共查询到20条相似文献，搜索用时 25 毫秒
1.
N. Chandra Wickramasinghe 《Astrophysics and Space Science》2013,343(1):1-5
The theory of cometary panspermia, developed by the late Sir Fred Hoyle and the present author argues that life originated cosmically as a unique event in one of a great multitude of comets or planetary bodies in the Universe. Life on Earth did not originate here but was introduced by impacting comets, and its further evolution was driven by the subsequent acquisition of cosmically derived genes. Explicit predictions of this theory published in 1979–1981, stating how the acquisition of new genes drives evolution, are compared with recent developments in relation to horizontal gene transfer, and the role of retroviruses in evolution. Precisely-stated predictions of the theory of cometary panspermia are shown to have been verified. 相似文献
2.
Virginia Trimble 《Astronomy and Astrophysics Review》1991,3(1):1-46
Summary The basic scheme of nucleosynthesis (building of heavy elements from light ones) has held up very well since it was first proposed more than 30 years ago by E.M. Burbidge, G.R. Burbidge, A.G.W. Cameron, W.A. Fowler, and F. Hoyle. Significant advances in the intervening years include (a) observations of elemental and a few isotopic ratios in many more extrasolar-system sites, including metal-poor dwarf irregular galaxies, where very little has happened, and supernovae and their remnants, where a great deal has happened, (b) recognition of the early universe as good for making all the elements up to helium, (c) resolution of heavy element burning in stars into separate carbon, neon, oxygen, and silicon burning, with fine tuning of the resulting abundances by explosive nucleosynthesis in outgoing supernova shock waves, (d) clarification of the role of Type I supernovae, (e) concordance between elements produced in short-lived and long-lived stars with those that increased quickly and slowly over the history of the galaxy, and (f) calibration of calculations of the evolution and explosion of massive stars against the detailed observations of SN 1987A. The discussion presupposes a reader (a) with some prior knowledge of astronomy at the level of recognizing what is meant by an A star and an AGB star and (b) with at least a mild interest in how we got to where we currently are. 相似文献
3.
Paul D. Kiel Jarrod R. Hurley 《Monthly notices of the Royal Astronomical Society》2006,369(3):1152-1166
We perform binary population-synthesis calculations to investigate the incidence of low-mass X-ray binaries (LMXBs) and their birth rate in the Galaxy. We use a binary-evolution algorithm that models all the relevant processes including tidal circularization and synchronization. Parameters in the evolution algorithm that are uncertain and may affect X-ray binary formation are allowed to vary during the investigation. We agree with previous studies that under standard assumptions of binary evolution the formation rate and number of black hole (BH) LMXBs predicted by the model are more than an order of magnitude less than what is indicated by observations. We find that the common-envelope process cannot be manipulated to produce significant numbers of BH LMXBs. However, by simply reducing the mass-loss rate from helium stars adopted in the standard model, to a rate that agrees with the latest data, we produce a good match to the observations. Including LMXBs that evolve from intermediate-mass systems also leads to favourable results. We stress that constraints on the X-ray binary population provided by observations are used here merely as a guide as surveys suffer from incompleteness and much uncertainty is involved in the interpretation of results. 相似文献
4.
Ľubor Kresák 《Earth, Moon, and Planets》1980,22(1):83-98
The dynamics of larger interplanetary bodies is reviewed, with emphasis on evolutionary problems, interrelations, and open questions. Observational biases distinguishing the sample of known objects from the whole population are briefly discussed. A schematic division of the solar system into zones with different regimes of motion, and a rough taxonomy of orbit types are attempted. The role of individual major planets in controlling the dynamical evolution of interplanetary objects, in particular by stabilizing resonances and destabilizing close encounters, is compared. There are significant evolution asymmetries due to boundary conditions and preferential evolutionary paths; individual major displacements in the phase space of orbital elements conserve the Tisserand invariant with respect to the planet responsible, thus favouring certain evolutionary sequences against others. Very limited lifetimes of some orbit types imply a long-term balance between source and sink, and require a continuous supply of objects from other types of orbits. In this respect, the ultimate fate of extinct comets is of particular interest. Under very specific conditions, nongravitational effects of mass loss can result in stabilization of a formerly unstable orbit. Since the dividing line between the two basic interplanetary populations distinguished by origin and composition — the asteroids and the comets — is essentially that between stable and unstable motion, orbital data can be used to specify which of the known asteroid-like objects may be devolatilized cometary nuclei.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy. 相似文献
5.
6.
