This paper studies the dynamical evolution of young groups/clusters, with N ¼ 100 1000 members, from their embedded stage out to ages of $10 Myr. We use N-body simulations to explore how their evolution depends on the system size N and the initial conditions. Motivated by recent observations suggesting that stellar groups begin their evolution with subvirial speeds, this study compares subvirial starting states with virial starting states. Multiple realizations of equivalent cases (100 simulations per initial condition) are used to build up a robust statistical description of these systems, e.g., the probability distribution of closest approaches, the mass profiles, and the probability distribution for the radial location of cluster members. These results provide a framework from which to assess the effects of groups/clusters on the processes of star and planet formation and to study cluster evolution. The distributions of radial positions are used in conjunction with the probability distributions of the expected far-ultraviolet (FUV ) luminosities (calculated here as a function of cluster size N ) to determine the radiation exposure of circumstellar disks. The distributions of closest approaches are used in conjunction with scattering cross sections (calculated here as a function of stellar mass using $10 5 Monte Carlo scattering experiments) to determine the probability of disruption for newly formed solar systems. We use the nearby cluster NGC 1333 as a test case in this investigation. The main conclusion of this study is that clusters in this size range have only a modest effect on forming planetary systems. The interaction rates are low, so that the typical solar system experiences a single encounter with closest approach distance b $ 1000 AU. The radiation exposure is also low, with median FUV flux G 0 $ 900 (1.4 ergs s À1 cm À2 ), so that photoevaporation of circumstellar disks is only important beyond 30 AU. Given the low interaction rates and modest radiation levels, we suggest that solar system disruption is a rare event in these clusters.
We present results from an X-ray imaging survey of the young cluster NGC 2264, carried out with the European Photon Imaging Cameras (EPIC) onboard the XMM-Newton spacecraft. The X-ray data are merged with extant optical and near-infrared photometry, spectral classifications, Hα emission strengths, and rotation periods to examine the interrelationships between coronal and chromospheric activity, rotation, stellar mass, and internal structure for a statistically significant sample of pre-main sequence stars. A total of 300 distinct X-ray sources can be identified with optical or near-infrared counterparts. The sources are concentrated within three regions of the cluster: in the vicinity of S Mon, within the large emission/reflection nebulosity southwest of S Mon, and along the broad ridge of molecular gas that extends from the Cone Nebula to the NGC 2264 IRS 2 field. From the extinction-corrected color-magnitude diagram of the cluster, ages and masses for the optically-identified X-ray sources are derived. A median age of ∼2.5 Myr and an apparent age dispersion of ∼ 5 Myr are suggested by pre-main sequence evolutionary models. The X-ray luminosity of the detected sources appears well-correlated with bolometric luminosity, although there is considerable scatter in the relationship. Stellar mass contributes significantly to this dispersion while isochronal age and rotation do not. X-ray luminosity and mass are well correlated such that L X ∝ (M/M) 1.5 , similar to the relationship found within the younger Orion Nebula cluster. No strong evidence is found for a correlation between E H−K , the near-infrared color excess, and the fractional X-ray luminosity, which suggests that optically thick dust disks have little direct influence upon the observed X-ray activity levels. Among the X-ray detected weak-line T Tauri stars, the fractional X-ray luminosity, L X /L bol , is moderately well correlated with the fractional Hα luminosity, L Hα /L bol , but only at the 2σ level of significance. The cumulative distribution functions for the
This paper explores the stability of an Earth-like planet orbiting a solar-mass star in the presence of a stellar companion using ∼ 400, 000 numerical integrations. Given the chaotic nature of the systems being considered, we perform a statistical analysis of the ensuing dynamics for ∼ 500 orbital configurations defined by the following set of orbital parameters: the companion mass M C ; the companion eccentricity e; the companion periastron p; and the planet's inclination angle i relative to the stellar binary plane. Specifically, we generate a large sample of survival times (τ s ) for each orbital configuration through the numerical integration of N ≫ 1 equivalent experiments (e.g., with the same orbital parameters but randomly selected initial orbital phases). We then construct distributions of survival time using the variable µ s ≡ log τ s (where τ s is in years) for each orbital configuration. The primary objective of this work is twofold. First, we use the mean of the distributions to gain a better understanding of what orbital configurations, while unstable, have sufficiently long survival times to make them interesting to the study of planet habitability. Second, we calculate the width, skew, and kurtosis of each µ s distribution and look for general features that may aid further understanding and numerical exploration of these chaotic systems. To leading order, most distributions are nearly Gaussian with a width σ ∼ 0.5, although the longest-lived systems display substantial (non-Gaussian) tails. As a result, many independent realizations of these systems must be considered in order to characterize the survival time. The situation is more complicated for orbital configurations with longer mean survival times, owing in part to the increasing importance of resonances. Subject headings: planetary systems
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