This week we look the the light curve of the young star AA Ori. It is situated in the southern part of the Orion Nebula (M42). It is known as an Orion Type variable. This usually means it is changing it’s brightness in a stochastic, random way on numerous time scales.

We show here the long term V-band light curve of the star and a zoom in to the 2019/20 observing season of the object. The other filters are not shown for clarity, but we have B and I data with the same cadence. As one can see, the light curve now almost spans 3000 days (it will do by the end of this month) and on average we have slightly more than one data point per night in B, V, and I. This allows a very detailed long term study of the sources in this field.

The long term behaviour shows that the object has a baseline brightness which it does not exceed. There are some longer term changes in this baseline – most notable during the observing seasons 2018-2020. There the baseline brightness dropped by about 20 percent. Additionally there is a large scatter towards fainter magnitudes visible during the entire data set. These are real drops in brightness which are up to two magnitudes deep – i.e. the brightness drops to about 15 percent of the baseline – and are usually of a short duration.

The zoomed part of the light curve shows that these dips usually are a few days to a few weeks long. They also, at least in part, seem to be semi-periodic. I.e. for a while they seem to repeat with the same intervals, even if they differ in depth and duration. These could be denser parts of material in the inner disk which are transported into the line of sight for a while before they dissolve or are moved away. A detailed study is however needed to verify this quasi periodicity. But if confirmed, the periods will allow us to determine the physical size of the occulting structures. What is already clear, the object can safely be classified as a dipper.