Ultra-deep 31.0-50.3 GHz spectral survey of IRC+10216
  Context. IRC+10216, the carbon-rich envelope of the asymptotic giant branch
(AGB) star CW Leo, is one of the richest molecular sources in the sky.
Available spectral surveys below 51 GHz are more than 25 years old and new work
is needed.
  Aims. Characterizing the rich molecular content of this source, specially for
heavy species, requires to carry out very sensitive spectral surveys at low
frequencies. In particular in this work we have achieved an rms in the range
0.2-0.6 mK per MHz.
  Methods. long Q-band (31.0-50.3 GHz) single dish integrations were carried
out with the Yebes 40m telescope using specifically built receivers. State of
the art line catalogs are used for line identification.
  Results. A total of 652 spectral features corresponding to 713 transitions
from 81 species (we count as different the isomers, isotopologues and
ortho/para species) are present in the data. Only 57 unidentified lines remain
with signal to noise ratios >3. Some new species and/or vibrational modes have
been discovered for the first time with this survey.
  Conclusions. This IRC+10216 spectral survey is, by far, the most sensitive
one carried out to this date in the Q-band. It therefore provides the most
complete view of IRC+10216 from 31.0 to 50.3 GHz, giving unique information on
its molecular content, specially for heavy species. Rotational diagrams built
from the data provide valuable information on the physical conditions and
chemical content of this circumstellar envelope.
