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DARKSTAR Team members
Chris
Flynn , Team Leader
Burkhard Fuchs, Astronomisches-Rechen Inst. Heidelberg, Corresponding member
Johan Holmberg, Former researcher, now at MPIfA Heidelberg
Laura Portinari, Researcher
Pasi Nurmi, Researcher
Erik Zackrisson, Researcher
Rami Rekola, Ph.D. student
Janne Holopainen, Ph.D. student
Luca Casagrande, Ph.D. student
Chris Thom, Former Ph.D. student, now at UChicago
Esko Gardner, Ph.D. student
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Research
Developments in 2005
Older research reports for 2002
- 2003 - 2004
The colours of the Sun
It's a nice quandary! The Sun is so bright and large on the
sky, that making the same measurements for the Sun as one does
for stars, is very difficult.
We would like to have colour data for the Sun
nevertheless, in order to compare stellar and solar properties.
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 DARKSTARians
Johan Holmberg, Laura Portinari and Chris
Flynn at Tuorla
Observatory. We have used the surface temperature of the stars
and the Sun to infer the colours.
We 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 have been estimated 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.
A Preprint is
available here.
Kinematics of the Milky Way halo
About 99 percent of the Galaxy's stars belong to the disk and bulge of the
Galaxy, with the rest forming an extended, faint and roughly spherical halo in
which it is embedded. The stars in this halo are on orbits which take
them to all corners of the Galaxy --- and in this sense they are messengers
about the amount of dark matter in the Galaxy, as their orbits are dominated by
the gravity of the dark matter, rather than the paltry amounts of visible
matter. Exactly how halo stars move has been investigated from a new large
sample of observations by Chris Thom, as part of his PhD thesis. Chris is
supervised by Chris Flynn at Tuorla Observatory and Brad Gibson at Swinburne University of Technology in
Melbourne, Australia.
About 2000 halo stars have been put together by the researchers in various
directions on the sky, mainly gazing outward from the center of the
Galaxy, using telescopes at the European Southern Observatory as well as
published samples from the Sloan Digital Sky Survey. The speeds relative to the
Sun were measured, and compared to a model of how such stars have been thought
to be moving in the Galaxy's dark matter. The model turned out to fit the data
quite well, but to the researchers' surprise, this wasn't the only model that
fit. It turned out that it was quite difficult to distinguish between their
physical model, which contained a few parameters, and an exceedingly simple
model in which the velocities can be fit by a single parameter only. The
researchers had been expecting to be able to rule out such a simple model with
confidence, with access to several thousand distant halo stars.
The models are much easier to distinguish from each other for halo stars
very close to the Sun -- in this case, only the simple model clearly does not
fit. This suggests a test -- we whould be looking at distant halo stars where
the difference between the two models is greatest -- and this turns out to be
in the inner parts of the Galaxy, toward the central bulge. The team is
planning to follow up their work by gazing inward instead!
The research has been published in the Monthly Notices of the Royal
Astronomical Society.
Thom et al, MNRAS, 360, 354 (2005)
The stars in the sample were originally put together for quite another purpose
--- that of trying to constrain the distances to so-called High-Velocity Clouds
of neutral hydrogen gas which cover much of the sky. Such distance limits have
been keenly sought for more than 40 years, yet remain elusive. Field Horizontal
Branch stars from the Hamburg/ESO Survey have been correlated with the HI
Parkes All-Sky Survey to identify suitable stars aligned with high
column-density HVC gas. Echelle spectra of these stars have provided distance
constraints to several HVCs. A distance limit to HVC Complex WB has been made.
Detection of this HVC in absorption gives a direct distance constraint, placing
it within the Milky Way halo.
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, the amount of light emitted may
be the only measurable quantity, and can only be converted into a stellar mass
indirectly by assuming some value for (M/L) *.
The only place where (M/L) * can be measured directly is near the
Sun, in the Milky Way disk. Chris
Flynn of Tuorla Observatory and
Johan Holmberg (then of Lund Observatory) have used data from the
European
Space Agency's Hipparcos
satellite on so-called K giant stars, to measure the total amount of
disk matter, M, near the Sun. Johan has now joined us at Tuorla Observatory
(late 2004), and together with Laura Portinari, we are now finishing a
careful measure of the integrated amount of light, L, emitted by the same disk
stellar populations.
How far the neighbours?
Determining distances to individual galaxies using a variety of methods is a
fundamental part of mapping out the distribution of galaxies in the Universe.
Rami Rekola, has just completed
a PhD thesis at Tuorla Observatory, which focussed on the issue of distances to
nearby galaxies. Rami used three very different techniques with which to
measure distances: the first based on the brightness of small, ionized spheres
of gas in the galaxies, called planetary nebula; the second, a classical
technique, of using the brightness of pulsating, luminous stars called
cepheids; and the third, a method based on the variations in brightness from
place to place within a high quality image of a galaxy. Rami Rekola, has used
all three methods in his thesis work, a very notable achievement.
Part of the
giant spiral galaxy IC 342. It lies just beyond our own Local Group of
Galaxies in another group called Maffei Group or IC342/Maffei Group.
