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1.
Trans-neptunian objects (TNOs) are a population of small objects orbiting the Sun beyond Neptune. Because of their distance they are difficult to observe spectroscopically, but a large body of photometric observations is available and growing. TNOs are important tracers of the evolution of the outer Solar System and key when testing current dynamical evolution theories. Previous statistical studies of the colors of TNOs have yielded useful but limited results regarding the chemical history and evolution of these bodies.With the aim at obtaining compositional information on the small and distant TNOs we introduce a statistical cluster analysis (labelled albedo) based on colors and published albedos of TNOs. We compare it to a previous taxonomy, to illustrate the significance of including the albedo information when determining the composition of the objects. When the albedo contribution is removed from the data, the new taxonomy (now labelled classical) is in general agreement with the published ones, supporting the applicability of our approach. Making use of modeled reflectance spectra of a variety of plausible mixtures found on the surface of TNOs, we extract the average surface composition of each taxon, for both the classical and the albedo taxonomy, in a statistically consistent fashion.Differently from previous and classical, the albedo taxonomy establishes a direct link between the colors and albedos of the objects and their surface composition, allowing, for the first time, a quick assessment of the chemical history of TNOs. In fact, under closer examination the taxa show trends in composition that might be evolutionary in nature. If a simple ‘snow lines’ model is adopted, we can infer that albedo taxa relate the current objects’ locations to their original ones, prior to the migration of the outer planets. We regard the large population that characterizes the darkest classes spread at a variety of semi-major axis distances as one of the intriguing results of this work.  相似文献   

2.
S. T. Wu  W. P. Guo  J. F. Wang 《Solar physics》1995,157(1-2):325-348
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3.
《New Astronomy Reviews》2002,46(2-7):263-271
We review the question of the age of Compact Symmetric Objects (CSOs); defined as lobe-dominated sources smaller than 1 kpc in overall size. We show that the evidence increasingly points to these objects being very young (<104 yr old). Evidence from spectral aging, energy supply arguments and, most convincingly, from long term VLBI kinematic studies, is all consistent with the ‘youth’ scenario for CSOs. From VLBI kinematic studies hotspot advance speeds in CSOs are found to be 0.1 to 0.3c and external densities estimated from ram pressure balance are ≈1 cm−3. The separate question of the subsequent evolution of CSOs and whether they are the progenitors of classical double sources is, in contrast, not yet definitively answered. However it is found that the numbers of CSOs in flux limited samples is to first order what would be expected under such a scenario. The detailed differences in CSO population density between the data and model predictions might be resolved in various ways. Possibly not all CSOs evolve into large sources, or some sources show recurrent activity or, most likely, the simplest source evolution models need modification.  相似文献   

4.
Abstract— We present results of thermal evolution calculations for objects originating in the Kuiper belt and transferring inwards, to the region of the outer planets. Kuiper belt objects (KBOs) are considered to be part of a reservoir that supplies the flux of small icy bodies, mainly Centaurs and Jupiter‐family comets, to regions interior to the orbit of Neptune. We study the internal thermal evolution, for ?108 yr, of three typical KBOs and use the end state of the simulation as initial conditions for evolutionary calculations of two typical Centaurs. Some evolutionary trends can be identified for the KBOs, depending on key physical parameters, such as size and composition. The subsequent evolution in the Centaur region results in both specific features for each modeled object (mainly surface and sub‐surface composition) and common characteristics of thermally evolved Centaurs.  相似文献   

5.
In this paper we shall consider the following question: ‘If the whole galactical system expands and if galaxies are separating away from one another just as much as they are separating from our galaxy, can we assume that such a reality will have repercussions on the formulation of basic physical laws in our surroundings?’ Considering a confirmative answer, we performed an attempt to find a physical interpretation of the perturbations of the metric tensor components. It was shown that they can be expressed in terms of the dilatation function which also appears to be one reason which prohibits the precise measurement of real time and distance. Considering that an observer in our wold knows nothing of the aperiodicity of atomic oscillations which ‘determine’ the unit of time, we discerned the meaning of time in two ways: (i) the time as it is accepted in special relativity and (ii) the ‘time’ based on the reading of clocks which suffer dilatation. The usual procedure for establishing equations of the gravitational field was then carried out. These equations reduce to those of the classical theory, if the dilatation function is proportional to Newtonian gravitational potential. On this basis, considering the Earth's expansion as a process which exists as a cosmological manifestation, we discussed the observed fact of the agreement between the rate of the Earth's radius and that of the universe, according to Hubble's law.  相似文献   

