This week we have a look at the young star RW Ori, which is situated in the sigma-Ori target field of HOYS. The object is known as an Orion type variable, which essentially means it varies in an unpredictable way. The parallax and proper motion values from Gaia indicate it clearly as a member of this cluster of young stars.

The light curve shown above is our data from the last observing season in the B, V, R, and I-band filters. Broadly speaking we see some longer term brightness fluctuations of up to one magnitude in the I-band and two magnitudes in the Blue filter. These ups and downs in the brightness appear to occur roughly every 55 days, but are clearly not strictly periodic.

On shorter timescales the data look quite noisy. However, if one zooms into the light curve (you can use the light curve plotting function on the server for that), one finds that there is an almost periodic fluctuation in brightness every 6-7 days. The amplitudes vary, but are generally up to 0.5 magnitudes. This could be due to spots on the surface or occultations from an inner disk warp caused by a mis-alignment of the star and disk rotational axis (AA-Tau like object).

This light curve is a good example of different types of variability occurring on the same star. We have the short term periodic fluctuations due to spots and rotation or occultation by an inner disk warp. But also longer term variability of a larger amplitude. The fact that these longer term variations appear almost  periodic, could hint at a temporary structure in the inner disk, potentially about 0.2-0.3 Astronomical Units from the star. We are in the process of adding this winter’s data for the source, hence we can check if this variability pattern continues.

At the very end of the displayed data, we see a single very deep dimming event that only lasts about one day (JD=2460056). This is not a separate deep, short dip, as can be seen in some other objects. It is part of the roughly one week long periodic fluctuations. This sudden decrease in brightness in one of the regular minima is a good indication that this pattern is caused by a warped inner disk and not a spot. Only a sudden increase in occulting disk material along the line of sight can cause such a short duration brightness decrease. We will investigate the colour changes in detail to verify this.