This week we revisit one of the old friends of the light curve of the week feature. The star 2MASS J21383981+5708470 projected onto the target region IC1396A (the Elephant Trunk Nebula).
The light curve shows the HOYS data in B, V, R and I of the object over roughly the last two years. We know that the source shows ‘eruptions’ roughly once a year. Sometimes however, there are larger gaps and the source remains quiet for longer. Based on the pattern the next eruption is imminent, and the latest data points seem to suggest it has started to brighten. Hence: Please keep an eye on this field with a higher priority than usual. Note that this field is currently only visible in the early morning hours before sunrise.
We have made some investigation of this source over the past few months. The unusual behaviour, in that it turns red when in outburst, is due to the nature of the star. It is a B-type background star, which has a very high surface temperature – of the order of 20,000K. The eruptions are mass accretion events in the disk surrounding the star. Such events usually also cause very high temperatures (10-15,000K) but these are lower than the star. Thus, the additional emission from the accretion event turns the colours of the object towards the red.
A student has done some very basic modelling of the light curve over the last few months. We tried to determine the temperature and emitting surface area that causes the ‘eruption’. Using this, one can estimate the additional luminosity coming from the mass accretion event. As this is solely determined by the mass accretion rate, one can roughly estimate how much mass is accreted in each of the bursts. One finds that this is of the order of an Earth mass each time. This might seem a lot, but is quite insignificant for these stars. They are typically more than 10 solar mass, and one needs about a million earth masses to even make one solar mass.