Showing papers in "The Astrophysical Journal in 1990"
TL;DR: In this article, a modele de masse for les galaxies elliptiques, which approche la loi R 1/4 de Vaucouleur, is developpe.
Abstract: Un modele de masse pour les galaxies elliptiques, qui approche la loi R 1/4 de Vaucouleur, est developpe. Il est demontre que les proprietes intrinseques et les distributions de densite de ce modele peuvent etre evaluees analytiquement. La brillance de surface et les dispersions de vitesse projetees, la fonction de distribution et la densite des etats sont exprimees en terme de fonctions elementaires.
2,934 citations
TL;DR: A wide variety of least-squares linear regression procedures used in observational astronomy, particularly investigations of the cosmic distance scale, are presented and discussed in this article, where a formula for the intercept offset between two parallel data sets, which propagates slope errors from one regression to the other, and a generalization of the Working-Hotelling confidence bands to nonstandard least squares lines.
Abstract: A wide variety of least-squares linear regression procedures used in observational astronomy, particularly investigations of the cosmic distance scale, are presented and discussed. The classes of linear models considered are (1) unweighted regression lines, with bootstrap and jackknife resampling; (2) regression solutions when measurement error, in one or both variables, dominates the scatter; (3) methods to apply a calibration line to new data; (4) truncated regression models, which apply to flux-limited data sets; and (5) censored regression models, which apply when nondetections are present. For the calibration problem we develop two new procedures: a formula for the intercept offset between two parallel data sets, which propagates slope errors from one regression to the other; and a generalization of the Working-Hotelling confidence bands to nonstandard least-squares lines. They can provide improved error analysis for Faber-Jackson, Tully-Fisher, and similar cosmic distance scale relations.
1,059 citations
TL;DR: In this article, the authors show that the solar wind speed at 1 AU and the rate of magnetic flux tube expansion in the corona are inversely correlated with observations extending over the last 22 years.
Abstract: The hypothesis that the solar wind speed at 1 AU and the rate of magnetic flux-tube expansion in the corona are inversely correlated is shown to be consistent with observations extending over the last 22 years. This empirical relationship allows the daily wind speeds at earth to be predicted from a current-free extrapolation of the observed photospheric field into the corona. The narrow boundaries of high-speed wind streams are attributed to steep gradients in the flux-tube expansion rates at the edges of coronal holes. When a heliospheric current sheet is included in the model, it is found that the flux tubes near the hole axis, although diverging more slowly than those near the hole boundary in the corona, have undergone the greatest net expansion at 1 AU, an effect consistent with the low densities within high-speed streams.
834 citations
TL;DR: In this article, the geometric and irradiation heating problems for the binary star reflection effect theory are developed in terms of equipotential level surfaces and are sufficiently general so as to include eccentric orbits and nonsynchronous (even centrifugally limited) rotation and to treat multiple reflection.
Abstract: The geometric and irradiation heating problems for the binary star reflection effect theory are developed in terms of equipotential level surfaces and are sufficiently general so as to include eccentric orbits and nonsynchronous (even centrifugally limited) rotation and to treat multiple reflection. The requisite physics, mathematics, and logic are then presented and the computations are organized so that a given quantity is computed only as often as necessary, emphasizing the distinction between local surface quantities and aspect-related quantities. The local geometric, bolometric, and wavelength-specific quantities are grouped for storage according to how often they need to be recomputed. Some tests of a computer program based on this reflection model are given in the form of graphs in which program results are compared to a special exact case, and with results from an earlier program. The new program gives intuitively reasonable output for all tests, and the tests give an idea of how accurate the old program is, adopting the detailed reflection computations of the new program as a standard for comparison. A table is given which shows the convergence of the multiple reflection computations to a constant distribution of surface effective temperature. 11 refs.
826 citations
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TL;DR: In this paper, the effect of neutrinos in the overlying shells of heavy elements becomes so great that, despite the small cross-section, substantial nuclear transmutation is induced, and a large number of rare isotopes including many odd-Z nuclei from boron through copper, owe much of their present abundance in nature to this process.
