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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.
In helium-burning horizontal-branch stars, transformation of helium into carbon increases opacity in the convective core. Such a situation can drive an increase of the core-mass extension via the mixing by overshooting. The efficiency of this mechanism is investigated in order to obtain an indication of the time scale for the propagation of the convective boundary. Schwarzschild's criterion is shown to be fulfilled within a few percent at the innermost interface of the core boundary.
Riassunto Nei primi stadi evolutivi di stelle di ramo orizzontale, la combustione di elio in carbonio aumenta l'opacità nel nucleo convettivo. Ne segue che il rimescolamento al bordo della convezione (overshooting) induce una progressiva instabilità convettiva negli strati contornanti il nucleo con conseguente incremento della massa del nucleo stesso. E' studiata l'efficienza di tale meccanismo al fine di ottenere indicazioni sui tempi scala caratteristici del processo di propagazione. Si mostra come il criterio di Schwarzschild risulti verificato entro qualche percento al bordo interno del nucleo convettivo.相似文献
3.
The problem of the energy released by interstellar matter in its accretion by low-luminosity stars is examined. It is found that nearly all the gravitational energy is available as the energy source for the accreting star. When this source of energy becomes predominant, a luminosity-temperature relation can be derived.Evolutionary consequences for the white dwarfs are discussed.The behaviour of selected classes of white dwarfs is recognized in the theoretical H.R. diagram.The evidence for a separation between field white dwarfs and Hyades cluster's white dwarfs is shortly discussed. For the Hyades white dwarfs a well defined sequence in good agreement with a constant accretion locus is found.A critical discussion is finally given on the efficiency of the accretion mechanism in the Hyades dwarfs.
Sommario Viene discusso il problema dell'energia ceduta dalla materia interstellare nei processi di cattura gravitazionale da parte di oggetti stellari di bassa luminosità. Si mostra come le maggior parte di questa energia sia effettivamente ceduta alla struttura stellare.Nei casi in cui l'energia di accrescimento predomini sulle altre sorgenti di energia è possibile ricavare una relazione luminosità-temperatura. Sono discusse possibili conseguenze sull'evoluzione delle nane bianche.E'esaminato il comportamento di alcune classi di nane bianche nel diagramma H.R. teorico, e viene brevemente discussa l'evidenza per una reale separazione delle nane bianche di campo da quelle nell'ammasso delle Iadi. Per queste ultime in particolare si trova una sequenza molto ben definita, in ottimo accordo con un luogo di costante accrescimento.Un esame critico è infine compiuto riguardo la possibile efficienza dell'accrescimento nelle nane delle Iadi.相似文献
4.
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
5.
Italo Mazzitelli 《Astrophysics and Space Science》1972,17(2):378-389
The luminosity function derived for main sequence stars in the neighbourhood of the Sun shows evidence for a flattening in
the interval 5≲M
v
≲8. An interpretation of this feature by means of theoretical models enables us to deduce the mass function for stars belonging
to the main sequence.
Sommario Viene ricavata la funzione di luminosità per stelle di sequenza principale vicine al Sole. Si evidenzia un pianerottolo per 5≲M v ≲8. II confronto con i modelli teorici consente di desumere la funzione di massa della sequenza principale.相似文献
6.
An analysis of the colour-magnitude diagram of stars in Draco dwarf Galaxy is performed on the basis of Baade and Swope observational results. Some evidence is found for the occurrence of a clump in the luminosity distribution of Red Giants stars. By combining this characteristic with the morphology of Horizontal Branch one obtains — in the frame of present knowledge of star-evolution —a suggestion for accepting high helium content (Y-0.30) with a relatively small age (1–2×109 yr) for the evolving stars.Some correlations among lifetimes of evolutionaries stages are derived.