6.
The Kuiper Belt zone is unique insofar as the major heat sources of objects a few tens of kilometers in size—solar radiation on the one hand and radioactive decay on the other—have comparable power. This leads to unique evolutionary patterns, with heat waves propagating inward from the irradiated surface and outward from the radioactively heated interior. A major radioactive source that is considered in this study is 26Al. The long-term evolution of several models with characteristics typical of Kuiper Belt objects is followed by means of a 1-D numerical code that solves the heat and mass balance equations on a spherically symmetric grid. The free parameters considered are radius (10-500 km), heliocentric distance (30-120 AU), and initial 26Al content (0-5×10−8 by mass). The initial composition assumed is a porous mixture of ices (H2O, CO, and CO2) and dust. Gases released in the interior are allowed to escape to the surface. It is shown that, depending on parameters, the interior may reach quite high temperatures (up to 180 K). The models suggest that Kuiper Belt objects are likely to lose the ices of very volatile species during early evolution; ices of less volatile species are retained in a surface layer, about 1 km thick. The models indicate that the amorphous ice crystallizes in the interior, and hence some objects may also lose part of the volatiles trapped in amorphous ice. Generally, the outer layers are far less affected than the inner part, resulting in a stratified composition and altered porosity distribution. These changes in structure and composition should have significant consequences for the short-period comets, which are believed to be descendants of Kuiper Belt objects.  相似文献   

7.
Saturn’s satellite Phoebe is the best-characterized representative of large outer Solar System planetesimals, thanks to the close flyby by the Cassini spacecraft in June 2004. We explore the information contained in Phoebe’s physical properties, density and shape, which are significantly different from those of other icy objects in its size range. Phoebe’s higher density has been interpreted as evidence that it was captured, probably from the proto-Kuiper-Belt. First, we demonstrate that Phoebe’s shape is globally relaxed and consistent with a spheroid in hydrostatic equilibrium with its rotation period. This distinguishes the satellite from ‘rubble-piles’ that are thought to result from the disruption of larger proto-satellites. We numerically model the geophysical evolution of Phoebe, accounting for the feedback between porosity and thermal state. We compare thermal evolution models for different assumptions on the formation of Phoebe, in particular the state of its water, amorphous or crystalline. We track the evolution of porosity and thermal conductivity as well as the destabilization of amorphous ice or clathrate hydrates. While rubble-piles may never reach temperatures suitable for porous ice to creep and relax, we argue that Phoebe’s shape could have relaxed due to heat from the decay of 26Al, provided that this object formed less than 3 Myr after the production of the calcium–aluminum inclusions. This is consistent with the idea that Phoebe could be an exemplar of planetesimals that formed in the transneptunian region and later accreted onto outer planet satellites, either during the satellite’s formation stage, or still later, during the late heavy bombardment.  相似文献   

8.
《Planetary and Space Science》2007,55(11):1541-1556
The exosphere of Mercury has been the object of many investigations and speculations regarding its composition, formation, depletion and dynamics. While vapourization of Mercurian surface materials by meteorite impacts has been often considered to be a less important contributor to the exosphere than other potential processes, larger objects coming from the Main Asteroid Belt could cause high local and transient enhancements in the density of the exosphere. Vapourization by such impacts is an almost stoichiometric process, and thus would contain valuable information about the surface composition. We investigate some exospheric effects of impact vapourization for meteorites with radii of 1, 10 cm, and 1 m, with particular reference to the missions that will explore Mercury during the next decade (MESSENGER and BepiColombo). Because of their higher probabilities, impacts of objects in the two smaller size ranges will surely occur during the lifetimes of the two missions. The enhancement of the exospheric density on the dayside of Mercury would be appreciable for the 10-cm and 1-m meteorites (some orders of magnitude, especially for Al, Mg, Si, and Ca). Such events could allow detection, for the first time, of refractory species like Al, Mg, and Si, which are expected to exist on the surface but have not yet been detected in the exosphere. Ca could be detectable in all cases, even if produced by impacting objects as small as 1 cm in radius. The lower exospheric background on the night side should allow easier identification of Na and K produced by impulsive events, even if their generally high background values make this eventuality less likely.  相似文献   