Abstract: As the core of a massive star collapses to form a neutron star, the flux of neutrinos in the overlying shells of heavy elements becomes so great that, despite the small cross section, substantial nuclear transmutation is induced. Neutrinos excite heavy elements and even helium to particle unbound levels. The evaporation of a single neutron or proton, and the back reaction of these nucleons on other species present, significantly alters the outcome of traditional nucleosynthesis calculations leading to a new process: nu-nucleosynthesis. Modifications to traditional hydrostatic and explosive varieties of helium, carbon, neon, oxygen, and silicon burning are considered. The results show that a large number of rare isotopes, including many of the odd-Z nuclei from boron through copper, owe much of their present abundance in nature to this process.
550 citations
TL;DR: In this article, the geometric structures of configurations with a pair of type A and B nulls permit reconnection across the null-null lines; these are the field lines which join the two nulls.
Abstract: The present investigation of three-dimensional reconnection of magnetic fields with nulls and of fields with closed lines gives attention to the geometry of the former, with a view to their gamma-line and Sigma-surface structures. The geometric structures of configurations with a pair of type A and B nulls permit reconnection across the null-null lines; these are the field lines which join the two nulls. Also noted is the case of magnetostatic reconnection, in which the magnetic field is time-independent and the electrostatic potential is constant along field lines.
525 citations
TL;DR: In this paper, a preliminary spectrum is presented of the background radiation between 1 and 20 cm from regions near the north Galactic pole, as observed by the FIRAS instrument on the COBE satellite.
Abstract: A preliminary spectrum is presented of the background radiation between 1 and 20/cm from regions near the north Galactic pole, as observed by the FIRAS instrument on the COBE satellite. The spectral resolution is 1/cm. The spectrum is well fitted by a blackbody with a temperature of 2.735 + or - 0.06 K, and the deviation from a blackbody is less than 1 percent of the peak intensity over the range 1-20/cm. These new data show no evidence for the submillimeter excess previously reported by Matsumoto et al. (1988) in the cosmic microwave background. Further analysis and additional data are expected to improve the sensitivity to deviations from a blackbody spectrum by an order of magnitude.
514 citations
TL;DR: In this article, three gravitational potentials differing in the content of dark matter in the Galactic plane are used to study the structure of the z-distribution of mass and pressure in the solar neighborhood.
Abstract: Three gravitational potentials differing in the content of dark matter in the Galactic plane are used to study the structure of the z-distribution of mass and pressure in the solar neighborhood. A P(0) of roughly (3.9 + or - 0.6) x 10 to the -12th dyn/sq cm is obtained, with roughly equal contributions from magnetic field, cosmic ray, and kinetic terms. This boundary condition restricts both the magnitude of gravity and the high z-pressure. It favors lower gravity and higher values for the cosmic ray, magnetic field, and probably the kinetic pressures than have been popular in the past. Inclusion of the warm H(+) distribution carries a significant mass component into the z about 1 kpc regime.
461 citations
TL;DR: In this paper, a simple model of an initially high-sigma striped MHD wind is developed which describes the reconnection annihilation of the opposite polarity stripes as the wind flows radially outward.
Abstract: Previous relativistic pair plasma MHD wind models of the Crab Nebula indicate that the ratio of the wind's Poynting flux to particle kinetic energy flux must be very small. However, many current theories of pulsar magnetospheres and pulsar observations suggest that the spin-down luminosity should be primarily electromagnetic in origin and outward transport. For an oblique rotator, the asymptotic wind magnetic field near the rotational equator should consist of stripes of alternating toroidal field. A simple model of an initially high-sigma striped MHD wind is developed which describes the reconnection annihilation of the opposite polarity stripes as the wind flows radially outward. The initially dominant Poynting flux is converted into particle thermal and directed kinetic energy well within the inner standing shock which terminates the superfast wind.
460 citations
TL;DR: In this article, high-angular-resolution maps of the M17 SW cloud core in the C(O-18) J = 2-1 (13 arcsec FWHM beam) and C(S-34) J < 2.5 km/s were obtained with the IRAM 30 m telescope.
