We stay in the sigma-Ori cluster for this weeks light curve. The object we look at is known as 2MASS J05385410-0249297. The Gaia data places the star at the correct distance for the cluster, and it is a known T Tauri star. The spectral energy distribution shows some scatter, indicating variability, as well as a weak hint of mid-infrared excess emission, which is indicative of a disk.
We show the V, R, and I data we have for the star from the last observing season. Similar to last weeks object, this source also shows more than one type of variability. We ignore the outlying data points, esp. in the I-Band, which need our colour correction applied or be removed before a detailed study of the object is made.
The at first sight most obvious change in brightness is the initial slow drop by about 0.1mag during the first 100days of the data. After that one sees a steady increase in brightness by a total of 0.2mag during the last 200days that are shown. Indeed, the long term light curve shows that the object had undergone a slow and steady fade of 0.2mag during the last five years, before the ‘rapid’ 0.2mag rise during the last 200days. These slow and smooth changes in brightness are usually caused by occulting disk material far out in the accretion disks, or small long term changes in the mass accretion rates.
The second obvious feature in the data is the apparent scatter of about 0.1mag from the ‘smooth’ long term behaviour. This is not noise in the data, as the typical photometric uncertainties are of the order of 0.05mag or slightly better. Zooming into the data shows short term variability that is correlated in all the filters. Indeed, as has been shown by the ASAS-SN survey, there is an underlying periodic signal with a period of 2.74days. This indicates there are spots on the surface and we are seeing the rotation period of the star. It becomes hence clear, that if one wants to study the amplitudes of the variations, one needs to correct for the longer term trends in the brightness of the star first.