Credit: IAC
New study lead by Aarhus astronomer Víctor Silva Aguirre to be published
in MNRAS: 33 Kepler stars have been selected for their solar like
oscillations and a set of basic parameters have been determined with
high precision showing that even stars older than 11 billion years have
Earth-like planets.
A new study of 33 Kepler stars with solar-like oscillations to be
published in Monthly Notices of the Royal Astronomical Society.
According to lead author of the article Víctor Silva Aguirre from the
Stellar Astrophysics Centre at Aarhus University, Denmark: " Our team
has determined ages for individual host stars before with similar levels
of accuracy, but this constitutes the best characterised set of
exoplanet host stars currently available."
Measuring the ages of stars is one of the very tough problems that
contemporary astronomers are faced with. Up to now only the age of the
Sun has been determined with high precision (it is 4.57 billion years,
with a precision of 10 million years to each side). The international
group of astronomers have determined ages, diameters, densities, masses
and distances for 33 stars better than ever before. As an extra, all of
these stars have earth-like planets, giving us a clear indication that
such planets have formed in our Milky Way Galaxy long before the Earth
and are still being formed out there.
The 33 stars have been carefully selected from the more than 1 200
stars with planets around them that have been observed with the highly
successful Kepler satellite. The stars have to be sufficiently bright to
give a good statistical basis for the results, and they have to show
some of the same characteristics similar to the Sun to make them
comparable.
Stars pulsate, vibrate and resonate just like sound waves in a
musical instrument. The advanced technique of measuring these starry
tunes is called asteroseismology - a method quite similar to the one
used by geologists to sound out the composition of the interior of the
Earth by means of earthquakes.
The NASA-launched Kepler satellite has constantly measured tiny
variations in the light from some 145 000 stars over a period of a
little more than four years. Analyzing these variations over time gives
the periods of the many simultaneous pulsations in each star, and from
that the scientists can derive the important basic properties of the
individual stars.
Why is it important
Knowing the ages, sizes and other basic parameters of the stars,
apart from being interesting in itself, is important if one wants to
study the large scale development of our galaxy and the Universe as a
whole - a relatively new discipline named "galactic archeology". We all
wish to know where we came from! On a more practical level the stars
function in largely the same way as a fusion reactor. Precise knowledge
of the internal machines in stars might help in future energy production
here on Earth.
It is not the first time that precise ages of individual stars have
been determined. But using a large sample and studying them with the
same instrument - the Kepler satellite - and the same theoretical and
statistical methods gives us a much higher confidence in the precision
of the results. Comparing the stars may also reveal unusual and so far
unknown stellar properties.
With a large, and hopefully growing, set of well-studied stars it
will be possible to expand our knowledge even to stars which are too
faint to obtain asteroseismological values for. The precise knowledge
especially of stellar ages can be related to the properties of the
light; the spectra, from the same stars. This gives us a set of
well-known calibration stars and thus it enables us to work backwards
from spectroscopy of faint stars to their ages.
How is it done
The 33 stars selected for the study are not all similar to the Sun,
but they behave in much the same way as the Sun does. They are what
technically is called "solar-like oscillators". Víctor Silva Aguirre
explains: "The term solar-like oscillators means that the stars exhibit
pulsations excited by the same mechanism as in the Sun: gas bubbles
moving up and down. These bubbles produce sound waves that travel across
the interior of stars, bouncing back and forth between the deep
interior and the surface producing tiny variations in the stellar
brightness."
How precise?
The new study gives us values for the selected stars with
uprecedented precision. On the average stellar properties are better
than the percentages below. If a star e.g. has a calculated age of 5
billion years, the 14% means that it's true age lies between 4.3 and
5.7 billion years:
1.2% (radius),
1.7% (density),
3.3% (mass),
4.4% (distance),
and 14% (age).
Are they representative?
All the stars studied by the Kepler satellite lie in a small area of
the sky, close to the constellation of Cygnus. The 33 stars in this
study span distances between 100 and 1600 light-years from the Sun. With
such a small area of the Milky Way Galaxy studied over such relatively
short time, one could wonder if the stars selected for the study are at
all representative for the more than 300 billion stars in our galaxy.
The answer is a qualified "yes". Certainly the astronomers would like to
study many more stars for much longer time, but for the time being and
compared to what was previously known this is a large first step. In the
future we will be able to study larger samples of stars, selected from a
larger area of sky with the current Kepler2 project and from 2017 on
hopefully from all over the sky with the TESS-satellite. Even better
results are expected from the PLATO-satellite due to be launched by the
European Space Agency in the mid-2020'ies.
What about those planets
The Kepler satellite is able to provide two very different types of
results with the same sort of measurements. From the small variations in
the intensity of starlight, one can both deduce asteroseismic values of
the stars and also discover any exoplanets circling the stars.
Determining the exact properties of these exoplanets is only possible if
we also know the basics of the host stars, and these come from
asteroseismology. The two fields of astronomy are closely connected.
Assistant Professor Silva Aguirre sums up:
"One of the biggest questions in astrophysics is: does life exists
beyond earth? To even begin answering this, we need to know how many
planets like ours exist out there, and when they formed. However
determining ages of stars (and thus of their orbiting planets) is
extremely difficult; precise ages are only available for a handful of
host stars thanks to asteroseismic observations made with the Kepler
satellite.
Our study provides the first sample of homogeneously determined ages
for tens of exoplanet host stars with a high level of precision. The
stars we studied harbour exoplanets of size comparable to earth (between
0.3 and 15 earth radii), and our results reveal a wide range of ages
for these host stars, both younger (down to half the solar age) and
older (up to 2.5 times the solar age) than the Sun. This is regardless
of the size of the exoplanets in the system or multiplicity, showing
that formation of exoplanets similar in size to earth has occurred
throughout the history of our Galaxy (and is still taking place!).
Actually some of these planets were of the same age as the Earth is now,
at the time when the Earth itself formed. This in itself is a
remarkable finding."
The title of the study is: "Ages and fundamental properties of Kepler
exoplanet host stars from asteroseismology". The full article can be
found in arXiv here.
Contact:
Víctor Silva Aguirre
0045 87155635
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