Star clusters Pleiades

PLEIADES Ast. Mediaeval aigil for the star cluster, sometimes called... 

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].

Composition Variations Li depletion is sensitive to interior opacities, which themselves depend on the stellar composition. Small star-to-star variations might cause an Li abundance scatter, which would grow towards lower masses. However, current limits on metallicity variations in the Pleiades (and other clusters) seem too small for this to be the dominant explanation of any scatter [38]. In addition, the correlation of Li-depletion with rotation is unexplained. 

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.

Solar-type stars start destroying Li soon after their arrival on the ZAMS. The difference in the average Li between the Pleiades and the 2 Gyr clusters implies depletion timescales of the order of 1 Gyr. After 2 Gyr they become extremely long for part of the stars (Li depletion virtually stops) or very short for another fraction. The Sun belongs to the second category and, as such, it is not representative of all stars with the same mass, age, and metallicity. Li depletion is clearly not a monotonic function of age and at least one additional... 

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... 

Jupiter, the book included the observations of the milky-way, stars in Orion constellation, the bright Pleiades and beehive cluster. 

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