共查询到20条相似文献，搜索用时 31 毫秒
1.
2.
M. Burbidge 《Astrophysics and Space Science》2003,285(2):303-307
The 150-inch telescope on Siding Spring mountain in New South Wales, Australia, stands as a reminder of Fred Hoyle's genius
in aspects of astronomy that surprise those who know him only as a theoretician, as one who did not himself use the telescopes
that produced the all-important data for his theoretical work. This article shows another side of Fred's abilities – that
he could immerse himself in the techniques of telescope building, from mount to mirror, and overcome the many hurdles that
had to be faced.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
3.
J. Faulkner 《Astrophysics and Space Science》2003,285(2):339-339
The impact of Fred Hoyle's work on the structure and evolution of red giants, particularly his breakthrough contribution with
Martin Schwartzschild (1955), is described and assessed. Working with his students in the early 1960s, Hoyle presented new
physical ways of understanding some of the approximations used, and results obtained, in that seminal paper. His initial viewpoint
on the critical role of the outer surface boundary condition was replaced by a more subtle, if related one, which emphasized
the peculiar difficulty of storing much mass outside a dense stellar core. That viewpoint that – low-mass red giants are essentially
white dwarfs with a serious mass-storage problem – is still extremely fruitful. Recently, I have extended Hoyle's approach
to explain not only many of the structural properties of red giants themselves, but also to link and unify the structures
of low-mass stars from the main sequence through both the red giant and horizontal branch phases of evolution. Many aspects
of these stars that had remained mysterious for decades have now fallen into place, and some questions have been answered
that were not even posed before. With red giants as the simplest example, this recent work emphasizes that stars, in general,
may have at least two distinct but very important centres: (i) a geometrical centre, and (ii) a separate nuclear centre, residing in a shell outside a zero-luminosity dense core for example. This two-centre perspective leads to an explicit,
analytic, asymptotic theory of low-mass red giant structure. In this theory, there arises a naturally important in situ measure of central compactness: the parameter
. That parameter, like others, is derived self-consistently and explicitly, and can be used to show how close a given model's
properties are to ultimate asymptotic relationships. The results obtained also imply that the problem of understanding why
such stars become red giants is one of anticipating a remarkable yet natural structural bifurcation which occurs in them.
In the resulting theory, both the ratio
and products like
prove to be important, self-consistently derived quantities. Two striking theorems involving such quantities express between
them the very essence of red giant behaviour, proving analytically for the first time that stars with dense cores are necessarily
(i) extremely luminous, and (ii) very large. Perhaps the most astonishingly unexpected single result is that for the very
value Nature provides for the relevant nuclear energy-generating temperature exponent (CNO's η=15), ρsh and
behave in a well-defined, precisely inverse manner. This emphasizes that the internal behaviour of such stars is definitely anti-homologous rather than homologous, thus showing how very unfortunate the term `shell homology' is. Finally, I sketch a viewpoint which (i) links the structural
and evolutionary behaviour of stars from the main-sequence through horizontal branch phases of evolution, and also (ii) has
implications for post-main-sequence developments in more massive stars.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
4.
5.
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
6.
7.
Jayant V. Narlikar Geoffrey Burbidge R. G. Vishwakarma 《Journal of Astrophysics and Astronomy》2007,28(2-3):67-99
In this paper we discuss the properties of the quasi-steady state cosmological model (QSSC) developed in 1993 in its role
as a cyclic model of the universe driven by a negative energy scalar field. We discuss the origin of such a scalar field in
the primary creation process first described by F. Hoyle & J. V. Narlikar forty years ago. It is shown that the creation processes
which take place in the nuclei of galaxies are closely linked to the high energy and explosive phenomena, which are commonly
observed in galaxies at all redshifts.
The cyclic nature of the universe provides a natural link between the places of origin of the microwave background radiation
(arising in hydrogen burning in stars), and the origin of the lightest nuclei (H, D, He3 and He4). It also allows us to relate the large scale cyclic properties of the universe to events taking place in the nuclei of galaxies.
Observational evidence shows that ejection of matter and energy from these centers in the form of compact objects, gas and
relativistic particles is responsible for the population of quasi-stellar objects (QSOs) and gamma-ray burst sources in the
universe.
In the later parts of the paper we briefly discuss the major unsolved problems of this integrated cosmological and cosmogonical
scheme — the understanding of the origin of the intrinsic redshifts, and the periodicities in the redshift distribution of
the QSOs. 相似文献
8.