9.
Merk  Rainer  Prialnik  Dina 《Earth, Moon, and Planets》2003,92(1-4):359-374
Early evolution of trans-Neptunian objects,commonly known as Kuiper Belt objects (KBOs),is the result of heating due to radioactive decay, the most important sourcebeing 26Al. Several studiesare reviewed, dealing with the long-termevolution of KBO models, calculatedby means of 1-D numerical codesthat solve the heat and mass balanceequations on a fixed spherically symmetric grid. It is shown that, depending on parameters, the interior may reachquite high temperatures. The modelsthus suggest that KBOs are likely to lose the ices of very volatile species during early evolution; ices of less volatile species are retained in the cold subsurface layer. As the initially amorphous ice isshown to crystallize in the interior, some objects may also lose part of the volatiles trapped in amorphous ice. Generally, the outer layers are far less affected than the inner part, resulting in a stratified composition and altered porosity distribution. It is further shown that the thermal evolution of KBOs cannot be treated separately from their accretional evolution, as the processes occur in parallel. A systematic attempt to calculate accretion and thermal evolution simultaneously is presented, based on a numerical moving grid scheme. The accretion rate is obtained from the solution of the coupled coagulation equations for gravitationally interacting planetesimals. The effect of planetesimal velocities on the accretion scheme is included by a simplified equipartition argument. The time dependent accretion rates serve as input for the numerical solution of the heat transport equation for growing bodies mainly heated by radioactive decay of 26Al, allowing for phase transitions. Calculations carried out over the parameter space [heliocentric distance; final radius; ice fraction] lead to conclusions regarding the structure of KBOs, such as melt fraction, or extent of crystalline ice region.  相似文献   

10.
In our present understanding of the Solar System, small bodies (asteroids, Jupiter Trojans, comets and TNOs) are the most direct remnants of the original building blocks that formed the planets. Jupiter Trojan and Hilda asteroids are small primitive bodies located beyond the ‘snow line’, around respectively the L4 and L5 Lagrange points of Jupiter at ～5.2?AU (Trojans) and in the 2:3 mean-motion resonance with Jupiter near 3.9?AU (Hildas). They are at the crux of several outstanding and still conflicting issues regarding the formation and evolution of the Solar System. They hold the potential to unlock the answers to fundamental questions about planetary migration, the late heavy bombardment, the formation of the Jovian system, the origin and evolution of trans-neptunian objects, and the delivery of water and organics to the inner planets. The proposed Trojans’ Odyssey mission is envisioned as a reconnaissance, multiple flyby mission aimed at visiting several objects, typically five Trojans and one Hilda. It will attempt exploring both large and small objects and sampling those with any known differences in photometric properties. The orbital strategy consists in a direct trajectory to one of the Trojan swarms. By carefully choosing the aphelion of the orbit (typically 5.3?AU), the trajectory will offer a long arc in the swarm thus maximizing the number of flybys. Initial gravity assists from Venus and Earth will help reducing the cruise time as well as the ΔV needed for injection thus offering enough capacity to navigate among Trojans. This solution further opens the unique possibility to flyby a Hilda asteroid when leaving the Trojan swarm. During the cruise phase, a Main Belt Asteroid could be targeted if requiring a modest ΔV. The specific science objectives of the mission will be best achieved with a payload that will perform high-resolution panchromatic and multispectral imaging, thermal-infrared imaging/ radiometry, near- and mid-infrared spectroscopy, and radio science/mass determination. The total mass of the payload amounts to 50?kg (including margins). The spacecraft is in the class of Mars-Express or a down-scaled version of Jupiter Ganymede Orbiter. It will have a dry mass of 1200?kg, a total mass at launch of 3070?kg and a ΔV capability of 700?m/s (after having reached the first Trojan) and can be launched by a Soyuz rocket. The mission operations concept (ground segment) and science operations are typical of a planetary mission as successfully implemented by ESA during, for instance, the recent flybys of Main Belt asteroids Steins and Lutetia.  相似文献   

11.
Earlier, under certain simplifying assumptions, on the basis of the General Theory of Relativity, it has been concluded by many authors that when the radius of a gravitationally collapsing spherical object of massM reaches the critical value of the Scharzschild radiusR s=2GM/c 2, then, in a co-moving frame, the object collapses catastrophically to a point. However, in drawing this conclusion due consideration has not been given to the nuclear forces between the nucleons. In particular, the very strong ‘hard-core’ repulsive interaction between the nucleons which has the range ～0.4×10?13 cm has been totally ignored. On taking into account this ‘hard-core’ repulsive interaction, it is found that no spherical object of massM g can collapse to a volume of radius smaller thanR min=(1.68×10?6)M 1/3 cm or to a density larger than ρmax=5.0 × 1016 g cm?3. It has also been pointed out that objects of mass smaller thanM c～1.21×1033 g can not cross the Schwarzschild barrier and gravitationally collapse. The only course left to the objects of mass less thanM cis to reach the equilibrium as either a white dwarf or a neutron star.  相似文献   