Sommario Sulla base dei risultati osservativi di Baade e Swope viene eseguita un'analisi del diagramma colore-magnitudine delle stelle della Galassia Nana in Draco. Viene così evidenziata la possibile esistenza di un picco nella distribuzione in luminosità delle Giganti Rosse. Combinando questa caratteristica con l'osservato andamento del Ramo Orizzontale si mostra come — nell'ambito delle attuali conoscenze sull'evoluzione stellare — un simile comportamento conduca a postulare un alto contenuto di elio (Y-0.30) e una età relativamente bassa (1–2×109 anni) per le stelle del sistema.Sono infine ricavate alcune correlazioni, tra le vite medie delle principali fasi evolutive.相似文献
7.
An effort is made to select, from the available observational data, information concerning the asymptotic branch stars. Particular care is devoted to derive luminosity functions for these stars in clusters representative of selected classes. Some theoretical predictions are briefly made. It is stressed that, from a theoretical point of view, the comparison between the one-shell H-burning evolutionary characteristics and the corresponding ones for the A.B. phase could be expected to provide a powerful means of investigation.
Sommario Sulla base dei dati osservativi disponibili sono raccolte ed esaminate le possibili informazioni riguardanti la fase di braccio asintotico. In particolare vengono investigate le funzioni di luminosità relative a questa fase evolutiva, e ne viene tentato un inquadramento nella classificazione multidimensionale degli ammassi globulari recentemente proposta. Sono fornite alcune valutazioni teoriche, ed è messo in evidenza come il confronto delle caratteristiche evolutive delle fasi di bruciamento ad una (H) e due (H, He) shell potrebbe rivelarsi come un potente mezzo di investigazione.相似文献
8.
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
9.
L. Angeletti 《Astrophysics and Space Science》1976,44(1):23-34
The characteristic features of particle motion in close binaries have been studied for sequences of evolutionary models with small or intermediate masses. The possible behaviour of the particle streams has been determined with regard to the change of systems with time as a result of mass-exchange between the two stellar components. A grid of values for the velocity and direction of ejection from the first Lagrangian point has been investigated. Among the various possibilities, impact of particles with one, or the other component, or orbits encircling the Main-Sequence star have been detailed. This latter possibility seems to lead to the formation of a disk-like structure around the Main-Sequence component. From the results obtained for semi-detached systems of both cases A and B of evolution it is found that the formation of disks is difficult in cases A, and in the early stages of the slow mass exchange in cases B.
Sommario Sono state studiate le caratteristiche principali delle traiettorie in sequenze di modelli evolutivi di sistemi binari stretti di massa totale piccola e intermedia. E' stato messo in evidenza l'andamento generale delle traiettorie in connessione con la variazione temporale delle proprietà dei sistemi binari dovuta allo scambio di massa tra le componenti. La velocità e la direzione iniziali di eiezione dal primo punto lagrangiano sono state variate con sistematicità in un ampio intervallo di valori. Tra le varie possibilità sono state messe in luce sia l'impatto con l'una o l'altra delle componenti sia la formazione di orbite attorno alla componente di sequenza principale. Quest'ultima eventualità potrebbe condurre alla formazione di una struttura di disco. Dai risultati ottenuti con modelli evolutivi di sistemi semistaccati di casi A e B è emerso che la formazione di una struttura di disco sembra difficile nei casi A e nelle prime fasi dello scambio lento di massa nei casi B.相似文献
10.
The critical accretion flow of gas onto compact stars with mass of 0.6M
is investigated by numerical integrations of the time-dependent hydrodynamic equations in the sphericallysymmetric and optically thick case. For the compact stars surrounded by such a dense cloud of gas, the radiation pressure force decelerates the infall gas significantly and free fall regime of the gas is not at all attained. This results in incident low velocities at the standing shock front close to the stellar surface, low temperatures of the gas around the compact stars, and no X-ray in white dwarfs but soft X-rays in neutron stars, respectively. Some applications of the results to the X-ray sources are discussed. 相似文献
11.
