Stellar models and Hyades: the Hipparcos test     

V. Castellani  S. Degl'Innocenti  P. G. Prada Moroni 《Monthly notices of the Royal Astronomical Society》2001,320(1):66-72

We compare theoretical stellar models for main sequence (MS) stars with the Hipparcos data base for the Hyades cluster to give a warning against the uncritical use of available theoretical scenarios and to show how formal MS fittings can be fortuitous if not fictitious. Moreover, we find that none of the current theoretical scenarios appears able to account for an observed mismatch between theoretical predictions and observations of the coolest Hyades MS stars. Finally, we show that current theoretical models probably give too faint He burning luminosities unlike the case of less massive He burning models, with degenerate progenitors, which have been suggested to suffer the opposite discrepancy.  相似文献   

On the rotational components of seismic motion     

A. Castellani  G. Boffi 《地震工程与结构动力学》1989,18(6):785-797

The surface motion during an earthquake is different from point to point depending on the propagation properties of the seismic waves. Rocking and torsion are thus present in the free field, in proportion to the spatial derivatives of the surface motion with respect to a given direction. These derivatives are inversely proportional to the apparent wave velocity in that direction, so the smaller the wave apparent velocity, the more important its contribution to the rotations. In this respect, a marked contribution to surface rotations from surface waves is expected. A mathematical model is presented, based on a detailed representation of soil impedance, an approximate identification of surface waves and a deconvolution of body waves in P and SV contributions. Through this model the surface motion obtained from the records of strong-motion accelerometers can be expressed as a superposition of plane waves of known wavelengths. Rocking response spectra are computed and results are compared with previously published spectra. A sensitivity analysis is performed on some parameters of the model.  相似文献   

Hydrological control of flooding: Tuscany,October 1992     

I. Becchi  E. Caporali  L. Castellani  E. Palmisano  F. Castelli 《Surveys in Geophysics》1995,16(2):227-252

In the study of flash-flood occurrence in small catchments the lack of flow measurements is often one of the main limiting factors. Prior to estimating the forecasting potentialities and techniques for such events, an accurate reconstruction of past event flood dynamics is first required. This issue is here addressed by analyzing, with the use of a distributed hydrological model, the hydrometeorological conditions in which a severe flash-flood occurred, on October 1992, on a 48 square kilometers catchment in the Arno basin. Such an event was caused by the persistence of intense convective clusters on the background of widespread rain bands of frontal origin. The distributed hydrological model here adopted is devoted to simulate the evolution and the variability of the primary processes involved in the runoff cycle. Together with the hydrological model structure, other particular aspects of the event reconstruction procedure are discussed: the managing and processing of the information coming from different sensors, with different temporal and spatial resolutions; the identification of local precipitation dynamics (frontal or convective) within small areas of integrated radar and rain gauges data fields; the interpolation of rain gauge data on the basis of the radar-estimated spatial correlation. The results of the distributed modeling, concerning the estimate of the flood wave at various sites, are compared with analogous results obtained with simpler lumped models.  相似文献   

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

Castellani  V.  Giannone  P.  Renzini  A. 《Astrophysics and Space Science》1971,10(3):355-362

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.

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Physical conditions in the convective envelope of stars     

V. Castellani  L. Puppi  A. Renzini 《Astrophysics and Space Science》1971,10(1):136-149

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.

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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.

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Stellar models for very low-mass main-sequence stars: the role of model atmospheres     

Brocato  Cassisi  & Castellani 《Monthly notices of the Royal Astronomical Society》1998,295(3):711-724

We present very low-mass stellar models as computed using non-grey model atmospheres for selected assumptions about the stellar metallicities. The role of atmospheres is discussed and the models are compared with models based on the Eddington approximation, and with similar models that have appeared in the recent literature. Theoretical predictions concerning both the HR diagram location and the mass–luminosity relation are presented and discussed in terms of expectations in selected photometric bands. Comparison with available observational data concerning both galactic globular clusters and dwarfs in the solar neighbourhood reveals a satisfactory agreement together with the existence of some residual mismatches.  相似文献   

On the models of core-helium-burning stars     

V. Castellani  P. Giannone  A. Renzini 《Astrophysics and Space Science》1972,17(1):80-86

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.

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On the structure of the globular cluster M4     

Vittorio Castellani  Francesca A. D'Antona  Antonella Natta 《Astrophysics and Space Science》1972,17(1):23-29

On the basis of the work by Greenstein on the globular cluster M 4, the spatial distribution for groups of cluster stars in different stages of evolution is investigated. The comparison between the various samples is made using the apparent distributions. The results are discussed.

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Riassunto Sulla base dello studio di Greenstein sull'ammasso globulare M4, è stata riesaminata la distribuzione spaziale di gruppi di stelle di questo ammasso evolutivamente significativi. Si puntualizza come sia possibile operare un confronto tra le distribuzioni dei vari gruppi sulla base delle distribuzioni apparenti anzichè su quelle reali senza alterare la sostanza dei risultati. Si discutono i risultati.

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The field of NGC 6397 as a test for Galactic models     

V. Castellani  S. Degl'Innocenti  S. Petroni  G. Piotto 《Monthly notices of the Royal Astronomical Society》2001,324(1):167-179

Taking advantage of recent Hubble Space Telescope ( HST ) data for field stars in the region of the Galactic globular cluster NGC 6397, we tested the predictions of several Galactic models with star counts reaching a largely unexplored range of magnitudes, down to V ∼26.5. After updating the input stellar ( V − I ) colours, we found that the two-component Bahcall–Soneira (B&S) model can be put into satisfactory agreement with observations for suitable choices of disc/spheroid luminosity functions (LFs). However, if one assumes the disc LF of Gould, Bahcall and Flynn together with the spheroid LF of Gould, Flynn and Bahcall, there is no way to reconcile the predicted and observed V -magnitude distribution. We also analysed the agreement between observed and predicted magnitude and colour distributions for two selected models with a thick disc component. Even in this case there are suitable combinations of model parameters and faint-magnitude LFs that can give a reasonable agreement with observational star counts in both magnitude and colour. However, the above-quoted combination of Gould et al. LFs again gives predictions in clear disagreement with observations.  相似文献   

Self-accretion of matter,red subluminous stars and early evolution of low-mass stars     

V. Castellani  N. Panagia 《Astrophysics and Space Science》1972,15(3):462-466

Low-mass stars during their pre-main sequence contraction phase are expected to be surrounded by a certain amount of the primordial gas. At low luminosities, accretion of this gas can make the stars to follow a peculiar evolutionary course which can account for certain types of red subluminous stars. The efficiency of the accretion mechanism can also account for some peculiarities in the spectra of the stars of low mass.

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Sommario Stelle di bassa massa in fase di contrazione pre-sequenza sono con ogni probabilità circondate da abbondante gas. In queste condizioni al diminuire della luminosità può divenire efficiente il meccanismo di accrescimento, cosi da condurre la stella su una linea evolutiva del tutto peculiare. Una simile evoluzione può rendere conto di oggetti sottoluminosi e a bassa temperatura, come pure di alcune peculiarità negli spettri di stelle di bassa massa.

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