FK Comae Berenices Variables
FK Comae Berenices is the prototype star for this class of variables. FK Com stars are giant stars that show periodic brightness variability that is thought to be caused by large, cool spots on the surface of the star that rotate across our line of sight. FK Com stars spin very rapidly and have very strong magnetic fields, which together lead astronomers to believe that they may be the result of the merger of a contact binary - two stars that were in a very close orbit.
The rotation of any star defines its axis of spin, the two poles of that axis on the stellar surface, and the equatorial region of the star. Detailed analysis of the light curves of FK Com stars seem to indicate that there is strong differential rotation, meaning that higher latitudes near the poles may be rotating at a very different rate than regions near the equator. The speed of rotation near the equator can be near 100 km/sec, causing the star to bulge at the equator so that it is elliptical in shape.
Because of the differential rotation, the cool spots may have slightly different rotation periods, so that the shape of the light curve, or even the best rotation periods to describe the variations, can change with time.
FK Comae Berenices
Below is an observed light curve for FK Comae Berenices. The video covers over 5 years of observations, and each plotted time interval corresponds to a week of real time. The magnitude of FK Com ranges from 8.4 magnitudes to 8.15 magnitudes during the video. Also interesting to note are the regular silent gaps in observations. These occur for a couple months each year due to Earth's orbital causing FK Com to pass out of view. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.
Unlike for most other objects, for FK Com, we do not show its full optical spectrum, but rather just the region between 6350-6650 Angstroms, centered on the strongest optical emission line of hydrogen, called H alpha. The H alpha line in FK Com is strong, very broad, and variable. Strong H alpha emission lines are typical in stars with powerful magnetic fields. The line appears to be broad because of the very rapid rotation, whereby the range of speeds relative to us of different parts of the stellar surface appears to stretch or shrink the apparent wavelength of light due to the Doppler effect. The H alpha line also shows two strong peaks, one on the blue (shorter wavelength) side and one on the red (longer wavelength) side. This is because the broad emission line is split in the middle by a narrower absorption line from cooler material in the outer layers of the star. Here we present 4 short consecutive videos that scan across the H alpha emission line from blue to red. Each spectrum is observed about a day apart. Note the broad rise of flux first on the blue side of the line, the narrow absorption line in the middle, and the strong rise again scanning the red side of the line. The shape and position of the line changes somewhat each day.
Light curve downloaded via CDS/Vizier from:
Hackman, T., Pelt, J., Mantere, M.~J., et al. 2013, Astronomy & Astrophysics, 553, A40 (link)
Spectra downloaded via CDS/Vizier from
Vida, K., Korhonen, H., Ilyin, I.~V., et al.\ 2015, Astronomy & Astrophysics, 580, A64 (link)