2015A&A...578A..98P

The variability of young stellar objects (YSO) changes their brightness and color preventing a proper classification in traditional color-color and color magnitude diagrams. We have explored the feasibility of the flux variation gradient (FVG) method for YSOs, using H and K band monitoring data of the star forming region RCW38 obtained at the University Observatory Bochum in Chile. Simultaneous multi-epoch flux measurements follow a linear relation F_H=α+β. F_K for almost all YSOs with large variability amplitude. The slope β gives the mean HK color temperature T_var of the varying component. Because T_var is hotter than the dust sublimation temperature, we have tentatively assigned it to stellar variations. If the gradient does not meet the origin of the flux-flux diagram, an additional non- or less-varying component may be required. If the variability amplitude is larger at the shorter wavelength, e.g. α<0, this component is cooler than the star (e.g. a circumstellar disk); vice versa, if α>0, the component is hotter like a scattering halo or even a companion star. We here present examples of two YSOs, where the HK FVG implies the presence of a circumstellar disk; this finding is consistent with additional data at J and L. One YSO shows a clear K-band excess in the JHK color-color diagram, while the significance of a K-excess in the other YSO depends on the measurement epoch. Disentangling the contributions of star and disk it turns out that the two YSOs have huge variability amplitudes (∼3-5mag). The HK FVG analysis is a powerful complementary tool to analyze the varying components of YSOs and worth further exploration of monitoring data at other wavelengths.