Induced semi-convection in helium-burning horizontal-branch stars

In the evolutionary phase of central helium burning, models of horizontal-branch stars reach a stage at which an intermediate region is unstable against convection. This feature is due to the progressive increase of the core mass by overshooting during the previous evolutionary stages and to the behaviour of the radiative temperature gradient at the core boundary. By consideration of the typical time scales of propagation of convection and nuclear burning, a partial mixing (semi-convection) can be induced in a region around the convective core.

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Riassunto Come conseguenza del progressivo aumento in massa per overshooting del nucleo convettivo nella fase iniziale di combustione centrale dell'elio in stelle di ramo orizzontale e per l'andamento del gradiente radiativo della temperatura al bordo del nucleo, viene raggiunto uno stadio nel quale una zona intermedia è instabile rispetto alla convezione. Dalla considerazione dei tempi caratteristici di propagazione della convezione e di combustione nucleare, si conclude che l'instabilità può indurre un mescolamento parziale (semiconvezione) in una regione esterna al nucleo.

     

On the models of core-helium-burning stars

Summary Some aspects concerning the core-mass increase and the appearance and development of an intermediate semiconvective zone in helium-burning stars are discussed. An iterative method of computation of core increase and semiconvection is also presented. Details of this procedure are given with regard to the horizontal-branch stars of globular clusters.

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Riassunto Sono presi in esame alcuni aspetti relativi all'aumento in massa del nucleo ed alla comparsa e sviluppo di una zona semiconvettiva intermedia in modelli di stelle con combustione centrale dell'elio. E'inoltre schematizzato il metodo iterativo di calcolo con particolare riferimento alle stelle di ramo orizzontale degli ammassi globulari.

     

Test computations on the dynamical evolution of star clusters

Test calculations have been carried out on the evolution of star clusters using the fluiddynamical method devised by Larson (1970a). Large systems of stars have been considered with specific concern with globular clusters. With reference to the analogous standard model by Larson, the influence of varying in turn the various free parameters (cluster mass, star mass, tidal radius, mass concentration of the initial model) has been studied for the results. Furthermore, the partial release of some simplifying assumptions with regard to the relaxation time and distribution of the target stars has been considered. The change of the structural properties is discussed, and the variation of the evolutionary time scale is outlined. An indicative agreement of the results obtained here with structural properties of globular clusters as deduced from previous theoretical models is pointed out.     

Tabular mass solutions for close binaries

The solutions for the mass of single-line close binaries with Main-Sequence primaries are given in a tabular form as a result of the application of the mass-luminosity relation. Linear interpolation in a compact bi-variate table suffices to give the values of mass of the two components for a wide range of masses and to a very good approximation. The indeterminacy of the solutions depending on the formulation of the mass-luminosity relation for the primaries is discussed, as well as the condition of filling up their Roche lobes by the secondaries.     

Absolute dimensions of 140 close binary systems

The absolute dimensions of 140 close binaries have been computed. These systems have been extracted from recent catalogues of photometric and spectroscopic elements.     

A multidimensional classification of globular clusters

Forty-two galactic globular clusters are classified according to the five observational parameters:S and (B-V)_{o, g}, used by Hartwick,P _ab, the mean period of theab-type RR Lyrae variables,n _c/n _ab, the relative number of thec-type to theab-type RR Lyrae variables and the red-dening-free parameterQ=(U-B)–0.72(B-V).Seven groups are isolated; a consistency between the Hartwick classification and a subdivision based on the RR Lyrae properties is shown.Globular clusters are confirmed to form a two parametric family of objects, what means that probably only another parameter in addition to the metal abundance must vary from a cluster to another.Evolutionary considerations in relation to the HB characteristics are given in detail. Implications concerning the initial galactic structure and the belium problem are briefly discussed.

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Sommario 42 ammassi globulari galattici sono stati classificati sulla base di cinque parametri osservativi: (B-V)_{o, g} edS, usati da Hartwick,P _ab (periodo medio delle variabili RR Lyrae di tipoab),n _c/n _ab (rapporto tra i numeri delle variabili di tipoc e di tipoab) eQ=(U-B)–0.72 (B-V).Sono state così selezionate sette classi e si è ottenuta una concordanza tra la classificazione basata sulle proprietà delle variabili RR Lyrae e quella di Hartwick. Resta in particolare confermato che gli ammassi globulari formano una famiglia di oggetti dipendente da due parametri, ciò che implica come probabilmente un solo ulteriore parametro in aggiunta all'abbondanza metallica vari da ammasso ad ammasso.Sono esaminate in dettaglio le caratteristiche evolutive di modelli stellari in relazione con le osservazioni di stelle di ramo orizzontale.Vengono infine discusse le implicazioni relative alla iniziale struttura galattica ed al problema dell'elio.

