This week we are looking again at a star that has recently been flagged up as variable by the Gaia photometric alerts. The object is called Gaia21uey, and also known as a variable TTauri star OY Mon. The star is part of the NGC2264 target region which contains the Christmas Tree Cluster and the Cone Nebula. The entire field is full of very interesting young stellar objects.

The object got flagged up as a Gaia alert, because it showed a 0.7mag drop in brightness. It can be seen in the Gaia light curve, that this is a significant drop compared to it’s past behaviour, which mostly consists of small variations at approximately the 10% (0.1mag) level. However, upon inspection one finds another dip in April 2019. At this time the object got not flagged as variable, because one of the criteria is that two consecutive data points need to be above or below a given threshold, but the dip was only detected once.

In the above plot we show the HOYS V, R, and I light curve of the object from the last observing season. While we too have gaps in the data, our light curve is much better sampled. And it becomes apparent that these dips in brightness are not unusual. There are at least four significant dips detected, most of them very short, i.e. at most a few days long. This explains that with the typical cadence of 30 days by Gaia, most of these events will be missed.

We can see that theses dips, while short, can be significant (1mag) in depth. Even if the material that causes the dips is in the very inner part of the accretion disk, hence moving very fast, these structures must be very compact and dense. They hence show that these disks are structured on very small spatial scales.