Abstract: This paper presents high-angular-resolution maps of the M17 SW cloud core in the C(O-18) J = 2-1 (13 arcsec FWHM beam) and C(S-34) J = 2-1 (27 arcsec FWHM), 3-2 (17 arcsec FWHM) lines obtained with the IRAM 30 m telescope. These maps directly reveal the clumpy structure of the entire cloud core down to linear scales of 13 arcsec and below. Individual clumps have line widths as small as 0.5 km/s, compared to a line width of 3-5 km/s observed with lower angular resolution in the same lines. The C(O-18) 2-1 line reaches a peak brightness temperature of 24 K in the 13-arcsec beam and is probably slightly optically thick in several locations. Spectra in the more abundant CO isotopes obtained toward several selected positions show strong self-absorption notches and peak brightness temperatures in between these notches of about 100 K. 65 refs.
457 citations
TL;DR: In this article, it is shown that the tidal field of the cluster as a whole is physically sufficient to trigger cluster spiral nuclear and disk activity, and that tidally induced collisions of disk gas clouds should be much more effective than ram pressure in causing activity.
Abstract: Gavazzi and Jaffe found that late-type spiral galaxies in rich clusters are radio sources 10 times more powerful than spirals outside clusters. Gavazzi and Jaffe propose that ram pressure from the cluster medium causes collapse of dense molecular clouds, star formation, and other activity in the spirals disks. Bothun and Dressler similarly explain blue H I poor disk galaxies found in the central regions of the Coma Cluster via ram pressure effects. With computer simulations, it is shown in the present work that the tidal field of the cluster as a whole is physically sufficient to trigger cluster spiral nuclear and disk activity. It is shown that tidally induced collisions of disk gas clouds should be much more effective than ram pressure in causing activity. The observed tendency for disk galaxies in centers of rich clusters to be barred and S0 is explained as due to the cluster tidal field. Metals in the cluster medium would be enhanced by the expulsion of spirals enriched gas via tidally induced activity. 36 refs.
TL;DR: In this paper, a polynomial time series representation for these three effects is developed that is appropriate for analysis of the present data with its limited degrees of freedom, and a pulsar timing array program is described that has been established at the National Radio Astronomy Observatory 43 m telescope with observations of PSR 1620 - 26, PSR 181 - 24, and PSR 1937 + 21.
Abstract: Arrival time data from a spatial array of millisecond pulsars can be used (1) to provide a time standard for long time scales, (2) to detect perturbations of the earth's orbit, and (3) to search for a cosmic background of gravitational radiation. A polynomial time series representation for these three effects is first developed that is appropriate for analysis of the present data with its limited degrees of freedom. A pulsar timing array program is then described that has been established at the National Radio Astronomy Observatory 43 m telescope with observations of PSR 1620 - 26, PSR 181 - 24, and PSR 1937 + 21. The results cover a 2 yr period beginning in July 1987. The influence of global parameters - clock, earth location, and effects of gravitational radiation is discussed in the context of the polynomial model. 45 refs.
TL;DR: In this paper, the authors investigated the dynamic mass exchange process in doubly degenerate binaries using a three-dimensional numerical simulation of the evolution of a binary system in which the primary is a 1.2-solar-mass white dwarf and the Roche lobe filling secondary is a 0.9 -solar mass dwarf.
Abstract: The dynamic mass exchange process in doubly degenerate binaries was investigated using a three-dimensional numerical simulation of the evolution of a doubly degenerate binary system in which the primary is a 1.2-solar-mass white dwarf and the Roche lobe filling secondary is a 0.9-solar-mass dwarf. The results show that, in a little more than two orbital periods, the secondary is completely destroyed and transformed into a thick disk orbiting about the primary. Since only a very small fraction of the mass (0.0063 solar mass) escapes the system, the evolution of the binary results in the formation of a massive object. This object is composed of three parts, the initial white dwarf primary, a very hot pressure-supported spherical envelope, and a rotationally supported outer disk. The evolution of the system can be understood in terms of a simple analytical model where it is shown that the angular momentum carried by the mass during the transfer and stored in the disk determines the evolution of the system.
