This week we look at one of the plots from the first HOYS paper: “A survey for variable young stars with small telescopes: First results from HOYS-CAPS” – note we changed the project name shortly after that. In that first paper we investigated the known young stars in a few of our HOYS fields and looked at their light curves to investigate their variability. Note that at this stage, the light curves were barely two years long, compared to the almost seven years of data we have now.

There are two light curve properties plotted in the graph shown above. On the y-axis we show the light curve asymmetry index. This describes if the light curve looks like it has dimming events (large positive values) or outbursts in brightness (large negative values). Objects that for example vary like a sine-function (e.g. many of the stars with spots) are in the middle, i.e. at y=0.

On the x-axis we show how the stars data points along the light curve behave in a colour vs. magnitude plot. This traces by how much the colour of a star changes when it varies in brightness. This can give us a lot of insight into the physical reason for the brightness changes. For example, if the brightness of a star decreases because some of the light is blocked by a completely opaque object (another star, i.e. eclipsing binary, or very dense, opaque dust), then typically the brightness of the star will drop in all filters by the same fraction. Thus, the star will dimm, but not change colour. This will place it near the very right hand side (90deg) of the plot. If the star gets occulted by small dust grains, then the blue light gets scattered more than the red light. Hence, in such a case the star will turn red while dimming. For typical interstellar dust the x-axis values would be between 55deg and 65deg. If the dust grains are larger than in the interstellar medium, the x-axis values can be larger than 65deg. Finally, changes in the mass accretion rate do typically cause larger colour changes in the stars and smaller x-axis values between 35deg and 55deg.

In the plot one can see that the young stars in the investigated HOYS fields cover a wide range of the parameter space. The colour coding in the plot shows different groups of young stars identified by a clustering algorithm (a story for another post, or read through the paper). We can see that the outbursting stars (green group 4) mostly display colour changes in agreement with changing accretion rates. There are only very few outbursting objects where the colour change indicates the brightness changes due to dust occulting the stars. Hence, this rules out that there are stars in the sample that are usually obscured by dust (e.g. from their disk), and occasionally there are holes in the occulting material, creating a brightness increase. On the other hand, the dipping stars (black group 1) mostly show colour changes that are in agreement with occultation from normal interstellar dust or slightly larger grains.  There are some objects that have a dipping light curve but seem to have colour changes indicating varying accretion rates. Thus, there might be a certain fraction of sources were the accretion rate drops for short amounts of time, creating dimming events. For the objects with symmetric light curves (y near zero), it is slightly harder to identify the reason for the variability. As additionally to the accretion changes and extinction variation, spots on the surface can also be the reason. These spots also change the colour of the stars, but in a less coherent way, because the colour change depends on the temperature contrast (hot or cold spots) to the star.

The latter can of course be investigated by identifying periodic variability in the stars. This is something we have now started to work on. We are planning to look again at all young stars in all hoys fields with now seven years of light curve data. This will give us a much better insight into the causes for variability and how it depends on properties of the stars such as their age or mass. Furthermore, we can identify stars that change their behaviour, i.e. turn from a dipper to a burster or periodic variable. We plan to do this analysis over the next year – stay tuned.