12.
Three types of meteoritic material are found on the Moon: micrometeorites, ancien planetesimal debris from the ‘early intense bombardment’, and debris of recent, crater-forming projectiles. Their amounts and compositions have been determined from trace element studies. The micrometeorite component is uniformly distributed over the entire lunar surface, but is seen most clearly in mare soils. It has a primitive, C1-chondrite-like composition, and comprises 1-1.5% of mature soils. Apparently it represents cometary debris. The mean annual influx rate is 2.4 × 10?9 g cm?2 yr?1. It shows no detectable time variation or dependence on selenographic position. The ancient component is seen in highland breccias and soils more than 3.9 AE old. It has a fractionated composition, with volatiles depleted relative to siderophiles. The abundance pattern does not match that of any known meteorite class. At least two varieties exist (LN and DN, with Ir/Au, Re/Au 0.25-0.5 and > 0.5 the C1 value). Both seem to represent the debris of planetesimals that produced the mare basins and highland craters during the first 700 Myr of the Moon's history. It appears that the LN and DN objects impacted at less then 10 km s?1, had diameters less than 100 km, contained more than 15% Fe, and were not internally differentiated. Both were depleted in volatiles; the LN objects also in refractories (Ir, Re). This makes it unlikely that the LN bodies served as important building blocks of the Moon. The crater-forming component has remained elusive. Only a possible hint of this component has been seen, in ejecta from Dune Crater and Apollo 12 KREEP glasses of Copernican (?) origin.  相似文献   

13.
A. Rivoldini  T. Van Hoolst 《Icarus》2011,213(2):451-472
Knowledge of the interior structure of Mars is of fundamental importance to the understanding of its past and present state as well as its future evolution. The most prominent interior structure properties are the state of the core, solid or liquid, its radius, and its composition in terms of light elements, the thickness of the mantle, its composition, the presence of a lower mantle, and the density of the crust. In the absence of seismic sounding only geodesy data allow reliably constraining the deep interior of Mars. Those data are the mass, moment of inertia, and tides. They are related to Mars’ composition, to its internal mass distribution, and to its deformational response to principally the tidal forcing of the Sun. Here we use the most recent estimates of the moment of inertia and tidal Love number k2 in order to infer knowledge about the interior structure of the Mars.We have built precise models of the interior structure of Mars that are parameterized by the crust density and thickness, the volume fractions of upper mantle mineral phases, the bulk mantle iron concentration, and the size and the sulfur concentration of the core. From the bulk mantle iron concentration and from the volume fractions of the upper mantle mineral phases, the depth dependent mineralogy is deduced by using experimentally determined phase diagrams. The thermoelastic properties at each depth inside the mantle are calculated by using equations of state. Since it is difficult to determine the temperature inside the mantle of Mars we here use two end-member temperature profiles that have been deduced from studies dedicated to the thermal evolution of Mars. We calculate the pressure and temperature dependent thermoelastic properties of the core constituents by using equations state and recent data about reference thermoelastic properties of liquid iron, liquid iron-sulfur, and solid iron. To determine the size of a possible inner core we use recent data on the melting temperature of iron-sulfur.Within our model assumptions the geodesy data imply that Mars has no solid inner core and that the liquid core contains a large fraction of sulfur. The absence of a solid inner is in agreement with the absence of a global magnetic field. We estimate the radius of the core to be 1794 ± 65 km and its core sulfur concentration to be 16 ± 2 wt%. We also show that it is possible for Mars to have a thin layer of perovskite at the bottom of the mantle if it has a hot mantle temperature. Moreover a chondritic Fe/Si ratio is shown to be consistent with the geodesy data, although significantly different value are also possible. Our results demonstrate that geodesy data alone, even if a mantle temperature is assumed, can almost not constrain the mineralogy of the mantle and the crust. In order to obtain stronger constraints on the mantle mineralogy bulk properties, like a fixed Fe/Si ratio, have to be assumed.  相似文献   

