HI Observations of Major-Merger Pairs at z = 0: atomic gas and star
  formation
  We present a study of the HI gas content of a large K-band selected sample of
88 close major-merger pairs of galaxies (H-KPAIR) which were observed by $\it
Herschel$. We obtained the 21 cm HI fine-structure emission line data for a
total of 70 pairs from this sample, by observing 58 pairs using the Green Bank
Telescope (GBT) and retrieving the HI data for an addition 12 pairs from the
literature. In this HI sample, 34 pairs are spiral-spiral (S+S) pairs, and 36
are spiral-elliptical (S+E). Based on these data, we studied the HI-to-stellar
mass ratio, the HI gas fraction and the HI star formation efficiency
(SFE$_{\mathrm{HI}}$ = star formation rate/$M_{\mathrm{HI}}$) and searched for
differences between S+S and S+E pairs, as well as between pairs with and
without signs for merger/interaction. Our results showed that the mean
HI-to-stellar mass ratio of spirals in these pairs is $=7.6\pm1.0 \%$,
consistent with the average HI gas fraction of spiral galaxies in general. The
differences in the HI gas fraction between spirals in S+S and in S+E pairs, and
between spirals in pairs with and without signs of merger/interaction are
insignificant ($< 1 \sigma$). On the other hand, the mean SFE$_{\mathrm{HI}}$
of S+S pairs is $\sim4.6\times$ higher than that of S+E pairs. This difference
is very significant ($\sim 4\sigma$) and is the main result of our study. There
is no significant difference in the mean SFE$_{\mathrm{HI}}$ between galaxies
with and without signs of merger/interaction. The mean SFE$_{\mathrm{HI}}$ of
the whole pair sample is $10^{-9.55\pm 0.09}\ \mathrm{yr}^{-1}$, corresponding
to a HI consumption time of $3.5\pm0.7$ Gyrs.
