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].

As first emphasized by [25], measuring Li in clusters older than the Hyades is a key tool to investigate MS Li depletion and its timescales. Now, more than 30 years later, Li surveys have been carried out for several old clusters including NGC 752, M 67, and the very old NGC 188 ([22], [9], [18] and references therein). 

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.

In Fig. 2 log n(Be) is plotted as a function of log n(Li) for the Hyades, IC 4651, and M 67. We recently obtained UVES spectra of a new sample of F-and G-type stars in M 67. Be abundances for these stars are shown in the figure together with those of the sample of [17]. As found and discussed by [1], the figure shows a tight correlation between Be and Li abundances for Hyades stars... 

Fig. 2. Be vs. Li abundances for the Hyades (circles -[1]), IC 4651 (triangles -[17]) and M 67 (squares -[17] and new UVES data). Filled and open symbols denote stars with masses below and above 1.1 M , respectively. The horizontal and vertical lines indicate initial Be and Li abundances. Log n(Be) derived by different authors have been put on the same scale.

Abstract. We report on preliminary results of VLT/FLAMES observations of the old open clusters NGC 2506, Mel 66 and Cr 261, obtained as part of our Guaranteed Time on this instrument. We focus in particular on the very old cluster Cr 261, one of the oldest open clusters in the Galaxy. We compare the derived Li abundances with those of other old clusters, and we discuss briefly Li depletion on the main-sequence from the age of the Hyades to 8 Gyr. 

Fig. 2. Li abundances vs. Tefi for MS stars in Cr 261 (filled circles), compared to the Hyades (asterisks, [22]) and M 67 (squares, [7]). A reddening E(B — F)=0.34 has been assumed.

These results, as well as those for NGC 2506 and Mel 66, will be compared with those being acquired in our on-going GO observations with FLAMES (P.I. S. Randich) in order to obtain a better understanding of Li depletion on the MS from the age of the Hyades to 8 Gyr. 

Fig. 1. Projected rotational velocity in the Hyades and lithium versus effective temperature in 3 open clusters of the same age (Hyades, ComaB, Praesepe). The open symbols are the observed data (Kraft 1965, Stauffer et al. 1987, Mermilliod 1992, Burkhart Coupry 1998, 2000, Boesgaard 1987). The black symbols are the predictions of the rotating models at the age of the Hyades (compilation of Talon Charbonnel 1998 and Charbonnel Talon 1999)...

What happens for cooler (i.e. less massive) stars on the red side of the Li dip As we shall see now, the stellar mass or the effective temperature of the dip is a transition point for stellar structure and evolution. First of all it is a transition as far as the rotation history of the stars is concerned. Indeed the physical processes responsible for surface velocity are different, or at least operate with different timescales on each side of the dip. At the age of the Hyades, the stars hotter than the dip still have their initial velocity while cooler stars have been efficiently spun down (Fig. 1). This behavior is linked to the variation of the thickness of the superficial H-He convection zone which gets rapidly deeper as Teff decreases from 7500 to 6000K (e.g. TC98). Below 6600 K, the stars have a sufficiently deep... 

Fig. 3. Measured Li abundances in the Hyades. Superposed is the pattern of AM transport required to produce the Li dip in terms of rotational mixing...

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.

Fig. 3.21. Two-colour plot of (U-B) against (B—V). The curve shows the main sequence for stars with the metallicity of the Hyades, loci of black bodies (with temperatures marked in units of 1000 K) and a deblanketing vector illustrating schematically the effect of metal deficiency in F and G stars. Adapted from Unsold (1977).

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 hydrogenase specific endopeptidases (HybD, HyaD, Hyl, HoxM, HupM, HupD)... 

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. 

Observations of other clusters can lead to a better understanding of the evolution of the Li abundance. In the Pleiades, Pilachowski and Hobbs (1987) have observed less than a factor of 1.5 decrease of the Li abundance in the gap (see also Duncan and Jones 1983, Duncan 1981). Since the Pleiades are about ten times younger than the Hyades, a scaling of the exponential dependence of the abundance reduction leads to exp(3.4/10)=1.4 this is reasonable agreement. 

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. 

Fig. 4 The Li underabundance caused by the matter brought, to the convection zone, by meridional circulation from the region where Li burns, is shown as a function of Teff for the Hyades (8 10s yr) and UMa (4 10s yr). It is compared to observations for these two clusters (Boesgaard, Budge and Burck 1988).

On Fig. 4 is shown the Li abundances to be expected from such a model at the age of the Hyades and of Uma. They are compared to the observations of Boesgaard, Budge and Burck (1988). To do these calculations the equatorial rotation velocity is needed. I used V= 50 km s"1 at Tetf=6700 K and V=25 km s 1 at Teff =6350 K, from an average of the observed rotational velocities in the appropriate Teff range taking the effect of sin i... 

Another explanation of the lithium gap in the Hyades could be found in terms of turbulent diffusion and nuclear destruction. Turbulence is definitely needed to explain the lithium abundance decrease in G stars. If this turbulence is due to the shear flow instability induced by meridional circulation (Baglin, Morel, Schatzman 1985, Zahn 1983), turbulence should also occur in F stars, which rotate more rapidly than G stars. Fig. 2 shows a comparison between the turbulent diffusion coefficient needed for lithium nuclear destruction and the one induced by turbulence. Li should indeed be destroyed in F stars This effect gives an alternative scenario to account for the Li gap in the Hyades. The fact that Li is normal in the hottest observed F stars could be due to their slow rotation. 

Stephanocoenia sp.) collected from Discovery Bay, Jamaica. Branching at about 25 cm from the top is clearly evident. Samples analyzed are designated with a letter and number, (b) Plot of activity of Ra/ Ra in the Jamaica coral head and the North Carolina coral (Solenastrea hyades) head vs. distance from outer surface. The estimated growth rates based on decay of excess Ra activity are 0.5 and 0.15 cm/yr, respectively. From Moore and Krishnaswami (1972). 

Moore and Krishnaswami (1972) studied the growth rate of head corals (Stephanocoenia sp. and Solenastrea hyades) by two radiometric methods. Assuming an initial incorporation of Ra (or b) to Ra, which has a long half-life (1600 years) as contrasted to Ra and Pb, they used the decay of the latter in the coral as an index of growth rate. Since Ra has a shorter half-life (5.77 years) than Pb (21 years), the presence of Ra would approach zero in the older areas of the coral, and if no additional material was incorporated into the coral, it would be measurable for about 30 years. The Pb would reach equilibrium with its parent, Ra, in older sections of the coral head. Figure 116 shows the procedure for sampling a coral head and the graphic results of the 2 Ra/ Ra ratio method. 

A NEAR-INFRARED IMAGING SEARCH FOR LOW MASS COMPANIONS TO HYADES STARS... 

Abstract. We axe conducting an in aied imaging search fox very low mass stars and sub-stellar objects in 90" by 90" fields around 141 known Hyades stars. The Hyades represents a nearly ideal location for such a seardi due to its moderate age and proximity to Earth. An imaging search for companions seems more likely to sample a distribution similar to the original Hyades mass fimction than a search of the Hyades field. 

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