This week we have look at the light curve of the young variable star GM Cep. It is situated in the elephant Trunk Nebula target region, and we have looked at it a few times before (e.g. in weeks 171 and 249). In the former we looked at the first 6 years of data and in the latter at the data from the 2023 observing season.
We have seen that the source usually does not vary a lot, i.e. the short term fluctuations are very small (below a few percent). However, the source then undergoes dimming events of various lengths and depths at irregular intervals. These can be up to two magnitudes deep and range from a few days to several months. Occasionally there are even longer phases where the brightness is below the ‘baseline’ and fluctuates on shorter timescales.
In the image above we show the data of the last two observing seasons. Ignoring the few outliers present, we see that in 2023 the source followed its traditional behaviour. There were only a few short term dimming events, which were all less than half a magnitude deep. In the last summer, the object has undergone a steady decrease in brightness, by about one magnitude so far. The short term variability has also increased, in line with previous behaviour in these longer term dimming phases.
We will keep an eye on the source and see what happens for the rest of this observing season and next year to see when it recovers to its normal brightness. The last time it went into such a phase it took almost two years to get back to ‘normal’. These phases are most likely caused by dust forming close to the star and causing the brightness fluctuations. Thus, from the frequency and duration of these phases we can learn about the conditions for dust formation in the inner disk, as well as how long it takes to then either accrete or disperse (sublimate) the dust after it has formed.