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We present CO observations of 78 spiral galaxies in local merger pairs. Thesegalaxies representa subsample of a Ks-band selected sample consisting of 88close major-merger pairs (HKPAIRs), 44 spiral-spiral (S+S) pairs and 44spiral-elliptical (S+E) pairs, with separation $<20 h^{-1}$ kpc and mass ratio<2.5. For all objects, the star formation rate (SFR) and dust mass were derivedfrom HERSCHEL PACS and SPIRE data, and the atomic gas mass, MHI, from the GreenBank Telescope HI observations. The complete data set allows us to study therelation between the gas (atomic and molecular) mass, dust mass and SFR inmerger galaxies. We derive the molecular gas fraction (MH2/M*),molecular-to-atomic gas mass ratio (MH2/MHI), gas-to-dust mass ratio and SFE(=SFR/MH2) and study their dependences on pair type (S+S compared to S+E),stellar mass and the presence of morphological interaction signs. We find anoverall moderate enhancements (~2x) in both molecular gas fraction (MH2/M*),and molecular-to-atomic gas ratio (MH2/MHI) for star-forming galaxies inmajor-merger pairs compared to non-interacting comparison samples, whereas noenhancement was found for the SFE nor for the total gas mass fraction(MHI+MH2)/M*. When divided into S+S and S+E, low mass and high mass, and withand without interaction signs, there is a small difference in SFE, moderatedifference in MH2/M*, and strong differences in MH2/MHI between subsamples. ForMH2/MHI, the difference between S+S and S+E subsamples is 0.69+-0.16 dex andbetween pairs with and without interaction signs is 0.53+-0.18 dex. Together,our results suggest (1) star formation enhancement in close major-merger pairsoccurs mainly in S+S pairs after the first close encounter (indicated byinteraction signs) because the HI gas is compressed into star-forming moleculargas by the tidal torque; (2) this effect is much weakened in the S+E pairs.

CO observations of major merger pairs at z=0: Molecular gas mass and star formation