N. R. Ikhsanov 《Astrophysics and Space Science》2007,308(1-4):137-140
Accretion of interstellar material by an isolated neutron star is discussed. The point I address here is the interaction between
the accretion flow and the stellar magnetosphere. I show that the interchange instabilities of the magnetospheric boundary
under the conditions of interest are basically suppressed. The entry of the material into the magnetosphere is governed by
diffusion. Due to this reason the persistent accretion luminosity of isolated neutron stars is limited to <4×1026 erg s−1. These objects can also appear as X-ray bursters with the burst durations of ∼30 min and repetition time of ∼105 yr. This indicates that the number of the accreting isolated neutron stars which could be observed with recent and modern
X-ray missions is a few orders of magnitude smaller than that previously estimated.
相似文献
12.
We elaborate the model of accretion disks of young stars with the fossil large-scale magnetic field in the frame of Shakura and Sunyaev approximation. Equations of the MHD model include Shakura and Sunyaev equations, induction equation and equations of ionization balance. Magnetic field is determined taking into account ohmic diffusion, magnetic ambipolar diffusion and buoyancy. Ionization fraction is calculated considering ionization by cosmic rays and X-rays, thermal ionization, radiative recombinations and recombinations on the dust grains. Analytical solution and numerical investigations show that the magnetic field is coupled to the gas in the case of radiative recombinations. Magnetic field is quasi-azimuthal close to accretion disk inner boundary and quasi-radial in the outer regions. Magnetic field is quasi-poloidal in the dusty “dead” zones with low ionization degree, where ohmic diffusion is efficient. Magnetic ambipolar diffusion reduces vertical magnetic field in 10 times comparing to the frozen-in field in this region. Magnetic field is quasi-azimuthal close to the outer boundary of accretion disks for standard ionization rates and dust grain size a d=0.1 μm. In the case of large dust grains (a d>0.1 μm) or enhanced ionization rates, the magnetic field is quasi-radial in the outer regions. It is shown that the inner boundary of dusty “dead” zone is placed at r=(0.1–0.6) AU for accretion disks of stars with M=(0.5–2)?M ⊙. Outer boundary of “dead” zone is placed at r=(3–21) AU and it is determined by magnetic ambipolar diffusion. Mass of solid material in the “dead” zone is more than 3?M ⊕ for stars with M≥1?M ⊙. 相似文献
13.
In the UV spectra of BP Tau, GW Ori, T Tau, and RY Tau obtained with the Hubble Space Telescope, we detected an inflection near 2000 Å in the F λ c (λ) curve that describes the continuum energy distribution. The inflection probably stems from the fact that the UV continuum in these stars consists of two components: the emission from an optically thick gas with T<8000 K and the emission from a gas with a much higher temperature. The total luminosity of the hot component is much lower than that of the cool component, but the hot-gas radiation dominates at λ<1800 Å. Previously, other authors have drawn a similar conclusion for several young stars from low-resolution IUE spectra. However, we show that the short-wavelength continuum is determined from these spectra with large errors. We also show that, for three of the stars studied (BP Tau, GW Ori, and T Tau), the accretion-shock radiation cannot account for the observed dependence F λ c (λ) in the ultraviolet. We argue that more than 90% of the emission continuum in BP Tau at λ>2000 Å originates not in the accretion shock but in the inner accretion disk. Previously, a similar conclusion was reached for six more classical T Tau stars. Therefore, we believe that the high-temperature continuum can be associated with the radiation from the disk chromosphere. However, it may well be that the stellar chromosphere is its source. 相似文献
14.
The importance in the blanketing effect of the wings of strong lines broadened by collisions is evaluated. It is found that in stars of spectral types F5-K5 the cumulative absorption by a group of lines is due half to the center of the lines and half to the wings.For the same stars it is evaluated the influence of the collisional broadening on the computed color indices (U-B), (B-V).
