At these pages I describe briefly some of my past and present research topics. Information on Planck satellite project can be found on separate pages that are accessed using a button on the left. You can find some additional pages in Finnish by first clicking on 'Suomeksi' on the left.

Interstellar clouds

Most of my research has involved observations and modelling of interstellar clouds. A small cloud may contain material of just one solar mass while largest giant molecular cloud complexes are millions times more massive. In dense clouds gas is mostly in molecular form - largely thanks to the presence of interstellar dust. Dust grains shields clouds from external stellar radiation that would otherwise destroy the molecules. The shielding is visible also at optical wavelengths. The dust can block all light from background stars and cloud shows as a dark patches against stellar background - hence the term 'dark cloud'. In molecular clouds one can see mass concentrations of different sizes and densities. Some of these cloud cores may eventually collapse under gravitation leading to the formation of one or more stars. One of the main reasons for studying interstellar clouds is that we want to understand how star-formation process works. Why do some clouds form stars and others don't? What determines the number and the mass of born stars?

Clouds consist of gas and dust. The gas component is studied with radio line observations. Dust can be studied by looking at how it absorbs and scatters star light or by observing radiation that dust grains emit at infrared wavelengths.

Radiative transfer

Radiative transfer calculations are used to predict observed radiation when physical properties of an astronomical source are known. Conversely, comparison of model predictions and observed data enables us to determine the true nature of these objects. This is an iterative process where models are improved until they reproduce observations. Unfortunately, the relation between physical state of a source and the observable radiation is not always unique. In other words, one may be able to build different models that all produce more or less similar radiation.

Line emission

Interstellar clouds consist mostly of gas which in dense regions is in in molecular form. Molecules have distinct energy levels and when molecules move spontaneously to lower energy states they send electromagnetic radiation. Radiation produced in such transitions can be observed with radiotelescopes as narrow spectral lines. The frequency of a line identifies the radiating species and the transition.

Intensity of observed lines depends on the number of molecules on the initial, upper energy level. Molecules are excited to higher levels either by external radiation or by collisions with other particles. The collisions depend on physical conditions of the gas. Once we know how molecules are distributed on different energy levels we know something about the density and temperature of the gas. At radio wavelengths we also can resolve line profiles that are widened by Doppler effect (observed frequency changes depending on the relative velocity of the source and the observer). Therefore, line profiles can be used to study gas motions inside interstellar clouds.

The modelling of line emission requires some assumption of density, velocity and temperature distributions in a cloud. With radiative transfer calculations one can solve how molecules are excited in different parts of the cloud and simulate line spectra that can be compared with the observed ones.

L183
Radio and infrared observations of dark cloud L183. The colours show the distribution of 200Ám emission from large dust grains. The contours show intensity of line emission from the C18O molecule. (Juvela et al. 2002, A&A 382, 583)

Dust emission

About one per cent of the interstellar matter is dust. Gas and dust are usually well mixed so that clouds can be studied also by observing dust emission. In normal interstellar radiation field large dust grains settle at a temperature slightly below 20K. Because of this low temperature emission reaches maximum in far-infrared at wavelengths longer than 0.1mm. Small grains do not, however, remain at one temperature. When a photon is absorbed by a very small dust grain the temperature of that grain may for a short period become hundreds of degrees higher. Higher temperature means that such grains radiate at shorter wavelengths, below 0.1mm. There are even very small particles (actually large molecules) that can be heated transiently to several thousand degrees and which explain the observed emission around 0.01mm.

With radiative transfer calculations one can first solve the strength of the radiation field in different parts of a model cloud. This information together with assumed dust properties (size distributions and other physical properties) determines temperature distribution of grains. Once grain temperatures are known one can calculate the resulting infrared emission.

L183
On the left the temperature distribution of large dust grains in cloud L183. Temperatures are calculated from observations made with the ISO satellite at 100Ám and 200Ám wavelengths. On the right is an optical image of this dark cloud. Dust obscures background stars but light scattered from dust grains is visible as faint surface brightness. (Juvela et al. 2002, A&A 382, 583)