The Rich Circumstellar Chemistry of SMP LMC 11

TL;DR

This paper investigates the complex molecular chemistry of the preplanetary nebula SMP LMC 11, revealing high abundances of certain molecules like benzene and propyne, and suggesting an unusual torus-shaped circumstellar environment.

Contribution

It provides the first detailed molecular absorption modeling of SMP LMC 11, highlighting its unique chemical composition and environment compared to typical carbon-rich evolved stars.

Findings

High molecular abundances, notably benzene and propyne.

Detection of HC3N and marginal HCN presence.

Unusual torus-shaped circumstellar environment.

Abstract

Carbon-rich evolved stars from the asymptotic giant branch to the planetary nebula phase are characterized by a rich and complex carbon chemistry in their circumstellar envelopes. A peculiar object is the preplanetary nebula SMP LMC 11, whose Spitzer-IRS spectrum shows remarkable and diverse molecular absorption bands. To study how the molecular composition in this object compares to our current understanding of circumstellar carbon chemistry, we modeled this molecular absorption. We find high abundances for a number of molecules, perhaps most notably benzene. We also confirm the presence of propyne (CH3C2H) in this spectrum. Of all the cyanopolyynes, only HC3N is evident; we can detect at best a marginal presence of HCN. From comparisons to various chemical models, we can conclude that SMP LMC 11 must have an unusual circumstellar environment (a torus rather than an outflow).