Volume 570, October 2014
|Number of page(s)||24|
|Section||Stellar structure and evolution|
|Published online||21 October 2014|
Mapping accretion and its variability in the young open cluster NGC 2264: a study based on u-band photometry⋆,⋆⋆
Univ. Grenoble Alpes, IPAG,
2 CNRS, IPAG, 38000 Grenoble, France
3 Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
4 Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Palermo G.S. Vaiana, Piazza del Parlamento 1, 90134 Palermo, Italy
5 Departamento de Física – ICEx – UFMG, Av. Antônio Carlos, 6627, 30270-901 Belo Horizonte, MG, Brazil
6 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
7 Spitzer Science Center, California Institute of Technology, 1200 E California Blvd., Pasadena, CA 91125, USA
8 Universität Wien, Institut für Astrophysik, Türkenschanzstrasse 17, 1180 Vienna, Austria
9 Canada–France–Hawaii Telescope Corporation, 65-1238 Mamalahoa Highway, Kamuela, HI 96743, USA
Accepted: 29 July 2014
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 (Hα, 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), yields Ṁacc ∝ 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 different Ṁacc 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.
Key words: accretion, accretion disks / stars: formation / stars: low-mass / stars: pre-main sequence / ultraviolet: stars / open clusters and associations: individual: NGC 2264
Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii.
Full Tables 2–4 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/570/A82
© ESO, 2014
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