Hints of pollution

Although, as we have seen in the previous section, the bulk stellar metallicity "excess" observed seems to have a "primordial" origin, some hints of stellar "pollution" related to planet-host stars have been discussed in the literature (e.g. Laws & Gonzalez 2001; Israelian et al. 2001,2003). One of the clearest of these has to do with the detection of significant amounts of 6Li in the atmosphere of the metal-rich Solar-type dwarf HD 82943.

6,100

6,140

6,180

6,220

'eff

6,260

6,300

6,340

3.2

2.8

2.4

2.0

1J

1.6

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'eff

6,300

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□ □i nifflpn □ "gc

A

A -A -

L

6,020

6,060

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6,060

6,100

5,600

5,640 5,680 5,720

5,760

5,800 5,840

Figure 2.3. Lithium (Li) abundances as a function of effective temperature for planet-host stars (filled symbols) and field stars in three different temperature regimes. In the temperature interval between ~5,600 and 5,850 K, planet-hosts seem to have systematically lower Li abundances. From Israelian et al. (2004).

5,600

5,640 5,680 5,720

5,760

5,800 5,840

Figure 2.3. Lithium (Li) abundances as a function of effective temperature for planet-host stars (filled symbols) and field stars in three different temperature regimes. In the temperature interval between ~5,600 and 5,850 K, planet-hosts seem to have systematically lower Li abundances. From Israelian et al. (2004).

The rare isotope 6Li provides a unique way of looking for traces of "pollution." Until recently, this isotope had been detected in only a few metal-poor halo and disk stars, but never with a high level of confidence in any metal-rich or even Solar-metallicity star. Standard models of stellar evolution predict that 6Li nuclei are efficiently destroyed during the early evolution of Solar-type (and -metallicity) stars and disappear from their atmospheres within a few million years. Planets, however, do not reach high enough temperatures to burn 6Li nuclei, and fully preserve their primordial content of this isotope. A planet engulfed by its parent star would boost the star's atmospheric abundance of 6Li.

In fact, as discussed in Israelian et al. (2003), planet (or planetesimal) engulfment seems to be the only convincing and the least speculative way of explaining the presence of this isotope in the atmosphere of HD 82943, a planet-host star known to harbor a system of two resonant giant planets.

It is important to note, however, that the quantity of material that we need to add to the atmosphere of HD 82943 in order to explain the lithium isotopic ratio would not be able to change the [Fe/H] of the star by more than a few hundreaths of a dex. Furthermore, the results presented above, supporting a "primordial" source for the high [Fe/H] of planet-host stars, are not dependent on these probably isolated cases of "pollution".

Note also that we are referring to the infall of planets or planetary material after the star has reached the main-sequence phase and fully developed a convective envelope; if engulfment happens before that, all planetary material will be deeply mixed, and no traces of "pollution" might be found. It is interesting to note that the whole giant-planet-formation phase must take place when a disk of gas (and debris) is present. Massive gas disks may not exist at all when a star like the Sun reaches the main-sequence phase; although it is still not known definitively, inner disks seem to disappear after ~10 Myr - e.g. Haisch et al. (2001). Thus, all the "massive" infall that would be capable of changing the measured elemental abundances if the star were already in the main sequence might simply occur too early, explaining why we do not see strong traces of "pollution" (in particular, concerning iron).

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