We are celebrating the 125th edition of the light curve of the week feature by looking back at an old friend – GM Cep. This is a known variable of Orion type, which basically means it randomly varies in a way that does not follow any classifiable patterns. The star is situated in the HOYS field centred on the Elephant Trunk nebula, and has an approximate distance of about 825pc. This places it into the IC1396 HII region of which the elephant trunk is a part.
The object has also been classified as an UX Ori type star. These are slightly more massive objects (spectral type A or B) which show deep occultation events by dense circumstellar disk material. Some authors have also claimed the detection of a protoplanetary clump in the light curve that orbits the star with a period of about 320days, causing semi-periodic occultation events.
We show the HOYS V and I data in the plot above. One can clearly see that the classification of the star is more or less spot on. There are numerous occultation events from a baseline brightness. The colour of the star typically turns redder during the occultation. The duration of the dimming events ranges from a few days to half a year, or longer, depending on the definition of the baseline. The dips can be as deep as 2mag in V, but 1mag is more typical. Some dips are nice and smooth, wile others are extremely structured, with very rapid changes – up to 0.5mag within one day can be seen. This indicates very small scale dense clumps in the inner disks. We hope to investigate these kind of objects in more detail in the future.
It seems like we do not see the proposed periodic dimming events every 320d. This could either mean the proposed protoplanetary clump was a transient structure, or the apparent periodicity was coincidentally caused by occultations of the star by different unrelated clumps in the disk.