共查询到20条相似文献，搜索用时 968 毫秒
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I. A. Bonnell C. J. Clarke M. R. Bate 《Monthly notices of the Royal Astronomical Society》2006,368(3):1296-1300
We use numerical simulations of the fragmentation of a 1000 M⊙ molecular cloud and the formation of a stellar cluster to study how the initial conditions for star formation affect the resulting initial mass function (IMF). In particular, we are interested in the relation between the thermal Jeans mass in a cloud and the knee of the IMF, i.e. the mass separating the region with a flat IMF slope from that typified by a steeper, Salpeter-like, slope. In three isothermal simulations with M Jeans = 1, 2 and 5 M⊙ , the number of stars formed, at comparable dynamical times, scales roughly with the number of initial Jeans masses in the cloud. The mean stellar mass also increases (though less than linearly) with the initial Jeans mass in the cloud. It is found that the IMF in each case displays a prominent knee, located roughly at the mass scale of the initial Jeans mass. Thus clouds with higher initial Jeans masses produce IMFs which are shallow to higher masses. This implies that a universal IMF requires a physical mechanism that sets the Jeans mass to be near 1 M⊙ . Simulations including a barotropic equation of state as suggested by Larson, with cooling at low densities followed by gentle heating at higher densities, are able to produce realistic IMFs with the knee located at ≈1 M⊙ , even with an initial M Jeans = 5 M⊙ . We therefore suggest that the observed universality of the IMF in the local Universe does not require any fine tuning of the initial conditions in star forming clouds but is instead imprinted by details of the cooling physics of the collapsing gas. 相似文献
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The hierarchical formation of a stellar cluster 总被引：1，自引：0，他引：1
Ian A. Bonnell Matthew R. Bate Stephen G. Vine 《Monthly notices of the Royal Astronomical Society》2003,343(2):413-418
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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
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Renato De Santis 《Monthly notices of the Royal Astronomical Society》2001,326(1):397-402
In this paper, by assuming the equilibrium temperatures of RRab Lyrae variables defined by Carney, Storm & Jones as correct we show that temperatures derived from ( B − V ) colour (mean colour over the pulsational cycle calculated on the magnitude scale) transformations by Bessel, Castelli & Plez are consistent with the Carney et al. equilibrium temperatures within a probable error of δ log T e =±0.003 . As a consequence, it is shown that the pulsational temperature scale temperature–period–blue amplitude [ T eff = f ( P , A B )] relation provided by De Santis, who studied the ( B − V ) colour of about 70 stars of Lub's sample, is a suitable relation, being reddening- and metallicity-free, to calculate equilibrium temperatures for RRab variables. This relation is independent of variable mass and luminosity within a large range of period-shift from the mean period–amplitude relation valid for Lub's sample of variables. On the contrary, it is also shown that a temperature–amplitude–metallicity relation is strictly dependent on the period–amplitude relation of the sample used for calibrating it: we prove that this means it is dependent on both the mass and luminosity variations of variables. 相似文献
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I. A. Bonnell M. R. Bate C. J. Clarke J. E. Pringle 《Monthly notices of the Royal Astronomical Society》2001,323(4):785-794
We investigate the physics of gas accretion in young stellar clusters. Accretion in clusters is a dynamic phenomenon as both the stars and the gas respond to the same gravitational potential. Accretion rates are highly non-uniform with stars nearer the centre of the cluster, where gas densities are higher, accreting more than others. This competitive accretion naturally results in both initial mass segregation and a spectrum of stellar masses. Accretion in gas-dominated clusters is well modelled using a tidal-lobe radius instead of the commonly used Bondi–Hoyle accretion radius. This works as both the stellar and gas velocities are under the influence of the same gravitational potential and are thus comparable. The low relative velocity which results means that R tidal < R BH in these systems. In contrast, when the stars dominate the potential and are virialized, R BH < R tidal and Bondi–Hoyle accretion is a better fit to the accretion rates. 相似文献
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The effect of gas ejection on the structure and binding energy of newly formed stellar clusters is investigated. The star formation efficiency (SFE), necessary for forming a gravitationally bound stellar cluster, is determined.
Two sets of numerical N -body simulations are presented. As a first simplified approach we treat the residual gas as an external potential. The gas expulsion is approximated by reducing the gas mass to zero on a given time-scale, which is treated as a free parameter. In a second set of simulations we use smoothed particle hydrodynamics (SPH) to follow the dynamics of the outflowing residual gas self-consistently. We investigate cases where gas outflow is induced by an outwards propagating shock front and where the whole gas cloud is heated homogeneously, leading to ejection.
If the stars are in virial equilibrium with the gaseous environment initially, bound clusters only form in regions where the local SFE is larger than 50 per cent or where the gas expulsion time-scale is long compared with the dynamical time-scale. A small initial velocity dispersion of the stars leads to a compaction of the cluster during the expulsion phase and reduces the SFE needed to form bound clusters to less than 10 per cent. 相似文献
Two sets of numerical N -body simulations are presented. As a first simplified approach we treat the residual gas as an external potential. The gas expulsion is approximated by reducing the gas mass to zero on a given time-scale, which is treated as a free parameter. In a second set of simulations we use smoothed particle hydrodynamics (SPH) to follow the dynamics of the outflowing residual gas self-consistently. We investigate cases where gas outflow is induced by an outwards propagating shock front and where the whole gas cloud is heated homogeneously, leading to ejection.
If the stars are in virial equilibrium with the gaseous environment initially, bound clusters only form in regions where the local SFE is larger than 50 per cent or where the gas expulsion time-scale is long compared with the dynamical time-scale. A small initial velocity dispersion of the stars leads to a compaction of the cluster during the expulsion phase and reduces the SFE needed to form bound clusters to less than 10 per cent. 相似文献
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S. Catalán J. Isern E. García-Berro I. Ribas 《Monthly notices of the Royal Astronomical Society》2008,387(4):1693-1706
The initial–final mass relationship connects the mass of a white dwarf with the mass of its progenitor in the main sequence. Although this function is of fundamental importance to several fields in modern astrophysics, it is not well constrained either from the theoretical or from the observational points of view. In this work, we revise the present semi-empirical initial–final mass relationship by re-evaluating the available data. The distribution obtained from grouping all our results presents a considerable dispersion, which is larger than the uncertainties. We have carried out a weighted least-squares linear fit of these data and a careful analysis to give some clues on the dependence of this relationship on some parameters such as metallicity or rotation. The semi-empirical initial–final mass relationship arising from our study covers the range of initial masses from 1.0 to 6.5 M⊙ , including in this way the low-mass domain, poorly studied until recently. Finally, we have also performed a test of the initial–final mass relationship by studying its effect on the luminosity function and on the mass distribution of white dwarfs. This was done by using different initial–final mass relationships from the literature, including the expression derived in this work, and comparing the results obtained with the observational data from the Palomar Green Survey and the Sloan Digital Sky Survey. We find that the semi-empirical initial–final mass relationship derived here gives results in good agreement with the observational data, especially in the case of the white dwarf mass distribution. 相似文献
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Babur M. Mirza 《Monthly notices of the Royal Astronomical Society》2009,395(4):2288-2291
The isothermal Lane–Emden equation arises in many astrophysical problems, in particular in modelling of a self-gravitating, polytropic gas in a spherically symmetric configuration. In this work, the isothermal Lane–Emden equation is investigated using the fractional approximation technique. The method provides an efficient and accurate way of obtaining approximate analytic solution to the Lane–Emden equation thus is useful in the modelling of self-gravitating gaseous spheres in astrophysics. 相似文献