It is obvious that Venusian volcanotectonic features, namely, coronae and novae, are somehow connected to each other. First, approximately half of the novae are located in the interior region of coronae. In addition, it is proposed that novae represent either the first or a later stage of corona evolution. However, the affects of novae structures on the properties of a corona have been uncertain. The analysis of the Hecate Chasma area shows that coronae with associated novae are different from those coronae with no associated novae structures: corona-nova joint structures are located at higher altitudes and show more prominent volcanism than single coronae. Furthermore, the latest morphologically recorded activity of corona-novae structures is, in general, younger than the latest morphologically recorded activity of the lone coronae. Therefore, it is apparent that the associated novae have a noticeable affect to the characteristics of the adjoining coronae. A stratigraphical analysis of the area has established that the region shows the age relationships expected from the global-scale stratigraphy model by Basilevsky and Head (1995a, 1995b, 2000). In addition, the analysis showed that the formation of coronae and novae cannot be exclusively connected to a certain time era in Venusian geological history, at least not in that particular area.__________From Astronomicheskii Vestnik, Vol. 39, No. 3, 2005, pp. 227—242.Original English Text Copyright © 2005 by Aittola, Raitala.This article was submitted by the authors in English. 相似文献
7.
Thierry Montmerle Jean-Charles Augereau Marc Chaussidon Mathieu Gounelle Bernard Marty Alessandro Morbidelli 《Earth, Moon, and Planets》2006,98(1-4):39-95
The solar system, as we know it today, is about 4.5 billion years old. It is widely believed that it was essentially completed 100 million years after the formation of the Sun, which itself took less than 1 million years, although the exact chronology remains highly uncertain. For instance: which, of the giant planets or the terrestrial planets, formed first, and how? How did they acquire their mass? What was the early evolution of the “primitive solar nebula” (solar nebula for short)? What is its relation with the circumstellar disks that are ubiquitous around young low-mass stars today? Is it possible to define a “time zero” (t 0), the epoch of the formation of the solar system? Is the solar system exceptional or common? This astronomical chapter focuses on the early stages, which determine in large part the subsequent evolution of the proto-solar system. This evolution is logarithmic, being very fast initially, then gradually slowing down. The chapter is thus divided in three parts: (1) The first million years: the stellar era. The dominant phase is the formation of the Sun in a stellar cluster, via accretion of material from a circumstellar disk, itself fed by a progressively vanishing circumstellar envelope. (2) The first 10 million years: the disk era. The dominant phase is the evolution and progressive disappearance of circumstellar disks around evolved young stars; planets will start to form at this stage. Important constraints on the solar nebula and on planet formation are drawn from the most primitive objects in the solar system, i.e., meteorites. (3) The first 100 million years: the “telluric” era. This phase is dominated by terrestrial (rocky) planet formation and differentiation, and the appearance of oceans and atmospheres. 相似文献
8.
A.K. Campbell 《Astrophysics and Space Science》2003,285(2):571-585
Throughout his life, Fred Hoyle had a keen interest in evolution. He argued that natural selection by small, random change,
as conceived by Charles Darwin and Alfred Russel Wallace, could not explain either the origin of life or the origin of a new
protein. The idea of natural selection, Hoyle told us, wasn't even Darwin's original idea in the first place. Here, in honour
of Hoyle's analysis, I propose a solution to Hoyle's dilemma. His solution was life from space – panspermia. But the real
key to understanding natural selection is `molecular biodiversity'. This explains the things Darwin missed – the origin of
species and the origin of extinction. It is also a beautiful example of the mystery disease that afflicted Darwin for over
40 years, for which we now have an answer.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
9.
The clumpy structure in the Vega's debris disk was reported at millimeter wavelengths previously, and attributed to the concentration of dust grains trapped in resonances with a potential high-eccentricity planet. However, current imaging at multi-wavelengths with higher sensitivity indicates that the Vega's debris disk has a smooth structure. But a planet orbiting Vega could not be neglected, and the present-day observations may place a severe constraint on the orbital parameters for the potential planet. Herein, we utilize the modi- fied MERCURY codes to numerically simulate the Vega system, which consists of a debris disk and a planet. In our simulations, the initial inner and outer boundaries of the debris disk are assumed to be 80 AU and 120 AU, respectively. The dust grains in the disk have the sizes from 10 μm to 100 μm, and the nearly coplanar orbits. From the outcomes, we show that the evolution of debris disk is consistent with recent observations, if there is no planet orbiting Vega. However, if Vega owns a planet with a high eccentricity (e.g., e = 0.6), the planet's semi- major axis cannot be larger than 60 AU, otherwise, an aggregation phenomenon will occur in the debris disk due to the existence of the postulated planet. In addition, the 2:1 mean motion resonances may play a significant role in forming the structure of debris disk. 相似文献
10.
