Star no. 1412 in M4, a post-AGB star with low carbon and enhanced TiO

Abstract

We have completed an abundnace analysis of 14 elements in the peculiar globular cluster star M4 star 1412. Star 1412 stands 0.7 mag above the red giant branch in the clusters color-magnitude diagram, is grossly deficient in carbon and shows TiO bands despite its effective temperature of at least 4100 K. No additional abundance anomalies are present. We can eliminate most causes for the presence of TiO. These include excess oxygen, excess titanium, a hydrogen deficiency, and excess gas pressure. The best remaining possibility is a reduced boundary temperature due to the absence of line blanketing by the CO and CN molecules. A deficiency in CO blanketing may also explain the higher abundances as derived from ionized lines than from neutral lines because the absence of the ultra-violet CO bands may permet an excess of radiation between 1300 and 1550 A to ionize species such as iron and titanium. Other aspects of non-standard model atmospheres are also discussed qualitatively.

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