Building Blocks of Dust: A Coordinated Laboratory and Astronomical Study of AGB Stars

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 (AGB) stars. 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 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 - 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 millimeters-wave rotational lines that are observed in the inner envelope of carbon rich AGB stars. As an illustration we refer to the example of an initial laboratory approach for establishing whether vibrationally excited SiC₂ and HCN are the carriers of some of the unassigned features observed in the prototypical carbon rich AGB star IRC+10216 with ALMA. Also highlighted are ongoing laboratory studies of the silicon carbides SiC₂ and SiCSi in their ground and excited vibrational states, and SiC₃ in the ground vibrational state. Following the initial detection of 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, K_a ≤ 3 and upper level energies E_u ≤ 484 K in the range from 178 - 391 GHz, and 35 new transitions of SiC₃ with J ≤ 38, K_a ≤ 20 and E_u ≤ 875 K between 315 and 440 GHz were measured in the laboratory. In addition five to 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 (ν ₁) of SiCC were measured in the normal and doubly substituted ¹³C isotopic species.