HiroyukiHirashita † Leslie K.Hunt reaFerrara 《Monthly notices of the Royal Astronomical Society》2002,330(1):L19-L23
During the early stages of galaxy evolution, the metallicity is generally low and nearby metal-poor star-forming galaxies may provide templates for primordial star formation. In particular, the dust content of such objects is of great importance, because early molecular formation can take place on grains. To gain insight into primeval galaxies at high redshift, we examine the dust content of the nearby extremely low-metallicity galaxy SBS 0335–052 which hosts a very young starburst (≲107 yr). In young galaxies, the dust formation rate in Type II supernovae governs the amount of dust, and by incorporating recent results on dust production in Type II supernovae we model the evolution of dust content. If the star-forming region is compact (≲100 pc), as suggested by observations of SBS 0335–052 , our models consistently explain the quantity of dust, far-infrared luminosity, and dust temperature in this low-metallicity object. We also discuss the H2 abundance. The compactness of the region is important to H2 formation, because the optical depth of dust for UV photons becomes large and H2 dissociation is suppressed. We finally focus on implications for damped Ly α systems. 相似文献
11.
V. V. Svetsov 《Solar System Research》2007,41(1):28-41
The atmospheric erosion induced by impacts of cosmic bodies with sizes from ～100 m to 10 km is calculated for the Earth with its present atmosphere and for Mars with a dense carbon dioxide atmosphere that could be at the early stages of planetary evolution. Numerical results are compared to simple analytic models and calculations performed by other authors; approximate formulas are suggested. The evolutions of early atmospheres, which could exist at the late stage of the planetary accumulation, are numerically simulated using an integral model of impact-induced atmospheric erosion and replenishment in the approximation of a one-component atmosphere with a composition determined by the basic atmosphile component of the bodies falling onto the planet. 相似文献
12.
Irini Sakelliou 《Monthly notices of the Royal Astronomical Society》2000,318(4):1164-1170
In clusters of galaxies, the reaction of the intracluster medium (ICM) to the motion of the co-existing galaxies in the cluster triggers the formation of unique features, which trace their position and motion. Galactic wakes, for example, are an apparent result of the ICM/galaxy interactions, and they constitute an important tool for deciphering the motion of the cluster galaxies.
In this paper we investigate whether Bondi–Hoyle accretion can create galactic wakes by focusing the ICM behind moving galaxies. The solution of the equations that describe this physical problem provides us with observable quantities along the wake at any time of its lifetime. We also investigate which are the best environmental conditions for the detectability of such structures in the X-ray images of clusters of galaxies.
We find that significant Bondi–Hoyle wakes can only be formed in low-temperature clusters, and that they are more pronounced behind slow-moving, relatively massive galaxies. The scalelength of these elongated structures is not very large: in the most favourable conditions a Bondi–Hoyle wake in a cluster at the redshift of z =0.05 is 12 arcsec long. However, the X-ray emission of the wake is noticeably strong: the X-ray flux can reach ∼30 times the flux of the surrounding medium. Such features will be easily detectable in the X-ray images of nearby, relatively poor clusters of galaxies by the Chandra and XMM-Newton satellites. 相似文献
In this paper we investigate whether Bondi–Hoyle accretion can create galactic wakes by focusing the ICM behind moving galaxies. The solution of the equations that describe this physical problem provides us with observable quantities along the wake at any time of its lifetime. We also investigate which are the best environmental conditions for the detectability of such structures in the X-ray images of clusters of galaxies.
We find that significant Bondi–Hoyle wakes can only be formed in low-temperature clusters, and that they are more pronounced behind slow-moving, relatively massive galaxies. The scalelength of these elongated structures is not very large: in the most favourable conditions a Bondi–Hoyle wake in a cluster at the redshift of z =0.05 is 12 arcsec long. However, the X-ray emission of the wake is noticeably strong: the X-ray flux can reach ∼30 times the flux of the surrounding medium. Such features will be easily detectable in the X-ray images of nearby, relatively poor clusters of galaxies by the Chandra and XMM-Newton satellites. 相似文献
13.