Sommario Viene valutata l'importanza nell'effetto blanketing delle ali delle righe forti allargate per collisioni. Si trova che per stelle dei tipi spettrali F5-K5 l'assorbimento complessivo di un gruppo di righe è all'incirca dovuto per metà all'assorbimento al centro delle righe e per metà alle ali.Per le stesse stelle viene anche data una stima dell'influenza dell'allargamento collisionale sugli indici di colore calcolati.相似文献
15.
Non-degenerate stars of essentially all spectral classes are soft X-ray sources. Their X-ray spectra have been important in
constraining physical processes that heat plasma in stellar environments to temperatures exceeding one million degrees. Low-mass
stars on the cooler part of the main sequence and their pre-main sequence predecessors define the dominant stellar population
in the galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense, of X-ray spectra from the solar corona.
The Sun itself as a typical example of a main-sequence cool star has been a pivotal testbed for physical models to be applied
to cool stars. X-ray emission from cool stars is indeed ascribed to magnetically trapped hot gas analogous to the solar coronal
plasma, although plasma parameters such as temperature, density, and element abundances vary widely. Coronal structure, its
thermal stratification and geometric extent can also be interpreted based on various spectral diagnostics. New features have
been identified in pre-main sequence stars; some of these may be related to accretion shocks on the stellar surface, fluorescence
on circumstellar disks due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot stars clearly dominate
the interaction with the galactic interstellar medium: they are the main sources of ionizing radiation, mechanical energy
and chemical enrichment in galaxies. High-energy emission permits to probe some of the most important processes at work in
these stars, and put constraints on their most peculiar feature: the stellar wind. Medium and high- resolution spectroscopy
have shed new light on these objects as well. Here, we review recent advances in our understanding of cool and hot stars through
the study of X-ray spectra, in particular high-resolution spectra now available from XMM-Newton and Chandra. We address issues related to coronal structure, flares, the composition of coronal plasma, X-ray production in accretion
streams and outflows, X-rays from single OB-type stars, massive binaries, magnetic hot objects and evolved WR stars. 相似文献
16.
V. Urpin U. Geppert & D. Konenkov 《Monthly notices of the Royal Astronomical Society》1998,295(4):907-920
The evolution of neutron stars in close binary systems with a low-mass companion is considered, assuming the magnetic field to be confined within the solid crust. We adopt the standard scenario for the evolution in a close binary system, in which the neutron star passes through four evolutionary phases ('isolated pulsar'–'propeller'– accretion from the wind of a companion – accretion resulting from Roche-lobe overflow). Calculations have been performed for a great variety of parameters characterizing the properties of both the neutron star and the low-mass companion. We find that neutron stars with more or less standard magnetic field and spin period that are processed in low-mass binaries can evolve to low-field rapidly rotating pulsars. Even if the main-sequence life of a companion is as long as 1010 yr, the neutron star can maintain a relatively strong magnetic field to the end of the accretion phase. The model that is considered can account well for the origin of millisecond pulsars. 相似文献
17.
We consider the problem of dust grain survival in the disk winds from T Tauri and Herbig Ae stars. For our analysis, we have chosen a disk wind model in which the gas component of the wind is heated through ambipolar diffusion to a temperature of ～104 K. We show that the heating of dust grains through their collisions with gas atoms is inefficient compared to their heating by stellar radiation and, hence, the grains survive even in the hot wind component. As a result, the disk wind can be opaque to the ultraviolet and optical stellar radiation and is capable of absorbing an appreciable fraction of it. Calculations show that the fraction of the wind-absorbed radiation for T Tauri stars can be from 20 to 40% of the total stellar luminosity at an accretion rate ? a = 10?8-10?6 M ⊙yr?1. This means that the disk winds from T Tauri stars can play the same role as the puffed-up inner rim in current accretion disk models. In Herbig Ae stars, the inner layers of the disk wind (r ≤ 0.5 AU) are dust-free, since the dust in this region sublimates under the effect of stellar radiation. Therefore, the fraction of the radiation absorbed by the disk wind in this case is considerably smaller and can be comparable to the effect from the puffed-up inner rim only at an accretion rate of the order of or higher than 10?6 M ⊙yr?1. Since the disk wind is structurally inhomogeneous, its optical depth toward the observer can be variable, which should be reflected in the photometric activity of young stars. For the same reason, moving shadows from gas and dust streams with a spiral-like shape can be observed in high-angular-resolution circumstellar disk images. 相似文献
18.
