Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...453..819r&link_type=abstract
Astrophysical Journal v.453, p.819
Astronomy and Astrophysics
Astronomy
63
Stars: Binaries: Close, Stars: Abundances, Stars: Evolution, Stars: Rotation
Scientific paper
Stellar evolution theory with rotation, as developed at Yale in recent years, makes some rather dramatic predictions about the evolution of the light-element tracers. This includes the prediction that primordial lithium (Li) has been significantly depleted to the abundance observed today in halo dwarfs, with implications for cosmology. More generally, the Yale models predict that rotational mixing (related to angular momentum loss and redistribution) is a cause of Li destruction in normal, single late-type dwarfs. In this study we test this through the complementary and striking prediction that certain short-period binaries should have preserved more Li than the mean trend of normal, single stars. We have observed seven short-period halo and disk binaries to discover the conditions under which Li rich binaries may be found. A halo dwarf and a halo subgiant with periods of 12.4 and 7.6 days have Li abundances on the mean halo Li abundance trend; these orbits probably synchronized too late to prevent mixing-induced Li depletion. A 5.6 day period halo subgiant, however, does have a higher Li abundance than otherwise similar stars, as also previously reported. An intermediate-metallicity 1.8 day subgiant is found with a measurable Li abundance at a Teff value where most stars yield only low upper limits. The remaining three disk stars, with 3.3 to 6.6 day periods, yielded only upper limits to their Li, and suitable single stars against which to compare them were lacking. We have also examined previously observed short-period binaries, not all of which have been appreciated as such. High Li abundances were found for an intermediate-metallicity cool dwarf and also for three Hyades cool dwarfs (where the Hyades is old enough for its single stars to be depleted), but not in the younger Pleiades, all as predicted. The Yale models further predict the gradual depletion of the Li peak region in open clusters, in analogy to the intermediate-metallicity and halo Li plateaus. Two short-period binaries were found to lie above the Hyades Li peak, and one above the Li peak of the much older M67, supporting the predicted depletion of the Li peak. High Li short-period binaries have thus been observed in a variety of contexts, consistent with the Yale rotational models combined with tidal considerations: young cool disk and old intermediate-metallicity dwarfs, young and old Li peak dwarfs, and old cool intermediate-metallicity and halo subgiants. We conclude that some synchronized binaries with periods less than about 6 days may be better able to preserve their Li abundances, in line with Yale rotational model predictions. Suitable short-period binaries in the Teff region of the halo Li plateau still await identification and observation.
Deliyannis Constantine P.
Ryan Sean G.
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