The image above is a combination of images taken with BVRI filters
(roughly blue, visual = green, red, and infrared) by Rami Rekola and
Kari Nilsson at the Nordic Optical Telescope. |
Distances have been obtained for dwarf elliptical and irregular galaxies of
the extended local group using the NOT,
in collaboration with Helmut Jerjen at
Mount Stromlo Observatory. A Cepheid based distance has been obtained for
IC 342, a large, starburst spiral galaxy, for which few (and contradictory)
distance estimates exist in the literature, 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).
Rami Rekola's work 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. The surface
brightness fluctuation method has been shown to be a particularly efficient
means of determining good distances, especially for dwarf galaxies for which
other methods cannot be easily applied. We are planning to use the method to
measure distances for large numbers of nearby dwarfs in an ongoing effort to
understand better the local Hubble flow.
"Distance to NGC 253 based on the planetary nebula luminosity function", MNRAS, 361, 330
"New distances of unresolved dwarf elliptical galaxies in the vicinity of the Local Group", A&A, 437, 823
The production of
cosmic Helium
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 to measure the amount of Helium which has been produced in stars
relative to the amount of heavier elements, i.e. DY/DZ. The physical effect
being exploited is the broadening of the lower Main Sequence caused by the
spread of metallicity and helium content in stars. 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. The research
appeared in the journal
Science in 2003. We are now intensively
following this work up at Tuorla Observatory. PhD student Luca Casagrande has been using the remotely
controlled 30 cm KVA telescope on the island of La Palma; he has obtained
photometry for a carefully selected set of K dwarfs, the stars used in the
earlier study, with accurate metallicities. Casagrande's sample of stars is now
about three times bigger than our earlier study; but more importantly, they are
much better selected; they have accurate metallicity data, in some cases for a
range of elements, and we have used a brand new method due to Wielen and
collaborators, termed "mu delta", to remove troublesome binary stars from the
sample. The multi-band photometry obtained allows us to recontruct the
bolometric flux and the effective temperature of the stars, so that the
analysys of the HR diagram of nearby K dwarfs can be carried out in the
theoretical plane, where the comparison to stellar models is more direct and
robust, and moreover the effect of helium content on the broadening of the
lower Main Sequence is more evident.
We are presently analysing this sample in order to produce a significantly
improved estimate of the rate of Helium production in the cosmos. Future plans
include the use of Portinari's chemical models for the Solar
Neighbourhood to simulate the local helium enrichment history and estimate the
amount of helium locked away in remnant stars. Since stars of different masses
produce helium and metals in different proportions and over different
timescales, DY/DZ as reflected in nearby K dwarfs may depend on the specific
star formation and chemical evolution history of the local environment, so that
chemical models are necessary to link the local estimate to the true overall
"cosmic production rate" and to infer YP. This latter quantity is
interesting in light of recent discrepancies between "astronomical" and "CMB"
determinations of YP.
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The Royal Swedish Academy's 60 cm telescope at
La Palma in the Canary Islands, Spain. The telescope has recently been upgraded
to allow fully remote observing from Tuorla Observatory, in Turku Finland. We
are using a 30 cm piggyback telescope to obtain BVRI colours of a sample of K
dwarf stars in order to determine DY/DZ, the rate of production of Helium
relative to metals in stars.
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Personnel Movements in 2005
Dr. Laura Portinari
joined took up funding as a Marie Curie Fellow at Tuorla in late 2005, from her
Academy of Finland position. Dr. Pasi Nurmi joined the DARKSTAR
team. Pasi works on the European Space Agency's GAIA mission, and is a member
of the Double and Multiple Stars Working Group and the Photometry Working
Group. PhD student Chris Thom visited
Tuorla Observatory several times in 2005, coming to us from the Swinburne in
Australia. PhD students Luca Casagrande and Janne Holopainen are
travelling in January; Luca to Pennsylvania to work with Professor Raul Jimenez
on Helium in K dwarf stars, while Janne went to Potsdam to work with
Dr. Alexander Knebe on dark matter in galactic halos. Esko Gardner
joined the team in late 2005 and is working toward a Masters thesis. Chris
Flynn departed in late 2005 for a one year sabbatical at Mount Stromlo
Observatory.
Visitors in 2005
- Professor Burkhard Fuchs, Astronomisches-Rechen Institut,
Heidelberg, October 2005; working on the disk luminosity
- Profs. Licia Verde and Raul Jimenez, University of
Pennsylvania, August 2005, talks on dark energy and the CMB
- Chris Thom, Swinburne Institute of Technology, July, 2005,
working on distances to high velocity gas clouds
- Professor Ali Talib, Baghdad, Iraq, July 2005. Discussions
on active imaging and terrestrial planet finding
- Dr. Jesper Sommer-Larsen, NORDITA/TAC, Copenhagen,
Denmark, working on the Galactic halo and the theory of galaxy
formation
- Erik Zackrisson, University of Uppsala, March
2005. Discussed dark matter in the Milky Way and gave a seminar
on clumpy dark matter
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