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The Abundances of the Elements in the Oldest Disk Stars

Nineteen old disk G stars in the solar neighbourhood have been selected which, according to their UBV photometry and their trigonometric parallaxes, have evolved above the zero-age main sequence so as to be apparently older than or nearly as old as the old galactic cluster NGC 188. The influence of microturbulence on the UBV colours has been computed, and it is concluded that the positions of these stars in the HR diagram cannot be due to abnormal microturbulence blanketing effects on their colours. Effective temperatures for these stars have been obtained from : a ) (R-I) colour s b) photoelectric spectral scans c) Ha wing profiles the latter two being calibrated by means of flux constant LTE model atmospheres, computed for this project. A coarse differential curve of growth analysis has been carried out with respect to the sun for twenty elements in each of the stars from measurements of 376 lines on high dispersion plates of the blue and red spectral regions. The element-to-iron ratios for several elements are discussed, and one of the conclusions is that iron-deficient old disk stars are generally no more than marginally carbon deficient, a result which is probably not valid for halo stars. An unusual anomaly concerning the s-process elements copper and zinc in a few of the stars is noted, and a tentative explanation is proposed. The abundance of lithium in old subgiants is discussed. The view that these stars replenish their surface lithium supply and then loose it again on a long time scale is consistent with the data. A model atmosphere analysis of neutral and ionised iron lines has been carried out, and a calibration set up between gravity and electron pressure. Hence spectroscopic gravities have been obtained, and it is found that these are significantly less than zero-age main sequence gravities in the majority of cases. This supports the conclusion that these stars are evolved, and hence probably old. Two visual binaries with ..known orbits were included in the nineteen stars observed, and the gravities obtained from their orbital parameters are compared with those obtained spectroscopically. By means of a grid of helium-rich model atmospheres, the effect of a high helium abundance on the gravity determinations is considered. Furthermore, it is shown that a high helium abundance can, in principle, be detected if the trigonometric parallax is accurately known. Although one star does show a high helium abundance by this method, the conclusion is treated with some caution. For the remainder of the stars, the spectroscopic data do not contradict the view that their helium abundances are normal. In this case, they must be very old to have evolved. The orbital parameters for one of the visual binaries suggest it is helium-rich, but a more precise determination of the orbit would be necessary to confirm this assertion. The ages of the stars have been derived by comparing their positions in the (M /log T ) diagram with isochrones interb o 1 e polated from evolutionary tracks, using the premiss that their helium contents are normal. Although the interpretation of the data is hampered by a lack of a large sample of stars, it is shown that an age-abundance relation probably exists among old disk stars. On this basis, the rates of formation of different elements during the old disk epoch are discussed. However, there appears to be a small group of very old metal-rich stars which lie significantly off the age-abundance relation for the majority of old disk stars. An explanation for their high abundances, other than that of gradual enrichment of the interstellar medium by heavy elements, may have to be sought. The data are shown to be consistent with the view that the Galaxy has undergone a rapid collapse, and inconsistent with some of the premisses which lead to the hypothesis of the ejection of halo stars from the Galactic nucleus. Furthermore, it is shown that, after the rapid collapse during the halo epoch, the Galaxy has since then been slowly collapsing for 9 some 6 x 10 years or more . Since the oldest stars observed apparently have ages of 9 about 11 x 10 years, the Galaxy must be at least this old. This figure is not inconsistent with ages frequently assigned to halo globular clusters, or with the age of the universe, if this is supposed to equal the inverse Hubble constant.