Observational Evidence Linking Interstellar UV Absorption to PAH Molecules

The 2175 UV extinction feature was discovered in the mid-1960s, yet its physical origin remains poorly understood. One suggestion is absorption by polycyclic aromatic hydrocarbon (PAH) molecules, which is supported by theoretical molecular structure computations and by laboratory experiments. PAHs a...

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Veröffentlicht in:The Astrophysical journal 2017-02, Vol.836 (2), p.173
Hauptverfasser: Blasberger, Avi, Behar, Ehud, Perets, Hagai B., Brosch, Noah, Tielens, Alexander G. G. M.
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container_issue 2
container_start_page 173
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creator Blasberger, Avi
Behar, Ehud
Perets, Hagai B.
Brosch, Noah
Tielens, Alexander G. G. M.
description The 2175 UV extinction feature was discovered in the mid-1960s, yet its physical origin remains poorly understood. One suggestion is absorption by polycyclic aromatic hydrocarbon (PAH) molecules, which is supported by theoretical molecular structure computations and by laboratory experiments. PAHs are positively detected by their 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 m IR emission bands, which are specified by their modes of vibration. A definitive empirical link between the 2175 UV extinction and the IR PAH emission bands, however, is still missing. We present a new sample of hot stars that have both 2175 absorption and IR PAH emission. We find significant shifts of the central wavelength of the UV absorption feature, up to 2350 , but predominantly in stars that also have IR PAH emission. These UV shifts depend on stellar temperature in a fashion that is similar to the shifts of the 6.2 and 7.7 m IR PAH bands, that is, the features are increasingly more redshifted as the stellar temperature decreases, but only below ∼15 kK. Above 15 kK both UV and IR features retain their nominal values. Moreover, we find a suggestive correlation between the UV and IR shifts. We hypothesize that these similar dependences of both the UV and IR features on stellar temperature hint at a common origin of the two in PAH molecules and may establish the missing link between the UV and IR observations. We further suggest that the shifts depend on molecular size, and that the critical temperature of ∼15 kK above which no shifts are observed is related to the onset of UV-driven hot-star winds and their associated shocks.
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These UV shifts depend on stellar temperature in a fashion that is similar to the shifts of the 6.2 and 7.7 m IR PAH bands, that is, the features are increasingly more redshifted as the stellar temperature decreases, but only below ∼15 kK. Above 15 kK both UV and IR features retain their nominal values. Moreover, we find a suggestive correlation between the UV and IR shifts. We hypothesize that these similar dependences of both the UV and IR features on stellar temperature hint at a common origin of the two in PAH molecules and may establish the missing link between the UV and IR observations. 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subjects ABSORPTION
Aromatic hydrocarbons
astrochemistry
Astrophysics
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
CORRELATIONS
CRITICAL TEMPERATURE
dust, extinction
EMISSION
Extinction
Hot stars
Interstellar chemistry
INTERSTELLAR SPACE
ISM: molecules
Laboratory experiments
MOLECULAR STRUCTURE
MOLECULES
Observational studies
POLYCYCLIC AROMATIC HYDROCARBONS
RED SHIFT
STARS
Stellar temperature
Stellar winds
Ultraviolet absorption
ULTRAVIOLET RADIATION
Vibration mode
WAVELENGTHS
title Observational Evidence Linking Interstellar UV Absorption to PAH Molecules
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