# Photometric modelling

## Photometric modeling

Thursday, 06 March 2008 19:17

The light curve shape due to rotational modulation is a classical tracer to study stellar surface inhomogeneities and its many related phenomena as pioneered many decades ago, by, e.g., Catalano & Rodono (1967) and Hall (1972). Full time- series light curves spanning continuously over many stellar rotations and even over many observing seasons were made possible with the advent of automated photoelectric telescopes (e.g. Henry et al. 1995, Strassmeier et al. 1997b). Small space telescopes like WIRE (Brunnt, Buzasi 2006), MOST (Matthews 2005) and COROT (Roxburgh & Baglin 2006) boosted the photometric precision by at least a factor of ten to below the 100 ppm level. However, such extremely high-precision time series require more and more sophisticated reduction and analysis tools (e.g. Tamuz et al. 2005Walker et al. 2007). Modern (model-independent) inversion techniques that linearize an ill-posed problem and uniquely convert a one-dimensional light curve into a spot map are now state-of-the art. However, applications with a pre-defined spot number and spot shape, e.g. with the assumption of two strictly circular spots, are not wrong per se but are likely bad approximations and suffer from the chance of being unrealistic. The modern approach is to let the data decide on the shape and the number of spots. If starspots were intrinsically circular, and the data would indeed preserve that information, then a model-independent inversion code would recover circular spots rather than anything else, as would a code that assumes circular spots. However, the alternate situation would be drastically incorrect. A fit to a light curve of a star with an intrinsically irregular spot distribution under the limitation of, e.g., circular spots could never - by definition - recover the true spot distribution. Erroneous spot locations, particularly in surface latitude, would be the result. Model-independent inversion techniques were originally formulated for spectroscopic Doppler imaging (for a summary see, e.g., Strassmeier et al. 2007) but were recently also employed for pure light-curve inversion (Lanza et al. 1998Rodonó et al. 2000Berdyugina et al. 2002Messina & Guinan 2003, Järvinen et al. 2005Jeffers 2005Ribárik et al. 2003Oláh et al. 2006).

In our recent paper, we adopted the new technique from our recently published Doppler-imaging inversion code based on quasi-optimal filtering of the object's principal components (Savanov & Strassmeier 2005). It is also applicable to multi-color light curves. We apply it for the first time to a decade-long time series of precise V and I-band photometry of the spotted star HD 291095 (V1355 Ori).

Last Updated on Wednesday, 19 March 2008 18:41