While HOYS is aiming to investigate young stars, and thus focusses on imaging star forming regions and young clusters, it is obvious that there are other interesting objects in our observed fields. This is the reason why the HOYS database is designed to measure and store the brightness of all stars in all submitted images, instead of just focusing on the known young stars, or even just the known variable stars. Thus, we can identify not just newly variable sources, which might have not been variable in the past, but also interesting variables that are not young stars.

This week we look at one such source, the Gaia alert object Gaia19eyy. This is our only southern field, not visible for most observers in Europe or North America. When the object was discovered, it was classified as an outbursting young star candidate, and we thus included it into our target list. While there are possibly some young stars in this area of the sky, Gaia19eyy certainly is not. However, it shows regular brightness increases, roughly once per year, in the longer term Gaia light curve.

Our HOYS data shows the last complete burst and the start of the next one in the B, V, R, and I filters. As you can see, the maxima in brightness in the different filters occur at different times. After an initially fast brightness increase, the blue light peaks first, then the green light, then the red and finally the infrared light. After that, the brightness drops fast in all filters back to ‘normal‘. The unusual burst behaviour and the fact that the star is redder during the burst gives some hints what the object could be.

It is actually not a young star, but a so-called Be-star. These are massive stars of spectral type B, with masses between 2 and 16 solar masses, which are typically on the main sequence. They are fast rotators and occasionally form disks around them from material that is lifted of their surface. The details why this happens are not fully understood. The material in the disk is then heated by star and pushed outwards, explaining the shape of the light curve in the different filters. These disks are in essence the opposite of the accretion disks around young stars, where material is moving inwards and accretes onto the star. One could hence call these disks decretion disks.