NEW LABORATORY MEASUREMENTS OF RHOMBOIDAL SiC$_3$

A.~J.~Apponi, M.~C.~McCarthy, C.~A.~Gottlieb, and P.~Thaddeus, {\sl Journ. Chem. Phys.A.~J.~Apponi, M.~C.~McCarthy, C.~A.~Gottlieb, and P.~Thaddeus, {\sl Astrophys. Journ. Lett.N.~A.~Patel, K.~H.~Young, S.~Brunken, R.~W.~Wilson, P.~Thaddeus, K.~M.~Menten, M.~Reid, M.~C.~McCarthy, Dinh-V~Trung, C.~A.~Gottlieb, and A.~Hedden, {\sl Astrophys. Journ.Author Institution: Harvard-Smithsonian Center for Astrophysics 60 Garden St. Cambridge, MA 02138; and School of Engineering \& Applied Sciences, Harvard University, 29 Oxford St.,Cambridge, MA 02138Rhomboidal SiC$_3$, the highly polar planar ring with C$_{2v}$ symmetry and a transannular C---C bond, was detected in our laboratory about 10~years ago, {\bf 111}, 3911 (1999).} and soon afterwards was identified with a radio telescope in the expanding envelope of IRC+10216. {\bf 516}, L103 (1999).} Recently a sensitive spectral line survey of IRC+10216 was made with the Submillimeter Array (SMA) in the $300 - 355$~GHz range with a $3^{\prime\prime} \times 2^{\prime\prime}$ synthesized beam. Many new lines were detected in this survey. Most are from high rotational transitions of molecules that are known in IRC+10216, but some of the lines are quite narrow and more than 10 of these are unassigned.nken, R.~W.~Wilson, P.~Thaddeus, K.~M.~Menten, M.~Reid, M.~C.~McCarthy, Dinh-V~Trung, C.~A.~Gottlieb, and A.~Hedden, {\sl Astrophys. Journ.}, in press (2009).} In support of the SMA observations we have extended the earlier laboratory measurements by Apponi {\it et al.} from 286~GHz and $K_a \le 6$, to 450~GHz and $K_a \le 20$ from rotational levels as high as 825~K above ground. As a result uncertainties in the predicted spectrum for lines with high $K_a$ have been reduced by as much as two orders of magnitude, which should aid the assignment of SiC$_3$ in the SMA survey and in future observations with ALMA. % ##########################################################################################