Abstract
At a distance of 2.4 kpc W33 is one of the most prolific sources of molecular line emission, and it is an excellent research target for a centimeter spectral line search. We carried out a 1.3 cm spectral line survey in the frequency range 18–26 GHz. The lines we identified include 44 radio recombination lines (RRLs) and 24 molecular lines, excluding transitions from the main isotopolog of NH 3 . The RRLs are associated with the ionized gas from W33 Main. Intensity ratios between RRL pairs with varying differences in the principal quantum number η (i.e., Δ n ) from the same element at adjacent frequencies agree with ratios expected under conditions of local thermodynamical equilibrium. In spite of a resulting helium-to-hydrogen abundance ratio (equal emitting volumes assumed) of (10.7±1.8)%, which is consistent with expectations, helium shows broader turbulent line widths than hydrogen. The difference amounts to a few kilometers per second, hinting that the spatial distributions are slightly different. The molecular lines are attributed to nine different species (CH 3 OH HC 3 N SiS, c-C 3 H 2 , CH 3 CN NH 2 D, HNCO, H 2 O and CCS). Rotation temperatures and column densities were derived from CH 3 OH transitions using rotational temperature diagram analysis. Maser emission produced by water vapor and methanol have been observed in W33 Main, W33 A, and W33 B. Our survey discovered a CH 3 OH (10 2,8 –10 1,9 E) maser in W33 Main. Toward W33 B1, the fractionated deuterium-to-hydrogen ratio (D/H) deduced from para-NH 2 D/NH 3 is estimated to be ≲(1.0 ± 0.2)×10 −3 . For the other molecular W33-hotspots, 3 σ upper limits are (5.0 ± 0.4)×10 −3 . At linear scales of (0.5 pc), fractional abundances and excitation temperatures do not reach values close to those in well-established hot cores, but higher-resolution measurements may alter this picture.