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1.
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.
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.相似文献
2.
Summary The mechanical flux originating in the convective envelope of stars is shown to depend critically by the treatment of convection. In particular, in the framework of the mixing-length theory, in passing from a mixing lengthl=H
P to a mixing lengthl=H
the presence of mechanical fluxes shifts from being a marginal phenomenon to a dominant one.Possible implications concerning atmospheric microturbulence in Main Sequence stars, as so asA
p andA
m stars, several types of variables and mass loss are briefly discussed.
Sommario Si mostra come il flusso cinetico originato negli inviluppi convettivi di una stella dipenda criticamente dalle assunzioni fatte nel trattamento della convezione superadiabatica. In particolare, sulla base della teoria della lunghezza di rimescolamento, a seconda che si assumal=H P o l=H i flussi cinetici passano dall'essere un fenomeno marginale a contributi determinanti.E' discussa una serie di possibili implicazioni riguardanti la microturbolenza atmosferica in stelle di sequenza principale, le stelleA p eA m, diversi tipi di variabili e la perdita di massa in fase di gigante.相似文献
3.
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.
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.相似文献
4.
R. Feldmann C. M. Carollo C. Porciani S. J. Lilly P. Capak Y. Taniguchi O. Le Fèvre A. Renzini N. Scoville M. Ajiki H. Aussel T. Contini H. McCracken B. Mobasher T. Murayama D. Sanders S. Sasaki C. Scarlata M. Scodeggio Y. Shioya J. Silverman M. Takahashi D. Thompson G. Zamorani 《Monthly notices of the Royal Astronomical Society》2006,372(2):565-577
5.
The actual knowledge on Horizontal Branch stars of the metal-poor Globular Clusters of our Galaxy suggests that these stars evolve along the Horizontal Branch from the very blue stars towards lower effective temperature.From this behaviour it follows that a substantial mass loss occurs at the He-flash. On the basis of a gravitational thermalization of Horizontal Branch stars any recent observational data concerning NGC 7006 are re-examined. It is pointed out that today it is not necessary to leave the hypothesis of a strong correlation between the metal and helium content for our Galaxy.At present at the Osservatorio Astronomico, Bologna. 相似文献
6.
Laura Greggio Alvio Renzini Emanuele Daddi 《Monthly notices of the Royal Astronomical Society》2008,388(2):829-837
Following an episode of star formation, Type Ia supernova events occur over an extended period of time, following a distribution of delay times (DDT). We critically discuss some empirically based DDT functions that have been proposed in recent years, some favouring very early (prompt) events, other very late (tardy) ones, and therefore being mutually exclusive. We point out that in both the cases the derived DDT functions are affected by dubious assumptions, and therefore there is currently no ground for claiming either a DDT strongly peaked at early times, or at late ones. Theoretical DDT functions are known to accommodate both prompt as well as late SN Ia events, and can account for all available observational constraints. Recent observational evidence exist that both single and double degenerate precursors may be able of producing SN Ia events. We then explore on the basis of plausible theoretical models the possible variation with cosmic time of the mix between the events produced by the two different channels, which, in principle, could lead to systematic effects on the SN Ia properties with redshift. 相似文献
7.
A theoretical check is performed to determine for the horizontal-branch stars the zone (in the H-R diagram) in which the surface convection is acting. A possible experimental test of the efficiency of the depletion mechanism is also proposed.At present at Osservatorio Astronomico, Bologna.Since RR Lyrae stars have surely extended convective envelopes we expected that at least a part of the blue H.B. stars would have surface convection. 相似文献
8.
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.
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.相似文献
9.
Results of calculations of main sequence stellar models for metal poor star are presented. A fictitious pre-main sequence evolution was computed in order to give the distribution of the primary elements H, He3, He4, C12, N14 and O16 in the stellar interior. The selected initial chemical composition wasX=.9,Z=.001.Comparison with previous results indicates a life-time of the main sequence phase shorter than it was believed before. 相似文献
10.