     

Evolutionary stages of visual binary components

In the assumption of a coeval origin of the components of visual binaries, the possible evolutionary stages of the stars forming a system can be studied on the basis of the differences in apparent visual magnitudes and color indices. According to such differences, a subdivision of systems in groups depending on the evolutionary stages of the components has been investigated. Some rough limiting estimates of the masses can be attempted. A comparison with observations is given.

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Résumé Dans l'hypothèse de l'origine contemporaine pour les étoiles composantes des systèmes binaires visuels on a recherché les phases évolutives possibles en se basant sur les différences des magnitudes apparentes visuelles et des indices de la couleur. En conséquence on a subdivisé les systèmes par groupes par rapport aux phases évolutives des composantes. On a donné des évaluations approximatives limites pour les masses et une comparaison avec les observations.

     

Accuracy assessment of high resolution satellite imagery orientation by leave-one-out method

Interest in high-resolution satellite imagery (HRSI) is spreading in several application fields, at both scientific and commercial levels. Fundamental and critical goals for the geometric use of this kind of imagery are their orientation and orthorectification, processes able to georeference the imagery and correct the geometric deformations they undergo during acquisition. In order to exploit the actual potentialities of orthorectified imagery in Geomatics applications, the definition of a methodology to assess the spatial accuracy achievable from oriented imagery is a crucial topic.In this paper we want to propose a new method for accuracy assessment based on the Leave-One-Out Cross-Validation (LOOCV), a model validation method already applied in different fields such as machine learning, bioinformatics and generally in any other field requiring an evaluation of the performance of a learning algorithm (e.g. in geostatistics), but never applied to HRSI orientation accuracy assessment.The proposed method exhibits interesting features which are able to overcome the most remarkable drawbacks involved by the commonly used method (Hold-Out Validation — HOV), based on the partitioning of the known ground points in two sets: the first is used in the orientation–orthorectification model (GCPs — Ground Control Points) and the second is used to validate the model itself (CPs — Check Points). In fact the HOV is generally not reliable and it is not applicable when a low number of ground points is available.To test the proposed method we implemented a new routine that performs the LOOCV in the software SISAR, developed by the Geodesy and Geomatics Team at the Sapienza University of Rome to perform the rigorous orientation of HRSI; this routine was tested on some EROS-A and QuickBird images. Moreover, these images were also oriented using the world recognized commercial software OrthoEngine v. 10 (included in the Geomatica suite by PCI), manually performing the LOOCV since only the HOV is implemented.The software comparison guaranteed about the overall correctness and good performances of the SISAR model, whereas the results showed the good features of the LOOCV method.     

Dynamical evolution of clusters with two stellar groups

The generalization of the fluid-dynamical approach from one-component star clusters to clusters with several stellar groups (as far as the star masses are concerned) has been applied to the study of two-component clusters. Rather extreme values of stellar masses and masses of groups were chosen in order to emphasize the different dynamical evolutions and asymptotic behaviours. Escape of stars from clusters and the problem of equipartition of kinetic energy among the two star groups are discussed. Comparisons of the main features of our results with those obtained by other authors have shown a good agreement. Some characteristic properties of the last computed models with an age of 18×10⁹ yr have been pointed out and discussed in relation with some observed features of galactic globular clusters.     

An investigation on H-R diagrams of globular clusters

The most characteristic features of H-R diagrams of blobular clusters are summarized with particular regard to the position and the extension of the horizontal branch. A comparison of observations with theory in the two cases of evolution with and without mass loss is then shortly given.A preliminary approach to the evolution of models with large mass fractions (q ₀) in the helium cores is presented. Inhomogeneous equilibrium models with double energy sources have been computed in a range of values ofq ₀ and of the total mass referring to possible horizontal-branch stars.

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Riassunto Sono indicate le principali caratteristiche dei diagrammi H-R degli ammassi globulari con particolare riferimento alla posizione ed alla estensione del ramo orizzontale. Sono esaminate relazioni tra la teoria ed i parametri indicati nei due casi di evoluzione con e senza perdita di massa.E' presentata infine una analisi di modelli con un'alta frazione di massa nel nucleo di elio (q ₀). I modelli di equilibrio, non omogenei ed a doppia sorgente di energia, sono stati calcolati in un intervallo di valori diq ₀ e della massa totale che sono in relazione alla regione delle stelle del ramo orizzontale.

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- . . . q ₀ . q ₀ .