The chemical composition of stars that host planets has persuaded astronomers that planetary debris falling onto the outer layer of the star produces a detectable effect in a dwarf star.
However, this pollution is diluted by the giant star and mixed into its interior, they believe.
According to the astronomers from the European Southern Observatory (ESO), dwarf stars often show iron enrichment on their surface, but giant stars do not.
'It is a little bit like a Tiramisu or a Capuccino,' said Luca Pasquini at the ESO, lead-author of the paper reporting the results. 'There is cocoa powder only on the top!'
Following the discovery of the first exoplanet (so named because it orbits a star other than the sun), astronomers established that planets are found around stars that are enriched in iron. So planet-hosting stars are on average almost twice as rich in metals as their counterparts with no planetary system.
The immediate question was whether or not this richness in metals enhances planet formation, or whether it is caused by the presence of planets.
According to the team of Italian and German astronomers, it is a classic chicken and egg problem. In the first case, the stars would be metal-rich down to their centre, and in the second, the debris from the planetary system would have polluted the star and only the external layers would be affected by this pollution.
When observing stars, astronomers only see the outer layers and cannot be sure that the whole star has the same composition. When planetary debris falls onto a star, the material will stay in the outer parts, polluting it and leaving traces in the spectra.
The team of astronomers decided to tackle this question by looking at a different kind of stars: red giants. These are stars that have exhausted the hydrogen in their core. As a result, they have puffed up, becoming much larger and cooler. This will happen to the Sun in several billion years from now.
Looking at the distribution of metals in 14 planet-hosting giants, the astronomers found that their distribution was rather different from normal planet-hosting stars.
'We find that evolved stars are not enriched in metals, even when hosting planets,' said Dr Pasquini. 'Thus, the anomalies found in planet-hosting stars seem to disappear when they get older and puff up!'
Looking at the various options, the astronomers concluded that the most likely explanation lies in the difference in the structure between red giants and solar-like stars: the size of the convective zone, the region where all the gas is completely mixed.
In the Sun, this convective zone comprises only 2% of the star's mass. But in red giants, the convective zone is huge, encompassing 35 times more mass. The polluting material would thus be 35 times more diluted in a red giant than in a solar-like star.
'Although the interpretation of the data is not straightforward, the simplest explanation is that solar-like stars appear metal-rich because of the pollution of their atmospheres,' said Artie Hatzes, co-author of the report.
European Southern Observatory (ESO)
Based on the report 'Evolved stars hint to an external origin of enhanced metallicity in planet-hosting stars'
Aerospace Technology; Scientific Research