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Titolo: Multicolor variability of young stars across the mass spectrum
Abstract: Recently, space telescope missions have provided unprecedented depictions of the manifold variability behaviors typical of low-mass, young stars (YSOs), with presumed origins in starspots, accretion, and circumstellar obscuration. However, more massive YSOs (2 to 4-5 Msun) have been largely omitted from those studies, and the lack of coordinated, multiwavelength data often limited the physical understanding of the observed luminosity patterns. We have conducted a comprehensive variability survey of 278 B-to-K YSOs in the 1-2 Myr-old cluster NGC 6530. Our sample was monitored within Kepler/K2 Campaign 9, and simultaneous u,g,r,i,Halpha time series were acquired with VST/OmegaCAM. We employed this dataset, plus archival infrared photometry, to determine individual stellar parameters and disk statuses. By adopting well-established metrics for light curve classification, we found significant mass-dependent trends, with B/A stars displaying substantially lower variability than G/K stars for any given light curve morphology. We also detected a dearth of some distinctive disk-driven behaviors, particularly dippers, among stars earlier than G. This suggests their higher temperatures and more chaotic magnetic fields prevent the formation and survival of inner disk dust structures co-rotating with the star. Finally, we examined the characteristic timescales of variability for each light curve pattern, to pinpoint the source of specific signatures within the star-inner disk environment. We found that in each case the day-to-week timescales are predominant in the K2 time series, but these reflect distinct mechanisms, from intense accretion triggered by instabilities in the innermost disk regions (bursting behaviors) to variable accretion efficiency in the outer magnetosphere (stochastic behaviors).