We are back to stars with spots this week. The above images is a crop from a slide I created for a talk last week. It simply shows what kind of young stars do we have in our sample of objects where we can measure the rotation period – and thus the spot properties.
The x-axis value (alpha) is the slope of the spectral energy distribution in the mid-infrared. This is usually used to classify young stars into protostars (positive alpha). I.e. they have a lot of cold material surrounding the object. Then we have TTauri stars with negative alpha. The classical TTauri stars (CTTS) do usually still have an accretion disk surrounding them, while the weak line TTauri stars (WTTS) have not, and they are hence not accreting material any longer. On the y-axis we we show the colour between the near and mid infrared, between 2.2 micron and 4.5 micron. If this is above 0.5mag the star usually has hot dust in the inner disk. This means dust heated to above ~1000K within ~0.1AU.
If we take all the young stars HOYS is observing we find that one in three has an inner dust disk. If we only consider the variable objects, then two thirds of those have an inner dust disk. This indicates that the disk, in particular the inner part, is responsible for a large fraction of the variability in young stars. This is different again when we are looking at the rotational variables. For these, very much again one in three has an inner dust disk. The reason for this change is that in most cases the variability caused by the disk material is far stronger than the change in brightness caused by spots. Thus, we simply cannot identify the rotation signal in the data if the object shows significant variation that are caused by other factors (the disk or accretion rate changes).