This week we look at the newly announced Gaia altert object Gaia22ehy, which is situated north of the Orion Nebula Cluster. The object is also know as the variable star TT-Ori. The spectral energy distribution shows an excess emission at infrared wavelengths, compared to normal stars. Thus, this indicates a disk surrounding the star. Together with the parallax and distance from Gaia, the object seems to be a bona-fide young member of the Orion star  forming region.

Gaia flagged the object as variable, because it turned slightly brighter than usual. Looking at the Gaia light curve, one can see that the source generally varies by about a magnitude around its average. At the time of the alert, the object was slightly brighter than its normal upper bound.

In the figure above, we show two weeks worth of HOYS data from a few years ago. We display the B, V, R and Halpha photometry in the plot. With the exception of one night, we have data once every 24 hours in that period (unfortunately the field is not covered that well all the time). One can see that even within this short time, the star varies by about one magnitude. Hence, the general scatter in the Gaia light curve is caused by very short timescale variations, which are happening as fast as one day. It is thus not surprising that Gaia will eventually catch the source in a slightly brighter state than seen before.

We have not done a detailed analysis of the colours yet to identify the most likely cause of the variations. As always, varying mass accretion rates or material in the very inner disk (short timescales) are on the list of culprits. Despite the larger error bars on the Halpha  data, we can see that in some nights the R-Halpha colour increases. This hints that there is at least some contribution of a changing mass accretion rate to the variability, because the strength of the Halpha line correlates with the mass accretion rate of young stars.