TL;DR: In this article, the authors consider the tidal disruption of a star by a moderately massive black hole such as might be found in the nucleus of a galaxy like M31 and study the evolution of the accretion disk produced by this torus using a time dependent α-disk model.
Abstract: We consider the tidal disruption of a star by a moderately massive black hole such as might be found in the nucleus of a galaxy like M31 The initial eccentric orbit of the stellar debris will circularize near the tidal radius after experiencing strong shocks We study the evolution of the accretion disk produced by this torus using a time dependent α-disk model We find that the light-to-mass ratio of the disk-plus-black-hole exceeds unity for several thousand years after disruption Some fraction of galaxies should be extremely bright at far UV wavelengths if they contain black holes of mass 106–8 M ⊙
TL;DR: In this article, it is argued that easily measured, reliable estimates of terminal velocities for early-type stars are provided by the central velocity asymptotically approached by narrow absorption features and by the violet limit of zero residual intensity in saturated P Cygni profiles.
Abstract: It is argued that easily measured, reliable estimates of terminal velocities for early-type stars are provided by the central velocity asymptotically approached by narrow absorption features and by the violet limit of zero residual intensity in saturated P Cygni profiles. These estimators are used to determine terminal velocities, v(infinity), for 181 O stars, 70 early B supergiants, and 35 Wolf-Rayet stars. For OB stars, the values are typically 15-20 percent smaller than the extreme violet edge velocities, v(edge), while for WR stars v(infinity) = 0.76 v(edge) on average. New mass-loss rates for WR stars which are thermal radio emitters are given, taking into account the new terminal velocities and recent revisions to estimates of distances and to the mean nuclear mass per electron. The relationships between v(infinity), the surface escape velocities, and effective temperatures are examined. 67 refs.
TL;DR: The caracteristiques les plus significatives of cette branche UV observee dans les galaxies elliptiques and S0 sont passees en revue, and the dispersion de la metallicite dans ces galaxies is discutee as discussed by the authors.
Abstract: Les galaxies elliptiques emettent un flux significatif de rayonnement aux longueurs d'onde au-dessous d'environ 2000 A, avec le flux croissant pour la longueur d'onde decroissante. Les caracteristiques les plus significatives de cette branche UV observee dans les galaxies elliptiques et S0 sont passees en revue, et la dispersion de la metallicite dans ces galaxies est discutee. Les problemes et complexites inherents aux candidats d'etoiles vieilles uniques sont ensuite discutes, divers scenarios unifies pour ces candidats sont exposes, et les manieres possibles de generer la correlation UV - metallicite observee sont discutees. Les candidats binaires sont egalement presentes. Tous les candidats sont reexamines dans le contexte des populations stellaires vieilles qui ont ete resolues en etoiles individuelles, i.e., les amas globulaires galactiques, le bulbe de la Voie Lactee et de M 31.
TL;DR: In this paper, basic dynamical mechanisms that produce an amplification of the accretion rate of gas clouds into the central regions of barred galaxies, and their subsequent effects on the evolution of the galaxy are discussed.
Abstract: Basic dynamical mechanisms that produce an amplification of the accretion rate of gas clouds into the central regions of barred galaxies, and their subsequent effects on the evolution of barred galaxies, are discussed. Weakly dissipative orbits, representing gas clouds, are computed in a barred galaxy model with a central mass concentration such as a black hole, or a secondary small inner bar. Amplified accretion across resonances that is especially rapid inside the inner Lindblad resonance, large excursions outside the galactic plane, and the existence of nontrivial attractors like strange (chaotic) attractors or limit cycles are found. The underlying physical mechanisms are, in general, due to the presence of broad horizontal and vertical resonances through which weakly dissipative particles can rapidly traverse. The principal physical implications of these findings are discussed. 65 refs.
TL;DR: The carbon isotopic ratio across the galaxy was studied by observing nine interstellar clouds in the doubly rare isotope of carbon monoxide, C-13/O-18, and in the rare isotopic species C-12/C-13 isotope species as mentioned in this paper.
