This week we look at the light curve of the star PR Ori (or so one might think). But let’s start at the beginning and work through the 30min of confusion I had last night – step by step.
Last night, just after 8:30pm the Gaia alert website published the alert Gaia21arx. As it is situated in one of our target fields, I checked our light curve (see image above). Usually the Gaia G-band magnitudes more or less resemble our Red filter, in terms of magnitude values and also variability. But this time neither the changes visible in the Gaia light curve, nor the magnitudes had anything to do with our R data. After multiple checks of our data, I finally submitted into accepting everything was correct and started looking into the details.
Searching in SIMBAD around the position reveals there are two stars that are about 3.5arcsec apart from each other – PR Ori and ESO-HA 1481, The latter one is the one matched to the Gaia alert. The DSS and 2MASS images on SIMBAD however only show one star. One can, however, display the PanSTARRS image on the Gaia alert page or download the images from UKIDSS. Those higher resolution images clearly show the two objects. Within the uncertainties, both stars have the same parallax of about 2.6 milliarcseconds, and they are thus most likely a physical binary system. The distance is about 380pc, i.e. they are nicely part of the Orion star forming region. This also means their projected separation is of the order of 1300 Astronomical Units.
The brighter of the two stars, by approximately 2mag is PR Ori. Interestingly, this brighter object has the larger error for the parallax. According to Gaia the fainter star has dimmed and now re-brightened by 4mag over 3 years and is now only about one magnitude (factor of two) fainter than the brighter star. This makes interpreting our light curve very difficult, as it resembles the sum of the brightness of the two stars. Over the six years we see a long, extended dimming event of about 0.2-0.3mag. And there are short term variations of the same order of magnitude. Given the brightness difference between the two stars, the long term trend in our light curve is indeed in agreement with the strong changes seen by Gaia for the fainter star. However, it is next to impossible to decide which of the stars imprints the short term variations on the light curve. During the faint state, most likely PR Ori is the main source, but during the bright state it is likely that both stars contribute.
To summarise: On needs to be careful interpreting HOYS light curves of stars, and indeed any light curves. HOYS typically resolves objects 3-5 arcseconds apart. Especially young stars tend to be in binary systems. In this case we can resolve the companion with other data (and indeed in some of the HOYS images), so we are aware of it. But all light curves have this potential caveat.