Protostars and Planets VI, Heidelberg, July 15-20, 2013

Poster 2S011

UV variability and accretion in PMS stars in NGC 2264

Venuti, Laura (Institut de Planétologie et d\'Astrophysique de Grenoble)
Bouvier, Jerome (Institut de Planétologie et d\'Astrophysique de Grenoble)
Irwin, Jonathan (Harvard-Smithsonian Center for Astrophysics)
Stauffer, John (Spitzer Science Center)
Cody, Ann Marie (Spitzer Science Center)
Micela, Giuseppina (INAF/Osservatorio Astronomico di Palermo)
CSI 2264 Team, ()

We present the results of an extensive u-band variability survey of the PMS population of the young open cluster NGC 2264 (3 Myr), performed at CFHT/MegaCam (1 sq. degree FOV) as a part of a wide project of simultaneous multi-wavelength (X-rays to IR) monitoring aimed at unambiguously characterizing YSO variability (the Coordinated Synoptic Investigation of NGC 2264, P.I. = J. Stauffer and G. Micela). The region has been imaged repeatedly over two full weeks in the u-band and r-band, with several measurements (temporal cadence ~1.5h) for each observing night; this monitoring survey is complemented by a deep u,g,r,i mapping of the region. We surveyed a population of >700 young stars, ranging in mass from 0.2 to 2 M_Sun and seen at different evolutionary stages (around half of the objects in the sample are actively accreting). The u-band observations offer the only direct probe to the excess luminosity emitted in the accretion shock, hence providing a unique clue to the accretion dynamics throughout the region. Combining the u-band with g-, r- and i-, we are able to investigate the photometric properties of different stellar groups on various color-color and color-magnitude diagrams and infer a straightforward identification of accreting sources thanks to the UV excess they display compared to the colors of non-accreting young stars. We investigate the u-band variability of PMS stars on week timescales and show that accreting stars (showing in many cases a large, variable UV excess) exhibit significantly higher levels of variability than non-accreting stars. Based on the light curve morphology, we identify three main classes of variables among the PMS population of NGC 2264, each dominated by a different physical component (cool magnetic spots, accretion bursts, disk occultation); we characterize the u-r color variations corresponding to the r-band magnitude variations for each different subgroup and demonstrate that well distinct color behaviors are specific to processes of different nature. From the UV excess diagnostic, we yield a straightforward characterization of mass accretion rates in NGC 2264. The distribution we infer traces an increasing trend in the average accretion rate with stellar mass; a large spread in the values of accretion rate, covering ~2 dex, is detected at each mass. Rates as large as 1E-7 M_Sun/yr and as small as 1E-10 M_Sun/yr are measured among the accreting population of the region. We investigate the variability of mass accretion rates on week timescales (accounting for both the geometric effects linked with stellar rotation and the intrinsic variability), by measuring the individual mass accretion rates at ~16 observing epochs distributed along the 2 week long CFHT monitoring; we find that this variability amounts on average to ~0.5 dex, i.e. much smaller than the spread observed in the mass accretion rates at each mass.

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