September 2015 • 2015A&A...581A..66V
Context. Photometric variability is a distinctive feature of young stellar objects; exploring variability signatures at different wavelengths provides insight into the physical processes at work in these sources.
Aims: We explore the variability signatures at ultraviolet (UV) and optical wavelengths for several hundred accreting and non-accreting members of the star-forming region NGC 2264 (~3 Myr).
Methods: We performed simultaneous monitoring of u- and r-band variability for the cluster population with CFHT/MegaCam. The survey extended over two full weeks, with several flux measurements per observing night. A sample of about 750 young stars is probed in our study, homogeneously calibrated and reduced, with internally consistently derived stellar parameters. Objects span the mass range 0.1-2 M⊙; about 40% of them show evidence for active accretion based on various diagnostics (Hα, UV, and IR excesses).
Results: Statistically distinct variability properties are observed for accreting and non-accreting cluster members. The accretors exhibit a significantly higher level of variability than the non-accretors, in the optical and especially in the UV. The amount of u-band variability is found to correlate statistically with the median amount of UV excess in disk-bearing objects, which suggests that mass accretion and star-disk interaction are the main sources of variability in the u band. Spot models are applied to account for the amplitudes of variability of accreting and non-accreting members, which yields different results for each group. Cool magnetic spots, several hundred degrees colder than the stellar photosphere and covering from 5 to 30% of the stellar surface, appear to be the leading factor of variability for the non-accreting stars. In contrast, accretion spots with a temperature a few thousand degrees higher than the photospheric temperature and that extend over a few percent of the stellar surface best reproduce the variability of accreting objects. The color behavior is also found to be different between accreting and non-accreting stars. While objects commonly become redder when fainter, typical amplitudes of variability for accreting members rapidly increase from the r to the u band, which indicates a much stronger contrast at short wavelengths; a lower color dependence in the photometric amplitudes is instead measured for diskless stars. Finally, we compare the u-band variability monitored here on two-week timescales with that measured on both shorter (hours) and longer (years) timescales. We find that variability on timescales of hours is typically ~10% of the peak-to-peak variability on day timescales, while longer term variability on a timescale of years is consistent with amplitudes measured over weeks.
Conclusions: We conclude that for both accreting and non-accreting stars, the mid-term rotational modulation by hot and cold spots is the leading timescale for a variability of up to several years. In turn, this suggests that the accretion process is essentially stable over years, although it exhibits low-level shorter term variations in single accretion events.