Light Element Discontinuities Suggest an Early Termination of Star
  Formation in the Globular Cluster NGC 6402 (M14)
  NGC 6402 is among the most massive globular clusters in the Galaxy, but
little is known about its detailed chemical composition. Therefore, we obtained
radial velocities and/or chemical abundances of 11 elements for 41 red giant
branch cluster members using high resolution spectra obtained with the
Magellan-M2FS instrument. We find NGC 6402 to be only moderately metal-poor
with $\langle$[Fe/H]$\rangle$ = $-$1.13 dex ($\sigma$ = 0.05 dex) and to have a
mean heliocentric radial velocity of $-$61.1 km s$^{\rm -1}$ ($\sigma$ = 8.5 km
s$^{\rm -1}$). In general, NGC 6402 exhibits mean composition properties that
are similar to other inner Galaxy clusters, such as [$\alpha$/Fe] $\sim$ $+$0.3
dex, [Cr,Ni/Fe] $\sim$ 0.0 dex, and $\langle$[La/Eu]$\rangle$ = $-$0.08 dex.
Similarly, we find large star-to-star abundance variations for O, Na, Mg, Al,
and Si that are indicative of gas that experienced high temperature
proton-capture burning. Interestingly, we detect three distinct populations,
but also find large gaps in the [O/Fe], [Na/Fe], and [Al/Fe] distributions that
may provide the first direct evidence of delayed formation for intermediate
composition stars. A qualitative enrichment model is discussed where clusters
form stars through an early ($<$ 5$-$10 Myr) phase, which results in first
generation and "extreme" composition stars, and a delayed phase ($>$ 40 Myr),
which results in the dilution of processed and pristine gas and the formation
of intermediate composition stars. For NGC 6402, the missing intermediate
composition stars suggest the delayed phase terminated prematurely, and as a
result the cluster may uniquely preserve details of the chemical enrichment
process.
