This weeks light curve is from our auxiliary dataset, introduced as part of our 100th light curve of the week video. This is a huge set of data taken as part of the Cygnus Project and has been made available to us. We do have now all data calibrated and started to explore it for science.

The light curve shown is for the star EM* CGHA 51, which is a known emission line star. Looking it up in Gaia shows that it is nearby with a distance of about 423pc. The magnitude and colours of the star indicate it is roughly of spectra type F0, and on the main sequence. This means the star has about 1.4 solar masses and 1.4 solar radii.

The light curve shows that the object is an eclipsing binary with a period of 1.54303 days. One can make a rough estimate of the properties of the secondary object. The depth of the light curve is about 0.135 of the flux. This roughly means the eclipsing body will have a radius of about half a solar radius. This means it is probably an early M type star, so about 0.4 solar masses.

The period and the estimated masses result in a semi major axis of 0.032 AU for the system. From this one can calculate the duration of the primary eclipse in phase space for a perfectly edge on orbit by using duration = R*/(π a) which results in 0.065. This is very close to the observed duration. Furthermore, such M-type stars are about a factor of 100 fainter than the F-type stars. Nicely indicating the secondary eclipse should only be 0.01 deep, again roughly what we can see in the plot. But note, that despite the huge amount of data we still seem to have some larger scatter in the light curve near that secondary minimum. Might be worth observing it again – but note that it requires the same accuracy as finding an exoplanet transit.

It seems odd that the star has Hydrogen emission lines while on the main sequence. But potentially there is some interaction between the two objects, after all the semi major axis of the orbit is of the order of 20 stellar radii. The rounded maxima in the light curve are already an indication that the primary star is not round but slightly elliptical due to the nearby companion. Most likely, however, the Hydrogen alpha emission comes from the M-type companion. It will have a rotation period identical to the orbital period. It is well known that low mass stars, especially fast rotators are more active. Faster rotation causes stronger magnetic activity and thus more Hydrogen alpha emission.