Abundances of sodium, magnesium and calcium in K-type giant stars

Narrow band photometric measures of the strengths of the magnesium b, calcium triplet, and sodium D lines in 312 G and K type giant stars have been analysed with a model atmosphere technique to determine abundances of magnesium, calcium and sodium. The following conclusions have been reached: (I)[Mg/H], [Ca/H], [Na/H] in low velocity stars do not differ significantly from their solar values. (2)High velocity stars are deficient in Ca and Na by varying amounts, but the mean ratio [Ca/Na] is approximately equal to its solar value. (3)Ultra-violet excesses show some correlation with [Ca/H] and [Na/H]. (4)[Mg/H] in high velocity stars is equal to or perhaps slightly greater than its solar value. (S)Almost all high velocity stars have [Mg/Ca] and [Mg/Na] greater than their solar values. The cut-off at the solar value is very abrupt. These results suggest that the magnesium content of the Galaxy increased at a more rapid rate than the calcium, sodium and heavy element contents, and that it reached its present value at an earlier stage. The formation of high velocity giant stars must have come to an abrupt end at a particular instant in the history of the Galaxy.