  NGC 4833 is a metal-poor Galactic globular cluster (GC) whose multiple
stellar populations present an extreme chemical composition. The Na-O
anti-correlation is quite extended, which is in agreement with the long tail on
the blue horizontal branch, and the large star-to-star variations in the
[Mg/Fe] ratio span more than 0.5 dex. Recently, significant excesses of Ca and
Sc with respect to field stars of a similar metallicity were also found,
signaling the production of species forged in H-burning at a very high
temperature in the polluters of the first generation in this cluster. Since an
enhancement of potassium is also expected under these conditions, we tested
this scenario by analysing intermediate resolution spectra of 59 cluster stars
including the K I resonance line at 7698.98 A. We found a wide spread of K
abundances, anti-correlated to Mg and O abundances, as previously also observed
in NGC 2808. The abundances of K are found to be correlated to those of Na, Ca,
and Sc. Overall, this chemical pattern confirms that NGC 4833 is one of the
relatively few GCs where the self-enrichment from first generation polluters
occurred at such high temperatures that proton-capture reactions were able to
proceed up to heavier species such as K and possibly Ca. The spread in K
observed in GCs appears to be a function of a linear combination of cluster
total luminosity and metallicity, as other chemical signatures of multiple
stellar populations in GCs.
