When an old star smolders

Abstract

Polycyclic aromatic hydrocarbons (PAHs) produce characteristic infrared emission bands that have been observed in a wide range of astrophysical environments, where carbonaceous material is subjected to ultraviolet (UV) radiation. Although PAHs are expected to form in carbon-rich AGB stars, they have up to now only been observed in binary systems where a hot companion provides a hard radiation field. In this letter, we present low-resolution infrared spectra of four S-type AGB stars, selected from a sample of 90 S-type AGB stars observed with the infrared spectrograph aboard the Spitzer satellite. The spectra of these four stars show the typical infrared features of PAH molecules. We confirm the correlation between the temperature of the central star and the centroid wavelength of the 7.9 μm feature, present in a wide variety of stars spanning a temperature range from 3000 to 12 000 K. Three of four sources presented in this paper extend this relation towards lower temperatures. We argue that the mixture of hydrocarbons we see in these S-stars has a rich aliphatic component. The fourth star, BZ CMa, deviates from this correlation. Based on the similarity with the evolved binary TU Tau, we predict that BZ CMa has a hot companion as well.

Optical constants of refractory oxides at high temperatures

“…It should be noted that this dust feature has also been detected in AGB stars outside our galaxy (e.g. Lebzelter et al 2006) and in the spectra of S-type AGB stars (Smolders et al 2012).…”

“…In short, the following comparisons confront normalized dust emissivities with mean residual dust emissions in the 11−15 μm range. Just for comparison, the residual emission spectrum of an S-type AGB star as seen with Spitzer -taken from Smolders et al (2012) -is also shown.…”

“…The coexistence of crystalline corundum and amorphous Al 2 O 3 in an oxygen-rich circumstellar shell with a low mass loss rate seems quite likely. Incomplete condensation of a gas of solar composition might at the same time result in the formation of amorphous alumina and in a relative lack of silicates (compared to shells with greater optical depths that had more time to form silicates as well, in addition to refractory oxides -see, e.g., Smolders et al 2012 for a short discussion of this idea).…”

Carbonaceous molecules in the oxygen-rich circumstellar environment of binary post-AGB stars

“…All our sources follow the trend between the effective temperature of the central star and the central wavelength of the 7-8 μm PAH feature (Sloan et al 2007;Smolders et al 2010). The carriers of the class C features are proposed to be aliphatic carbonaceous material (Sloan et al 2007).…”

“…For example, S-type stars, which are undergoing the transition from oxygen-rich to carbon-rich and thus have C/O ∼ 1, do not yet form carbon-rich dust. However, due to non-equilibrium chemistry effects carbon-rich molecules, such as PAHs, HCN, and C 2 H 2 can be seen (Hony et al 2009;Smolders et al 2010). In the carbon-rich case, the excess carbon is used to form, for example, CN, C 2 , C 2 H 2 , CH 4 , polycyclic aromatic hydrocarbons (PAHs), and SiC.…”