This week we look at the light curve of the T-Tauri Star 2MASS J21395813+5728335, which is situated in the Elephant Trunk Nebula target region of HOYS. It is near the centre of the main cluster in that field and based on the distance and proper motion clearly a member of the region. The colours in the near and mid infrared indicate a presence of material in the inner disk around the star.

The light curve shown above is one of the more unremarkable ones. But there is some variation that is interesting. Starting about half way through the data available, there are three ‘humps’ lasting about half to one year. These variations are about twice as high in the visual filter (green symbols) than in the infrared (black symbols) filter. The most likely explanation is a small increase of the mass accretion rate in the star during those periods. It is however also possible that the line of sight extinction dropped during these times. The noise in the photometry is too large to check if one of these explanations is more likely than the other, without a thorough and detailed look at the data.

However, it shows nicely one of the challenges this project phases. Variations in brightness are almost always larger at shorter (bluer) wavelength. This is true for accretion rate changes and line of sight extinction. However, the young stars are almost always much fainter at these shorter wavelengths, and thus the photometry errors get larger. Thus, usually during the observations we aim to spend more of the integration time in the bluer filters than in the redder ones, to try to counteract this effect. Thus, have a look at your observing programs for HOYS and maybe adapt the split of time spend in each filter if you can.