Darwin is a cornerstone mission of the Horizons 2000+ program of the European Space Agency. It has the express purpose of
carrying out the first direct search for terrestrial exoplanets, and to achieve unprecedented spatial resolution in the infrared wavelength region. The
detection and study of terrestrial exoplanets promises to usher in a new era in science and will affect a broad spectrum of
disciplines. Further, the time line for implementation of such an instrument is now likely to be of the close order of 10
years, leading to possible answers to one of mankind's most fundamental questions in the second decade of the 21st century.
It has been found that in order to realize am interferometer in space in the next 10–15 years, it is necessary to prepare
the way through a number of intermediary steps – both on the ground and in space. In this context, we here describe GENIE
– a ground based nulling interferometry experiment to be implemented at the VLTI in a partnership between ESA and ESO.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
Michael Efroimsky 《Celestial Mechanics and Dynamical Astronomy》2012,112(3):283-330
Spin–orbit coupling can be described in two approaches. The first method, known as the “MacDonald torque”, is often combined with a convenient assumption that the quality factor Q is frequency-independent. This makes the method inconsistent, because derivation of the expression for the MacDonald torque
tacitly fixes the rheology of the mantle by making Q scale as the inverse tidal frequency. Spin–orbit coupling can be treated also in an approach called “the Darwin torque”. While this theory is general enough to accommodate an arbitrary frequency-dependence of Q, this advantage has not yet been fully exploited in the literature, where Q is often assumed constant or is set to scale as inverse tidal frequency, the latter assertion making the Darwin torque equivalent
to a corrected version of the MacDonald torque. However neither a constant nor an inverse-frequency Q reflect the properties of realistic mantles and crusts, because the actual frequency-dependence is more complex. Hence it
is necessary to enrich the theory of spin–orbit interaction with the right frequency-dependence. We accomplish this programme
for the Darwin-torque-based model near resonances. We derive the frequency-dependence of the tidal torque from the first principles
of solid-state mechanics, i.e., from the expression for the mantle’s compliance in the time domain. We also explain that the
tidal torque includes not only the customary, secular part, but also an oscillating part. We demonstrate that the lmpq term of the Darwin–Kaula expansion for the tidal torque smoothly passes zero, when the secondary traverses the lmpq resonance (e.g., the principal tidal torque smoothly goes through nil as the secondary crosses the synchronous orbit). Thus,
we prepare a foundation for modeling entrapment of a despinning primary into a resonance with its secondary. The roles of
the primary and secondary may be played, e.g., by Mercury and the Sun, correspondingly, or by an icy moon and a Jovian planet.
We also offer a possible explanation for the “improper” frequency-dependence of the tidal dissipation rate in the Moon, discovered
by LLR. 相似文献
10.
There is a growing interest among cosmologists for theories with negative energy scalar fields and creation, in order to model
a repulsive gravity. The classical steady state cosmology proposed by Bondi, Gold & Hoyle in 1948, was the first such theory
which used a negative kinetic energy creation field to invoke creation of matter. We emphasize that creation plays a very
crucial role in cosmology and provides a natural explanation to the various explosive phenomena occurring in local (z < 0.1) and extra galactic universe. We exemplify this point of view by considering the resurrected version of this theory
— the quasi-steady state theory, which tries to relate creation events directly to the large scale dynamics of the universe
and supplies more natural explanations of the observed phenomena.
Although the theory predicts a decelerating universe at the present era, it explains successfully the recent SNe Ia observations
(which require an accelerating universe in the standard cosmology), as we show in this paper by performing a Bayesian analysis
of the data. 相似文献
11.
The general solution of the Henon–Heiles system is approximated inside a domain of the (x, C) of initial conditions (C is the energy constant). The method applied is that described by Poincaré as ‘the only “crack” permitting penetration into
the non-integrable problems’ and involves calculation of a dense set of families of periodic solutions that covers the solution
space of the problem. In the case of the Henon–Heiles potential we calculated the families of periodic solutions that re-enter
after 1–108 oscillations. The density of the set of such families is defined by a pre-assigned parameter ε (Poincaré parameter),
which ascertains that at least one periodic solution is computed and available within a distance ε from any point of the domain
(x, C) for which the approximate general solution computed. The approximate general solution presented here corresponds to ε =
0.07. The same solution is further improved by “zooming” into four square sub-domain of (x, C), i.e. by computing sufficient number of families that reduce the density parameter to ε = 0.003. Further zooming to reduce
the density parameter, say to ε = 10−6, or even smaller, although easily performable in both areas occupied by stable as well as unstable solutions, was found unnecessary.
The stability of all members of each and all families computed was calculated and presented in this paper for both the large
solution domain and for the sub-domains. The correspondence between areas of the approximate general solution occupied by
stable periodic solutions and Poincaré sections with well-aligned section points and also correspondence between areas occupied
by unstable solutions and Poincaré sections with randomly scattered section points is shown by calculating such sections.