The physical conditions in the convective envelopes of Pop. II main-sequence stars are described with some detail. A particular care has been devoted to the effects due to the formation of hydrogen molecules and to the pressure ionization.
Riassunto Vengono descritte in dettaglio le condizioni fisiche nelle zone convettive esterne delle stelle di sequenza principale di Popolazione II. In particolare vengono studiati gli effetti dovuti alla formazione della molecola dell'idrogeno ed alla ionizzazione per pressione.相似文献
19.
Fracassini Massimo Gilardoni Giorgio Pasinetti Laura E. 《Astrophysics and Space Science》1973,22(1):141-152
The spectrophotometric method for computing the apparent stellar diameters, proposed by Chalonge and Divan (1950) and modified by Fracassini and Pasinetti (1967) has been applied to 172 B5V-A5V stars (single and belonging to galactic clusters) of the Catalogue of Geneva Observatory (Rufener, 1971). In the straight line diagrams log vsm, drawn for the stars of the same spectral type, it is possible to identify some peculiar stars (with peculiarities in the diameter or monochromatic brightness). The agreement with the values of the apparent diameters of Leo, Ori, Lyr and CMa, obtained by Hanbury Brownet al. (1967), is very good. The comparison with the diagramss
v
vs (B-V)0,R vs (B-V)0 deduced by Wesselink (1969), seems satisfactory. The comparison with the results obtained by other authors and the great number of stars which will be observed in the photometric system in seven colors of Geneva, demonstrate the wide possibilities of this spectrophotometric method for determining the apparent stellar diameters.
Thesis for the degree on Applied Physics. 相似文献
Sommario Il metodo spettrofotometrico di Chalonge e Divan (1950) per il calcolo dei diametri stellari apparenti, modificato da Fracassini e Pasinetti (1967), è stato applicato a 172 stelle B5V-A5V (isolate e in ammassi galattici) del Catalogo dell'Osservatorio di Ginevra (Rufener, 1971). Costruiti i diagrammi lineari log -m v per i vari gruppi di stelle dello stesso tipo spettrale, è possibile identificare alcune stelle peculiari con diametri e brillanze monochromatiche anomale. Il confronto con i diametti apparenti di Leo, Ori, Lyr e CMa determinati da Hanbury Brownet al. (1967) mostra un accordo molto buono. Il confronto con le relazionis v -(B-V)0eR-(B-V) 0 ottenute da Wesselink (1969) appare soddisfacente. Il confronto con i risultati ottenuti da altri autori e il grande numero di stelle che saranno osservate nel sistema fotometrico in sette colori di Ginevra, dimostrano le ampie possibilità di questo metodo spettrofotometrico per la determinazione dei diametri stellari apparenti.
Thesis for the degree on Applied Physics. 相似文献
20.
Ian A. Bonnell Matthew R. Bate & Hans Zinnecker 《Monthly notices of the Royal Astronomical Society》1998,298(1):93-102
We present a model for the formation of massive ( M ≳10 M⊙) stars through accretion-induced collisions in the cores of embedded dense stellar clusters. This model circumvents the problem of accreting on to a star whose luminosity is sufficient to reverse the infall of gas. Instead, the central core of the cluster accretes from the surrounding gas, thereby decreasing its radius until collisions between individual components become sufficient. These components are, in general, intermediate-mass stars that have formed through accretion on to low-mass protostars. Once a sufficiently massive star has formed to expel the remaining gas, the cluster expands in accordance with this loss of mass, halting further collisions. This process implies a critical stellar density for the formation of massive stars, and a high rate of binaries formed by tidal capture. 相似文献