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Showing papers in "Astronomy and Astrophysics in 1993"


Journal Article
TL;DR: In this paper, a solution for the precession and obliquity of the Earth, issued from the orbital solution La90 (Laskar 1990), is presented, which provides the necessary data for the computation of insolation at the surface of Earth from −20 Myr to +10 Myr.
Abstract: The solution for the precession and obliquity of the Earth, issued from the orbital solution La90 (Laskar 1990) is presented. This solution provides the necessary data for the computation of insolation at the surface of the Earth from −20 Myr to +10 Myr. When taking into account the tidal dissipation, this solution presents very good agreement with the 3Myr numerical integration of (Quinn et al. 1991). The main source of uncertainty in the computation of precession and obliquity of the Earth is found to arise from the changes of dynamical ellipticity of the Earth which can occur during an ice age. This change is especially important because of the existence of a resonant effect with a secular term of frequency s 6 − g 6 + g 5 resulting from the perturbations of Jupiter and Saturn

509 citations


Journal Article
TL;DR: In this paper, relativistic protons are considered in the context of the Blandford & Konigl (1979) model of compact radio jets which are assumed to be responsible for the nonthermal emission component of flat-spectrum quasars and BL Lacertids.
Abstract: Recent gamma ray detections and the radio/X-ray correlation of extragalactic flat-spectrum radio sources make the existence of an ultra-relativistic proton population in jets very probable. The protons with maximum Lorentz factors in the range 10 9 -10 11 generate hard photons with energies from keV to TeV via pion and pair photoproduction and subsequent synchrotron cascade reprocessing. In this paper relativistic protons are considered in the context of the Blandford & Konigl (1979) model of compact radio jets which are assumed to be responsible for the nonthermal emission component of flat-spectrum quasars and BL Lacertids. The baryons are simply added to the electrons considered as the only radiative agent in the original work

217 citations


Journal Article
TL;DR: In this paper, the authors extend the numerical calculations of Choudhuri (1989) to study the tilts produced on the rising flux loops by the Coriolis force, and find that the theoretically calculated tilts match the observations only if the magnetic field of the flux loops lies in the range between 60 and 160 kG.
Abstract: Joy's law (Hale et al. 1919) states that bipolar magnetic regions (BMRs) are inclined to the latitudinal line, with the p-shot (preceding spot) of the BMR closer to the equator and the tilt angle increasing with latitude. It is believed that the solar dynamo operates in the overshoot region just below the convection zone and the BMRs are produced by the flux loops rising from there due to magnetic buoyancy. These rising loops are expected to be twisted by the Coriolis force so that they eventually emerge on the solar surface with a tilt. The authors extend the numerical calculations of Choudhuri (1989) to study the tilts produced on the rising flux loops by the Coriolis force. They find that the theoretically calculated tilts match the observations only if the magnetic field of the flux loops lies in the range between 60 and 160 kG. For such flux loops, the tilt has the correct magnitude and also varies correctly with the latitude. If the magnetic fields were stronger than 160 kG, then Coriolis force is much weaker than magnetic buoyancy and is only able to produce tilts which are very small in overall magnitude, though they still vary correctly with latitude. On the other hand, if the fields were smaller than 60 kG, then the Coriolis force would have been so overpowering that the flux loops would move parallel to the rotation axis as found earlier (Choudhuri 1989). Such flux loops appear only in high latitudes and do not obey Joy's law. On changing the drag on the flux tube, these conclusions are not changed. If we change the footpoint separation of the flux loop, then we find that magnetic tension may halt the rise of the flux loop if the footpoint separation is below a critical value. However, for flux tubes which are able to reach the surface, the range from 60 to 160kG for the magnetic field still approximately holds. Thus our calculations seem to rule out either equipartition fields (about lOkG) or very strong megagauss fields.

