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DARKSTAR Team members
Chris Flynn , Team Leader
Burkhard Fuchs, Corresponding consultant
Johan Holmberg, Researcher
Laura Portinari, Researcher
Rami Rekola, Ph.D. student
Janne Holopainen, Ph.D. student
Luca Casagrande, Ph.D. student
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Research Developments in 2003
Flynn has led the DARKSTAR project, ``Space Based Studies of Dark
Matter'', which is funded under the
ANTARES program of the Academy of
Finland and TEKES. DARKSTAR acheived its first year of operation
in 2002 and continues until early 2004. The Academy of Finland and
Tekes' ANTARES program held the final seminar of the three year
program at Finlandia Talo in Helsinki. DARKSTAR presented the results
of the program highlighting baryonic dark matter searches; massive
black holes in the Galactic halo and Helium production in the
cosmos. We also showed our results at the Space 2003 exhibition at the
old Cable Factory in Helsinki.
Older research report for 2002
Dark Matter as black holes
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Jyrki Hänninen
and Chris Flynn completed a
numerical study of the orbits of stars in the disk of the
Galaxy by masive black holes (dark matter) and giant molecular clouds.
The work is a follow up of a study of the effect of such dark matter
and clouds on stars in the Solar Neighbourhood. In the new work, the
effect of these gravitational perturbers has been studied for disk
stars from the inner to the outer disk, and the results compared to
observations of the velocity dispersion of disk K giants over a wide
range of Galactocentric radius. We
conclude that black holes can be coaxed into reproducing the observed
velocities of the disk giants from the inner to the outer disk; in
other words they remain a possible but
somewhat unlikely source of disk heating. The figure shows a patch of
the Galactic disk,
seen close to edge-on, and surrounded by massive balck
holes.
MOVIE (in gif
format).
The simulation shows stars (shown in green) in the disk being heated by
black holes (shown in red) as they orbit in a Galactic potential.
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Dark Matter as white dwarfs in a Galactic "shroud"
Part of the solution to this
so-called "dark matter problem" would be that
there are a lot of very dim stars out there which we have yet to detect
with our telescopes. A very thick disk of stars , "shrouding" the
galaxy's disk, has been proposed as a solution to the problem.
The
shroud is shown schematically below, in which the shroud
appears in grey and the visible Milky Way in red.
Schematic
representation of the Galactic shroud.
The visible galaxy is
shown in red, and the shroud in grey.
If such a shroud
of stars actually envelopes the Galaxy, then the stars must be very
faint in order never to have been seen before; the best candidate for
these stars are the so-called "white dwarfs", stars which have run out
of fuel and are slowly cooling away to near invisibility. Whether the
proposed shroud could be made of such stars has now been addressed by Janne Holopainen and Chris
Flynn of
Tuorla Observatory by creating
a model of
the distribution of the low-mass stars around the Sun, including the
colours, luminosities and space motions of the stars. They compared
their model to two very large surveys of the fastest moving and
faintest detectible stars on the sky.
The team concluded that,
although a few quite interesting stars have turned up in these two
surveys, practically all the observations are well understood in terms
of our present knowledge of the Milky Way. Furthermore, they were able
to put strong constraints on how bright the putative white dwarfs could
be before significant numbers of them would have been detected in the
surveys; the results indicate that the white dwarfs must be very faint
indeed to have avoided detection. But there is still hope for
white dwarfs; they need only be a bit dimmer than the limits of the
existing surveys to have been missed. New large surveys are being
planned and may yet find them, if they are there.
Elemental abundances for K dwarf stars
In 2002
Eira Kotoneva completed a study of the metal composition of a large
uniform set of K dwarf
stars in the European Space Agency's Hipparcos satellite in order to
make a very precise measurement to be made of the distribution of
``heavy'' elements (i.e. elements heavier than Helium) in a
representative sample of stars near the Sun. This is a major constraint
on models of the evolution of galaxies, and can be used inderectly to
constrain the density of matter (dark or otherwise) near the Sun. The
work has been extended to spectral analysis of many of the stars in a
collaboration with National Astronomical Observatories of China, part
of the Chinese Academy of Sciences; high resolution spectra were
obtained by Kotoneva; effective temperatures, surface
gravities and abundances for a wide range of elements are currently
being computed. These data will probe the nature of chemical enrichment
in the Galaxy using K dwarfs for the first time; accurate abundances
for K dwarfs will also be very useful for constraining the production
of Helium over successive stellar generations (see "Helium" below). Kotoneva has also been working in
collaboration with the scientists at York University, Toronto. The
main interest has concerned "Kapteyn's star". This star is one of
our most interesting stellar neighbourhood, e.g. its proper motion is
very large and it is one of the few stars with retrograde stellar
orbit. We have observed the IR-spectrum of the star in order to compare
it with the synthetic spectra (so called NextGen models). Also
theoretical Galactic simulations have been run to explain reasons for
its peculiar spectrum as well as its strange orbit.
Distances to galaxies in the extended local group
Rami Rekola
has
obtained observations of dwarf elliptical and irregular galaxies of the
extended local group using the NOT
. Distances to the galaxies are being determined via the surface
brightness fluctuation method (with Helmut Jerjen, Mount Stromlo Observatory ).
Cepheid based distances for IC 342, a large, starburst spiral galaxy,
are being determined, using observations with the NOT over a long baseline (5 years).
