Building blocks of dust: A coordinated laboratory and astronomical study of the archtype AGB carbon star IRC+10216

Abstract

This article provides an overview of recent astronomical studies and a closely coordinated laboratory program devoted to the study of the physics and chemistry of carbon-rich Asymptotic Giant Branch stars, as illustrated by the archtype star IRC+10216. The increased sensitivity and angular resolution of high altitude, ground-based millimeter-wave interferometers in the past few years has enabled molecular astronomers to determine the excitation and spatial distribution of molecules within a few stellar radii of the central star where the molecular seeds of dust are formed, and thus to critically assess the physicochemical mechanisms of dust formation and growth. However the astronomical studies are crucially dependent on precise laboratory measurements of the rotational spectra — both in the ground and vibrationally excited states of the normal and rare isotopic species as well — of the principal molecules in the inner region which appear to contain only two or three heavy atoms.Much remains to be done by laboratory spectroscopists as evidenced by the large number of unassigned millimeter-wave rotational lines that are observed in the inner envelope of IRC+10216. To assess whether vibrationally excited SiC2 and HCN are the carriers of some of the unassigned features observed in this source, millimeter-wave laboratory spectra are compared with Cycle 0 observations from the Atacama Large Millimeter-wave Array (ALMA) under experimental conditions that produce both species. Also highlighted are ongoing laboratory studies of the silicon carbides SiC2 and SiCSi in their ground and excited vibrational states, and c-SiC3 in its ground vibrational state. Following the initial detection of c-SiC3 and SiCSi in the outer molecular envelope of IRC+10216, the laboratory spectroscopy was extended to higher frequency in support of the recent interferometric measurements. Thirty-two new millimeter-wave rotational transitions of SiCSi with J⩽48,Ka⩽3 and upper level energies Eu⩽484 K in the range from 178 to 391 GHz, and 35 new transitions of c-SiC3 with J⩽38,Ka⩽20 and Eu⩽875 K between 315 and 440 GHz were measured in the laboratory. In addition, five or six rotational transitions in one quanta of each of the three fundamental vibrational modes of SiCSi and the two lowest rotational transitions in the previously unexplored C–C stretching mode (ν1) of SiC2 were measured in the normal and doubly substituted 13C isotopic species. Areas of continued emphasis and the importance of complementary experimental methods and production techniques to detect and assign vibrational transitions are also highlighted.