189 citations



Journal Article
TL;DR: In this paper, a collection of quantitative sunspot observations recorded at the Observatoire de Paris from 1660-1719 is presented, and the comparison with modern observations of the 11-year solar cycle provides a better understanding of the solar cycles over the Maunder Minimum.
Abstract: We present a unique collection of quantitative sunspot observations recorded at the Observatoire de Paris from 1660- 1719. These data contrast significantly with that of earlier observers who bequeathed us a sporadic set of drawings from before the Maunder Minimum in that they are quantitative measurements, they span most of the Maunder minimum uninterrupted. About 8000 daily observations were made from 1660 to 1719, on which we base our description of solar activity properties. Sunspot numbers, butterfly diagrams, active longitudes and rotation rates are all reconstructed and compared to modern ones, and the comparison with modern observations of the 11-year solar cycle provides us with a better understanding of the solar cycles over the Maunder Minimum

166 citations


Journal Article
TL;DR: The IRON project as mentioned in this paper aims to compute electron excitation cross sections and rates of astrophysical and technological importance using the most reliable procedures currently available, e.g. from the Opacity Project.
Abstract: The IRON Project has the goal of computing on a large scale electron excitation cross sections and rates of astrophysical and technological importance, using the most reliable procedures currently available. Radiative transition probabilities and photoionization cross sections not known from other sources e.g. from the Opacity Project, will also be presented. Although the major effort will be for ions of the iron-group elements, other ions of astrophysical interest will also be included. In this introductory paper models and procedures to be used are summarized and the approximations are discussed. As an example of our computational procedures, typical results for fine structure transitions involving electron collisions with Fe XVIII ions and radiative data from Fe XVII are presented

160 citations


Journal Article
TL;DR: GamGamma ray production in supernova remnants is discussed on the basis of current ideas about cosmic ray acceleration in this paper, where the authors consider the case of a supernova with a single supernova core.
Abstract: Gamma ray production in supernova remnants is discussed on the basis of current ideas about cosmic ray acceleration.

129 citations


Journal Article
TL;DR: A detailed model of the structure of Titan's atmosphere in the altitude range 250 to 450 km is developed, giving the distribution of temperature, pressure, haze optical depth, and zonal wind velocity as a function of altitude and latitude as discussed by the authors.
Abstract: We present a comprehensive analysis of data obtained during the 1989 July 3 occultation of 28 Sgr by Titan. The data set includes 23 lightcurves from 15 separate stations, spanning wavelengths from 0.36 to 0.89 micron. A detailed model of the structure of Titan's atmosphere in the altitude range 250 to 450 km is developed, giving the distribution of temperature, pressure, haze optical depth, and zonal wind velocity as a function of altitude and latitude. Haze layers detected in Titan's stratosphere are about one scale height higher than inferred from Voyager data, and show a wavelength dependence indicative of particle sizes on the order of 0.1 micron. A marked north-south dichotomy in haze density is observed with a transition to lower density south of about -20 deg latitude. Zonal wind speeds are inferred from global distortions from spherical symmetry and are of the order of 100 m/s with significant increase toward higher latitudes. Titan's high atmosphere shows substantial axial symmetry; the position angle of the symmetry axis is equal to the position angle of Saturn's spin axis to within about 1 deg.

119 citations


Journal Article
TL;DR: In this paper, a new set of equations and a new numerical method for computing stationary axisymmetric rapidly rotating star models within general relativity is presented, where the source term of the gravitational field is not restricted to a perfect fluid one but represents the most general one, including anisotropic stresses, permitted under the assumptions of stationarity, axismmetry and absence of meridional currents.
Abstract: A new set of equations and a new numerical method for computing stationary axisymmetric rapidly rotating star models within general relativity is presented. The source term of the gravitational field is not restricted to a perfect fluid one but represents the most general one, including anisotropic stresses, permitted under the assumptions of stationarity, axisymmetry and absence of meridional currents. This allows us to derive the equations governing the equilibrium of rotating neutron stars with strong magnetic fields in a self-consistent and fully relativistic way. As regards the numerics, the improvement lies in the exact computation of the metric in the whole space outside the star, thanks to a numerical grid which extends to infinity (due to a suitable compactification of the space external to the star)

