This week we look at an interesting source in the only real southern HOYS field. The object is Gaia19eyy. It was originally picked up by Gaia as an outburst from a candidate young star. We did add the source as a new HOYS field, as it resembled another object we identified in the Pelican Nebula. Both objects stand out from all the other objects in our fields in the sense that they turn red during the outbursts. Usually if objects get brighter for almost any reason, they turn blue. Those two do not and hence the outbursts are stronger in the longer wavelengths filters than in the short wavelengths filters.
The light curve at the top shows the HOYS data of the source in B, V, R, I during the last observing season. We have shifted the magnitudes in the filters by the amount indicated in the plot to make all features visible. As one can see, the amplitude in the I-band is much higher (0.6mag) than in the B filter (0.3mag). The entire outburst lasted about 150days and after that the object returned to its normal brightness. The Gaia light curve shows that the object has repeated bursts of varying strengths roughly every 12-13 months. The last one, shown in our data, has been the strongest of the observed ones.
The reason for the ‘strange’ behaviour of this object can be easily explained. These are very massive and hence young stars of spectral type B. They do still have a disk of material around them. In those disks there are mass accretion events where material is moved towards smaller separation, i.e. closer to the central star. This frees potential energy which is in part converted into heating of the disk material, which we see as the outbursts in brightness. These heating events create temperatures of 10-12 thousand degrees in the disk material. But this is still cooler than the surface temperature of the star. Hence, the object becomes redder during the outbursts.
Detailed analysis of the photometry and some spectroscopy will allow the determination of how much energy is released in each event, as well as where in the disk it occurs. Combining these allows to calculate how much material gets accreted in the event. Using some basic assumptions one finds that of the order of one Earth mass is accreted per burst. We plan to do a much more detailed analysis of the object over the next few months. Stay tuned……