A dynamical study of Galactic globular clusters under different relaxation conditions

We perform a systematic combined photometric and kinematic analysis of a sample of globular clusters under different relaxation conditions, based on their core relaxation time (as listed in available catalogs), by means of two well-known families of spherical stellar dynamical models. Systems characterized by shorter relaxation time scales are expected to be better described by isotropic King models, while less relaxed systems might be interpreted by means of non-truncated, radially-biased anisotropic f^(\nu) models, originally designed to represent stellar systems produced by a violent relaxation formation process and applied here for the first time to the study of globular clusters. The comparison between dynamical models and observations is performed by fitting simultaneously surface brightness and velocity dispersion profiles. For each globular cluster, the best-fit model in each family is identified, along with a full error analysis on the relevant parameters. Detailed structural properties and mass-to-light ratios are also explicitly derived. We find that King models usually offer a good representation of the observed photometric profiles, but often lead to less satisfactory fits to the kinematic profiles, independently of the relaxation condition of the systems. For some less relaxed clusters, f^(\nu) models provide a good description of both observed profiles. Some derived structural characteristics, such as the total mass or the half-mass radius, turn out to be significantly model-dependent. The analysis confirms that, to answer some important dynamical questions that bear on the formation and evolution of globular clusters, it would be highly desirable to acquire larger numbers of accurate kinematic data-points, well distributed over the cluster field.Comment: 18 pages, 7 figures. Accepted for publication in Astronomy & Astrophysic

Rotating Globular Clusters

Internal rotation is considered to play a major role in the dynamics of some globular clusters. However, in only few cases it has been studied by quantitative application of realistic and physically justified global models. Here we present a dynamical analysis of the photometry and three-dimensional kinematics of omega Cen, 47 Tuc, and M15, by means of a recently introduced family of self-consistent axisymmetric rotating models. The three clusters, characterized by different relaxation conditions, show evidence of differential rotation and deviations from sphericity. The combination of line-of-sight velocities and proper motions allows us to determine their internal dynamics, predict their morphology, and estimate their dynamical distance. The well-relaxed cluster 47 Tuc is very well interpreted by our model; internal rotation is found to explain the observed morphology. For M15, we provide a global model in good agreement with the data, including the central behavior of the rotation profile and the shape of the ellipticity profile. For the partially relaxed cluster omega Cen, the selected model reproduces the complex three-dimensional kinematics; in particular the observed anisotropy profile, characterized by a transition from isotropy, to weakly-radial anisotropy, and then to tangential anisotropy in the outer parts. The discrepancy found for the steep central gradient in the observed line-of-sight velocity dispersion profile and for the ellipticity profile is ascribed to the condition of only partial relaxation of this cluster and the interplay between rotation and radial anisotropy.Comment: 19 pages, 14 figures, accepted for publication in the Astrophysical Journa

Biases in the determination of dynamical parameters of star clusters: today and in the Gaia era

The structural and dynamical properties of star clusters are generally derived by means of the comparison between steady-state analytic models and the available observables. With the aim of studying the biases of this approach, we fitted different analytic models to simulated observations obtained from a suite of direct N-body simulations of star clusters in different stages of their evolution and under different levels of tidal stress to derive mass, mass function and degree of anisotropy. We find that masses can be under/over-estimated up to 50% depending on the degree of relaxation reached by the cluster, the available range of observed masses and distances of radial velocity measures from the cluster center and the strength of the tidal field. The mass function slope appears to be better constrainable and less sensitive to model inadequacies unless strongly dynamically evolved clusters and a non-optimal location of the measured luminosity function are considered. The degree and the characteristics of the anisotropy developed in the N-body simulations are not adequately reproduced by popular analytic models and can be detected only if accurate proper motions are available. We show how to reduce the uncertainties in the mass, mass-function and anisotropy estimation and provide predictions for the improvements expected when Gaia proper motions will be available in the near future.Comment: 14 pages, 8 figures, accepted for publication by MNRA

Modelling Molecular Motors as Folding-Unfolding Cycles

We propose a model for motor proteins based on a hierarchical Hamiltonian that we have previously introduced to describe protein folding. The proposed motor model has high efficiency and is consistent with a linear load-velocity response. The main improvement with respect to previous models is that this description suggests a connection between folding and function of allosteric proteins.Comment: 5 pages RevTeX, 2 Postscript figures, replaced due to LaTeX proble