14.
Bulk chemical analyses of six E-chondrites (Daniel's kuil, Khairpur, Kota Kota, Saint-Sauveur, South Oman and St Mark's) are given, together with partial analyses of a further five (Blithfield, Hvittis, Indarch, Jajh deh Kot Lalu and Pillistfer). The distribution of some normally lithophile elements (Al, Ca, Cr, K, Mg, Na, P and Ti) between silicate and sulphide groups of minerals was determined using the selective attack by dry chlorine (350°C) on magnetically separated fractions. Subdivision of the E-chondrites into types I and II (Yavnel;, 1963; Anders, 1964) is accepted and it is shown using chemical data that St Mark's and Saint-Sauveur should be included in type I. Sulphides contribute an unexpectedly high proportion of several elements to the bulk: e.g. Ca (av. 88.5% type I, 66.3% type II); Ti(av. 77.1% type I, 84.8% type II) and P as phosphide (av. 44.4% type I, > 83.2% type II). The proportion of Ti contributed to the bulk composition by the sulphides in types I and II increases with increae in ‘thermal metamorphic effect’ (Easton, 1983b) within each type. There is marked variability in the relative abundances of metal, phosphide, silicate and sulphide among the members of each type in keeping with their aggregate nature. The chemical composition of the ‘silicate’ and ‘sulphide’ in type IE-chondrites differs from that in type II (e.g. CaO in the silicates, Mg in the bulk sulphides) which therefore precludes the isochemical evolution of all E-chondrites from a common parent material. Partition of Ti between silicate and sulphide groups of minerals indicates that types I and II E-chondrites originated in separate, chemically distinct bodies.  相似文献   

15.
It appears possible to establish a preliminary geological model for the origin and evolution of the breccias of Boulder 1 at Station 2 in the Valley of Taurus-Littrow based on firm and probable geological constraints. The crystallization of plagioclase and other ANT-suite phases now present as clasts appears to have occurred in the lunar crust about 4.5 b.y. ago during the ‘melted shell stage’ of lunar history as that history is presently modeled. The original rocks containing these phases, which now make up the gray competent breccias of Boulder 1, were greatly modified by impact processes during the ‘cratered highland stage’ and the early part of the ‘large basin stage’, up to about 4.0 b.y. ago. About 4.0 b.y. ago, pigeonite basalts with KREEP affinities appear to have been intruded into the pre-Serenitatis crust from which the light friable breccias of Boulder 1 were later derived. During the large basin stage, three major dynamic events profoundly influenced the present character of the Boulder 1 materials. These events probably occurred as follows: (1) formation of gray competent breccia containing ANT-suite clasts in the hot ejecta blanket of an old large basin event, such as Tranquillitatis, that took place about 4.0 b.y. ago; (2) rebrecciation and redeposition of the gray competent breccia, mixed with light friable breccia and pigeonite basalt, in a relatively cool ejecta deposit, possibly produced by the northern Serenitatis event; (3) uplift and exposure of the Boulder 1 materials in the South Massif by the southern Serenitatis event about 3.90 b.y. ago.  相似文献   

16.
Two CCD spectra of the star V1016 Ori were obtained with the echelle spectrograph of the 6-m (BTA) telescope. An analysis of these spectra allowed us to estimate the star's atmospheric parameters (T eff=29700 K, logg=4.4) and projected rotational velocity (Vsini=60 km s?1) and to determine its chemical composition. Chemical anomalies were found. The Fe abundance is nearly solar; He, C, O, Mg, Al, Si are underabundant; and Ne, S, Zn are overabundant. The “spectroscopic” radius of the primary is in satisfactory agreement with its radius determined from the light and radial-velocity curves if the small star is assumed to lie in front of the giant star during an eclipse. The paradox of the primary's anomalous radius is thus resolved. A table of line equivalent widths and a portion of the star's spectrum are given in Appendices 1 and 2.  相似文献   

17.
A time-dependent, nonplanar, two-dimensional magnetohydrodynamic computer model is used to simulate a series, separately examined, of solar flare-generated shock waves and their subsequent disturbances in interplanetary space between the Sun and the Earth's magnetosphere. The ‘canonical’ or ansatz series of shock waves include initial velocities near the Sun over the range 500 to 3500 km s?1. The ambient solar wind, through which they propagate, is taken to be a steady-state homogeneous plasma (that is, independent of heliolongitude) with a representative set of plasma and magnetic field parameters. Complete sets of solar wind plasma and magnetic field parameters are presented and discussed. Particular attention is addressed to the MHD model's ability to address fundamental operational questions vis-à-vis the long-range forecasting of geomagnetic disturbances. These questions are: (i) will a disturbance (such as the present canonical series of solar flare shock waves) produce a magnetospheric and ionospheric disturbance, and, if so, (ii) when will it start, (iii) how severe will it be, and (iv) how long will it last? The model's output is used to compute various solar wind indices of current interest as a demonstration of the model's potential for providing ‘answers’ to these questions.  相似文献   

