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The chemistry of astronomical hydrocarbons, responsible for the well-knowninfrared emission features detected in a wide variety of targets, remainsenigmatic. Here we focus on the group of young intermediate-mass Herbig Aestars. We have analyzed the aliphatic and polycyclic aromatic hydrocarbon (PAH)emission features in the infrared spectra of a sample of 53 Herbig Ae stars,obtained with the Infrared Spectrograph aboard the Spitzer Space Telescope. Weconfirm that the PAH-to-stellar luminosity ratio is higher in targets with aflared dust disk. However, a few sources with a flattened dust disk still showrelatively strong PAH emission. Since PAH molecules trace the gas disk, thisindicates that gas disks may still be flared, while the dust disk has settleddue to grain growth. There are indications that the strength of the 11.3 umfeature also depends on dust disk structure, with flattened disks being lessbright in this feature. We confirm that the CC bond features at 6.2 and 7.8 umshift to redder wavelengths with decreasing stellar effective temperature.Moreover, we show that this redshift is accompanied by a relative increase ofaliphatic CH emission and a decrease of the aromatic 8.6 um CH featurestrength. Cool stars in our sample are surrounded by hydrocarbons with a highaliphatic/aromatic CH ratio and a low aromatic CH/CC ratio, and vice versa forthe hot stars. We conclude that, while the overall hydrocarbon emissionstrength depends on the dust disk's geometry, the relative differences seen inthe IR emission features in disks around Herbig Ae stars are mainly due tochemical differences of the hydrocarbon molecules induced by the stellar UVfield. Strong UV flux reduces the aliphatic component and emphasizes thespectral signature of the aromatic molecules in the IR spectra.

Spitzer's view on aromatic and aliphatic hydrocarbon emission in Herbig Ae stars