As mentioned a few times in the past, one of the goals of HOYS is to characterise how the typical young star is varying. We have started to prepare the analysis for a large overview paper. In a first step we used the latest data release from Gaia – DR3 – to identify all potential cluster members in the star forming regions and clusters that we are observing. The candidate member stars are selected purely based on their astrometry, i.e. they have a distance (parallax) and proper motion that is in agreement with them belonging to a cluster in the HOYS field. There are about 3000 such stars with HOYS light curves of at least 100 data points.

One of our collaborators has now determined two parameters for each light curve, which characterise the variability. These are called the quasi-periodicity (Q) and asymmetry  (M) parameters. These are shown in the plot above. If you are interested in the equations for Q and M, please look at section 5 in this paper.

The Q-parameter basically characterises how periodic the variability is. In other words, Q=0 means the star is completely periodic. On the other hand, Q=1 indicates that the brightness varies completely randomly. For values in between zero and one, either the period is changing over time, or the amplitudes of the variations are changing. Or both.

The M-parameter describes how the variations manifest. Values of zero indicate objects where the variations are symmetric around the average. For example a light curve that looks like a sine-function would have M=0, and of course Q=0. Outbursting objects have negative M-values and dipping objects have positive M-values. The more pronounced the bursts or dips are, the bigger the absolute value of M.

In the plot above we show the determined Q and M values for all cluster members determined from the V-band photometry. These are preliminary results at the moment and we note that it includes all cluster members, even if these clusters are not young – so, not all objects are young stars. But we can clearly see that the majority of stars shows stochastic and symmetric variability. This is in part because we have not removed any stars that are non-variable. We see already that there seem to be more periodic dippers than periodic bursters – something that has been found before.

We will continue to work on the analysis of these data and investigate other light curve parameters, as well as how e.g. the Q vs. M plot looks for objects in different clusters or for stars in different evolutionary stages or properties. For example it will be interesting to see if young cluster members with a detectable disk behave differently to stars without disk. This work will be part of our next general publication that will contain all people who delivered data as co-authors – stay tuned.