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 ^ { -1 } ( \sigma = 8.5 km s ^ { -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 ( \la 5 - 10 Myr ) phase , which results in first generation and ‘ ‘ extreme '' composition stars , and a delayed phase ( \ga 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 .