This week we continue to look at our analysis of the entire sample of young stars in HOYS. This time we look at their mid-infrared colours measured by the WISE satellite. This mission measured the brightness of stars at four wavelengths: 3.4, 4.6, 12, and 22 micrometer, which are usually referred to as W1, W2, W3, ad W4. The colours are simply the magnitude differences. Typically, normal stars without a disk have close to zero colour.

In the plot above (either panel) we plot two of the WISE colours against each other. We only include stars which are actually variable. In this case, ‘variable’ is defined as average variations bigger than the uncertainty of the brightness measurement. We can nicely see that the young stars split into two main groups. One has a relatively large W1-W2 colour of about 0.4-0.8mag, while the other one has basically 0.0mag colour. The first group has a roughly homogeneous W3-W4 colour of about 2mag, while the second group has a much wider range from 0-4mag (or higher) for that colour. In all cases, the circle size of each star is scaled with its variability. I.e. more variable objects have bigger circles.

The 1st group with large W1-W2 shows hence an excess of emission at shorter wavelength. This indicates a disk with high temperatures, i.e. closer to the star. The 2nd group has not much inner disk material, but still material that is colder, i.e. further away from the star. In the panel on the left we have used only the V-Band light curves, and in the panel on the right only the I-band light curves of the stars. The colour code of the symbols indicates how asymmetric the light curve is. Red points inticate dipping light curves, while blue points indicate outburst like light curves. Green points are indicative of symmetric variability.

One can see in both panels that the objects with warm/hot inner disk material have a larger fraction of dipping light curves (caused by disk material moving in and out of the line of sight). Why is this effect much stronger in the I-band data? And why are there many more strong dippers in the I-band plot and more bursters in the V-band plot? This is a selection effect. Remember that young stars are usually quite red in colour. this means they are bright in the I-band and much fainter in the V-band. Thus, the in the V-band light curve of a dipping star, the star will often not be detected during the dimming events and it thus might not be recognised as a (strong) dipper. This is not the case for bursters, as their brightness increases will always be fully detected. In the I-band data we have the opposite effect. Most stars are much brighter, hence almost all dippers can be observed even during a deep dimming event. Bursters, however, might become so bright that they saturate in the images and the bursts are thus in part removed from the light curve as the stars brightness cannot be calibrated.