Star Surface Polluted by Planetary Debris (Forwarded)

ESO Education and Public Relations Dept.

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Contacts:

Luca Pasquini
ESO Garching, Germany
Phone: +49 89 3200 6792

Artie Hatzes
Thüringer Landessternwarte Tautenburg, Germany
Phone: +49-36427-863-51

For Immediate Release: 6 July 2007

ESO Science Release 29/07

Star Surface Polluted by Planetary Debris

Looking at the chemical composition of stars that host planets,
astronomers have found that while dwarf stars often show iron enrichment
on their surface, giant stars do not. The astronomers think that the
planetary debris falling onto the outer layer of the star produces a
detectable effect in a dwarf star, but this pollution is diluted by the
giant star and mixed into its interior.

"It is a little bit like a Tiramisu or a Capuccino," says Luca Pasquini
from ESO, lead-author of the paper reporting the results. "There is cocoa
powder only on the top!"

Just a few years after the discovery of the first exoplanet it became
evident that planets are preferentially found around stars that are
enriched in iron. Planet-hosting stars are on average almost twice as rich
in metals than their counterparts with no planetary system.

The immediate question is whether this richness in metals enhances planet
formation, or whether it is caused by the presence of planets. The classic
chicken and egg problem. In the first case, the stars would be metal-rich
down to their centre. In the second case, debris from the planetary system
would have polluted the star and only the external layers would be
affected by this pollution.

When observing stars and taking spectra, astronomers indeed only see the
outer layers and can't make sure the whole star has the same composition.
When planetary debris fall onto a star, the material will stay in the
outer parts, polluting it and leaving traces in the spectra taken.

A team of astronomers has decided to tackle this question by looking at a
different kind of stars: red giants. These are stars that, as will the Sun
in several billion years, have exhausted the hydrogen in their core. As a
result, they have puffed up, becoming much larger and cooler.

Looking at the distribution of metals in fourteen 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," says 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 conclude 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," says co-author Artie Hatzes, Director
of the Thüringer Landessternwarte Tautenburg (Germany) where some of the
data were obtained.

When the star was still surrounded by a proto-planetary disc, material
enriched in more heavy elements would fall onto the star, thereby
polluting its surface. The metal excess produced by this pollution, while
visible in the thin atmospheres of solar-like stars, is completely diluted
in the extended, massive atmospheres of the giants.

More Information

"Evolved stars hint to an external origin of enhanced metallicity in
planet-hosting stars", by L. Pasquini et al. To appear in Astronomy and
Astrophysics. The paper is available on astro-ph at
http://arxiv.org/abs/0707.0788

The team is composed of L. Pasquini and M.P. Döllinger (ESO), A. Weiss
(Max-Planck-Institut für Astrophysik, Garching, Germany), L. Girardi
(INAF-Osservatorio Astronomico di Padova, Italy), C. Chavero (Instituto de
Astrofisica de Canarias, Tenerife, Spain, and Observatorio Nacional/MCT,
Rio de Janeiro, Brasil), A. P. Hatzes (Thüringer Landessternwarte
Tautenburg, Germany), L. da Silva (Observatorio Nacional/MCT, Rio de
Janeiro, Brasil), and J. Setiawan (Max Planck Institute für Astronomie,
Heidelberg, Germany).

The data have been partially collected at ESO, and partially at the 2-m
telescope of the Thüringer Landessternwarte Tautenburg (TLS).

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