RESPONSE OF STRAWBERRY PLANTS TO SHORTENING DAY LENGTH , SHADING AND COLD STORAGE UNDER EGYPTIAN CONDITIONS

This experiment was performed in 2004/2005 and 2005/2006 seasons at El-Bosaly Protected Cultivation Experimental Site, of the Central Laboratory for Agricultural Climate (CLAC), 15 Km west of Rosetta. The current study was conducted in open field to investigate the growth and productivity of strawberry plants under the shading and cold storage methods. Two strawberry cultivars were used, i.e., Camarosa (cv.) and Yael (cv.). Transplants were set up on 15 of March to 15 of September in both two seasons of 2004-2005 and 2005-2006, respectively under El-Bosaly conditions. Seven treatments were used i.e., short day, 40% shading, 73% shading, three periods cold storage at 5Co, i.e., (24h, 48h and 73h) and control. Results indicated that the application of 73% shading treatment was the most effective in reducing maximum and minimum of air temperature and radiation, followed by 40% shading, whereas, the highest value of maximum and minimum air temperature, radiation were observed in control treatment throughout the two growing seasons. In addition, 73% shading treatment produced the tallest plants and the highest number of leaves per plant and leaf area per plant. Moreover, flowering and chlorophyll in leaves were increased by application of short day treatment. Also, fruit characters such as average weight and number of fruits per plant were the highest values with 40% shading. Chemical components of fruits, i.e., T.S.S were increased with 73% shading treatment. When plants were grown under 40% shading treatment, followed by 73% shading, while cold storage for 72 h treatment gave the lowest values. Neither cultivars nor the interaction between cultivars and tested factors had any significant effects on many studied characters

Statistical mechanics of base stacking and pairing in DNA melting

We propose a statistical mechanics model for DNA melting in which base stacking and pairing are explicitly introduced as distinct degrees of freedom. Unlike previous approaches, this model describes thermal denaturation of DNA secondary structure in the whole experimentally accessible temperature range. Base pairing is described through a zipper model, base stacking through an Ising model. We present experimental data on the unstacking transition, obtained exploiting the observation that at moderately low pH this transition is moved down to experimentally accessible temperatures. These measurements confirm that the Ising model approach is indeed a good description of base stacking. On the other hand, comparison with the experiments points to the limitations of the simple zipper model description of base pairing.Comment: 13 pages with figure

OBSERVATIONAL TESTS ON THE DYNAMICS OF GLOBULAR CLUSTERS

Galactic globular clusters are nearly gas-free, self-gravitating stellar systems characterized by an apparently simple geometry, with finite size probably determined by tidal truncation. These unique properties make them excellent laboratories for studies of stellar dynamics, and ideal targets for N-body simulations. For a long time, they have been treated as spherically symmetric, nonrotating, isotropic systems. The spherical King (1966) models, constructed to match this physical picture, are usually considered as the correct zeroth-order dynamical reference model, and are sometimes successful in representing the observed characteristics of these systems. In reality, this simple physical picture suffers from a number of limitations, which become more evident now that much improved observations have become available. In particular, deviations from sphericity have been observed and should be explained. Three physical ingredients are expected to affect the observed morphologies of stellar systems: internal rotation, pressure anisotropy, and external tides. A proper identification of the physical ingredients that shape the internal dynamics of globular clusters will lead to draw conclusions on their origin, and on the origin of their host systems. In particular, a detailed characterization of the role played by internal rotation and pressure anisotropy in present-day globular clusters would be a crucial element to discriminate among different formation scenarios for this class of stellar systems; indeed, the main goal of this Thesis is to clarify the role of these two important dynamical factors. For the purpose of giving a detailed and more realistic description of globular clusters, dynamical studies are an important counterpart to the stellar populations analyses often carried out for these systems. Dynamical studies are meaningful only when both photometric and kinematic data are taken into account, but unfortunately for globular clusters the application of dynamical models is frequently carried out only in relation to the available photometric profiles. This Thesis addresses this issue, and strongly supports the view that accurate kinematic data are crucial to provide a satisfactory description of these systems