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Fig. 1:
This figure shows the relationship between age and metal content for
the stars in our sample, where the colour coding indicates the number
of stars. The peak of the metallicity distribution hardly shifts with
age, but the width of the distribution increases significantly. This
signature can be ascribed to radial migration.
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Fig. 2:
Mean rotational velocity of stars with different chemical composition
in a model calculation. The horizontal axis gives the metal content of
stars, the vertical axis the relative amount of the alpha-element
oxygen (O) compared to iron (Fe). Object high in oxygen (top) are
generally the oldest. The colours indicate the mean rotational
velocity of the objects. There is a clear contrast between metal rich
and metal poor stars. Black lines indicate the origin of stars in the
model; they trace the evolution of the star forming gas in distances
of 10 kpc (outer disc), 7.5 kpc (about our Sun), 5 kpc and 2,5 kpc
(inner disc) from the centre.
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Fig. 3:
These plots show various properties versus metal content. In the central
panel, stars are selected that have higher (black) or lower (yellow)
rotational velocity relative to the Sun (v = 232 km/s). The top panel shows
the amount alpha elements for slow stars (yellow) and fast stars (black),
the coloured lines show the mean trends of both populations. Slower stars
(the ”thick disc“) have a higher abundance in alpha elements, which points
to a higher age. The bottom panel shows the distribution of metal content in
the slow (yellow) and fast (black) stars; slow stars have a higher metal
content on average. In contrast to the classical belief, where older and
thick disc stars should be more metal poor, this can naturally be explained
in the migration model.
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Stars are both drivers of the evolution of our Galaxy and witnesses of
its past. At their centres, during most of their life they burn
hydrogen and helium via nuclear fusion reactions into heavier
elements, which astronomers broadly refer to as metals. The most
massive stars burn their nuclear fuel very quickly on a cosmic
timescale: they live only millions of years, while our Galactic disc
already exists for about 10 billion years. During their violent death
they expel most of their nuclear processed products. This mixes with
the pristine gas clouds, from which new stars are continuously being
formed, and enrich them with metals. Stars with masses similar to our
Sun act as witnesses to this history: a star's outer layer hardly
mixes with the material at their cores and so the stellar atmosphere
largely reflects the chemical composition of the gas from which the
star was formed. Their longer lifetimes, of the order of several
billions of years, make them fossils of early cosmic epochs.
The enrichment of the star-forming interstellar gas is not a sudden
process, metals will accumulate with time. Moreover, the relative
composition of the metals changes: very old stars, i.e. stars that
have been born during the youth of our Galactic disc, carry a surplus
of so called "alpha elements" compared to iron. These alpha elements
are multiples of helium nuclei such as oxygen, magnesium, silicon and
calcium. The ratio of alpha elements to iron serves as a natural time
indicator hinting at when a star was born.
Until recently, it was thought that nearby stars could be regarded as
a book from which to read the local history of star formation and
enrichment. This information alone is, however, not enough to fully
characterize the history of the Milky Way disc: as shown by
researchers at MPA and the University of Oxford, stars migrate heavily
within the disc. They do not orbit around the centre of the Galaxy
with a roughly constant radius but can shift both further-in and
further-out with time, making the chemical signature very difficult to
interpret. Stars found in the solar neighbourhood did not necessarily
get formed there; they can be immigrants from elsewhere within the
disc. There is an abundance gradient within the Galaxy, the star
forming gas in the dense inner disc develops more quickly and is more
metal rich than in the outskirts, and so the metal content of a star
is linked to where it was born.