18.
The 1982 model for the formation of Hulse–Taylor binary radio pulsar PSR B1913+16 is described, which since has become the ‘standard model’ for the formation of the double neutron stars, confirmed by the 2003 discovery of the double pulsar system PSR J0737-3039AB. A brief overview is given of the present status of our knowledge of the double neutron stars, of which 15 systems are presently known. The binary-recycling model for the formation of millisecond pulsars is described, as put forward independently by Alpar et al. (1982), Radhakrishnan & Srinivasan (1982) and Fabian et al. (1983). This now is the ‘standard model’ for the formation of these objects, confirmed by the discovery in 1998 of the accreting millisecond X-ray pulsars. It is noticed that the formation process of close double black holes has analogies to that of close double neutron stars, extended to binaries with larger initial component masses, although there are also considerable differences in the physics of the binary evolution at these larger masses.  相似文献   

19.
Classical methods to analyze the surface composition of atmosphereless planetary objects from an orbiter are IR and gamma ray spectroscopy and neutron backscatter measurements. The idea to analyze surface properties with an in-situ instrument has been proposed by Johnson et al. (1998). There, it was suggested to analyze Europa's thin atmosphere with an ion and neutral gas spectrometer. Since the atmospheric components are released by sputtering of the moon's surface, they provide a link to surface composition. Here we present an improved, complementary method to analyze rocky or icy dust particles as samples of planetary objects from which they were ejected. Such particles, generated by the ambient meteoroid bombardment that erodes the surface, are naturally present on all atmosphereless moons and planets. The planetary bodies are enshrouded in clouds of ballistic dust particles, which are characteristic samples of their surfaces. In situ mass spectroscopic analysis of these dust particles impacting onto a detector of an orbiting spacecraft reveals their composition. Recent instrumental developments and tests allow the chemical characterization of ice and dust particles encountered at speeds as low as 1 km/s and an accurate reconstruction of their trajectories. Depending on the sampling altitude, a dust trajectory sensor can trace back the origin of each analyzed grain with about 10 km accuracy at the surface. Since the detection rates are of the order of thousand per orbit, a spatially resolved mapping of the surface composition can be achieved. Certain bodies (e.g., Europa) with particularly dense dust clouds, could provide impact statistics that allow for compositional mapping even on single flybys. Dust impact velocities are in general sufficiently high at orbiters about planetary objects with a radius >1000 km and with only a thin or no atmosphere. In this work we focus on the scientific benefit of a dust spectrometer on a spacecraft orbiting Earth's Moon as well as Jupiter's Galilean satellites. This ‘dust spectrometer' approach provides key chemical and isotopic constraints for varying provinces or geological formations on the surfaces, leading to better understanding of the body's geological evolution.  相似文献   

20.
We present models in which the photoevaporation of discs around young stars by an external ultraviolet source (as computed by Adams et al.) is coupled with the internal viscous evolution of the discs. These models are applied to the case of the Orion Nebula Cluster (ONC), where the presence of a strong ultraviolet field from the central OB stars, together with a detailed census of circumstellar discs and photoevaporative flows, is well established. In particular we investigate the constraints that are placed on the initial disc properties in the ONC by the twin requirement that most stars possess a disc on a scale of a few astronomical unit (au), but that only a minority (<20 per cent) are resolved by Hubble Space Telescope ( HST ) at a scale of 50 au. We find that these requirements place very weak constraints on the initial radius distribution of circumstellar discs: the resulting size distribution readily forgets the initial radius distribution, owing to the strong positive dependence of the photoevaporation rate on disc radius. Instead, the scarcity of large discs reflects the relative scarcity of initially massive discs (with mass  >0.1 M⊙  ). The ubiquity of discs on a small scale, on the other hand, mainly constrains the time-span over which the discs have been exposed to the ultraviolet field (<2 Myr). We argue that the discs that are resolved by HST represent a population of discs in which self-gravity was important at the time that the dominant central OB star switched on, but that, according to our models, self-gravity is unlikely to be important in these discs at the present time. We discuss the implications of our results for the so-called proplyd lifetime problem.  相似文献   

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