This week we look at a plot from a recently submitted HOYS paper. In the paper we determine the properties of star spots on young stellar objects in the Pelican Nebula. This is based on the amplitude of variability in the HOYS light curves in the V, R, and I filters. We assume that there is only one single spot on the star that rotates in and out of view due to the stellar rotation and thus causes the observed brightness variations. There are ‘imposters’ that can mimic a similar behaviour and the paper discussed how we identify those. The plot is quite busy. Hence, lets have a look at the details step by step.
We show the spot coverage on the x-axis against the K-W2 colour of the star on the y-axis. The coverage is the fraction the spot covers on the visible surface. Thus, 0.2 means that 20% of the visible surface, or 10% of the entire star are covered by the spot. It is immediately apparent that the spots on young stars are much larger than the sunspots – they typically cover 0.01% of the visible surface. The size of the plotted symbols is also set to be proportional to the spot coverage.
The spot temperatures are indicated by the colour of the symbols. The scale is shown on the right hand side. It shows the temperature difference of the spot and the stellar surface. Thus, blue symbols indicate hot spots, and red symbols show the cold spots. It is evident that the majority of the spots we have found are cold spots – which are similar in their nature (just larger) to the above mentioned sunspots. The hot spots are usually caused by shocks on the stellar surface when material falling (accreting) from the disk onto the star crashes into the star at high velocities.
The K-W2 colour is the magnitude difference of the star measured between 2micron (K) and 5micron (W2). Normal stars have very small such colours, and the K-W2 colour is typically used to identify stars that have a hot inner accretion disk. In other words, if a young star has a disk that extends close to the star, and is hence quite hot (up to ~1500K), then the K-W2 colour increases due to the increased emission in W2 compared to K from the disk material. Typically a colour of 0.5mag or larger is used to identify objects with inner disks (dashed horizontal line).
There are other ways to identify disks around the stars, which use even longer wavelengths data. If this data is available with sufficient quality (high accuracy) for a star, the symbols are circles, otherwise squares. For the good data (circles) we highlight the objects with disks by a black ring around the symbol. As one can see, with the exception of two small hotspots and one cold spot, all the stars do have a disk – which is what is to be expected based on the age (1 million years) of the stars in this cluster.
There are a few interesting things we find: There are very few hot spots, despite the fact that all stars have disks and should hence have some form of mass accretion. But, this process is typically not stable over a long time. Thus, hot spots do appear and disappear on short timescales (days) and hence such objects do not show nice periodic light curves. The cold spots, however, typically are stable over longer times (weeks – months) and can be identified much easier. Furthermore, there are a few objects with hot spots that seem to have no disk. Either, the disks in those cases are very low mass, and hence are not detectable in the colours, or these hot spots have a different origin. There are hot spots on the Sun, called plage, which might also be present on these young stars.