This is the first time we are looking at the same star twice in two weeks, i.e. we have another more detailed look at the light curve of the variable star V1787Ori. No, we have not run out of ideas, but there are some nice details in the light curve of this object which we did not discuss in last week’s post.

The plot above shows the BVRI HOYS data for the star during the last three months. The star is quite red, so we have shifted the magnitudes a bit to make the behaviour much better visible. On first sight, the VRI light curves shown are mostly flat with a bit of noise. Some of it seems correlated, i.e. appears in all filters at the same time, hence represents real, small amplitude changes in brightness. What stands out is that the B data, despite its larger scatter and errors, shows a significant drop (by about 0.2mag, or 20%) over the course of 4days. There is no such drop in the VRI filters.

However, when one investigates the data in detail, one finds a small drop in the VRI data after the large drop in B. Indeed there are 5, 5, 3 percent drops in the average brightness in the VRI filters, respectively. These are smaller than the typical photometric uncertainties and are hence difficult to see. But if you place a horizontal ruler over the image you can spot it quite easily.

How can we explain this behaviour? This cannot be caused by  change in the line of sight extinction to the star, i.e. movement of some gas and dust in the disk blocking some light. While this is usually wavelength dependent, and stronger at shorter wavelengths, the difference in drop from the B to the V filter are too large. What seems to have happened is a significant drop in the mass accretion rate. Accretion hot spots emit a large amount of light in the U and B filters, due to their high temperatures. The star itself is very cold and hence most of its light is emitted in the R and I filters. Thus, if the contribution from hot spots suddenly drops, or vanishes, the change in the longer wavelengths filters can be minimal, while large changes in the short wavelengths filters can be seen. It should certainly be possible to create a simple two-temperature model to explain this behaviour. Some of the larger sudden drops in brightness discussed in last weeks post, where the VRI magnitudes change by much larger amounts, could simply represent more extreme cases of this event. Unfortunately we do not have B data during any of these other drops.

The above explanation does have one additional observable feature. The amount of emission in the Halpha filter should also drop with the B-band magnitude. This is due to the fact that the Halpha emission usually correlates with the accretion rates. Again, we do not have any data in the Halpha filters covering one of the episodes of brightness changes. But, if we keep observing in all the filters all the time, we should be able to catch one. After all the star seems to have a least one of these larger brightness drops per year – catching it is more tricky as it only lasts a few days 😉