  We present the results on millimeter interferometric observations of four
luminous infrared galaxies (LIRGs), Arp 220, Mrk 231, IRAS 08572+3915, and VV
114, and one Wolf-Rayet galaxy, He 2-10, using the Nobeyama Millimeter Array
(NMA). Both the HCN(1-0) and HCO+(1-0) molecular lines were observed
simultaneously and their brightness-temperature ratios were derived.
High-quality infrared L-band (2.8-4.1 micron) spectra were also obtained for
the four LIRGs to better constrain their energy sources deeply buried in dust
and molecular gas. When combined with other LIRGs we have previously observed
with NMA, the final sample comprised nine LIRGs (12 LIRGs' nuclei) with
available interferometric HCN(1-0) and HCO+(1-0) data-sufficient to investigate
the overall trend in comparison with known AGNs and starburst galaxies. We
found that LIRGs with luminous buried AGN signatures at other wavelengths tend
to show high HCN(1-0)/HCO+(1-0) brightness-temperature ratios as seen in
AGN-dominated galaxies, while the Wolf-Rayet galaxy He 2-10 displays a small
ratio. An enhanced HCN abundance in the interstellar gas surrounding a strongly
X-ray-emitting AGN, as predicted by some chemical calculations, is a natural
explanation of our results.
