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Physics & Astronomy Department

Astr384 Class Projects, Spring 2008

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Further Study of PPM 706067

If PPM 706067 were merely a typical eclipsing binary star system, having fully characterized it we would have moved on with our lives. However, in the course of characterizing PPM706067, we discovered more variability beyond the eclipses, themselves. The light curve here shows all of our data for 2008 in B. The curves do not perfectly overlap; instead we get several different possible magnitudes for a single phase. This variability we attribute to star spots.

Light Curve Showing Star Spots

Star spots are essentially sunspots on stars that are not the sun. In the photosphere of stars, energy is added to magnetic field lines through the rotation of the star and subsequent twisting of the field lines. These field lines enter and exit the photosphere at star spots, suppressing the energy flow at that area. This results in cooler and darker spots on the surface of the star. As the stars rotate, these spots move in and out of view, resulting in variability in the brightness of the system.

Optical Data

Using the computer software, BinaryMaker3, we have been able to model the location and relative brightness of the star spots. These models agree fairly well with our optical data as shown in the graph below and suggest that the star spots are located on the secondary star, at least in 2006, and cover roughly 10% of the surface area of the star. Each spot is 10% cooler than the rest of star.

Light Curve Showing the Model

Furthermore, we were able to calculate the star spot period, which is a measure of how fast the stars are rotating, using the 2006 data. We found it to be 2.098 days, which is 2% different from the orbital period. Similar systems only have differences of approximately 0.2%. Since the stars are rotating at a different rate than they are orbiting, this suggests that over time the orbital period may change to match the rotational period. However, this change will happen on such long time scales that we do not yet have enough years of data to see it.

When we attempted to fold 2008’s data on the star spot period calculated in 2006, we found the phase of the brightest peak to change from month to month. There are two possible explanations. First, our star spots may be changing rapidly so that in the course of a month we would see different spots.

Otherwise, we may be observing a star spot cycle, analogous to the sunspot cycle. Over the course of 22 years spots on the sun change location, steadily moving closer to the equator. As they change longitude, the period changes since they are now rotating about a different distance. We may be seeing something similar on PPM 706067. Thus, 2006’s period would no longer fit the data from 2008. This period difference would explain the movement of the bright peak.

Unfortunately, due to constraints on our observing on 2008, we do not yet know which of these two explanations best fits the data.

X-Ray Data