Structure of the Praesepe Star Cluster

We have used the Two Micron All Sky Survey and Palomar Observatory Sky Survey photographic plates, digitized by the US Naval Observatory's Precision Measuring Machine program, to derive proper motions over a 100 square degree, spatially complete region centered on the Praesepe open cluster. Proper-motion measurements spanned the magnitude range R ∼ 12–19, which covers most of the lower main sequence in Praesepe. The incidence of Hα emission from moderate-resolution (∼2600), red spectroscopy of 434 faint candidate members and 126 field control stars demonstrates that ∼60%–80% of candidates within 2° of the cluster center are genuine Praesepe members. Spectral index ratios from TiO and CaH absorption features show a well-defined sequence with J magnitude for high-probability candidates, relative to that of field stars. The inferred membership data have allowed us to estimate Praesepe's mass function and general projected structure for stars with mass 0.1–1.0 M ⊙ . The mass function, when fitted with a power law of the form dN / dm ∝ m -α , rises (α ≈ 1.6) from 1.0 to 0.4 M ⊙ , where it distinctly becomes flat (α ≈ 0) down to 0.1 M ⊙ . The mass function also exhibits a marginal (∼1 σ) radial dependence, which, when compared with recent N -body simulations, suggests that the core is depleted of low-mass stars. The outer halo of Praesepe is diffuse and shows a slightly elliptical shape among the highest probability members. We place an upper limit on the surface density of escaped members at 2.2 stars deg -2 outside the limiting radius.