Abstract: The carbon isotopic ratio across the Galaxy was studied by observing nine interstellar clouds in the doubly rare isotope of carbon monoxide, C-13/O-18, and in the rare isotopic species C-12/O-18 The goal in this study was to improve the data from optically thin carbon monoxide emissions A systematic gradient is found in the C-12/C-13 isotopic ratio across the Galaxy, ranging from about 30 in the inner part at 5 kpc to about 70 at 12 kpc, with a Galactic center value of 24 Near the solar radius the average ratio is 57, but it is somewhat larger, about 67, in Orion Carbon monoxide isotopic ratios are smaller than those of formaldehyde, but they have roughly the same slope across the Galaxy The result of 57 for the local CO ratio lies between the values from various CH(+) absorption measurements 48 refs
TL;DR: In this article, the core-component widths of pulsars with interpulses were studied in an effort to define the geometric properties of the core emission region, and the results were then applied to a large population of core-single, triple, and five-component pulsars which all have core components.
Abstract: The core-component widths of pulsars with interpulses are studied in an effort to define the geometric properties of the core emission region. The results are then applied to a large population of core-single, triple, and five-component pulsars which all have core components. Core-component widths are intimately related to the polar-cap geometry at the stellar surface. A simple mathematical expression, established through the study of two-pole interpulsars, indicates that the core-component widths depend only upon the pulsar period and alpha, the angle between the rotation and magnetic axes of the star. The relationship can then be used to estimate alpha in any pulsar with a core component. Values of alpha are estimated for about 110 core single (St), triple (T), and five-component (M) pulsars. These results have important implications for the nature of the core radiation process. Core emission appears to come essentially from the stellar surface, filling the entire polar-cap 'gap' region where particle acceleration is thought to take place. 70 refs.
TL;DR: In this paper, a systematic alignment of 20-60 faint background galaxy images has been detected, centered on foreground galaxy clusters of high velocity dispersion, and the background galaxy population is selected by its extreme blue B-R color.
Abstract: A gravitational lens distorts most background galaxies by stretching along a circle centered on the lens. This systematic alignment of 20-60 faint background galaxy images has been detected, centered on foreground galaxy clusters of high velocity dispersion. The background galaxy population is selected by its extreme blue B-R color. At a limiting surface brightness of 29 B mag/sq arcsec, there are over 30 background galaxies/sq arcmin per mag anywhere in the sky, which is sufficient to map statistically the dark matter distribution in a foreground cluster. The dark matter is apparently correlated (center and radial extent) with the cluster red light, suggestive of a baryonic origin or dissipative coupling. The existence of a high percentage of lens-distorted faint blue galaxies sets a lower limit of approximately 0.9 to this background galaxy population mean redshift. 13 refs.
TL;DR: The influence of matter on a radiation fireball is examined in this paper, where it is shown that even when a small amount of baryonic material is present, most of the radiation energy will be converted to a kinetic energy of the matter, and almost no electromagnetic signal will be observed.
Abstract: The influence of matter on a radiation fireball is examined. Even when a small amount of baryonic material is present, most of the radiation energy of the fireball will be converted to a kinetic energy of the matter, and almost no electromagnetic signal will be observed. The implications of this result for several processes in which such a fireball might occur are discussed. 10 refs.
TL;DR: In this paper, a simplified model of the vertical structure of accretion disks is derived and analytical expressions for the temperature and density structure, which represent a generalization of the gray model long known in the theory of classical stellar atmospheres, are presented.
Abstract: A simplified model of the vertical structure of accretion disks is derived. Analytical expressions for the temperature and density structure, which represent a generalization of the gray model long known in the theory of classical stellar atmospheres, are presented. The formalism naturally explains similarities and differences between the structure of a disk and a stellar atmosphere. In particular, the influence of viscous dissipation and external irradiation of the disk by the central star, as well as of the finite optical thickness of the disk, may be easily accounted for and explained by the present model.
TL;DR: An analytic solution for the mean intensity of resonance-line radiation within an absorbing medium of large scattering optical depth but low density is derived in this article, where the line profile, mean number of scatterings before absorption or escape, the mean path length traveled, and the fraction of the photons which escape the medium are derived.