In recent years there has been much debate, both observational and theoretical, about the nature of star formation at high
redshift. In particular, there seems to be strong evidence of a greatly enhanced star formation rate early in the Universe’s
evolution. Simulations investigating the nature of the first stars indicate that these were large, with masses in excess of
100 solar masses. By the use of a chemical model, we have simulated the molecular signature of massive star formation for
a range of redshifts, using different input models of metallicity in the early Universe. We find that, as long as the number
of massive stars exceeds that in the Milky Way by factor of at least 1000, then several ‘hot-core’ like molecules should have
detectable emission. Although we predict that such signatures should already be partly detectable with current instruments
(e.g. with the VLA), facilities such as ALMA will make this kind of observation possible at the highest redshifts. 相似文献
14.
《New Astronomy Reviews》2004,48(10):843-859
If a point mass moves through a uniform gas cloud, at what rate does it accrete material? This is the question studied by Bondi, Hoyle and Lyttleton. This paper draws together the work performed in this area since the problem was first studied. Time has shown that, despite the simplifications made, Bondi, Hoyle and Lyttleton made quite accurate predictions for the accretion rate. Bondi–Hoyle–Lyttleton accretion has found application in many fields of astronomy, and these are also discussed. 相似文献
15.
J. V. Narlikar 《Journal of Astrophysics and Astronomy》1984,5(1):67-78
The concept of the steady-state universe discussed by Hoyle & Narlikar two decades ago is revived in the light of the present
discussions of the phase transition in the early big-bang universe. It is shown that with suitable scaling the bubble universe
solution bears a striking similarity to the inflationary scenarios being discussed today. The currently discussed idea of
cosmic baldness was also anticipated in the C-field cosmology of the steady-state universe.
An erratum to this article is available at . 相似文献
16.
S. Collander-Brown M. Maran I. P. Williams 《Monthly notices of the Royal Astronomical Society》2000,318(1):101-108
We investigate the orbital evolution of both real and hypothetical Edgeworth–Kuiper Objects in order to determine whether any conclusions can be drawn regarding the existence, or otherwise, of the tenth planet postulated by Murray. We find no qualitative difference in the orbital evolution, and so conclude that the hypothetical planet has been placed on an orbit at such a large heliocentric distance that no evidence for the existence, or non-existence, can be found from a study of the known Edgeworth–Kuiper Objects. 相似文献
17.
Hsien Shang 《Astrophysics and Space Science》2007,311(1-3):25-34
We review status of theoretical development for jets and molecular outflows from young stellar objects. A particular framework
for explaining these phenomena is one based on the X-wind theory in an environment of magnetized collapsing molecular cloud
cores. The magnetized gravitational collapse follows the standard picture of isolated low-mass star formation, from quasi-static
evolution of the parent molecular cloud cores. The outflow phenomena operate throughout the early evolution of young stars
as a result of star-disk interaction. We discuss emission mechanisms of jets and formation of molecular outflows in this general
framework. The general theoretical framework provides room for self-consistent interpretations for recent observations. Jets
and outflows are integral part of earliest evolution of young stellar objects. 相似文献
18.
Shyamal K. Banerjee Jayant V. Narlikar 《Monthly notices of the Royal Astronomical Society》1999,307(1):73-78
The data on angular sizes and redshifts of ultracompact radio sources used by Jackson & Dodgson in a recent paper have been applied to the various theoretical models in the framework of the quasi-steady-state cosmology proposed by Hoyle, Burbidge & Narlikar. It is found that although acceptable fits to the data are available for the flat models, those with a negative curvature of spatial sections provide a better fit. These latter models require low densities of matter and as such do not demand too high a proportion of dark matter. A comparison is made with the work of Jackson & Dodgson and theoretical implications of the test are discussed. 相似文献
19.
E. Ellingson 《Astrophysics and Space Science》2003,285(1):9-18
The galaxy populations in present-day clusters are distinctly different from those of the field, indicating that environment
plays a strong role in galaxy evolution. This review discusses some of the recent observations of moderate to high redshift
clusters. A consistent picture of galaxy evolution in clusters appears to be emerging, which includes a population of galaxies
which formed early in the cluster history, as well as field galaxies which have had their star formation truncated upon falling
into the cluster potential. Galaxy interactions probably play an important role in exhausting star formation in some of these
galaxies. However, there is significant variation in the populations of different cluster samples, with substantial evidence
that some galaxies have their star formation terminated more gradually. This suggests that different mechanisms may dominate
in different clusters, perhaps because of the recent merging history of the clusters. We also present a recent analysis of
population gradients in clusters which suggests that the observed evolution in cluster populations is consistent with a scenario
where changing infall rates drive the fraction of star forming galaxies in clusters, rather than a changing physical mechanism
within the cluster. Thus, galaxy populations may provide a fundamental measure of the growth of large scale structure.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献