This week we look at a figure from an upcoming paper. In that paper we try to characterise the sample of young stars in the HOYS clusters which are young and within 3000 light years from Earth. Gaia data from the latest data release has been used to identify members of these clusters based on their distance and proper motion. This provides a well defined, almost contamination free sample of nearby young stars we can investigate. We find that for about 1700 of these we have HOYS light curves with at least 100 data points in at least the V, R, and I-band filters.

We determined the Stetson Variability Index (J), which in essence tells us if the variations we see in a star’s light curve are larger or smaller than what is expected from the uncertainties in the photometry. We then only select the variable stars and look up their brightness at infrared wavelengths. We also determine if the light curve looks more like a dipper, a burster, or is symmetric around a mean (M-index). All of these data are put together in the above figure, so there is a lot of information to digest. Hence, just a few thoughts about this here.

Along the x-axis we show the colour in the near infrared, in particular between the H-band (1.6micron) and the K-band (2.1micron). Usually, almost all normal stars have roughly the same H-K colour (~0.1-0.3mag), independent of their temperature, as long as it is above about 3000 Kelvin. Anything redder, i.e. larger colour, is usually caused by extinction – scattering of light along the line of sight.

Along the y-axis we show the near/mid-infrared colour between the K-band (2.1micron) and the W2 -band (3.3micron). Again most normal stars usually have very similar values for this colour (0.1-0.4mag). This is, unless they are surrounded by a disk of material. The dust in these disks gets heated to temperatures of ~1500 Kelvin (any higher and it sublimates). The blackbody radiation emitted by such hot dust shows up at wavelengths longer than about 2microns (see Wiens Law). Thus, one usually considers objects with K-W2 colours above 0.5mag as having a disk close to the star which contains hot dust. One can see that also in H-K the disks become detectable, but the K-W2 colour is much better at separating the two groups.

We can see that our sample of stars is a mix of disks and no disk objects. In particular about 2/3 stars have a detectable disk, and 1/3 do not. This is contrary to the entire sample of cluster members, where only 1/3 has a disk. Thus, this clearly shows that the presence of a disk is inextricably linked to the variability of the young stars.

The symbol size in the figure is proportional to the variability index (J). While there is a mix of values, the fraction of extremely variable sources (large circles) is much higher amongst the objects with disks. Finally the M-index is the colour code. Red means the stars have more dipper like light curves, and blue means they are more burster like. Again, there is a mix of sources in the sample and even stars without detectable inner disk can be dippers or bursters. This might sound strange, however no disk, simply means no (hot) dust close to the star. Thus, there is still a lot of gas that can cause outbursts. And there is still colder dust further out in the disk that can cause occultations/dips.

More on this soon ….