NOEMA Observations of CO Emission in Arp 142 and Arp 238
  Previous studies have shown significant differences in the enhancement of the
star-formation rate (SFR) and the star-formation efficiency (SFE=SFR/M_mol)
between spiral-spiral and spiral-elliptical mergers. In order to shed light on
the physical mechanism of these differences, we present NOEMA observations of
the molecular gas distribution and kinematics (linear resolutions of ~ 2kpc) in
two representative close major-merger star-forming pairs: the spiral-elliptical
pair Arp142 and the spiral-spiral pair Arp238. The CO in Arp142 is widely
distributed over a highly distorted disk without any nuclear concentration, and
an off-centric ring-like structure is discovered in channel maps. The SFE
varies significantly within Arp142, with a starburst region (Region 1) near the
eastern tip of the distorted disk showing an SFE ~0.3 dex above the mean of the
control sample of isolated galaxies, and the SFE of the main disk (Region 4)
0.43 dex lower than the mean of the control sample. In contrast, the CO
emission in Arp238 is detected only in two compact sources at the galactic
centers. Compared to the control sample, Arp238-E shows an SFE enhancement of
more than 1 dex whereas Arp238-W has an enhancement of ~0.7 dex. We suggest
that the extended CO distribution and the large SFE variation in Arp142 are due
to an expanding large-scale ring triggered by a recent high-speed head-on
collision between the spiral galaxy and the elliptical galaxy, and the compact
CO sources with high SFEs in Arp238 are associated with nuclear starbursts
induced by gravitational tidal torques in a low-speed coplanar interaction.
