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DARKSTAR Team members
Chris Flynn , Team Leader
Burkhard Fuchs, Corresponding member
Johan Holmberg (Now at MPIA, Heidelberg)
Laura Portinari, Researcher
Pasi Nurmi, Researcher
Erik Zackrisson, Researcher
Laura Dunn, Researcher
Rami Rekola, Ph.D. student
Janne Holopainen, Ph.D. student
Luca Casagrande, Ph.D. student
Chris Thom, former Ph.D. student, now at the University of Chicago
Esko Gardner, Pd.D. student
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Research
Developments in 2006
Older research reports for
2002
- 2003
- 2004
- 2005
Galactic disk mass-to-light ratio
The amount of light emitted by groups of stars, relative to their total mass,
is an important quantity in astronomy; it is called the stellar mass-to-light
ratio, or (M/L)*. For many studies of galaxies, the amount of light
emitted is the measurable quantity, and can only be converted into a stellar
mass indirectly by assuming some value for (M/L)*.
Broadly speaking, stars fall into two categories from the point of view of
lighting up the universe. Some stars are highly luminous -- they can be seen at
great distances from the Earth -- and produce a lot of light. But such stars
are rare. Most stars are actually very dim -- quite a lot dimmer than the Sun,
and produce very little light in interstellar space.
Chris Flynn, Johan Holmberg and Laura Portinari of Tuorla Observatory and Burkhard
Fuchs and Hartmut Jahreiss of the Astronomisches Rechen-Institut in
Heidelberg have combined forces to work out how much light is produced by all
the stars in space around the Sun, and which stellar types do most of the
effort.
The team put together a 'luminosity budget' for nearby stellar space. They
found that about half the light near the Sun is generated by giant stars, while
the other half comes from so-called 'turnoff' stars, which are stars coming to
the end of their supply of Hydrogen fuel, and will eventually become giant
stars themselves.
The amount of light emitted can be compared to the amount of mass contained
in the same stars. Armed with the ratio of the two, astronomers can look to
distant galaxies, where the individual stars cannot be resolved, and make
estimates of the amount of matter galaxies contain in stars from the amount of
light they emit.
One of the big surprises of studies of this latter type, is that stars make
up only a very small part of the mass of galaxies -- the so-called 'dark
matter' problem, in which galaxies are dominated by a kind of matter which
emits no (or very little) light, and quite unlike their stellar content.
The team was also able to estimate the total light emitted by the Milky Way
as a galaxy -- about 40 billion times the light emitted by the Sun, give or
take a few billion.
The study has been accepted for publication in the
Monthly Notices of the Royal Astronomical Society.
Preprint
is available here
Solar colours
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Astronomers at Tuorla Observatory have made a new, indirect
measurement of the "colours" of the Sun.
Stars have been known to have come in various colours since antiquity,
with stars such as Betelgeuse being termed reddish, while Rigel appears as
a blueish, although stars are such pin-prick sources of light, that the
colours are difficult to perceive in all but the brightest of them. It was
the introduction of photography and spectroscopy into astronomy, over 100
years ago, which showed that star colour is closely related to its
temperatures, in the same manner that a metal glows first red and finally
blue-white as it is heated up. The surfaces of stars range in temperature
from a few thousand degrees (red) to a few 100,000 degrees celsius
(blue-white).
Nowadays, stellar colour is measured by comparing how bright a star appears
when viewed through a red and a blue filter. The redder, or cooler, the star,
the greater the amount of red light it will emit compared to blue. Conversely,
hotter stars emit relatively more blue light than red.
Over many decades, astronomers have put together systems of light filters
for measuring the properties of stars just from their colours; physical
properties, such as temperature, surface gravity, chemical composition and
intrinsic luminosity.
One major problem has been to measure the colours of the Sun in the same
system. It's a nice quandary that the Sun is so bright and large on the sky,
making the same measurements with telescopes designed to detect extremely
faint, point-like stars, is next to impossible. One way around this is to
find stars, which are as very similar in other properties to the Sun, and
from their colours infer the colours of the Sun. This is the technique used
by Johan Holmberg, Laura Portinari and Chris Flynn at
Tuorla Observatory. The researchers use the surface
temperature of the stars and the Sun to infer the colours.
