We present the results of a deep optical 2.6 deg² survey with near-infrared (NIR) follow-up measurements of the intermediate-aged Praesepe open cluster. The survey is complete to I_c = 21.3, Z = 20.5, corresponding to M ∼ 0.06 M⊙ assuming a cluster age of 0.5 Gyr. Using three to five passbands to constrain cluster membership, we identify 32 new low-mass cluster members, at least four of which are likely to be substellar. We use the low-mass census to trace the region where the sequence moves away from the NEXTGEN towards the DUSTY regime at T_eff = 2200 K. In doing so, we identify four unresolved binaries, yielding a substellar binary fraction (BF) of ∼30 per cent. The BFs appear to decrease below 0.1 M⊙, in contrast to the rising fractions found in the Pleiades. We also identify a paucity of late M dwarfs, thought to be due to a steepening in the mass–luminosity relation at these spectral types, and compare the properties of this gap in the sequence to those observed in younger clusters. We note an overdensity of faint sources in the region of the so-called subcluster (possibly an older smaller cluster within Praesepe), and subsequently derive the luminosity and mass functions (MFs) for the main Praesepe cluster, revealing a turnover near the substellar boundary. We conclude by presenting astrometric measurements for low-mass Praesepe candidates from the literature and rule out as a likely foreground dwarf RPr1, hitherto thought to be a substellar member.

Item Type:	Refereed Article
Keywords:	stars: low-mass, brown dwarfs; stars: luminosity function, mass function; open clusters and associations: individual: Praesepe
Research Division:	Physical Sciences
Research Group:	Astronomical and Space Sciences
Research Field:	Stellar Astronomy and Planetary Systems
Objective Division:	Expanding Knowledge
Objective Group:	Expanding Knowledge
Objective Field:	Expanding Knowledge in the Physical Sciences
Author:	Dobbie, PD (Dr Paul Dobbie)
ID Code:	81106
Year Published:	2005
Web of Science® Times Cited:	23
Deposited By:	Mathematics and Physics
Deposited On:	2012-11-22
Last Modified:	2013-06-26
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