118 citations


Journal Article
TL;DR: In this paper, the single body theory of meteoroid interaction with the atmosphere in the form of l = l(t), where l is the relative distance along the meteoroid trajectory and t the relative time, is generalized by allowing for points of sudden gross-fragmentation.
Abstract: The single body theory of meteoroid interaction with the atmosphere in the form of l = l(t), where l is the relative distance along the meteoroid trajectory and t the relative time, is generalized by allowing for points of sudden gross-fragmentation. This theory can be directly applied to photographic observations of meteors, comparing the measured distance, l obs , with the model computed distance, l, by means of the least-squares method. The no-fragmentation case has 4 free parameters to be determined from observations (l 0 , v 0 , v∞, σ), distance and velocity at time t = 0, velocity at t = −∞ and ablation coefficient, respectively

108 citations


Journal Article
TL;DR: In this paper, a new simple model for meteor spectra has been developed, where thermal equilibrium is assumed and self-absorption is taken into account, and free parameters of the model are the temperature, the column density of Fe I atoms, the relative abundances of other atoms, and the visible surface area of meteor radiating volume.
Abstract: A grating spectrum (45 A mm −1 ) of a fireball of −9th absolute magnitude and velocity of 18 km s −1 has been analyzed. A new simple model for meteor spectra has been developed. Thermal equilibrium is assumed and self-absorption is taken into account. The free parameters of the model are the temperature, the column density of Fe I atoms, the relative abundances of other atoms, and the visible surface area of meteor radiating volume. The synthetic spectrum based on this model was computed and compared with the observed spectrum. The values of the free parameters were calculated by the least squares method. Then the abundances of neutral atoms were corrected for ionization to obtain the true ratios of chemical elements

Journal Article
TL;DR: In this paper, the authors presented a thorough study of the north-south asymmetry of solar activity made with the daily sunspot areas, since they are a good indicator of magnetic activity.
Abstract: Although the existence of a north-south asymmetry in solar activity is generally accepted, a detailed study of its significance and behaviour is still missing. In this paper, we present a thorough study of the north-south asymmetry of solar activity made with the daily sunspot areas, since they are a good indicator of magnetic activity. To perform the study, we have constructed an asymmetry time series and analysed it by different methods. First of all, its significance and statistical behaviour has been studied. Secondly, using a Montecarlo simulation we have generated synthetic asymmetries and compared them to the real one by means of an χ 2 -test of goodness of fit, and thirdly, we have used modern techniques, developed to study the chaotic behaviour of time series, to search for low-dimensional deterministic chaos in the asymmetry time series

Journal Article
TL;DR: In this paper, a detailed model for the dust coagulation process in dense cores of molecular clouds without beginning star formation is presented. But the model does not take into account the effects of thermal, turbulent, gravitational motion, motion from incidential particle asymmetries, grain rotation, charges and the accretion of molecules onto the particles.
Abstract: Dust extinction observations and the calculation of gas-dust dynamics indicate that in dense clumps of molecular clouds dust grains coagulate efficiently We set up a detailed model for the dust coagulation process in dense cores of molecular clouds without beginning star formation We took into account the effects of thermal, turbulent, gravitational motion, motion from incidential particle asymmetries, grain rotation, charges, and the accretion of molecules onto the particles For most effects, we developed new formalisms For the first time, we explicitly considered the irregularity and changing fluffiness of the clusters produced in the coagulation process The basis for this treatment was an independent numerical simulation of the structure of such aggregates

Journal Article
TL;DR: A general criterion for the existence of sections of the photospheric polarity inversion line where the overlying magnetic field lines are parallel to the photosphere is presented in this paper.
Abstract: A general criterion for the existence of sections of the photospheric polarity inversion line where the overlying magnetic field lines are parallel to the photosphere is presented. It is shown that such sections, called bald patches, exist for a wide range of parameters in potential and constant-α force-free fields created by four concentrated sources of magnetic flux. Bald patches appear when the polarity inversion line is bent too much in an S-like manner. The appearance of bald patches may be important for the formation of prominences and possibly for solar flares

Journal Article
TL;DR: In this paper, a set of laboratory experiments is presented that study the evolution of solid organic matter at low temperature as a result of exposure to photons and ions, with the aim of simulating the formation and evolution of organic refractory mantles on interstellar grains.
Abstract: A set of laboratory experiments is presented that study the evolution of solid organic matter at low temperature as a result of exposure to photons and ions, with the aim of simulating the formation and evolution of organic refractory mantles on interstellar grains. Organic residue is a product of UV photolysed ice mantles in dense clouds and consists of 2-3 C atom containing compounds. From laboratory experiments, the efficiency of organic residue formation is given as a function of UV radiation dose. The yield of organic residue is so high that it probably is the major source of solid carbonaceous matter in the interstellar medium

