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High-mass X-ray Binaries as a Star Formation Rate Indicator

Scientist at the Max-Planck-Institute for Astrophysics suggest to use the number and brightness of X-ray binaries to measure the star formation rate in galaxies. This may allow in the future new insights into the nature of the star formation process.

Molecular cloud Barnard 68

Figure 1: Picture of the molecular cloud Barnard 68 taklen by the VLT. In gas and dust clouds like this star formation takes place. The cloud is impenetrable for visible light and UV radiation.

Galaxy M83

Figure 2: Galaxy M 83. The upper panel shows a picture in visible light (VLT, FORS team), the lower panle shows an X-ray image taken by the Chandra X-ray telescope (R.Soria & K.Wu).

star formation rate / x-ray brightness

Figure 3: The relation between star formation rate and X-ray brightness from X-ray binaries with a massive companion. for nearby galaxies (blue symbols) and distant galaxies (red symbols).

Although visible matter in form of stars contains only a small part of the total matter in the universe, stars are of utmost importance for life on Earth. The elements that Earth and other planets consist of are the ashes of exploded stars. Moreover, the Sun is the energy source for all life on Earth. For the understanding of the development of the Solar System in particular and galaxies in general, it is of great interestto understand how and where stars form.

From observations we know that star formation happens in different ways. Either many stars form in a short time, e.g. in a collision of galaxies, or stars form in a rather slow, continuous process like in the Milky Way. We call star fomration rate the mass of gas and dust that is converted into stars per year. This is measured in solar masses per year.

The measurement of the star formation rate is very difficult. Star formation takes place in dense gas and dust clouds that strongly hamper observations, see Fig. 1. All measurements of star formation also rely on not well understood details of the star formation process.

Scientist at the Max-Planck-Institute for Astrophysics now suggest to use a particular type of double star system to meassure the star formation rate. These double star systems are consist of a massive star, at least 10 times more massive than the Sun, and a so-called compact object, a neutron star or black hole. The neutron star or black holetakes away matter form the normal star due to its strong gravitational attraction. When the amtter falls onto the compact object it heats up very strongly and eventually sends out X-radiation. The X-radiation of these double star systems can nowadays be measured by satellites even in other galaxies. An example is given in Fig. 2, the upper panel shows an optical light image of the galaxy M 83, and the lower panel shows an X-ray image of the same region. One can clearly distinguish the single X-ray binaries.

These particular double star systems are well suited to measure the star formation rate since their lifetime is very short: The more massive a star is the shorter it lives. Therefore one obsreves these systems in regions of active star formation. This new method has several advantages. X-rays penetrate easily the dense gas and dust clouds. Moreover, this method is independent of the details of the star formation process. And also the conditions to create these systems are hard to fulfill so that they are relatively scarce. This makes possible their observation even at large distances without confusion due to the number of systems.

Based on the analysis of data from the Chandra X-ray telescope a research group at Max-Planck-Institute for Astrophysics has shown that the number and brightness of these double star systems are well suited to measure the star fomration rate in galaxies. For that the X-ray emission of galaxies was comapred with other measurements of star formation rate as shown in Fig. 3. There is a good agreement between the brightness and the star formation rate even for distant galaxies inthe socalled Hubble Deep Field (HDF) when the universe was only about half as old as now.

This method allows now to measure the star formation rate independently. Moreover, in the future it might be possible to make more precise statements about star formation since the existence of these particular double star systems depends strongly on the conditions of star formation.

Hans-Jakob Grimm

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