CO observations of major merger pairs at z=0: Molecular gas mass and
  star formation
  We present CO observations of 78 spiral galaxies in local merger pairs. These
galaxies representa subsample of a Ks-band selected sample consisting of 88
close major-merger pairs (HKPAIRs), 44 spiral-spiral (S+S) pairs and 44
spiral-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 derived
from HERSCHEL PACS and SPIRE data, and the atomic gas mass, MHI, from the Green
Bank Telescope HI observations. The complete data set allows us to study the
relation between the gas (atomic and molecular) mass, dust mass and SFR in
merger 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 an
overall moderate enhancements (~2x) in both molecular gas fraction (MH2/M*),
and molecular-to-atomic gas ratio (MH2/MHI) for star-forming galaxies in
major-merger pairs compared to non-interacting comparison samples, whereas no
enhancement 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 with
and without interaction signs, there is a small difference in SFE, moderate
difference in MH2/M*, and strong differences in MH2/MHI between subsamples. For
MH2/MHI, the difference between S+S and S+E subsamples is 0.69+-0.16 dex and
between pairs with and without interaction signs is 0.53+-0.18 dex. Together,
our results suggest (1) star formation enhancement in close major-merger pairs
occurs mainly in S+S pairs after the first close encounter (indicated by
interaction signs) because the HI gas is compressed into star-forming molecular
gas by the tidal torque; (2) this effect is much weakened in the S+E pairs.