All calculations were performed using the Runge-Kutta (R-K) 8th order direct integration method and the large output received,
consisting of many thousands of families is saved as “Atlas of the General Solution of the Henon–Heiles Problem,” including
their stability and is available at request. It is concluded that approximation of the general solution of this system is
straightforward and that the chaotic character of its Poincaré sections imposes no limitations or difficulties. 相似文献
12.
M. Stupar M. D. Filipović Q. A. Parker G. L. White T. G. Pannuti P. A. Jones 《Astrophysics and Space Science》2007,307(4):423-435
The Parkes–MIT–NRAO (PMN) radio survey has been used to generate a quasi all-sky study of Galactic Supernova Remnants (SNRs)
at a common frequency of 4.85 GHz (λ=6 cm). We present flux densities estimated for the sample of 110 Southern Galactic SNRs (up to δ=−65°) observed with the Parkes 64-m radio telescope and an additional sample of 54 from the Northern PMN (up to δ=+64°) survey undertaken with the Green Bank 43-m (20 SNRs) and 91-m (34 SNRs) radio telescopes. Out of this total sample
of 164 selected SNRs (representing 71% of the currently 231 known SNRs in the Green catalogue) we consider 138 to provide
reliable estimates of flux density and surface brightness distribution. This sub-sample represents those SNRs which fall within
carefully chosen selection criteria which minimises the effects of the known problems in establishing reliable fluxes from
the PMN survey data. Our selection criteria are based on a judicious restriction of source angular size and telescope beam
together with careful evaluation of fluxes on a case by case basis. Direct comparison of our new fluxes with independent literature
values gives excellent overall agreement. This gives confidence in the newly derived PMN fluxes when the selection criteria
are respected. We find a sharp drop off in the flux densities for Galactic SNRs beyond 4 Jy and then a fairly flat distribution
from 5 to 9 Jy, a slight decline and a further flat distribution from 9 to 20 Jy though the numbers of SNR in each Jy bin
are low. We also re-visit the contentious Σ–D (radio surface brightness–SNRs diameter) relation to determine a new power law index for a sub-sample of shell type SNRs
which yields β=−2.2±0.6. This new evaluation of the Σ–D relation, applied to the restricted sample, provides new distance estimates and their Galactic scale height distribution.
We find a peak in the SNR distribution between 7–11 kpc with most restricted to ±100 pc Galactic scale height. 相似文献
13.
We present new ultraviolet spectra of the hottest known, peculiar white dwarf H1504+65, obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. H1504+65 is the hottest known white dwarf (T
eff=200 000 K) and has an atmosphere mainly composed by carbon and oxygen, augmented with high amounts of neon and magnesium.
This object is unique and the origin of its surface chemistry is completely unclear. We probably see the naked core of either
a C–O white dwarf or even a O–Ne–Mg white dwarf. In the latter case, this would be the first proof that such white dwarfs
can be the outcome of single-star evolution. The new observations were performed to shed light on the origin of this mysterious
object. 相似文献
14.
Eva K. Grebel 《Astrophysics and Space Science》2003,284(2):947-956
A selection of future observing facilities relevant to galaxy evolution science covering the wavelength range from X-rays
to the radio regime are summarized. Scientific aspects that can be investigated with these next generation telescopes are
briefly discussed. While prospects look bright for highly advanced and innovative future facilities, it will be important
to ensure that human resources in observational astronomy and theory – both in terms of expertise, funding, and available
positions – are keeping pace with the technological developments.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
15.
Yasuhiro Shioya Kenji Bekki Alexandre Vazdekis 《Astrophysics and Space Science》2001,276(2-4):823-829
We investigate a one-zone chemo-photometric evolutionary model of truncated spiral galaxies with and without starbursts in
order to explore the origin of the spectrophotometric properties of S0s. First, we show that 1) the tight U-V colour–magnitude (CM) relation cannot reject the model with a starburst and 2) the model with a starburst can explain the
difference in the I-K colour–magnitude relation between S0 galaxies and spiral galaxies. Next, we demonstrate how we can distinguish the truncated
spirals with a starburst from those without a starburst by using the CaII–Hδ/λ4045 diagram (proposed by Leonardi and Rose,
1996) and the 1550-V colour.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
16.
