symmetry of the observed flux variations: periodic, quasi-periodic, stochastic, bursters, dippers. These observations point to at least two distinct paradigms of star-disk interaction: i) an unstable regime, characterized by stochastic, short-lived accretion bursts and rapidly evolving accretion features; ii) a stable regime, with ordered accretion streams from the inner disk to the star that persist over many rotational cycles. The dependence of these scenarios on stellar mass, age, and environment still remains to be explored. In this contribution, we present a new study on the time domain behavior of very young stars in the Lagoon Nebula. The region, monitored during K2 Campaign 9, hosts a younger (~2 Myr) and more massive population than other clusters investigated with CoRoT and Kepler. Auxiliary data, coordinated with the K2 observations, were obtained at different telescopes, including multi-band (u,g,r,i,Hα) ground-based photometry from the Vista Survey Telescope. We extracted light curves for around 300 Lagoon Nebula members, of spectral types between B and K, and with a disk fraction ~40%. We detect statistically distinct variability behaviors between early-type and later-type stars, with the former being characterized by comparable levels of variability to field stars, while the latter stand out clearly with respect to the field population in all variability indicators. The light curve morphology classes identified in previous surveys are also found among Lagoon Nebula stars, but with different occurrence rates and some indications of a mass dependence. Different light curve classes appear to be associated with different characteristic timescales of variability, suggesting a diverse physical origin of the corresponding variability signatures, which may arise from different locations in the star-inner disk environment.