Journal Article
TL;DR: In this article, the Second-Order Correlation Approximation with both gradients of density and turbulence intensity simultaneously involved was used to make determinations of the important, but poorly known α-effect.
Abstract: New determinations of the important, but poorly known α-effect are presented in the Second-Order Correlation Approximation with both gradients of density and turbulence intensity simultaneously involved. In both of the two approaches new results appear which are - if they are confirmed by numerical simulations - relevant to the design of new dynamo models. In our first approach the magnetic back-reaction is ignored but no restrictions on the rotation rate are imposed. The density stratification as well as the turbulence intensity inhomogeneity contribute with similar power to the production of the α-effect. The known equality of both contributions appears for fast rotation only

Journal Article
TL;DR: In this paper, the propagation of Alfven waves through isothermal, radially stratified, spherically symmetrical models of stellar atmospheres and winds is discussed, and the transmission coefficient for the waves is calculated as a function of frequency, magnetic field base intensity, surface gravity and atmospheric temperature.
Abstract: The propagation of Alfven waves through isothermal, radially stratified, spherically symmetrical models of stellar atmospheres and winds is discussed. The transmission coefficient for the waves is calculated as a function of frequency, magnetic field base intensity, surface gravity and atmospheric temperature. When a wind is present, the wave energy flux is no longer conserved, but the conservation of the wave-action flux (Heinemann & Olbert 1980) allows the definition of an analogous transmission coefficient, giving the relative amount of waves reaching the super-Alfvenic regions of the wind. It is shown that for high-frequency waves the transmission coefficient for static and wind models is identical, while for low-frequency waves the presence of a wind enhances the transmission considerably

Journal Article
TL;DR: TOPbase, the Opacity Project atomic database, has been set up at the Centre de Donnees Astronomiques de Strasbourg (CDS), France, for general access via Internet as mentioned in this paper.
Abstract: TOPbase, the Opacity Project atomic database, has been set up at the Centre de Donnees Astronomiques de Strasbourg (CDS), France, for general access via Internet This database contains accurately calculated energy levels, f-values and photoionisation cross sections for astrophysically abundant ions We briefly describe the physical model and computational method used, the atomic data specifications, the database management system and the network access arrangements The new facility should be of value to the astronomical community

Journal Article
TL;DR: In this article, the budget of free oxygen and organic carbon on Earth is studied, and it is shown that O 2 is a very reactive gas whose massive presence in a telluric planet atmosphere implies continuous production.
Abstract: Considering the future importance of the search for evidence of primitive life on a distant planet, we have revisited some points of the O 2 and O 3 detection criteria. The budget of free oxygen and organic carbon on Earth is studied. If one includes the organic carbon in sediments, it confirms that O 2 is a very reactive gas whose massive presence in a telluric planet atmosphere implies continuous production. Its detection would be a strong indication for photosynthetic activity, provided the planet is not in a runaway greenhouse phase. In principle, the direct detection of O 2 could be possible in the visible flux of the planet at 760 nm (oxygen A-band) but it would be extremely difficult, considering the much larger flux from the star

Journal Article
TL;DR: In this paper, an account is given of the instrumental setup and observational procedure that led to the qualifying of Zeeman-Doppler imaging and to new detections of magnetic fields on four stars HR 1099, σ 2 CrB, II Peg and UX Ari.
Abstract: An account is given of the instrumental setup and observational procedure that led to the qualifying of Zeeman-Doppler imaging and to new detections of magnetic fields on four stars HR 1099, σ 2 CrB, II Peg and UX Ari

