Showing papers by "N. C. Wickramasinghe published in 1968"
TL;DR: Graphite grain formation in cool stars examined on basis of molecular equilibrium data for stellar atmospheres was examined in this paper. But the results were limited to a single star and were not applicable to other stars.
Abstract: Graphite grain formation in cool stars examined on basis of molecular equilibrium data for stellar atmospheres
49 citations
TL;DR: The early overluminous phase during the condensation of the Sun can be a great help in understanding why the inner planets are largely made up of iron and magnesium silicate as discussed by the authors, which can be found in the inner solar disk.
Abstract: The early overluminous phase, during the condensation of the Sun can be a great help in understanding why the inner planets are largely made up of iron and magnesium silicate.
44 citations
TL;DR: In this paper, solid state processes may be a controlling factor in the evolution of the universe and may account for similarities in the energy densities of several astrophysical phenomena, which may explain the similarities of the two types of processes.
Abstract: Solid state processes may be a controlling factor in the evolution of the universe and may account for similarities in the energy densities of several astrophysical phenomena.
39 citations
TL;DR: The absence of detectable emission nebulae is a feature characteristic of many clusters and associations, such as Cygnus OB2, which contains several hundred OB stars and ∼ 100 M⊙ of dust in circumstellar shells, but no H II region.
Abstract: THE absence of detectable emission nebulae is a feature characteristic of many clusters and associations1,2. Among such associations is Cygnus OB2, which contains several hundred OB stars and ∼ 100 M⊙ of dust in circumstellar shells, but no H II region1,3. Assuming a normal gas to dust ratio, the association must contain ∼ 104 M⊙ of interstellar hydrogen which should give rise to an H II region with an emission measure ∼ 105–106.
32 citations
TL;DR: The 4430 absorption band has a significantly asymmetric profile, being somewhat steeper on the red side than on the blue as mentioned in this paper, which is a prediction of the solid state model proposed for the band.
Abstract: Unidentified diffuse interstellar bands may be caused by electronic transitions of impurity ions in grains which are coupled to the lattice vibrations. The 4430 absorption band has a significantly asymmetric profile, being somewhat steeper on the red side than on the blue. This feature is a prediction of the solid state model proposed for the band.
26 citations
TL;DR: In the case of galaxies, large quantities of solid particles with sizes of the order of the wavelength of light may form in explosions of massive objects in the nuclei of galaxies as mentioned in this paper.
Abstract: Large quantities of solid particles with sizes of the order of the wavelength of light may form in explosions of massive objects in the nuclei of galaxies.
26 citations
TL;DR: In this article, a graphite core with a dielectric mantle of refractive index different from that of ice may produce better agreement with the observations, including the apparent hump in the observed extinction at ∼ 2400 A.
Abstract: MANY attempts have been made recently to match the wavelength dependence of the observed interstellar extinction curves with calculations for theoretical models. These attempts (reviewed in refs. 1 and 2) have met with varying degrees of partial success. It is now generally recognized that a pure dielectric particle will not be able to explain the entire wavelength range of the extinction observations. In particular the very strictly linear portion (0.8µ−1 < λ−1 < 2.3µ−1) of the mean extinction curves of Nandy3,4 and the continued increase in extinction towards the far ultraviolet5 would seem to rule out homogeneous dielectric particles as a significant constituent of interstellar material. Calculations by Stecher and Donn6 and by ourselves7 have indicated that a size-distribution of pure graphite particles could produce general agreement with observations in the visible and ultraviolet spectral regions up to ∼ 2200 A, but no agreement is possible further in the ultraviolet. This agreement includes the reproduction of an apparent hump in the observed extinction at ∼ 2400 A—which Stecher and Donn have referred to as the “signature of graphite”. Of the grain models so far proposed the graphite core–dice mantle grain seems most favourable in being able to reproduce the detailed extinction curves in the visible spectral region as well as the overall features of the extinction up to ∼ 1500 A (ref. 8). For such particles, however, the hump in the extinction at ∼ 2400 A characteristic of graphite is washed out because of the effect of the outer ice mantle. If the structure in the extinction curve at the same wavelength is confirmed by future observations, the graphite core–ice mantle grain cannot be regarded as an entirely satisfactory model. A graphite component of the grains is, however, strongly indicated both by this ultraviolet feature as well as by the kink at λ−1 = 2.3µ−1 of the visible extinction observations, the latter being probably associated with a change in the refractive index of graphite. It is therefore interesting to explore the possibility that a graphite core with a dielectric mantle of refractive index different from that of ice may produce better agreement with the observations.
11 citations
TL;DR: Temperatures of grain models using interstellar radiation field in solar neighborhood and optical data for graphite and ice were derived by as mentioned in this paper, where they were used to estimate the temperature of graphite.
Abstract: Temperatures of grain models using interstellar radiation field in solar neighborhood and optical data for graphite and ice
9 citations
TL;DR: In this paper, the IR radiation of NML Cygnus with O/C ratio close to unity is attributed to the circumstellar shell of graphite particles, which is the same as the one described in this paper.
Abstract: IR radiation of NML Cygnus with O/C ratio close to unity ascribed to circumstellar shell of graphite particles
6 citations
TL;DR: In this article, it is claimed that the exact black-body intensity curve fits the observations better than the grey-body approximation at long wavelengths, and corresponds to a temperature T = 2.68° K.
