This week we look at the variable young star AH Ori, which is situated north of the Orion Nebula (M42). The data shown are for last winter only. We did look at the object before and have discussed the long-term light curve of it in May last year (light curve of the week 173).

We only show the B and R-band data for clarity, but including the measurement uncertainties for the magnitudes. As mentioned in the discussion last year, the source is usually ‘calm’, i.e. varies slowly in a range up of about one magnitude, and with very small day to day changes. However, it can switch to a completely different mode of variability suddenly, which then can look like noise.

The behaviour shown is a good example. With the exception of a one day, 1.2mag dimming dip (very left of the plot), the variations are ‘smooth’. However, for the last ~50d of data, the sources has switched to (for the lack of a better word) crazy mode. There we see variations of up to or over one magnitude on time scales of one day. Note that this is not noise. The uncertainties are very small compared to the changes and they are correlated in all filters.

This can best be explained by highly structured (lots of density fluctuations) material passing in front of the star in the inner disk. With our very long light curve we will be able to investigate how often this happens. If some of these erratic phases repeat semi-periodically, we can estimate the separation of the material from the central star and thus the physical sizes of the structures. If the material does not complete more than one orbit due the shearing or being accreted, then the frequency of these events can tell us about how often material is moved into the inner part of the disk and we can investigate if disk models predict the same rates.