This week we have a brief look at some of the preliminary analysis for the next overview paper. For all cluster members in the HOYS target fields we have identified objects that show a strong periodic signal in the photometry in the R-band, and their periods are determined. We plot these periods against the K-W2 colour for the stars. We also determine the light curve asymmetry and colour code the symbols with it. Finally, we also determine the amount of variability (in this case the Stetson Index) and the symbol size is coded by this property (larger symbols mean higher variability amplitudes).
The K-W2 colour typically indicates if an object has an inner accretion disk or not. Values above 0.5mag are usually used to identify sources with inner disks, while object with smaller K-W2 colours have no or only weak inner disks. We have discussed this before in weeks 218 and 193. The asymmetry index can identify objects with outbursts (large negative values) and objects that show dimming events (large positive values). Sources with more symmetric light curves have values around zero. A good example being a pure sine-function.
As you can see, there are two groups of objects in our sample that show periodic behaviour. We added a vertical line at about 5.5d to distinguish slow and fast rotators. One can see that the fraction of slow rotators is larger for the objects with an inner disk (K-W2>0.5mag) than for the objects without an inner disk. Generally there are also many more objects with measured periods if there is no inner disk present. This is easy to understand, because an inner disk very often leads to changes of extinction along the line of sight and more variable accretion. Both of these effects cause non-periodic variability that hides any periodic changes in brightness due spots on the rotating stars. Furthermore, the evolutionary times of objects without inner disk are longer than for the other group, thus there are simply more such sources.
Most of the periodic objects are identified as more or less symmetric light curves. These are most likely stars with spots on their surface, which cause a small amount of variation in brightness. This is further supported by their small symbol size, i.e. low variability amplitudes. There are a few notable exceptions of apparent periodic bursters and dippers. The latter might be AA-Tau like sources with warped inner disks. We will investigate those source in detail and verify and clean our sample from any anomalous light curves before publication, but the general trends observed in these preliminary plots will remain in place and already allow a detailed insight into the properties of the population of young stars in the HOYS sample.