1,720,991 research outputs found
Dehydrogenation study of cosmic-dust analogue grains
No full textNew infrared and optical-absorption measurements performed on hydrogenated amorphous-carbon grains are presented. In particular, the spectral modifications produced by dehydrogenation of the samples are analysed. We interpret the observed variations in terms of an increasing dimension of the graphitic clusters disordely arranged to form carbon grains. The results support the hypothesis that the behaviour of the interstellar extinction in the ultraviolet can be attributed to dehydrogenated amorphous-carbon grains. © 1993 Società Italiana di Fisica
Interstellar extinction: a parametrical study by using laboratory data
No full textIn this paper we present a comparative study between laboratory data, astronomical observations and theoretical models in order to provide a contribution to the characterization of cosmic-dust properties. In particular, we propose a parametrization of the amorphous-carbon extinction curve. The laboratory extinction measurements, performed in the extreme ultraviolet spectral region, are compared to the UV interstellar extinction curves provided by the IUE satellite. The aim of our work was to define a model that could describe both observational and laboratory data. To this purpose we referred to the model developed by Fitzpatrick and Massa. The second step of the study was the determination of extinction coefficients, for two different size distributions of amorphous-carbon grains, by means of the optical constants, n and k, derived by Rouleau and Martin. © 1993 Società Italiana di Fisica
Interstellar-dust properties as deduced from FIR and millimetric observations
No full textWe have considered some millimetric observations of our Galaxy and of the Large Magellanic Cloud (LMC) taken at the Italian Base in Antarctica. Using other submm data and fitting all the observations with a modified Planck curve, we have obtained useful information about the temperaure T and the spectral index α of the dust grains both for the Galaxy and the LMC. For the galactic spectrum two different couples of T and α match the detected fluxes:a)T=7K, α=2;b)T=15K, α=1.1. We have tried to identify the nature of the dust grains responsible for the submm and mm emission by comparing these results with the spectral trend of some cosmic analogues studied in laboratory. © 1993 Società Italiana di Fisica
Comet P/grigg-Skjellerup: Ground-based observations after the encounter with the Giotto spacecraft
No full textAfter the encounter with the Giotto spacecraft, the comet P/Grigg-Skjellerup was observed with the 2.2 m ESO telescope. Imaging in the V and R bands and infrared photometry with the J and H filters were performed. From the radial enhancement of the images we derived that the R frame mainly concerns the dust tail of the comet. Thus we applied the inverse numerical method of dust tail analysis in order to estimate the following cometary-grain parameters: ejection velocity, anisotropy of the emission, size distribution and loss rate. This from the observations back in time to the Giotto encounter. From infrared observation we could calculate the J and H magnitudes and the colour index J-H=0.4 consistent with previous measurements of the comet. © 1993 Società a Italiana di Fisica
Extinction signatures of amorphous carbon grains from the vacuum UV to the far-IR
Get PDFAmong the various components of cosmic dust, amorphous carbon grains play a relevant role. Together with other carbon-based materials. they have been invoked to interpret a number of spectral features observed in the interstellar and circumstellar environments. In order to validate, on a more quantitative basis, the reliability of such hypotheses. it is necessary to analyse in detail the optical properties of "analogue" grains produced in the laboratory. In the present paper, we report, for the first time, a complete spectral analysis of the extinction properties measured on various kinds of amorphous carbon grains, from the vacuum UV to the far-IR. These data can be complemented with other experimental results obtained on the same samples by means of different analytical techniques. This approach allows us to propose a self-consistent interpretation about the actual physico-chemical properties of our cosmic dust analogues. The resulting scenario provides a powerful tool to interpret astronomical observations concerning cosmic dust. © 1995
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
