Dr. Robert Harmon (Department of Physics and Astronomy)

LI is a technique for mapping the surfaces of late-type stars from their rotational light curves [1]. Of particular interest are starspots, which are analogous to sunspots on the Sun and are known to be present on certain classes of stars. Like sunspots, starspots are manifestations of stellar magnetic fields. The study of starspots can thus provide valuable insights into the physics of the magnetic dynamos operating on the Sun and other stars [2]. Recently, LI was used to study differential rotation, a crucial ingredient of stellar dynamos, on the star II Pegasi [3], and on the Kepler target star KIC 5110407 [4]. It is also being used to study starspots on the primary of the RS CVn binary system sigma Geminorum in conjunction with interferometry [5].

A spotted star’s brightness varies as its rotation brings the dark starspots into and out of view of Earth. If we knew in detail the appearance of the star’s surface, a relatively straightforward calculation would allow us to predict the star’s brightness as a function of time, i.e., its light curve. With LI we attempt to go in the other direction: knowing the light curve, determine the appearance of the star’s surface. This is not a simple matter, because the problem is ill-posed, in that very different surfaces can give rise to nearly identical light curves. This arises in part because the effects of a large number of small bright and dark patches on the surface would nearly but not completely cancel, such that their presence would impart a low-amplitude, high-frequency “ripple” on the light curve as the star rotates. This ripple would look very similar to random noise, with the result that a straightforward attempt to find the surface which best replicates the observed light curve will produce a surface peppered with spurious bright and dark spots which are merely noise artifacts. LI circumvents this problem by constraining the solution so as to favor surfaces that are “smooth” and thus free of noise artifacts in a well-defined sense.

While there are many potential spotted stars that could serve as targets of hands-on investigation, in practice since the inception of the OWU REU Program the star LO Pegasi has played that role. LO Pegasi is well-known to exhibit large spots [6]; it is well-placed for observation during the summer months; its short rotation period of 10.15 hours means that complete coverage of rotational phase can be achieved without great difficulty; and it exhibits significant year-to-year variations in the shape and amplitude of its light curve, making it an interesting target for long-term monitoring.

Students obtain images of a star field surrounding the target star using a CCD camera and B, V, R and I photometric filters at OWU’s Perkins Observatory. Standard reductions (dark subtraction and flat fielding) are performed on the images in order to reduce random noise and systematic errors. Then differential aperture photometry is used to obtain the light curve of the target star as seen through each filter. Using multiple filters significantly improves the latitude resolution of the technique by taking advantage of the wavelength dependence of the limb darkening (center-to-edge dimming) of the stellar surface. The light curve data are then analyzed via LI to produce maps of the stellar surface.

 

References

[1] Harmon, R. and Crews, L. J., Astronomical Journal 120, 3274 (2000).

[2] Strassmeier, K. G., Astronomy and Astrophysics Reviews 17, 251 (2009).

[3] Roettenbacher, R., Harmon, R. O., Vutisalchavakul, N. and Henry, G. W., Astronomical Journal 141, 138 (2011).

[4] Roettenbacher, R., Monnier, J., Harmon, R., Barclay, T. and Still, Martin, Astrophysical Journal 767, 60 (2013).

[5] Roettenbacher, R. et al., arXiv:1504.06628, accepted for publication in Astrophysical Journal (2015).

[6] Lister, T. A., Collier Cameron, A., and Bartus, J., Monthly Notices of the Royal Astronomical Society 307, 685 (1999).