Suprathermal rotation of PAHs in the ISM II. Observational evidence for the rotational broadening of lambda 5797 DIB in reflection nebulae - implication for the carrier size

Details Suprathermal rotation of PAHs in the ISM II. Observational evidence for the rotational broadening of lambda 5797 DIB in reflection nebulae - implication for the carrier size Suprathermal rotation of PAHs in the ISM II. Observational evidence for the rotational broadening of lambda 5797 DIB in reflection nebulae - implication for the carrier size

Jul 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999a%26a...347..669l&link_type=abstract

Astronomy and Astrophysics, v.347, p.669-675 (1999)

Astronomy and Astrophysics

Astrophysics

10

Line: Profiles, Molecular Processes, Ism: Molecules, Ism: Reflection Nebulae

Scientific paper

In a previous paper, we described a model which can explain the lambda5797 diffuse interstellar band (DIB) profile as seen in absorption in the diffuse interstellar medium and in emission in the Red Rectangle (RR), as a rotational envelope of electronic transitions where the molecular carrier is a free PAH of size ~ 40 atoms. One of the strongest predictions is the behaviour of the rotational temperature of PAH in the case of regions rich in UV such as Reflection Nebulae: it must be suprathermal with respect to the gas temperature but clamped to ~ 100 K for any PAH size. The width of the DIB, in such regions, can then be broader than in the classical ISM (T_rot ~ 30 K) if the values of the molecular transition are favorable. In order to test this prediction, we have obtained high resolution spectra of 27 reddened early type stars, mostly in reflection nebulae, in order to compare their lambda 5797 DIB width to those of stars in classical diffuse interstellar medium. These spectra were made for several DIBs such as lambda 5797, lambda6379 and lambda6613 , with a spectral resolving power of about 60 000. The analysis of the results agrees with our predic tion since the width of the lambda 5797 DIB is broader in a majority of areas with strong UV radiation. Moreover, the broadening is not observed on DIBs lambda6379 and lambda6613 , indicating that the molecular parameters of the electronic transitions at the origin of the different DIBs are pretty variable from one DIB to another and confirming that the measured broadening on lambda 5797 is not due to an instrumental bias. The statistical measurement of the lambda 5797 width in this medium permits the derivation of new constraints on the size of the carrier of this DIB, a molecule that should have 30 to 45 carbon atoms if, as we propose, it is indeed a PAH. Based on observations made at Observatoire de Haute Provence (CNRS), France.

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