This week we look at the light curve of the young star BB Ori. It is situated in the L1641N target field, just south of the Orion Nebula (M42). The object has been flagged up a week ago by Gaia to have undergone a one magnitude dip in brightness. This shows that Gaia can already see it, and indeed Orion now becomes visible again just before sunrise in the morning. It helps if you live further south.

The light curve shows that the star does vary to some extend, and a detailed look shows that there were more such events in the past, which Gaia missed due to the cadence. One occurred about 100 days before the one Gaia detected, only a handful of days after the last Gaia datapoint. The light curve also shows an important problem we can encounter when analysing our light curves. As one can see there is a second light curve shifted by about 1.7mag. Indeed there is a second fainter object (2MASS J05361563-0617388) visible in the images, about 8.6 arcsec to the East. While BB Ori is at a distance of 397pc, the other star is at 250pc. Thus, they are not in a binary system, and this is a simple chance alignment – blend.

Thus, if we analyse light curves, especially a large number, where it is impossible to look at them all by eye, we need to ensure they are free of such blends. In this case it will be simple to dissect the two because they are clearly separated. I used a large search radius to make the point. But if the blends are a bit closer together, then they might ‘merge’ in some images but not others, depending on the resolution/seeing. This will look in the light curves as if the stars are extremely variable on short timescales. Thus, we need to quality control the data and remove any points where the seeing is larger than half the separation between the two stars.

Luckily, Gaia provides us with a high resolution catalogue of all stars potentially visible in our data. So we know if there is another star near the one we are interested in. However, at small separations, below 3 arcseconds, this will remove all the data points, because almost all our data has seeing worse than 1.5 arcseconds. For separations below one arcsecond, even Gaia cannot separate two sources. However, it can measure their position much more accurate – hence the accuracy in parallax and proper motion measurements of up to 20 micro arcseconds. One can identify some of these tight binaries or blends because their point spread function is not the same as for the other, single stars. But once they get closer than maybe 0.1 arcseconds, they cannot be identified.

Thus, if there are close binaries (physical ones and chance alignments) and one of the sources is variable, we will underestimate the variability because the amount of light from the other star does not change, and we only observe the sum of both. Given that many young stars are binaries, that is something to keep in mind when investigating the light curves.