This weeks light curve is timely in two ways. First of all we are looking at the object Gaia21egu, (which is a known young star) that got flagged up as an alert for variability in the photometry from the Gaia satellite only last week – Sept. 25, 2021, 10:50 p.m. to be precise. Both, the Gaia light curve and our data (V, R and I shown above), display clearly the variability of the object. In particular in the last year there has been a deep (about 2mag) dimming event, with a brief recovery half way through. Our data additionally shows a more shallow (1mag) one year event some time ago. Note that one magnitude dimming means only slightly less than half of the light reaches the observer from the star. At two magnitude dimming the light drops to just about 15%. Furthermore, the light curve also shows typical variations of about half a magnitude at all times. While this might look like noise in the data, these are real very short term (days) variations of the light, something that almost all young stars do.

Without a detailed analysis of the data, and in particular the colours, it is not certain what causes the variability. However, in most cases like this, the short term variations are caused by small structures in the inner disk that orbit the star fast and block parts of the light for short durations. The larger deeper dimming events are caused by much larger structures further out in the disk that are orbiting slower around the star and thus block the light for longer. Thus, with data like this for many stars (that is what HOYS is collecting) we can determine how many structures of what size are where in the disks of the stars.

One might ask why the object has not been flagged up as an alert earlier, about one year ago when the first deep dip occurred. Well, I don’t know – despite me being a co-author on the paper describing the details of how the Gaia alerts are created. Most likely it just missed one of the selection thresholds due to its general short term variability.

The second reason why the object is timely, is that it is a member of the Cocoon Nebula (IC5146). This is one of our summer targets and during most of last year, the participants in our HOYS-LCO project observed this target region in B, V and R filters with the LCO network of telescopes. Indeed, a large number of the V and R data points one can see in our light curve originates from this project. We have started last week with our second season of being an LCO Global Sky Partner, where we will be working with School teachers. They will learn how to take data for HOYS with the LCO, process them into our data base and most importantly then use our data for science projects with pupils in their respective Schools. For sure, some of them going to image this target as well – but there are about 25 targets to pick from.