A lambda 3mm and 1mm line survey toward the yellow hypergiant IRC +10420
  Our knowledge of the chemical properties of the circumstellar ejecta of the
most massive evolved stars is particularly poor. We aim to study the chemical
characteristics of the prototypical yellow hypergiant star, IRC +10420. For
this purpose, we obtained full line surveys at 1 and 3mm atmospheric windows.
  We have identified 106 molecular emission lines from 22 molecular species.
Approximately half of the molecules detected are N-bearing species, in
particular HCN, HNC, CN, NO, NS, PN, and N2H+. We used rotational diagrams to
derive the density and rotational temperature of the different molecular
species detected. We introduced an iterative method that allows us to take
moderate line opacities into account.
  We have found that IRC +10420 presents high abundances of the N-bearing
molecules compared with O-rich evolved stars. This result supports the presence
of a N-rich chemistry, expected for massive stars. Our analysis also suggests a
decrease of the 12C/13C ratio from \gtrsim 7 to \sim 3.7 in the last 3800
years, which can be directly related to the nitrogen enrichment observed. In
addition, we found that SiO emission presents a significant intensity decrease
for high-J lines when compared with older observations. Radiative transfer
modeling shows that this variation can be explained by a decrease in the
infrared (IR) flux of the dust. The origin of this decrease might be an
expansion of the dust shell or a lower stellar temperature due to the pulsation
of the star.