Abstract: THE discovery of microwave background radiation by Penzias and Wilson1 is one of the most interesting events in observational astronomy in recent years. On the basis of the first observations at a single wavelength, the radiation was interpreted2 as being of the black-body type, and it was concluded that this was the remnant of the primordial radiation of a big-bang universe. Later results by different observers have strengthened this interpretation. Indeed it is now claimed3,4 that the black-body curve applies for a wavelength range of 1 : 90, and corresponds to a temperature T = 2.68° K. The observers5 go as far as to state that the exact black-body intensity curve fits the observations better than the “grey-body” approximation at long wavelengths—namely
6 citations
TL;DR: The role of impurity induced absorption bands has been discussed recently in connexion with the 4430 a absorption band as discussed by the authors, which is caused by electrons trapped at vacancies in any crystalline solid at a wavelength defined by the lattice.
Abstract: ONE of us has shown1,2 that the interstellar extinction curve in the visible spectral region may be represented to a high degree of accuracy by two straight lines intersecting at λ = 4300 A. Although graphite flakes or graphite cores with dielectric mantles come close to fitting the observational curve3, it is clearly not possible to explain a very sharp knee on the basis of Mie scattering by solid particles. The role of impurity induced absorption bands has been discussed recently4,5 in connexion with the 4430 a absorption band. The most widely studied impurity bands are the so-called F centres6,7 in alkali halides which occur in the visible and near ultraviolet. Fig. 1 shows a typical absorption profile due to an F centre in KBr (ref. 7). Such a band, which could occur in any crystalline solid at a wavelength defined by the lattice, is caused by electrons trapped at vacancies. Features common to all such bands are: (a) f values close to unity; (b) symmetrical profiles; (c) widths of the order 1000 A. We have already suggested5 that such a broad diffuse absorption band may be present, but escape resolution in stellar spectra. We consider here the possibility that the knee in the interstellar extinction at λ = 4300 A may be caused by such a band superposed on the extinction curve derived from Mie scattering.
TL;DR: This paper calculated extinction cross sections for quartz particles, results indicate quartz is not appreciable component of interstellar grains, and concluded that quartz is a relatively small component of the entire interstellar community.
Abstract: Interstellar extinction cross sections calculated for quartz particles, results indicate quartz is not appreciable component of interstellar grains
TL;DR: In this paper, Wolstencroft and Rose, combining their own data from a sounding rocket at λ = 4500 A with earlier observations, conclude that the average albedo of the grains γ > 0.49 ± 0.11.
Abstract: THE diffuse galactic radiation, which results from the scattering of starlight by interstellar grains, may contain valuable information about the scattering properties of the grains. The great difficulties associated with its measurement combined with the lack of a realistic scattering model of the galaxy have, however, usually made it difficult to reach any firm conclusions about the interstellar grains. Thus, Wolstencroft and Rose1, combining their own data from a sounding rocket at λ = 4500 A with earlier observations, conclude that the average albedo of the grains γ > 0.49 ± 0.11.
TL;DR: In this paper, the critical grain temperature for extensive H2 formation and deposition of solid hydrogen mantles in dense clouds is T 2.3° K, which is lower than Stecher and Williams' estimate of ≈ 3° K based on thermodynamic arguments.
Abstract: Stecher and Williams1 have claimed that the critical grain temperature for extensive H2 formation and deposition of solid hydrogen mantles in dense clouds is T≃2.3° K—lower than our estimate2 of ≈3° K based on thermodynamic arguments. We wish to point out that their conclusion is far from definitive and depends critically on several arbitrary assumptions. First, they presume that the surfaces of interstellar grains duplicate closely the properties of pure graphite surfaces in regard to physical absorption of hydrogen molecules. It is to be expected that interstellar graphite surfaces, if they are free, are exceptionally rich in dislocations and high energy impurity sites which would drastically alter their surface chemistry and adsorption properties. Such imperfections would make a surface much more efficient for trapping adsorbed atoms and molecules. Indeed, the surface chemistry involved in the formation of the first two or three monomolecular layers must in reality be expected to deviate considerably from the “ideal” situation described by Stecher and Williams1. Another possibility they did not consider is that the surfaces on which the hydrogen condenses in dense clouds are of an icy nature, appropriate to the composite graphite/ice grains.
TL;DR: In this paper, it was suggested that the formation and stability of the interstellar OH molecule under the conditions prevailing in the emitting region of the X2II→A2Σ+ bands was also investigated.
Abstract: THE attempts which have so far been made to explain the observed features of the interstellar OH molecule cannot be regarded as entirely successful1 The most interesting observations are the OH sources in emission, where it is clear that some form of maser action is involved Any theoretical model to explain these data must provide a mechanism for producing stimulated emission while also accounting for the formation and stability of the OH molecule under the conditions prevailing in the emitting region An optical pumping device involving absorption and re-emission at λ=3080 A in the X2II→A2Σ+ bands was suggested by Perkins et al2, but this process has recently been criticized by Solomon3 It has been argued3 that the OH would be dissociated faster than it would be pumped by ultraviolet absorption Solomon has in fact suggested that OH may form in the upper level of the Λ doublet by inverse pre-dissociation reactions taking place at a few thousand degrees Kelvin; but the possibility of other formation and stimulation mechanisms cannot be ruled out at the present moment
30 Mar 1968
TL;DR: In this article, it is claimed that the exact black-body intensity curve fits the observations better than the grey-body approximation at long wavelengths, and corresponds to a temperature T = 2.68° K.
Abstract: THE discovery of microwave background radiation by Penzias and Wilson1 is one of the most interesting events in observational astronomy in recent years. On the basis of the first observations at a single wavelength, the radiation was interpreted2 as being of the black-body type, and it was concluded that this was the remnant of the primordial radiation of a big-bang universe. Later results by different observers have strengthened this interpretation. Indeed it is now claimed3,4 that the black-body curve applies for a wavelength range of 1 : 90, and corresponds to a temperature T = 2.68° K. The observers5 go as far as to state that the exact black-body intensity curve fits the observations better than the “grey-body” approximation at long wavelengths—namely