Taking migration of stars into account greatly modifies the
interpretation of the history of the Milky Way. The xenophobic view
without migration needs to invoke catastrophic happenings in the
Galactic history, such as the impact of a smaller galaxy or at least a
period when star formation ceased almost completely. In contrast, the
new models with migration explain observations with a quite calm and
simple Galactic history. The two models also imply very different
relationships between the age of the stars and their metal content
(”metallicity“): the classical perspective needs a pronounced
change in the local metal content over time to build up the observed
broad distribution of metallicities in the solar vicinity. The
migration models do not require this as diversity is imported with the
immigrants. So, while the classical perspective demands a strong
evolution of the metal content over time, i.e. a significantly lower
metallicity for older stars, combined with no significant development
of the width of the metallicity distribution, our new models can allow
for a nearly constant average metal content over large spans of time,
but predict instead an increased spread of metallicity with age.
Whether stellar migration is indeed prominent can be decided by
studying local samples. The MPA scientists therefore embarked upon a
revision of the stellar parameters in the Geneva-Copenhagen survey,
the most comprehensive catalogue of the solar neighbourhood,
containing some 16,000 stars. The metal content of stars can be
derived by studying the stellar spectra and comparing them with model
spectra. As this approach is very time-consuming it is not feasible
when dealing with thousands of stars. Some abundance information can,
however, also be retrieved simply from colours: just as tiny amounts
of dye can colour wine red, the presence of metals in a stellar
atmosphere alters the star's colours. Using an improved scheme to
derive stellar physical parameters from colours, the researchers at
MPA have shown that the average metallicity of stars in the vicinity
of the Sun is higher than previously thought, thus making the Sun a
more common object.
The scientists were also able to re-derive the relationship between
age and metallicity of the stars. As can be seen from Figure 1, the
peak of the metallicity distribution hardly moves with age of the
objects, but the width of the distribution increases. This is in
conflict with the classical view, while it is naturally explained in
the radial migration model.
The new view of migration simplifies our picture of Galactic
history. To further discriminate this scenario from the classical view
and to improve our understanding of the mixing process in the disc,
more information is required. As noted above, the amount of alpha
element is related to the age of a star. As stars get scattered around
in the Galaxy, some information about their origin still remains in
the concerned populations.
The model developed at MPA and the University of Oxford is the first
analytical model that can relate chemical information with the motions
of stars. The explanations for observed relationships are easy and
straightforward in this model, while the classical model fails to
explain them. Further correlations can be predicted, such as the mean
rotational velocity in the solar neighbourhood as shown in Figure
2. Old stars with a content of alpha-elements are at the top, young
stars at the bottom. Superimposed on this distribution is the velocity
information: stars from the inner disc rotate more slowly (blue) and
are less metal rich (right) than stars from the outer disc. This
prediction was confirmed by observational data for young stars (the
prediction for old stars is more complex). The international research
group led by MPA could thus detect a systematic error in determination
of the velocities and to recalculate them.
Moreover, the MPA astronomers have been able to estimate the content
of alpha elements from colours. Thus they could explore the subtle
links between the detailed chemistry and the movements of stars in the
Galaxy with an unprecedented large sample.
Most excitingly, the new radial migration model also provides a
natural explanation for the so called thick disc, a puffed up stellar
population in the disc. Rather than postulating a cosmic collision
between the Milky Way and another galaxy as in the classical view, the
migration model explains its existence with the immigration of
relatively old stars from the inner Galactic disc. In most
definitions, stars with slow rotation are predominantly ascribed to
this thick disc, which is linked to the stars' origin in the inner
disc, while stars with higher rotation velocity are linked to an
origin from the outer, more metal poor disc (see Figure 3). The slowly
rotating stars clearly have a higher average content in alpha
elements, which points to their higher age. At the same time, they are
also more metal rich than the fast rotating population. This again
points to a problem in the classical picture without migration.
The SAGA-Team
Publications
Schönrich, R. & Binney, J.,
"Origin and Structure of the Galactic disc(s)",
2009, MNRAS, 396, 203
Schönrich, R. & Binney, J.,
"Chemical evolution with radial mixing",
2009, MNRAS, 399, 1145
Schönrich, R.,
"Unruhe im Ruhestandard",
Sterne und Weltraum, August 2010
Casagrande, L., Schönrich, R., Asplund, M., Ramírez, I., et al.
2010, A&A, submitted
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