An age-old problem with this technique has always been to make sure that
the temperature of the stars and of the Sun are measured consistently in the
same scale — as different approaches can be used to define and measure
stellar temperatures. To do this, the researchers have used the tremendous
advances being made in the last two years at the
European Southern Observatory's
Very Large Telescope,
which has been making direct measurements of the surface temperatures of
stars using interferometry.
The technique shows that surface temperature can be measured consistently
for faint stars and the Sun. This breaks through the old impasse to using
this technique for measuring the Sun's colours.
The colours of the Sun in the Johnson/Cousins, Tycho, Strömgren,
2MASS, and
SDSS systems.
The study has been accepted for publication in the Monthly Notices of the
Royal Astronomical Society.
Preprint here.
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Measuring the colours, or distribution of energy in various wavelength
passbands, is easy for stars, but next to impossible for the Sun. Too big
and too bright, the Sun overwhelms the sensitive instruments installed
around the globe on astronomical telescopes. One way around the impasse
is to find stars as similar to the Sun as possible and use their properties
to infer the Sun's colours. Image of the Sun from
NASA.
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Physical parameters for low mass stars
Stars are basic to astronomy, and determining their masses,
temperatures and luminosities a key task for astronomers. Cool stars are
surprisingly difficult to get right in this regard, as much of their luminosity
is releases in the infrared, where data have been traditionally more difficult
to obtain than in the optical. This situation has altered with the coming
on-line of large area surveys of the sky in the IR, such as 2MASS, meaning that colours are
available for large numbers of cool, low mass stars. In addition, a decade ago,
accurate parallaxes are available for the stars from the European Space
Agency's Hipparcos mission, so that distances are now measureed with very high
precision. This turns out to be the two things one really needs to put together
an accurate temperature and luminosity calibration for stars cooler than the
Sun.
Luca Casagrande, Laura Portinari and Chris Flynn of the
University of Turku's Tuorla Observatory have obtained a sample
of about 100 bright G and K dwarfs with accurate parallax and photometric data
in BVRIJH and K. New data for the stars have been obtained using the remotely
controled 30 cm KVA telescope at La Palma.
We have derived an empirical effective temperature and bolometric luminosity
calibration for G and K dwarfs, by applying our own implementation of the
Infrared Flux Method to multiband photometry.
The colours computed from the most recent synthetic libraries (ATLAS9 and
MARCS) for such stars are found to be in good agreement with the data in the
optical, but discrepancies remain in the infrared.
Despite very careful work, our study shows that one cannot yet get temperatures
and luminosities for such stars to much better than a few percent accuracy.
Our temperature scale is 100 Kelvin hotter than recent analogous determinations
in the literature, but is in agreement with spectroscopically calibrated
temperature scales and fits well the colours of the Sun. Our angular diameters
are typically 3 per cent smaller when compared to other (indirect)
determinations of angular diameter for such stars, but are consistent with the
limb-darkening corrected predictions of the latest 3D model atmospheres and
also with the results of asteroseismology. Clearly there are some wrinkles
still to be ironed out!
On the other hand, we find very tight empirical relations for bolometric
luminosity, effective temperature and angular diameters from photometric
indices.
The research has been published in the Monthly Notices of the Royal
Astronomical Society, and a preprint is available here
Personnel Movements in 2006
Esko Gardner has joined the team in 2006 and is working toward a PhD --
he is studying the orbit of the Sun. Masters thesis. Chris Flynn spent
most of 2006 on sabbatical leave in Australia at Mount Stromlo Observatory. He
was visited at Stromlo by DARKSTARians Burkhard Fuchs, Luca
Casagrande and Johan Holmberg. Erik Zackrisson joined the
DARKSTAR team as a postdoc in late 2006 -- Erik is funded for three years by
the Academy of Finland. Johan Holmberg left us and has taken up a
position at the Max Planck Institute for Astronomy in Heidelberg, working on
the European Space Agnecy's GAIA mission, among other things. Chris Thom
completed his PhD studies (jointly at Tuorla Observatory and Swinburne
University in Melbourne Australia) and has moved on to a postdoc position at
the University of Chicago.
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