Abstract: An analytic solution is derived for the mean intensity of resonance-line radiation within an absorbing medium of large scattering optical depth but low density. The line profile, the mean number of scatterings before absorption or escape, the mean path length traveled, and the fraction of the photons which escape the medium are derived. Approximate methods are also developed which permit the treatment of media of intermediate optical depth, where scattering takes place in the Doppler core. Astrophysical applications include the absorption of H Ly-alpha radiation by stellar dust, the pumping of H2 Lyman band lines by H Ly-alpha and the Bowen fluorescence.
TL;DR: In this article, the prepeak smooth rise and broad peak of the light curve are well modeled by assuming some mixing of Ni-56 into the hydrogen-rich envelope and of hydrogen into the core.
Abstract: Earlier light curve models for SN 1987A have been improved by adopting more realistic presupernova models and adopting a postexplosion elemental distribution consistent with X-ray and gamma-ray observations. The light curve shape is sensitive to the distributions of abundance and expansion velocity in the ejecta. It is found that the prepeak smooth rise and the broad peak of the light curve, respectively, are well modeled by assuming some mixing of Ni-56 into the hydrogen-rich envelope and of hydrogen into the core. The existence of the hydrogen recombination front is responsible for the formation of the plateaulike peak. For explosion energy, good agreement with the observed light curve is obtained for E/M(env) = (1.1 + or - 0./3) x 10 to the 50th ergs/solar mass. Further improvements on the early light curve, the dependence of the light curve on the distribution of Ni-56 and hydrogen, and the relation with other type II supernovae are discussed. 72 refs.
TL;DR: In this article, a comprehensive survey of Galactic globular cluster evolution until core collapse is presented, where two evolutionary endpoints are studied in detail: (1) the disruption of weakly bound clusters by mass loss due to stellar evolution, and (2) core collapse of multimass clusters.
Abstract: Globular cluster evolution is studied, modeling relaxation and energy exchange between the stars. A comprehensive survey is presented of Galactic globular cluster evolution until core collapse. Mass loss during the first five billion years is sufficiently strong to disrupt weakly bound clusters with a Salpeter initial mass function (IMF). The slope of the IMF is the critical parameter governing cluster survival during the mass-loss phase. Cluster which survive this initial phase then begin to collapse. Those with smaller mass and/or smaller Galactocentric orbital radii complete their core collapse by the present time. Two evolutionary endpoints are studied in detail: (1) the disruption of weakly bound clusters by mass loss due to stellar evolution, and (2) core collapse of multimass clusters. The state in which the multimass clusters exist today, if they have neither disrupted nor collapsed, is discussed. A number of observables based on stellar evolution calculations are presented. 83 refs.
TL;DR: In this paper, the distribution of old neutron stars in the Galaxy was calculated by integrating numerically some 90,000 orbits in the Galactic gravitational potential for up to 10 to 10th yr.
Abstract: Models in which gamma-ray bursters are young neutron stars may be difficult to reconcile with the apparently isotropic distribution of observed bursts. To justify this statement, the distribution of old neutron stars in the Galaxy was calculated by integrating numerically some 90,000 orbits in the Galactic gravitational potential for up to 10 to the 10th yr. Other calculations included the dipole moment in the angular distribution of neutron stars as a convenient measure of their concentration toward the Galactic center, the quadrupole moment in their angular distribution as a convenient measure of their concentration toward the Galactic plane, and the average value of V/Vmax as a convenient measure of the radial distribution of neutron stars. Models that require accretion from a cold circumstellar disk or from a close companion are only marginally consistent with the isotropic distribution of the observed bursts. Models that require accretion of interstellar matter are ruled out due to a very strong dipole anisotropy. Models that suggest the Galactic halo origin of gamma-ray bursts are briefly discussed. The GRANAT and GRO missions should provide evidence for or against the association of gamma-ray bursts with the Galactic disk neutron stars.
TL;DR: In this article, the authors used spectropolarimetry to study eight high-polarization Seyfert 2 galaxies and found evidence for a hidden broad-line region (BLR), visible only in the polarized flux spectrum.
