This week we have a look at some light curves presented in our latest paper “A survey for variable young stars with small telescopes: VI – Analysis of the outbursting Be stars NSW284, Gaia19eyy, and VES263“. You can find the full version of the paper, and all the others here.
The above image shows the detailed light curves in the B, V, R, and I filters of outbursts of two so-called Be-stars. These are not really the kind of objects we did set out to investigate in our project. But whenever one does a large, long term survey for something (variable young stars in our case), other interesting objects pop up. The story of the object in the left panel is quite a long one. We found the unusual behavior of the source in 2017, because it was the only object in the entire HOYS data sample at that time, that changed to a redder colour while increasing it’s brightness. Usually all young stars turn blue when getting brighter, no matter the reason (changing accretion rate, lowering of extinction). There is quarter page of description of this source in our very first HOYS paper (listed here too) from 2018.
It has taken us quite a while to understand that the object is not a young star as such, but rather a more massive spectral type B star, either in the late stages of its main sequence life or early on during its post-main sequence evolution. These stars do have disks around them. But in those disks mass is not flowing towards the star (the accretion disks we have around the young stars), but rather the material is ejected from the stars themself. Many of the type of stars rotate very fast. The rotation speeds are usually above 70% of the breakup velocity. This would be the velocity above which the centrifugal forces exceed gravity on the surface and matter would be ejected radially. The stars further have pulsations in their interior. And it seems to be the combination of these and the fast, near breakup rotation, hat occasionally ejects material from near the equator into a ‘decretion’ disk (the opposite to an accretion disk).
Many details of this process are not fully understood. Hence, detailed observations of such sources are important. However, it is usually not exactly known when these bursts occur. Thus, the three sources we identified in our paper, which do show almost regular bursts, are extremely important. They hence allow us to predict quite accurately when the bursts occur and plan detailed observations such as spectroscopy at the precise onset of the burst in order to learn more about these source.
The plot does not only show the light curves for two of the bursts in our usual colour coding, but also a solid line for each filter. This is a fit to the data for a simple analytical model how these bursts should look like. In the bottom panel we show the residuals of this fit. In other words we subtract the fit from the data. You can see that there are no real systematic deviations of the data from the model.