Multicolor Variability of Young Stars in the Lagoon Nebula: Driving Causes and Intrinsic Timescales

High-precision time series photometry provides a unique window into the dynamics of the inner disk regions around young stars. Exquisite surveys with CoRoT and Kepler/K2 have revealed a huge variety of photometric behaviors for young stars with disks, from purely periodic to stochastic, bursting, or dipping. By exploring the color dependence of each variability pattern, it is possible to identify the underlying physical drivers: stellar magnetic activity, star-disk interaction, variable mass accretion, rapidly evolving inner disk structures. Here, we present a new study on the time domain behavior of very young stars in the Lagoon Nebula. The region, monitored with K2, hosts a very young (~2 Myr) population distributed over a wide mass range, including dozens of OB stars. These characteristics render the region an ideal target to probe the impact of stellar mass on the dynamics of young star-disk systems at the age when planets are born. From the K2 mosaic we extracted light curves for around 300 Lagoon Nebula members, of spectral types between B and K. For the same stars, we gathered auxiliary multi-band (u,g,r,i,Halpha) photometry from the VLT Survey Telescope, to investigate the nature of the observed variability signatures. Our study reveals a clear distinction between weakly variable higher-mass stars and strongly variable lower-mass stars, with disk-related variability signatures that tend to disappear at spectral types earlier than G. These findings point to distinct magnetic field properties, and to varying physical conditions in the inner disk, for young stars of different masses. Distinct characteristic timescales of variability are found for different light curve classes, suggesting a diverse origin of the corresponding variability signatures within the star-disk environment. However, all stars exhibit the largest amount of variability on rotational (days) timescales, suggesting an overall stability of the inner disk processes over many dynamical periods.
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