Low Mass Stars and Brown Dwarfs in Praesepe

Presented are the results of a large and deep optical-near-infrared multi-epoch survey of the Praesepe open star cluster using data from the UKIDSS Galactic Clusters Survey. Multiple colour magnitude diagrams were used to select potential members and proper motions were used to assign levels of membership probability. From our sample, 145 objects were designated as high probability members (p >= 0.6) with most of these having been found by previous surveys although 14 new cluster members are also identified. Our membership assignment is restricted to the bright sample of objects (Z < 18). From the fainter sample, 39 candidates were found from an examination of multiple colour magnitude plots. Of these, 2 have small but significant membership probabilities. Finally, using theoretical models, cluster luminosity and mass functions were plotted with the later being fitted with a power law of alpha = 1.11 +/- 0.37 for the mass range 0.6 to 0.125 Msun and an assumed cluster age of 500 Myrs in the UKIDSS Z photometric band. Likewise taking an assumed cluster age of 1 Gyr we find alpha = 1.10 +/- 0.37. Similar values were also found for the J and K bands. These results compare favourably with the result of Kraus & Hillenbrand (2007) (alpha = 1.4 +/- 0.2) but are significantly lower than that of the more recent study conducted by Boudreault et al. (2009) (alpha = 1.8 +/- 0.1).Comment: 21 pages, 11 figures, 3 tables and 4 appendices. Accepted for publication in MNRAS, corrected a missing referenc

Brown dwarfs and very low mass stars in the Praesepe open cluster: a dynamically unevolved mass function?

[Abridged] In this paper, we present the results of a photometric survey to identify low mass and brown dwarf members of the old open cluster Praesepe (age of 590[+150][-120]Myr and distance of 190[+6.0][-5.8]pc) and use this to infer its mass function which we compare with that of other clusters. We have performed an optical (Ic-band) and near-infrared (J and Ks-band) photometric survey of Praesepe with a spatial coverage of 3.1deg^2. With 5sigma detection limits of Ic=23.4 and J=20.0, our survey is sensitive to objects with masses from about 0.6 to 0.05Msol. The mass function of Praesepe rises from 0.6Msol down to 0.1Msol and then turns-over at ~0.1Msol. The rise observed is in agreement with the mass function derived by previous studies, including a survey based on proper motion and photometry. Comparing our mass function with that for another open cluster with a similar age, the Hyades (age ~ 600Myr), we see a significant difference. Possible reasons are that dynamical evaporation has not influenced the Hyades and Praesepe in the same way, or that the clusters did not have the same initial mass function, or that dynamical interactions have modified the evolution of one or both clusters. Although a difference in the binary fractions of the clusters could cause the observed (i.e. system) mass functions to differ, measurements in the literature give no evidence for a significant difference in the binary fractions of the two clusters. Of our cluster candidates, six have masses predicted to be equal to or below the stellar/substellar boundary at 0.072Msol.Comment: 11 pages, 11 figures, accepted for publication in A&A. Higher resolution of Figures 2-3-4-5 in A&A published version. Revised version corrected for Englis

Composition and structure of the shallow subsurface of Ceres revealed by crater morphology

Before NASA’s Dawn mission, the dwarf planet Ceres was widely believed to contain a substantial ice-rich layer below its rocky surface. The existence of such a layer has significant implications for Ceres’s formation, evolution, and astrobiological potential. Ceres is warmer than icy worlds in the outer Solar System and, if its shallow subsurface is ice-rich, large impact craters are expected to be erased by viscous flow on short geologic timescales. Here we use digital terrain models derived from Dawn Framing Camera images to show that most of Ceres’s largest craters are several kilometres deep, and are therefore inconsistent with the existence of an ice-rich subsurface. We further show from numerical simulations that the absence of viscous relaxation over billion-year timescales implies a subsurface viscosity that is at least one thousand times greater than that of pure water ice. We conclude that Ceres’s shallow subsurface is no more than 30% to 40% ice by volume, with a mixture of rock, salts and/or clathrates accounting for the other 60% to 70%. However, several anomalously shallow craters are consistent with limited viscous relaxation and may indicate spatial variations in subsurface ice content