Journal Article
TL;DR: In this article, it was shown that drift (Hall diffusion) of particles in the global regular magnetic field of the Galaxy may be responsible for the observed "knee" in the cosmic ray energy spectrum at 3 10 15 eV.
Abstract: We consider diffusive propagation of galactic cosmic rays with energies up to 10 17 eV. It is shown that drift (Hall diffusion) of particles in the global regular magnetic field of the Galaxy may be responsible for the observed «knee» in the cosmic ray energy spectrum at 3 10 15 eV. The model implies weak energy dependence of cosmic ray diffusion along the magnetic field lines D ∞ E m , m = 0.15-0.20, for a unique power law in the entire energy range 10 9 -10 17 eV

Journal Article
TL;DR: In this article, the authors derived the total production of formaldehyde and the strength of its extended source within the contact surface located at 4660 km from the neutral mass spectrometer on the Giotto spacecraft.
Abstract: The Neutral Mass Spectrometer on the Giotto spacecraft measured the neutral and ion composition in the coma of comet P/Halley. In the ion mass spectra, the 31 amu/e peak is dominated by the protonated formaldehyde ion H 3 CO + and can be used to calculate the H 2 CO density. The radial profile of the amplitude of the 31 amu/e peak can not be reconciled with H 2 CO being only a parent molecule evaporating from the nucleus. From the radial profile of the H 3 CO + density, we derive the total production of formaldehyde and the strength of its extended source within the contact surface located at 4660 km. The total H 2 CO production inside the contact surface is Q(H 2 CO)/Q(H 2 O)=0.029, essentially independent of model assumptions

Journal Article
TL;DR: Using the concept developed in this paper that the cosmic rays originate in three different main sites, a) the normal supernova explosions into the interstellar medium, b) the supernova explosion into a stellar wind, and c) powerful radio galaxies, they demonstrate in this paper that the spectrum and chemical abundances above 10 4 GeV can be well understood.
Abstract: Using the concept developed in earlier papers, that the cosmic rays originate in three different main sites, a) the normal supernova explosions into the interstellar medium, b) the supernova explosions into a stellar wind, and c) powerful radio galaxies, we demonstrate in this paper that the spectrum and chemical abundances above 10 4 GeV can be well understood. Using existing data on the chemical composition of cosmic rays near TeV energies as a constraint, we adjust the parameters of the model to fit the shower size data from the Akeno experiment; this is necessary since the derivation of an all particle spectrum involves an assumption about the chemical composition of the cosmic rays and so we have to fit the shower size data first and then derive the all particle spectrum

Journal Article
TL;DR: In this article, a theory to account for the Cosmic Ray spectrum between 10 4 GeV and 3 10 9 GeV is proposed. But the model is based on a conjecture about the diffusion tensor of relativistic particles perpendicular to the magnetic field at a supernova shock, and considering particle drifts.
Abstract: Based on a conjecture about the diffusion tensor of relativistic particles perpendicular to the magnetic field at a shock, and considering particle drifts, I develop a theory to account for the Cosmic Ray spectrum between 10 4 GeV and 3 10 9 GeV. The essential assumption is that the free mean path perpendicular to the magnetic field is independent of energy and has the scale of the thickness of the shocked layer. I then use the basic concept, that the energetic Cosmic Ray particles are accelerated in a Supernova shock that travels down the density gradient of a stellar wind; as an example I use a Wolf Rayet star wind. Physically important ingredients beside the presence of a strong shock are diffusion, drifts, convection, adiabatic cooling, the injection history, and the topology of the magnetic field, assumed to behave similarly to the solar wind

Journal Article
TL;DR: In this article, the magnetic field topology in an active region was studied for three consecutive days, June 13-15 1980, and evidence was derived that solar flares are produced by magnetic reconnection.
Abstract: From a study of the magnetic field topology in an active region, evidence is derived that solar flares are produced by magnetic reconnection. We study a complex group with two active regions (AR 2511 and AR 2512) for three consecutive days, June 13-15 1980. The observed longitudinal magnetic field is used to model the coronal magnetic field by the potential field created by a series of magnetic charges. This computed field matches satisfactorily the Hα fibrils and the observed transverse field direction. A set of flares occurs principally in the northern group (AR 2511). During these three days, two different magnetic configurations in succession are responsible for the occurrence of these flares: first the intrusion of a new opposite flux in the following polarity, secondly the emergence of new flux between the main polarities

