Pleiades

Abstract: New measurements of lithium in more than 100 Pleiades F, G, and K dwarfs are reported. Abundances are determined from spectrum synthesis fits to the data as well as from use of new covers of growth from the Li 6708-A feature. It is argued that most Late-F and early-G dwarfs in the Pleiades are consistent with the tight N(Li) vs mass relation seen in the Hyades in the same mass range. Most Li-rich stars have abundances at or near the primordial level for Population I, and none exceed that level by a significant amount. At any given color the stars that rotate fast have the most Li and have the strongest chromospheric activity. Ways in which an apparent spread in N(Li) could arise from an intrinsically tight n(Li)-mass relation are considered, and it is concluded that the spread is probably real and is not an artifact of line formation conditions or inhomogeneous atmospheres on the stars.

Abstract: The availability of the Hipparcos Catalogue has triggered many kinematic and dynamical studies of the solar neighbourhood. Nevertheless, those studies generally lacked the third component of the space velocities, i.e., the radial velocities. This work presents the kinematic analysis of 5952 K and 739 M giants in the solar neighbourhood which includes for the first time radial velocity data from a large survey performed with the CORAVEL spectrovelocimeter. It also uses proper motions from the Tycho-2 catalogue, which are expected to be more accurate than the Hipparcos ones. An important by-product of this study is the observed fraction of only 5.7% of spectroscopic binaries among M giants as compared to 13.7% for K giants. After excluding the binaries for which no center-of-mass velocity could be estimated, 5311 K and 719 M giants remain in the final sample. The UV -plane constructed from these data for the stars with precise parallaxes (%) reveals a rich small-scale structure, with several clumps corresponding to the Hercules stream, the Sirius moving group, and the Hyades and Pleiades superclusters. A maximum-likelihood method, based on a Bayesian approach, has been applied to the data, in order to make full use of all the available stars (not only those with precise parallaxes) and to derive the kinematic properties of these subgroups. Isochrones in the Hertzsprung-Russell diagram reveal a very wide range of ages for stars belonging to these groups. These groups are most probably related to the dynamical perturbation by transient spiral waves (as recently modelled by De Simone et al. [CITE]) rather than to cluster remnants. A possible explanation for the presence of young group/clusters in the same area of the UV -plane is that they have been put there by the spiral wave associated with their formation, while the kinematics of the older stars of our sample has also been disturbed by the same wave. The emerging picture is thus one of dynamical streams pervading the solar neighbourhood and travelling in the Galaxy with similar space velocities. The term dynamical stream is more appropriate than the traditional term supercluster since it involves stars of different ages, not born at the same place nor at the same time. The position of those streams in the UV -plane is responsible for the vertex deviation of for the whole sample. Our study suggests that the vertex deviation for younger populations could have the same dynamical origin. The underlying velocity ellipsoid, extracted by the maximum-likelihood method after removal of the streams, is not centered on the value commonly accepted for the radial antisolar motion: it is centered on km s-1 . However, the full data set (including the various streams) does yield the usual value for the radial solar motion, when properly accounting for the biases inherent to this kind of analysis (namely, km s-1 ). This discrepancy clearly raises the essential question of how to derive the solar motion in the presence of dynamical perturbations altering the kinematics of the solar neighbourhood: does there exist in the solar neighbourhood a subset of stars having no net radial motion which can be used as a reference against which to measure the solar motion?

Abstract: Summary Direct N-body calculations are presented of the formation of Galactic clusters using GasEx, which is a variant of the code Nbody6. The calculations focus on the possible evolution of the Orion Nebula Cluster (ONC) by assuming that the embedded OB stars explosively drove out 2/3 of its mass in the form of gas about 0.4 Myr ago. A bound cluster forms readily and survives for 150 Myr despite additional mass loss from the large number of massive stars, and the Galactic tidal field. This is the very first time that cluster formation is obtained under such realistic conditions. The cluster contains about 1/3 of the initial 10 4 stars, and resembles the Pleiades Cluster to a remarkable degree, implying that an ONC-like cluster may have been a precursor of the Pleiades. This scenario predicts the present expansion velocity of the ONC, which will be measurable by upcoming astrometric space missions. These missions should also detect the original Pleiades members as an associated expanding young Galactic-field sub-population. The results arrived at here suggest that Galactic clusters form as the nuclei of expanding OB associations. The results have wide implications, also for the formation of globular clusters and the Galactic field and halo stellar populations. In view of this, the distribution of binary orbital periods and the mass function within and outside the model ONC and Pleiades is quantified, finding consistency with observational constraints. Advanced mass segregation is evident in one of the ONC models. The calculations show that the primordial binary population of both clusters could have been much the same as is observed in the Taurus–Auriga star forming region. The computations also demonstrate that the binary proportion of brown dwarfs is depleted significantly for all periods, whereas massive stars attain a high binary fraction.

Abstract: By combining a deep optical imaging (I; z 0 ) survey of 8 deg 2 in the Taurus star-forming region with data from the Two-Micron All-Sky Survey (2MASS) and follow-up spectroscopy, we have performed a search for low-mass Taurus members that is complete to 0.02 Mfor reddenings of AV d4. We report the discovery of nine new members with spectral types of M5.75-M9.5, corresponding to masses of 0.1-0.015 Mby recent evolutionary models. The new M9.5 member is the least massive brown dwarf found to date in the Taurus star-forming region. We derive an initial mass function (IMF) for the fields surveyed in this work and in our previous studies, which encompass 54% of the known Taurus membership. We compare the Taurus IMF with a similarly derived one for the Trapezium Cluster and to mass functions for the M35 and Pleiades open clusters. While the IMFs in all of these regions flatten near � 0.8 M� , the mass function in Taurus is more nar- row and sharply peaked at this mass. Our survey indicates that Taurus has � 2 � fewer brown dwarfs at 0.02- 0.08 Mthan the Trapezium. We discuss the implications of these results for theories of the IMF, and suggest that the lower frequency of brown dwarfs in Taurus relative to the Trapezium may result from the low-den- sity star-forming environment, leading to larger minimum Jeans masses. Subject headings: infrared: stars — stars: evolution — stars: formation — stars: low-mass, brown dwarfs — stars: luminosity function, mass function — stars: pre-main-sequence