G.A. Gurzadyan 《Astrophysics and Space Science》1997,257(2):223-236
The IUE observations do not favor the chromospheric origin of typical chromospheric emission lines, of the doublet 2800 MgII
first of all, in close binary systems SX Cas and 22 Vul, both with a cool giant-supergiant as primary, and a hot B7–B8 star
as secondary. Here, it is shown that the concept of common chromospheres – roundchroms can explain the unusual fluxes of chromospheric
emission lines in these systems. The hot components of these binaries – B type stars, themselves are assumed to represent
local binary systems with a white dwarf and an accretion disc: the latter being the source of generation of transition zone
type high excitation emission lines 1240 NV, 1400 SiIV, 1550 CIV, etc. It is concluded, that roundchroms of giant sizes, of
order of 100 R⊙ and larger, are not rare events for the RS CVn type binary systems.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
Wilfred H. Sorrell 《Astrophysics and Space Science》1997,253(1):27-41
Some two decades ago, Hoyle and Wickramasinghe (1976) proposed that the physical conditions inside dense molecular clouds
favour the formation of amino acids and complex organic polymers. There now exists both astronomical and laboratory evidence
supporting this idea. Recent millimeter array observations have discovered the amino acid glycine (NH2CH2COOH) in the gas phase of the dense star-forming cloud Sagittarius B2. These observations would pose serious problems for
present-day theories of molecule formation in space because it is unlikely that glycline can form by the gas-phase reaction
schemes normally considered for dense cloud chemistry. Several laboratory experiments suggest a new paradigm in which amino
acids and other large organic molecules are chemically manufactured inside the bulk interior of icy grain mantles photoprocessed
by direct and scattered ultraviolet starlight. Frequent chemical explosions of the processed mantles would eject large fragments
of organic dust into the ambient cloud. Large dust fragments break up into smaller ones by sputtering and ultimately by photodissociation
of individual molecules. Hence, a sizeable column density (N≈ 1010−1015 cm-2) of amino acids would be present in the gaseous medium as a consequence of balancing the rate of supply from exploding mantles
with the rate of molecule destruction. Exploding mantles can therefore solve the longstanding molecule desorption problem
for interstellar dense cloud chemistry. A sizeable fraction of the organic dust population can survive destruction and seed
primitive planetary systems throughout our galaxy with prebiological organic molecules needed for proteins and nucleic acids
in living organisms. This possibility provides fresh grounds for a new version of the old panspermia hypothesis first introduced
by Anaxagoras. It is shown that panspermia is more important than asteroid and cometary organic depositions onto primitive
Earth. Furthermore, no appeal to Miller-Urey synthesis in a nonoxidizing atmosphere of primitive Earth is then needed to seed
terrestrial life.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
We present a further investigation of the periodogram resulting from the photometric data by Rodríguez et al. (1997) for comet
C/1995 O1 Hale–Bopp and interpret that the main period in the data is 11.23 ± 0.01 h, but not 7.19 days. The latter is now
attributed to an alias of the 11.23-h period. Assuming that the periodicity observed in the photometry is the solar day, the
11.23-h period is consistent with estimates of the sidereal rotation period by Farnham et al. (1998), and Jorda et al. (1997–1999)
provided that the obliquity of the comet‘s equatorial plane to its orbital plane is larger than 75° and 80°, respectively.
This result is in agreement with estimates of the obliquity by Sekanina (1997–1999) and Jorda et al. (1998). A weaker periodic
signal in the light curve could be 5.48 ± 0.01 h, but we suggest that this is an alias of a 3.25 ± 0.01 h period of unknown
origin.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
L. N. Dzhimbeeva 《Solar System Research》2011,45(1):84-91
The use of integral characteristics of radio spots showed that in addition to the known 3–5-min oscillations, the above-spot
regions of sunspots exhibit quasiperiodic oscillations of intensity, perimeter, sectional area, and area of the surface of
the above-spot source radio image with periods in the ranges of 25–80, 90–110, 120–140, and 160–210 min. These periods are
observed for all spots studied using the different characteristics of a radio source, which does not contradict the interpretation
of these low-frequency oscillations as natural oscillations of spots near a certain position of stable equilibrium. 相似文献
20.
Mohammadmahdi Rezaei Yousefi Babak Salehi Kasmaei Abdolhossein Vahabie Caro Lucas Babak Nadjar Araabi 《Solar physics》2009,258(2):297-318
Various phenomena with solar origin and their mutual dependence must be studied in order to predict behaviors in solar – terrestrial
system. Linear statistical methods prevalent in analyzing natural systems may not be able to detect nonlinear dependencies
among solar and geomagnetic processes. When relations, whether linear or nonlinear, between indices and their changes over
time are revealed, better predictions can be made through appropriate modeling techniques. Selection of nonredundant input
variables to build suitable models for prediction of solar and geomagnetic activity is of utmost importance. Mutual information
is a tool that is capable of capturing all dependencies for detecting nonlinear relations and selecting the best subset of
input variables by means of an applicable algorithm that maximizes information about the output and minimizes the shared information
between inputs. High generalization power and improved interpretability of the selected inputs are the consequences of this
analysis. 相似文献