Chris Flynn : Tuorla Observatory

 disk mass
 black holes
 white dwarfs
 K dwarfs
  local group

Research at Tuorla on Dark Matter is partly funded by the Academy of Finland  
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  

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.
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.

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