Star clusters Hyades

Fig. 1. Oxygen abundances as a function of the activity index, Rx, derived from X-ray data (left-hand panels) and the excitation temperature Texc (right-hand panels). The bottom panels show the difference between [O/Fe] yielded by the OI triplet at about 7774 A and the [OI] A6300 line. Filled circles RS CVn binaries ([2] and [3]), filled squares field subgiants [3], filled triangles Pleiades stars, open triangles Hyades stars, open circles, squares and hexagons disk dwarfs. The source of the literature data for the open cluster and Galactic disk stars can be found in [4].

Fig. 1. Mean Li abundance as a function of age. Li abundances are in the usual notation log n(Li)= N(Li)/N(H)+12. Different symbols indicate stars in different mass bins, namely 1 0.02 M0 (circles), 1.05 0.02 M0 (squares) and 1.1 0.02 M0 (triangles). The Sun is also shown. The horizontal line denotes the initial log n(Li). The following clusters have been considered 120 Myr Pleiades 600 Myr Hyades 2 Gyr IC 4651, NGC 3680, NGC 752 4.5 Gyr M 67 (only the upper envelope of the Li vs. Teff distribution -see text) 7 Gyr NGC 188. The cluster samples have all been analysed with the same method. Error bars correspond to lcr deviations from the mean.

Fig. 3.20. Idealized continuum and actual fluxes measured in 50-A-wide bands of A7 stars in the Hyades open cluster with Teff = 8000 K, plotted against inverse wavelength in turn-1. Horizontal lines above the spectrum show the locations of the Johnson U, B, V pass bands and the vertical boxes show schematically the corresponding properties of the Stromgren system with central wavelengths in A. (In that system, there are actually two H/3 pass bands, one narrow and one broad, so that comparison of the two gives a measure of the strength of the line.) Some prominent spectral features are marked.

As stars become older, lithium at their surface becomes gradually depleted by mixing with deeper layers at temperatures above 2.5 x 106 K where it is destroyed by the (p, a) reaction, Eq. (4.49). This destruction takes place more rapidly in cooler stars with deeper outer convection zones, so that there is a trend for lithium abundance to decrease with both stellar age and diminishing surface temperature in cooler stars some depletion takes place already in the pre-main-sequence stage. Thus, in the young Pleiades cluster ( 108 yr), lithium has its standard abundance down to Teff = 5500 K (type G5), whereas in the older Hyades cluster ( 6 x 108yr) it is noticeably depleted below Tc t = 6300 K (F7) and also in... 

The position and width of the Li abundance gap observed in Hyades and other open clusters is explained by diffusion. A detailed reproduction of the Li(Teff) curve seems to require a mass loss rate of slightly more than 10"15 Mo yr-1, of the same order as the mass loss rate required by the FmAm stars. In the presence of such a mass loss only small overabundances of heavy elements are expected. The observed variations in the Li abundance as a function of the age of clusters suggests that the Li abundance observed in old halo stars does not represent the cosmological abundance. 

One can similarly use the meridional circulation fields to test its effect on the diffusion of Li in the F stars of clusters (Charbonneau and Michaud (1987). It turns out however that the upper limit of the equatorial rotation velocity is much smaller. This can be traced to the increase in the depth of the convection zone. The diffusion velocity decreases considerably due to the p 1 dependence of the diffusion coefficient while the meridional circulation velocity is nearly constant as one goes deeper in the star. While the critical velocity in the middle of the gap is about 15 km s 1, there are stars in the middle of the gap of the Hyades with a V sin i of 50 km s 1 (Boesgaard 1987). These stars have very low Li abundance and if the low Li abundance in the gap is to be explained by diffusion it is clear that the calculations of Tassoul and Tassoul (1982) do not apply to F stars. 

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