Journal Article
TL;DR: In this paper, the effect of fluctuations in the mean helicity α 0 on a plane dynamo wave was considered and the authors derived simple equations for the phases and the logarithmic amplitudes of the mean field.
Abstract: We consider the effect of fluctuations δα(t) in the mean helicity α 0 (both assumed independent of position) on a plane dynamo wave. The time scale in, of the fluctuations is much shorter than the diffusion time 1/β 0 k 2 (β 0 k 2 τ c 1; β 0 = turbulent diffusion coefficient; k = wave number). We distinguish weak and strong random forcing, according to whether (δα r.m.s. /α 0 )√β 0 k 2 τ c is small or large with respect to 1, and we present a detailed analysis of the weak forcing case. Simple equations are derived for the phases and the logarithmic amplitudes of the poloidal and toroidal mean field, in which the forcing terms appear as additive noise

Journal Article
TL;DR: In this article, the authors discuss the use of multi-frequency microwave links in Doppler tracking of interplanetary spacecraft for scientific purposes, especially in relation to the measurement and the elimination of the contribution due to the plasma in the solar wind and the ionosphere.
Abstract: We discuss the use of multi-frequency microwave links in Doppler tracking of interplanetary spacecraft for scientific purposes, especially in relation to the measurement and the elimination of the contribution due to the plasma in the solar wind and the ionosphere We show how the two contributions in the up- and the down-link can be separately measured with a five links scheme; for the ordinary case in which we have two carriers in the down-link, but only one in the up-link, (for which the plasma contribution can be directly measured only for the former) we find the optimum algorism for computing the latter contribution We also discuss in detail Doppler measurements near opposition, which can provide information on the dynamics of the turbulence in the solar wind and, in particular, study the dispersion of its modes

Journal Article
TL;DR: In this paper, the separated first integrals of Carter governing geodesic motion of light-like particles in Kerr geometry are removed from the uncertainties of sign, enabling a thorough discussion of the image generation of arbitrary axisymmetric, emitting objects at arbitrary observer locations.
Abstract: The stationary emitter-observer problem in Kerr geometry requires the systematic study of all lightlike geodesics connecting a given emitter with a given observer when emission is independent of coordinate time. To this aim, the separated first integrals of Carter governing geodesic motion of lightlike particles in Kerr geometry are removed from the uncertainties of sign. This enables a thorough discussion of the image generation of arbitrary axisymmetric, emitting objects at arbitrary observer locations, yielding a classification of all possible image orders by two parameters with only discrete values: one of them counts transits of the light path through the equatorial plane of the rotating hole, the second distinguishes two possible directions of the initial latitudinal emission

Journal Article
TL;DR: In this article, the authors apply two different methods for the numerical treatment of the hydrodynamical equations to the problem of stellar core collapse in one dimension, starting the simulations with the same initial model and using the same equation of state.
Abstract: We apply two different methods for the numerical treatment of the hydrodynamical equations to the problem of stellar core collapse in one dimension, starting the simulations with the same initial model and using the same equation of state. We follow the collapse with a standard Lagrangian finite difference scheme with artificial viscosity and compare it with a second order Godunov-type scheme (PPM), which works in Eulerian coordinates. Different from Marti et al. (1990) our results demonstrate that with a careful use of an up-to-date tensor form for the artificial viscosity the results of both codes are in excellent agreement

Journal Article
TL;DR: In this paper, the perturbative effect of the force due to direct solar radiation pressure on the dynamics of artificial satellites in a more comprehensive way than has been done so far is studied.
Abstract: We have studied the perturbative effect of the force due to direct solar radiation pressure on the dynamics of artificial satellites in a more comprehensive way than has been done so far. We have included a general model for the atmospheric refraction and extinction and have computed the illumination of a satellite for any geometric arrangement between it, the Earth and the Sun. This allowed us to investigate in detail the transition between the full sunlight and the shadow. We have considered separately the influence of the atmospheric refraction (treated according to geometrical optics) and the various physical processes in the atmosphere (absorption and scattering), to conclude that in general the flux attenuation during penumbra is most easily interpreted as due to a compression of the solar disk when viewed from the satellite through the Earth's atmosphere