Atypical dust species in the ejecta of classicalnovae

Abstract

A classical nova outburst arises from a thermonuclear runaway in the hydrogen-richmaterial accreted onto the surface of a white dwarf in a binary system. These explosionscan produce copious amounts of heavy element enriched material that are ejected violentlyinto the surrounding interstellar medium. In some novae, conditions in the ejecta aresuitable for the formation of dust of various compositions, including silicates, amorphouscarbon, silicon carbide, and hydrocarbons. Multiple dust grain types are sometimesproduced in the same system. CO formation in novae may not reach saturation, thusinvalidating the usual paradigm in which the C:O ratio determines the dust species. A fewnovae, such as V705 Cas and DZ Cru, have exhibited emission features near 6, 8, and 11μmthat are similar to “Unidentified Infrared” (UIR) features, but withsignificant differences in position and band structure. Here, we present SpitzerIRS spectra of two recent dusty novae, V2361 Cyg and V2362 Cyg, that harborsimilar peculiar emission structures superimposed on features arising from carbonaceousgrains. In other astronomical objects, such as star forming regions and young stellarobjects, emission peaks at 6.2, 7.7, and 11.3 μmhave been associated withpolycyclic aromatic hydrocarbon (PAH) complexes. We suggest that hydrogenated amorphouscarbon (HAC) may be the source of these features in novae based upon the spectral behaviorof the emission features and the conditions under which the dust formed.