This week we revisit an object we looked at before (in April 2022) – IP Mon, a T-Tauri variable in the Christmas Tree Cluster.

The object has not changed its typical behaviour, compared to the last time we looked at it. Indeed, the way it fluctuates in brightness has followed the same pattern throughout our entire dataset – which is now about nine years long. We can see small day to day changes at the 0.1ma level for a while (up to a few weeks) and then the source dims by up to a magnitude within a few days, and it then returns to the bright state.

Some of the changes of 0.5mag occur as quickly as from one night to the next. This indicates that they are caused by structures in the inner disk – they need to move fast enough to cover the star in this short amount of time. They also have to be quite compact, i.e. only a few times the size of the star itself (at least in the direction of the orbital motion), due to the short duration of the dips.

This is an ideal object to see if some of these structures create repeated dimming events (i.e. the dips from them occur periodically). This would allow us to determine their separation from the central star (using Keplers third Law). Thus, we will be able to directly measure the size of the structures and study their evolution – note that shear forces in the disk should dissolve any structures that are not bound by gravity within very few orbits.