When I decided to write a “series” on how to prepare an astronomical publication I knew that the data analysis part would be by far the most difficult to write. Because this step really depends on what you are doing. There are so many different options that I cannot cover them all. So, the only thing I can really do is to tell about a method I often use for my science.

Of course this is not the only thing I do, but it is a method I already used in my master thesis more than 12 years ago (is it really that long ago?) and have continued to use ever since. It is called Doppler imaging. It is named thus because it is based on Doppler effect. But let’s start from the beginning.

Stars are in principle gas balls with uniform temperature distribution. The interior of the Sun is hot and the outer layers cool. The “visible surface” of the Sun has temperature of approximately 6000 C. When the radiation of the hotter interior passed through these cooler outer layers, solar photosphere, you create an absorption spectrum. If you on the other hand have hot gas and you don’t see the original radiation source you create and emission spectrum. The whole creation of spectral lines is very complicated. For now it is sufficient to remember that stars in general produce absorption lines (not always though ;-).

The spectral lines in stars tell many things about the star itself. They tell what the star is made of, how hot it is, how large the star is (by telling about the gravity in the star), how rapidly it rotates, even hint at how old it is, etc. Also, if the stellar surface is the uniform disk you see in the image of the Sun without sunspots, you get a regular nicely shaped line profile. If on the other hand, you have large spots of different temperature on the surface, you start to create wiggles in the spectral line profiles.

If you then observe a rotating star with a spot on its surface at many different times, you can see these wiggles move across the spectral line. By combining many observations and using inversion techniques you can recover a map of the stellar surface. This is Doppler imaging. With Doppler imaging one can investigate the surface temperatures, inhomogeneities in the surface chemical composition of the star and even magnetic fields if one uses spectropolarimetric observations. It is a powerful tool, but it requires very high precision observations, many of them and from many different time points.

So, what did this have to do with the Doppler effect? When the star rotates, the edge coming towards you will get the wavelength of the absorption line moved towards shorter (bluer) wavelengths due to the Doppler effect, and the edge rotating away from you towards the longer (redder) wavelengths. So, faster the star is rotating broader the lines are, because the light from the edges gets shifted more. This is also why the wiggle caused by the spot moves across the spectral line. When it comes into the view at the edge rotating towards us it appears at the blue edge of the line profile, in the so-called wing of the spectral line. Then when the star rotates more and the spot moves more towards the centre of the stellar disk its Doppler shift gets less and it appears closer to the middle of the spectral line, so-called line core. At the centre of the stellar disk it will have Doppler shift 0 and it will be seen exactly at the line core. How high on the star the spot is, in other words at which latitude it is, can be determined based on where in the spectral line the wiggle is first seen. If the spot is at the equator, it will arrive in the spectral line at the maximum Doppler shift, really at the edge of the wing of the line. If it is higher up, it will never be seen at the maximum Doppler shift, but on a bit smaller shift. So, it will appear first somewhere between the really edge of the line and the line core. A spot at the rotational pole would always be seen in the line core.

I hope this was clear… ;-P Please ask if it was not!

This entry was posted on Tuesday, October 13th, 2009 at 10:41 am and is filed under astronomy, observations. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.