Photometry and membership for low mass stars in the young open cluster NGC 2516

Abstract

We present the results of a 0.86 square degree CCD photometric survey of the open cluster NGC 2516, which has an age of about 150 Myr and may have a much lower metallicity than the similarly-aged Pleiades. Our BVIc survey of cluster members is complete to V ≃ 20 and is used to select a preliminary catalogue of 1254 low mass (0.2 < M < 2.0 M⊙) cluster candidates, of which about 70-80 percent are expected to be genuine. After applying corrections for contamination by non-members and adding data for higher mass stars from the literature, we investigate the cluster binarity, luminosity and mass function, mass segregation and total mass. We find a binary fraction of 26 ± 5 percent, for A to M-type systems with mass ratios between 0.6 and 1, which is very similar to the Pleiades. The mass function is metallicity and evolutionary-model dependent, but consistent with a Salpeter-like law (dN/d log M ∝ M-α, α = +1.47 ± 0.11 or α = +1.67 ± 0.11 for the solar and half-solar metallicity models of Siess et al. 2000, and α = +1.58 ± 0.10 for the solar metallicity models of D'Antona & Mazzitelli 1997), for 0.7 < M < 3.0 M⊙. At lower masses (0.3 < M < 0.7 M⊙) there is a sharp fall in the mass function, with α = -0.75 ± 0.20 or α = -0.49±0.13 (for the solar and half-solar metallicity models of Siess et al.), and α = -1.00 ± 0.18 (for the solar metallicity models of D'Antona & Mazzitelli). The true stellar mass function might have α values up to 0.4 larger if account were taken of low mass stars in unresolved binary systems with mass ratios less than 0.6. The falling mass function of NGC 2516 at lower masses seems inconsistent with the much flatter mass functions derived from comparable data in the Pleiades and field populations. This deficit of lower mass, fainter stars is also seen in the observed luminosity function. We rule out incompleteness as the cause of this discrepancy, but demonstrate that mass segregation is clearly present in NGC 2516, with more than half the low-mass (<0.6 M⊙) stars likely to lie outside our survey area, but the vast majority of high-mass (>1.5 M⊙) stars included. Taking this into account, it is probable that the whole-cluster mass functions for NGC 2516 and the Pleiades are similar down to 0.3 M⊙. The mass of NGC 2516 stars with M > 0.3 M⊙ inside our survey is 950-1200 M⊙, depending on metallicity and what corrections are applied for unresolved binarity. Correcting for mass segregation increases this to ∼1240-1560 M⊙, about twice the total mass of the Pleiades. If NGC 2516 and the Pleiades do have similar mass functions, then less massive stars and brown dwarfs contribute about a further 15 percent to the mass of NGC 2516 and we predict a cluster population of about 360-440 brown dwarfs with 0.030 < M < 0.075 M⊙.