Mapping accretion and its variability in the young open cluster NGC 2264: a study based onu-band photometry

Abstract

Context. The accretion process has a central role in the formation of stars and planets. Aims. We aim at characterizing the accretion properties of several hundred members of the star-forming cluster NGC 2264 (3 Myr).Methods. We performed a deep ugri mapping as well as a simultaneous u-band+r-band monitoring of the star-forming region with CFHT/MegaCam in order to directly probe the accretion process onto the star from UV excess measurements. Photometric properties and stellar parameters are determined homogeneously for about 750 monitored young objects, spanning the mass range ∼0.1-2 M . About 40% of the sample are classical (accreting) T Tauri stars, based on various diagnostics (H α , UV and IR excesses). The remaining non-accreting members define the (photospheric + chromospheric) reference UV emission level over which flux excess is detected and measured. Results. We revise the membership status of cluster members based on UV accretion signatures, and report a new population of 50 classical T Tauri star (CTTS) candidates. A large range of UV excess is measured for the CTTS population, varying from a few times 0.1 to ∼3 mag. We convert these values to accretion luminosities and accretion rates, via a phenomenological description of the accretion shock emission. We thus obtain mass accretion rates ranging from a few 10 −10 to ∼10 −7 M /yr. Taking into account a mass-dependent detection threshold for weakly accreting objects, we find a >6σ correlation between mass accretion rate and stellar mass. A power-law fit, properly accounting for censored data (upper limits), yieldsṀ acc ∝ M 1.4±0.3 * . At any given stellar mass, we find a large spread of accretion rates, extending over about 2 orders of magnitude. The monitoring of the UV excess on a timescale of a couple of weeks indicates that its variability typically amounts to 0.5 dex, i.e., much smaller than the observed spread in accretion rates. We suggest that a non-negligible age spread across the star-forming region may effectively contribute to the observed spread in accretion rates at a given mass. In addition, different accretion mechanisms (like, e.g., short-lived accretion bursts vs. more stable funnel-flow accretion) may be associated to differentṀ acc regimes. Conclusions. A huge variety of accretion properties is observed for young stellar objects in the NGC 2264 cluster. While a definite correlation seems to hold between mass accretion rate and stellar mass over the mass range probed here, the origin of the large intrinsic spread observed in mass accretion rates at any given mass remains to be explored.

Stellar models simulating the disk-locking mechanism and the evolutionary history of the Orion Nebula cluster and NGC 2264

“…Although u-band photometry data have been obtained on December 2010, some years after Δ[R c − H α ] data, they agree reasonably well as accretion diagnostics. In addition, all 170 objects of our sample that have α IRAC measurements are present in the u-band survey, and the two diagnostics agree (Venuti et al 2014). In this subsample, 89 stars have no u-band excess and α IRAC ≤ −2.56 (no dust in the inner disk), 40 objects do not show either u-band excess or an IRAC inner disk, and only 6 objects show u-band excess and do not have dusty inner disk.…”

“…15. Mass (left) and age (right) distributions for NGC 2264 stars in common in our sample and that of Venuti et al (2014). Solid curves refer to masses and ages obtained with our models, dotted curves refer to masses and ages obtained with the models of Siess et al (2000).…”

“…One source of the differences in mass and age estimates is the different evolutionary models used in each work. Venuti et al (2014) used models of Siess et al (2000), while we used the non-gray models described in Sect. 2, which are more suitable for p-MS than the gray atmospheric boundary conditions used by Siess et al (2000).…”

“…These IRAC data were obtained in two epochs, March and October 2004, and can help us to distinguish the stars with a disk from diskless objects. In addition, we investigated W λ (Hα), U − V and near-infrared excesses (I − K and H − K) by Rebull et al (2002), near-infrared excess and W λ (Hα) by Dahm & Simon (2005), mid-infrared excess by Cieza & Baliber (2007), and uband exccess by Venuti et al (2014). For disk identification purposes, we used the following values to indicate the presence of a disk: .15, [3.8 − 8.0] ≥ 0.7, and α IRAC > −2.56.…”

“…We now compare the u-band survey by Venuti et al (2014) with Hα-indexes and α IRAC measurements and with our disklocking criterion. Of the 405 stars of our sample, 256 objects have u-band photometric measurements, 59 objects have u-band excess (compatible with ongoing accretion), and 197 stars have no u-band excess.…”