Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...285.1003g&link_type=abstract
Astronomy and Astrophysics 285, 1003-1011 (1994)
Astronomy and Astrophysics
Astrophysics
20
Interstellar Medium: Dust, Reflection Nebulae, Stars: Circumstellar Matter, Molecular Data
Scientific paper
Recent laboratory developments and measurements have led to a better knowledge of the optical absorptivity of coals from ~0.04 to ~50 μm. This allows us to take up the problem of the heating mechanism of coal grains on a quantitative basis. It is shown that highly graphitized coal grains at a steady-state temperature of ~250-400 K have near-infrared colour temperatures of 500-1000 K and adequately reproduce the general spectral characteristics of UIB emission. These temperatures can be reached and maintained if the grains are small (10-1000 A) and subjected to radiative fluxes of ~10^-6^-10^-4^ W/cm^2^, which is also the range of fluxes estimated in UIB emitting nebulae. The heating efficiency of the exciting source does not decrease dramatically with photon energy until the colour temperature of the source falls below ~2000 K: thus, hard UV photons are not necessary to excite the UIBs. These results are derived from measured properties of coal and can all be traced back to the semiconducting properties of this material, particularly to the dramatic decrease of absorptivity between the visible and the near infrared. Laboratory data already available on coals allow quantitative predictions of observables, such as flux ratios [3.3]/[Tot], [UIB]/[Tot], [12]/[Tot], to be made. They compare favourably with observations from IRAS and other sources.
Guillois Olivier
Nenner I.
Papoular Renaud
Reynaud Cécile
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