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INTERSTELLAR EXTINCTION Broadly speaking, interstellar extinction varies with wavelength in a way which is generally surprisingly uniform along different lines of sight within the Milky Way , and can be characterised by a ''standard extinction curve''. However, the amount of extinction varies greatly, from almost no absorption at all to many cases where objects are entirely invisible at optical wavelengths and can only be seen in Infrared or Radio wavelengths. Superimposed on the standard extinction curve are many small absorption features, which have various origins and can give clues as to the composition of the ISM. One of the most important types of absorption features are the Diffuse Interstellar Band s, about 100 of which are seen in typical stellar spectra. The origin of these bands has been a hotly disputed topic for many years, but current ideas suggest that molecular Polycyclic Aromatic Hydrocarbon s (PAH) may be responsible for most or all of them. Calculating a standard extinction curve Several methods can be used to calculate a standard extinction curve. One way is by comparing the spectra of stars thought to be very similar, but at different distances. The difference between the shape of the spectra will then be due only to extinction. Another way is by calculating a theoretical unreddened spectrum, and comparing it to the observed spectrum. The 2175Å feature One prominent feature in derived standard extinction curves of objects within the Milky Way is a broad 'bump' at about 2175 Å , well into the Ultraviolet region of the Electromagnetic Spectrum . This feature was first observed in the 1960s but its origin is still not well understood. Several models have been presented to account for this bump which include Graphitic grains with a mixture of PAH molecules. Measuring extinction towards an object Once a standard extinction curve has been obtained, the amount of extinction towards an individual object can be determined. With Star s, theoretical spectra can be compared to the observed spectra to determine the amount of reddening. In the case of Emission Nebula e, it is common to look at the ratio of two Emission Line s which should not be affected by the Temperature and Density in the nebula. For example, the ratio of Hydrogen Alpha to Hydrogen Beta emission is always around 2.85 under a wide range of conditions prevailing in nebulae. A ratio other than 2.85 must therefore be due to extinction, and the amount of extinction can thus be calculated. Extinction curves of other galaxies The form of the standard extinction curve depends on the composition of the ISM, which varies from , while the SMC's is about 10%. ATMOSPHERIC EXTINCTION Atmospheric extinction varies with location and altitude. Astronomical observatories generally are able to characterise the local extinction curve very accurately, to allow observations to be corrected for the effect. Nevertheless, the atmosphere is completely opaque to many wavelengths requiring the use of Satellite s to make observations. Atmospheric extinction has three main components: Rayleigh Scattering by air molecules, scattering by Aerosols , and molecular absorption. Molecular absorption is often referred to as 'telluric absorption', as it is caused by the Earth ("telluric" is a Synonym of "terrestrial"). The most important source of telluric absorption is Ozone , which absorbs strongly in the near- Infrared . The amount of atmospheric extinction depends on the Altitude of an object, being lowest at the Zenith and at a maximum near the Horizon . It is calculated by multiplying the standard atmospheric extinction curve by the mean Airmass calculated over the duration of the observation. REFERENCES # Howarth I.D. (1983), ''LMC and galactic extinction'', Royal Astronomical Society, Monthly Notices, vol. 203, Apr. 1983, p. 301-304. # King D.L. (1985), ''Atmospheric Extinction at the Roque de los Muchachos Observatory, La Palma'', RGO/La Palma technical note 31 # Rouleau F., Henning T., Stognienko R. (1997), ''Constraints on the properties of the 2175Å interstellar feature carrier'', Astronomy and Astrophysics, v.322, p.633-645 |
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