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High resolution spectra of the Spitzer Space Telescope showvibration-rotation absorption bands of gaseous C2H2, HCN, and CO2 moleculestoward a sample of deeply obscured (U)LIRG nuclei. The observed bands revealthe presence of dense (n>~ 10^7 cm^-3), warm (T_ex = 200-700 K) molecular gaswith high column densities of these molecules ranging from a few 10^15 - 10^17cm^-2. Abundances relative to H2, inferred from the silicate optical depth,range from ~10^-7 to 10^-6 and show no correlation with temperature.Theoretical studies show that the high abundances of both C2H2 and HCN excludea X-ray dominated region (XDR) associated with the toroid surrounding an AGN asthe origin of this dense warm molecular gas. Galactic massive protostars in theso-called Hot Core phase have similar physical characteristics with comparablehigh abundances of C2H2, HCN, and CO2 in the hot phase. However, the abundancesof C2H2 and HCN and the C2H2/CO2 and HCN/CO2 ratios are much higher toward the(U)LIRGs in the cooler (T_ex <= 400 K) phase. We suggest that the warm densemolecular gas revealed by the mid-IR absorption lines is associated with aphase of deeply embedded star formation where the extreme pressures anddensities of the nuclear starburst environment have inhibited the expansion ofHII regions and the global disruption of the star forming molecular cloudcores, and `trapped' the star formation process in an `extended' Hot Corephase.Comment: 22 pages, 4 figures, published in ApJ, 659, 296 v2. updated hydrogen  column densitie

Infrared Molecular Starburst Fingerprints in Deeply Obscured (Ultra)Luminous Infrared Galaxy Nuclei