ALMA Multiple-Transition Molecular Line Observations of the
  Ultraluminous Infrared Galaxy IRAS 20551-4250: Different HCN, HCO+, HNC
  Excitation and Implications for Infrared Radiative Pumping
  We present our ALMA multi-transition molecular line observational results for
the ultraluminous infrared galaxy, IRAS 20551-4250, which is known to contain a
luminous buried AGN and shows detectable vibrationally excited (v2=1f) HCN and
HNC emission lines. The rotational J=1-0, 4-3, and 8-7 of HCN, HCO+, and HNC
emission lines were clearly detected at a vibrational ground level (v=0).
Vibrationally excited (v2=1f) J=4-3 emission lines were detected for HCN and
HNC, but not for HCO+. Their observed flux ratios further support our
previously obtained suggestion, based on J=3-2 data, that (1) infrared
radiative pumping plays a role in rotational excitation at v=0, at least for
HCN and HNC, and (2) HCN abundance is higher than HCO+ and HNC. The flux
measurements of the isotopologue H13CN, H13CO+, and HN13C J=3-2 emission lines
support the higher HCN abundance scenario. Based on modeling with collisional
excitation, we constrain the physical properties of these line-emitting
molecular gas, but find that higher HNC rotational excitation than HCN and HCO+
is difficult to explain, due to the higher effective critical density of HNC.
We consider the effects of infrared radiative pumping using the available 5-30
micron infrared spectrum and find that our observational results are well
explained if the radiation source is located at 30-100 pc from the molecular
gas. The simultaneously covered very bright CO J=3-2 emission line displays a
broad emission wing, which we interpret as being due to molecular outflow
activity with the estimated rate of ~150 Msun/yr.