Planetary nebulae have been used to make a distance determination to
NGC
253 (using imaging data from the ESO 3.6 meter). This is part
of a longer term program to assemble reliable masses and distances for
the extended local group galaxies with a view to simulating their
dynamics. Many of these galaxies are at distances where the effects of
"dark energy" on the Hubble flow are first noticable; accurate
distances to the galaxies are therefore of particular interest.
Disk surface mass density determination
The thickness of the Milky Way's disk is a balance between the total
gravity of all the stars in it, and how fast they are individually
moving. A
given star, moving with a certain speed near the Sun, will rise upwards
through the surrounding disk stars until the total gravity of all the
stars below it pulls it back down again.
To measure the amount of matter above and below the Sun in the Galactic
disk, Chris Flynn of Tuorla Observatory and Johan
Holmberg of Lund Observatory have used data from the European Space
Agency's Hipparcos
satellite on so-called K giant stars. These stars are cooler
but much brighter than the Sun; indeed, the Sun is expected to become a
K giant itself some 5 billion years from now.
The
Hipparcos satellite measures very accurately
the distances and speeds of nearby K giant stars; the research team
have applied these measurements to much more distant K giants directly
'above' the Sun (i.e. perpendicular to the Milky Way disk) in order to
determine accurate distances to these stars too. The data they used was
first collected in the mid-1980's using a 100 year old brass and
clockwork 5" telescope on Mount Stromlo Observatory, the Oddie
Telescope (sadly destroyed along with all the other telescopes in the
bushfire which swept over Mount Stromlo in January 2003). It wasn't
until now that the results of the Hipparcos
satellite could be utilised to measure the distances to the stars with
real precision.
Distribution
of red giants perpendicular to the Milky Way disk. The
plot shows the number of K giant stars above the Sun seen in a survey
taken
with telescopes at Mount Stromlo Observatory. The distance of the
stars above the Sun is expressed in parsecs (a little over 3 light
years); the survey reaches stars more than 3000 light years away.
The curve shows the expected number of stars based on a theoretical
computation using the gravitational attraction of all the known stars
around the Sun; it is a very good match to the data.
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Their new precise
results confirmed their analysis from almost 20 years ago: the
thickness of the disk is exactly what is expected if only the visible
matter contributes to the Galactic potential. There is no need to
invoke putative dark matter in the Galactic disk.
The production of
cosmic Helium
Hydrogen and helium are the
most abundant elements - together they account for
about 98% of the mass of all the atoms in the Universe today.
The remaining 2% consists of all the other elements put
together - for example, all the iron, calcium, nitrogen and carbon
present in our bodies, all the silicon in the rocks beneath our feet
and the oxygen in the air we breath. However, in the oldest stars
known,
the amount of these heavier-than-helium elements is very small, much
less than the 2% found in the youngest stars. In really old stars,
hydrogen and helium together account for some 99.99% of their
composition.
Most stars shine by fusing Hydrogen atoms into Helium atoms, as the Sun
does; other stars,
having used up their Hydrogen supply, convert helium into carbon,
or carbon into heavier elements still, and release energy in
that way. As stars are born, grow old and die, the amount of helium and
the amount of the other heavier elements has been slowly increasing in
the Universe. Measuring the amount of helium and heavier elements tells
us much about
the stars: the number which have been born and have died; the processes
which cause them to shine; and how they enrich the Universe with the
elements they have created.
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of
Helium. The plot shows how the luminosity of K dwarfs increases as
the amount of Helium and heaver elements increases. The data points
represent real K dwarf stars observed with the Hipparcos satellite.
Three computer calculations are shown by the dark blue, light blue and
green lines: they represent different amounts of Helium produced in
stars relative to the amount of heavier metals. The data indicate that
the amount of Helium
produced has been twice the amount of all the heavier than Helium
elements combined (purple line). |
Chris Flynn
of Tuorla Observatory; Raul Jimenez of
the University of Pennsylvania;
James MacDonald of the University of Delaware and Brad Gibson of
Swinburne University of Technology have used data from the European Space
Agency Hipparcos
satellite and so-called K dwarf stars. These stars are cooler
and fainter than the Sun and are essentially stellar fossils. They have
changed very little of their initial supply of hydrogen into helium
during their long lives; in other words the
hydrogen, helium and heavy elements we see in them today is they same
as when they were born. We can follow the production of helium and
heavy metals with a set of these stars.
The Hipparcos
satellite measures very accurately the real energy output of these
stars. The research team
have used computer calculations to predict how brightly such
stars should shine depending on how much hydrogen, helium and heavier
elements they contain. Measuring the amount of heavier elements
using telescopes can be done very easily -- it is the amount of
helium in stars which has been very difficult to measure. Now, the
comparison of the model computations with the real stars reveals,
indirectly, the amount of helium they contain.
The team have found that over the billions of years since the Universe
was born, stars have produced just about exactly twice as much helium
as everything else. Stars are primarily helium factories!
The research appeared in the March 7th, 2003 issue of the journal
Science .
We are now intensively following this work up at Tuorla Observatory.
PhD student Luca Casagrande
will obtain phtometry for a carefully selected set of K dwarfs with
accurate metallicities; straightforward observations will triple the
size of the basic sample and lead to an improved measure of the Helium
production rate. Dr. Laura Portinari
(arrived Feb 2004) has a post-doctoral position at Tuorla; she has
worked extensively with galactic chemical evolution models; we are
interested in constraining the production site of Helium --- what type
of star is the main source of Helium as returned to the interstellar
gas in the Galaxy.
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