It is time to have a new look at the object 2MASS 21383981+5708470, situated in the Elephant trunk nebula field. The object might be projected onto the IC1396 HII region, but the Gaia parallax clearly identifies it as a background star. The most likely distance based on these data is about 5100pc. Hence, this is clearly an unrelated background star.
We have identified this star some years back as an erupting object. Eruptions happen roughly once a year, but quiet intervals of up to 2.5yrs are seen in our data. The standout property of the eruptions is the fact that the colour of the star turns red during the outbursts. This was the only object that did this in the entire HOYS data set when we identified it. There is now a second object, Gaia19eyy, which seems to show a similar behaviour. But that source is in the southern hemisphere and thus hard to observe for most of our participants.
The HOYS light curve shown here covers the data we have over the last ~1.5yrs. One can see that the source is currently on its way back to quiescence, after a second outburst in that period. The two outbursts shown are quite different. The first one was clearly brighter at the peak. But also the shape of the light curve is very different. In the first burst a sudden very steep increase in brightness is seen at the start of the burst. On the other hand the second burst showed a much more gentle increase in brightness at its start.
The also show V-I colour curve shows clearly how the object turns red – increasing V-I colour – during the bursts. It is also clear that the object did not fully return to its pre-outburst colour after the first burst. The colour always remained at least 0.05mag above the values before the first burst. It seems that at the moment the source is quickly returning to it’s quiescent state.
We are not very sure what causes the eruptions and working on the interpretation. The star itself seems to be a B-type star. One can successfully model the brightness increase as some form of eruption/explosion either on the surface of the star or in a potential disk around the star. This does explain the HOS photometry data, but we also do have spectra taken during the previous outburst which need investigating. Even if the explosion model explains the data, it does not tell us the underlying reason for the behaviour. Some work to do.
Continued data, following more of these eruptions will help to understand what is going on. It might tell us about the frequency of these eruptions and distribution of strengths, duration and light curve shape. All of which might help to constrain any models trying to explain this behaviour. Furthermore, trying to find other sources with similar behaviour might shed further light onto what is going on.