Session 4: First Lightcurve

Sunday December 15: Observing Session 4

Once again, weather interfered. Had to throw away at least an hour’s worth of images due to clouds. Nevertheless, we’ve now accumulated a grand total of 1,040 usable images, each of which provides one brightness measurement. Let’s see what we can do with that.

This (unimpressive) graph shows the raw data. (Remember that star brightness measured in magnitudes seems to be backwards: bigger magnitude numbers are fainter than smaller magnitude numbers.) Each of the 1,040 data points is plotted in green as a brightness (y axis) against the time the image was exposed (x axis). The numbers on the x axis are “Julian Dates,” the standard way that astronomers manage observation times. A day’s Julian Date is simply the number of days that have elapsed since the Julian Calendar reference date: Monday, January 1, 4713 BC. There’s nothing useful to be gleaned from this graph.

But, there’s another way to graph this data, one that takes advantage of KIC9832227’s orbital period of about 11 hours. Instead of plotting each point in “absolute time,” we can plot each point in “orbital time,” which goes from 0 to 11 hours. That results in the following graph:

Each observing run is plotted in a different color. These four runs give us almost complete coverage of an entire orbit. (Although there’s still a lot of data scatter as a result of all the poor weather we’ve encountered.) Observations and things to ponder:

• The two minima don’t match brightness. The minimum at 3 hours has a brightness of about 12.48 and the minimum at 8.2 hours has a brightness of about 12.51. (That makes the minimum at 3 hours the secondary eclipse and the minimum at 8.2 hours the primary eclipse.) Because the two minima don’t match, the two stars must have different brightness/size.
• The maximum that’s around 5.7 hours isn’t in the “middle” of the two minima. It takes longer to get from the 3-hour secondary eclipse to the maximum than it takes to get from the maximum to the 8.2 hour primary eclipse. This means that the two stars are in an elliptical orbit; if the orbit was circular, the timing would match. [Update: the more I stare a this, the more I’m uncertain about where the “middle” is. Maybe need more data before this becomes clear??]
• There are several parts of the orbit where measurements from two observing sessions overlap. This provides an opportunity for our own measurement of orbital period, which I’ll address in a subsequent post.
• There’s no part of the lightcurve where brightness “stands still.” This is further confirmation that these two stars are a contact binary, actually touching each other.

All in all, this is pretty cool. Despite all the bad weather and problems with fuzzy, blurred, near-the-horizon, out-of-focus images, we’ve succeeded in creating a graph showing how brightness changes over time for this close binary star. That’s neat.