Other
Scientific paper
Nov 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aj....106.1858c&link_type=abstract
Astronomical Journal (ISSN 0004-6256), vol. 106, no. 5, p. 1858-1869
Other
32
Abundance, Age Factor, Astronomical Catalogs, Chronology, Globular Clusters, Horizontal Branch Stars, Metallicity, Morphology, Stellar Models, Galactic Bulge, Iron, Oxygen, Ratios, Red Giant Stars
Scientific paper
Isochrones in the horizontal-branch (HB) morphology-metallicity diagram have been constructed on the basis of (1) detailed synthetic HB models, as recently presented by Catelan (1993), and herein extended to competely red and blue HB morphologies; and (2) the red giant branch (RGB) models of Sweigart & Gross (1978). The effect of variations in the several relevant input parameters that characterize the present (standard) isochrones -- namely, main-sequence helium content (YMS), abundances of the alpha-capture elements ((alpha/Fe)), and mean overall mass loss on the RGB (Delta script M) -- has been extensively studied. We find that the age difference which may be estimated between any two (old) stellar populations from their distributions in the HB morphology-(Fe/H) plane, holding YMS, (alpha/Fe), and Delta script M fixed, depends strongly on the absolute values which are assumed for the latter quantities. In other words, unless the absolute values for such parameters can be accurately determined for the objects subject to examination, the HB morphology-(Fe/H) diagram does not constrain age differences in a reliable way. As examples, we have analyzed several following cases. Our isochrones are for fixed YMS, (alpha/Fe), and Delta script M. The identification of the true second parameter that controls HB morphology (besides (Fe/H)) is thus seen to depend on the accurate determination of cluster-to-cluster differences in (at least) the three parameters listed above, whereas the magnitude of the problem itself depends on the absolute values of those parameters which may have been excluded as second-parameter candidates. We also find that similar age differences between any two populations, as obtained for different (YMS, (alpha/Fe), Delta script M) combinations, imply similar absolute ages for these populations. On the basis of this result, we show that the classical second-parameter pair NGC 288-NGC 362 can only be explained in terms of an age difference as small as approximately equal to 3 Gyr, if both clusters are younger than approximately 10 Gyr. This follows irrespective of the (YMS, (alpha/Fe), Delta script M) combination chosen which is capable of reproducing the reported age difference of 3 Gyr between these two clusters. The far-reaching implications of this result must clearly be further explored.
Catelan Márcio
de Freitas Pacheco José Antonio
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