Abstract: Spectropolarimetry was used to study eight high-polarization Seyfert 2 galaxies. In four of them (Mrk 3, Mrk 348, Mrk 463E, and NGC 7674), evidence was found for a hidden broad-line region (BLR), visible only in the polarized flux spectrum. The other four objects, Mrk 78, Mrk 1066, NGC 591, and NGC 7672, may also have hidden BLRs, but at a flux level below the detection threshold. The properties of the hidden BLRs are consistent with their normal Seyfert 1 nuclei. If all Seyfert 2 galaxies are in reality Seyfert 1 galaxies in which the nuclei are hidden from direct view and seen only in scattered light, then one might expect that many Seyfert 2 galaxies have polarizations of their featureless continua greater than 50 percent. Such high polarization are not seen, perhaps indicating a particular geometry, in which nearly edge-on obscuring tori also block light from the scattering region. This model predicts that, in edge-on cases, little or no featureless continua in the spectra of the objects will be detected. Arguments are presented that objects with high luminosity, such as QSOs, have geometrically thin tori and hence are much less likely to have obscured continua and BLRs. 49 refs.
TL;DR: In this article, the porosity of comets was derived from observations of Comet Halley and derived using a method based on the 3.4 and 9.7 micron emission bands.
Abstract: The interstellar dust model of comets is numerically worked out to satisfy simultaneously several basic constraints provided by observations of Comet Halley, and to derive the porosity of coma dust. The observational constraints are (1) the strengths of the 3.4 and 9.7 micron emission bands, (2) the shape of the 9.7 micron band, (3) the amount of silicates relative to organic materials, and (4) the mass distribution of the dust. The method used involves precise calculations of temperatures and the emission characteristics of porous aggregates of interstellar dust as a function of their mass, porosity, and distance to the sun and the wavelength. The results indicate that coma dust has a porosity in the range 0.93-0.975, i.e., a packing factor of 0.07 or less, consistent with independent observations of comet densities of 0.6 to 0.26 g/cu cm and meteor densities of less than 0.2 g/cu cm.
TL;DR: In this article, the radio emission from the Coma cluster of galaxies has revealed the most detailed image of an intergalactic halo yet seen in a cluster of the galaxies.
Abstract: New observations of the radio emission from the Coma cluster of galaxies have revealed the most detailed image of an intergalactic halo yet seen in a cluster of galaxies. The centroid of the halo emission is significantly displaced from that of the X-ray emission. The Faraday rotation of polarized emission from background radio sources has been measured. There is a significant contribution to the rotation measure (RM) of sources seen through the cluster. The excess RM, when combined with the X-ray data, has allowed direct measurement of the magnetic-field strength, at about 2 microG, providing an independent check of the widely used assumption of equipartition between the energy in relativistic particles and that in the magnetic field. The observations also allow an estimate of the degree of tangling of the cluster magnetic field. The magnetic field reversals seem to occur on a scale size of 13-40 kpc. The extent of the halo emission and the degree of field tangling indicate that the radio halo source must be sustained by particle reacceleration in the intracluster medium. 50 refs.
TL;DR: In this article, a theoretical parameter study of dense photodissociation regions is presented, where it is found that when the gas density is sufficiently high relative to the FUV flux, self-shielding of the molecules can move the C(+)/CO and H/H2 atomic-molecular transitions close to the surfaces of the molecular cloud, where they can feel the full effect of heating by the FU radiation field.
Abstract: A theoretical parameter study of dense photodissociation regions is presented. It is found that when the gas density is sufficiently high relative to the FUV flux, self-shielding of the molecules can move the C(+)/CO and H/H2 atomic-molecular transitions close to the surfaces of the molecular cloud, where they can feel the full effect of heating by the FUV radiation field. For n of 100,000/cu cm, collisional deexcitation of the FUV-pumped H2 can move the lower levels toward the LTE, producing line ratio resembling those of shocked regions for these low-v levels, while the high-v level line ratios retain a 'fluorescent' value. Appreciable emission in high-J transitions of CO originates in this warm molecular gas. Comparison with observations suggests that a small volume filling factor of high density clumps embedded within a moderate density interclump medium are a common phenomenon in photodissociation regions.