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The age–metallicity distribution of earthharbouring stars Helio J. RochaPinto (OV/UFRJ) Galactic Chemical Evolution We can divide the Galactic content in “stars” and “gas”. There is constant exchange of matter between these two components. However, this cycle enriches the matter with new elements produced in the stellar interiors. gas stars Abundances and Metallicities • Stellar abundances are relative measurements with respect to some element, like H, C or Si. The Fe abundance is commonly used to represent the stellar ‘metallicity’, i.e., the abundance of all chemical species heavier than He. It is usual to represent stellar abundances in the [X/H] notation [ X / H ] = log n(X) n(H ) star n( X ) − log n(H ) sun Solar Fe abundance: [Fe/H]= log(1) = +0.0 2x Solar Fe abundance : [Fe/H]= log(2) = +0.3 0.5x Solar Fe abundance : [Fe/H]= log(½) = 0.3 Goldilocks rule for forming Earths • Earthlike planets cannot form in metalpoor environ ments. • Hot jupiters are mainly found around very metal rich stars. There must be a preferential metallicity for the formation of stars that can harbour lifebearing earths. Lineweaver 2001, Icarus, 151, 307 Chromospheric ages HD 206860 log R' =4.42 HK HD 144287 log R' =5.06 HK 3920 3930 3940 3950 3960 3970 3980 Wavelength (Å) Stellar ages can be estimated from the flux in the emission lines of some elements, like the + H and K lines of Ca . Skumanich 1972, ApJ 171, 565 Selection of the solar stars sample • Our sample is based on a compilation of several surveys of chromospheric activity and Strömgren photometry among solartype stars. A total of 1188 stars were selected. • Stars were binned according to age and metallicity. Several corrections to account for stellar evolution, volume sampling and scale height were applied. Age– metallicity relation • The density of stars in the age– metallicity plane provides valuable constraints for the chemical evolution theory. Our result confirms that younger stars are, on average, more metalrich than older stars. There is a very small scatter around a mean relation, although the number of stars formed at different epochs is not constant. 0.4 0.2 0.0 0.2 [Fe/H] From the solar position in the age– metallicity plane, compared to coeval stars, we can see that the Sun was formed with a metallicity higher than the average metallicity of the interstellar medium at it birth time. 0.4 0.6 0.8 1.0 20 1 2 3 4 5 6 age (Gyr) 40 7 60 80 100 120 140 160 180 200 220 8 9 10 11 The Sun among its coeval stars 1.0 0.8 0.6 z > 0.25 z z > 0.50 z fraction of stars • We have found that more than 85% of the stars born 4.5 Gyr ago have metallicity smaller than the Sun (and the protosolar planetary disk). How could this affect the chances of life having developed in the Earth? z > 0.75 z 0.4 z > 1.00 z z > 1.50 z 0.2 0.0 0 2 4 6 age (Gyr) 8 10 12 EarthHarbouring Star Formation • If we use Lineweaver’s estimate for the probability of harbouring earths as a function of [FeH], we can estimate the density of earthharbouring stars in the age– metallicity plane. 0.4 0.2 0.0 [Fe/H] 0.2 0.4 0.6 0.8 1.0 1 2 3 4 5 6 age (Gyr) 1 10 20 7 8 9 40 10 11 • We have seen that although the Sun was born during one major star formation episode, this plot shows the Sun does not took part in the bulk stellar generation likely to harbour earths. Seager et al. 2005, astroph/0503302 F irs t M et az oa F ir st E uk ar yo te s C ru st S ol id ifi ca tio n F ir st P ro ka ry ot es Where to choose targets? 0.4 1 0.2 10 20 40 [Fe/H] 0.0 0.2 0.4 0.6 0.5 1.0 reducing oxidizing atmosphere atmosphere 1.5 Pictures from Kaltenegger et al. 2005, astroph/0512053 2.0 2.5 3.0 3.5 age (Gyr) 4.0 4.5 5.0 5.5 6.0 6.5 Conclusions • The relation between age and metallicity is very tight and agrees with the general predictions of the chemical evolution theory. • The Sun is more metalrich than 85% of its coeval stars. • Direct spectroscopic findings of terrestrial planets would be more efficient if young stars (having 3 to 4 Gyr, and solar metallicity) are surveyed.