This week we have a look at some ongoing research – something that will hopefully get published in the medium term future. In the past we have shown how to identify rotation periods of young stars (paper) and then how to use the amplitudes of the variation in different filters to determine the spot properties on the surface (paper).
The next logical step is to look at the evolution of the spot properties over time. In the above image we show a phase plot (R-band data) for one of the periodic variables in the Pelican Nebula region. This means on the x-axis we show where in the period a given data point is situated. In principle this works as follows: The date of the very 1st data point in the light curve is subtracted from every data point. This value is then divided by the period of the star (in this case a bit over three days). We then only plot the number after the comma (the phase) against the magnitude – and show every data point twice, once with its phase and once with phase+1 (trust me, this looks better 🙂 ).
In the plot we have colour coded the data points based on where in the light curve they occurred. I.e. purple points are at the very start of the light curve, and red data points are from the very end of the light curve. In total the light curve covers about 7-8 years now. While there is much more detail in the data, there is one obvious change visible. There are clearly high amplitude variations observable in this source roughly in the middle (light blue points) of the light curve. At the very end of the light curve (red points), the variations have a lower amplitude, but the period is still the same.
However, the red points are shifted in phase space by about one third of the phase. The change in amplitude means that there are surface spots (we know that in this case these are cold spots) which vary in size and/or temperature. The shift in phase suggests that the spot that is visible towards the end of our light curve, is placed on a different part of the star. Based on the numbers, the spot position has shifted by 120deg in longitude on the stellar surface. We are now investigating these plots (and others) for a number of stars with periodic variations. This will allow us to determine the lifetime of the spots, how they change their properties (size and temperature) and their position on the stellar surface.