Observational Evidence Linking Interstellar UV Absorption to PAH Molecules

The 2175 \AA\ UV extinction feature was discovered in the mid-1960s, yet its physical origin remains poorly understood. One suggestion is absorption by Polycyclic Aromatic Hydrocarbons (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 & 12.7 $\mu$m IR emission bands, which are specified by their modes of vibration. A definitive empirical link between the 2175 \AA\ UV extinction, and the PAH IR emission bands, however, is still missing. We present a new sample of hot stars that have both 2175 \AA\ absorption and PAH IR emission. We find significant shifts of the central wavelength of the UV absorption feature, up to 2350 \AA, 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$\mu$m PAH IR bands, namely the features are increasingly more red-shifted as the stellar temperature decreases, but only below $\sim 15$ 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 to 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 $\sim 15$ 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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