Download The age–metallicity distribution of earth-harbouring stars

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
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F
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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.