Photometric biases as a result of stellar blending: implications for measuring distances, constraining binarity and detecting exoplanetary transits

Abstract

We investigate blending, binarity and photometric biases in crowded‐field CCD imaging. For this, we consider random blend losses, which correspond to the total number of stars left undetected in unresolved blends. We present a simple formula to estimate blend losses, which can be converted to apparent magnitude biases using the luminosity function (LF) of the analysed sample. Because of the assumptions used, our results give lower limits of the total bias and we show that in some cases even these limits point towards significant limitations in measuring apparent brightnesses of standard candle stars and thus distances to nearby galaxies. A special application is presented for the OGLE‐II BVI maps of the Large Magellanic Cloud (LMC). We find a previously neglected systematic bias up to 0.2–0.3 min for faint stars (V∼ 18.0 –19.0 min) in the OGLE‐II sample, which affects LMC distance measurements using RR Lyrae and red clump stars. We also consider the effects of intrinsic stellar correlations, i.e. binarity, via calculating two‐point correlation functions for stellar fields around seven recently exploded classical novae. In two cases, for V1494 Aql and V705 Cas, the reported close optical companions seem to be physically correlated with the cataclysmic systems. Finally, we find significant blend frequencies up to 50–60 per cent in the samples of wide‐field exoplanetary surveys, which suggests that blending calculations are highly advisable to be included into the regular reduction procedure.

Survey for Transiting Extrasolar Planets in Stellar Systems. III. A Limit on the Fraction of Stars with Planets in the Open Cluster NGC 1245

“…This study uses a variant of the original pipeline developed at Ohio State University and currently in use by the Microlensing Follow Up Network (Yoo et al 2004). Due to low stellar crowding, we estimate that chance blends have a negligible impact on the photometry (Kiss & Bedding 2005). Finding charts in Figure 6 (discussed in x 5) demonstrate the stellar crowding conditions of the survey.…”

“…Modeling such details is not possible without knowing the binary nature for each object, but we can estimate the number of stars affected by assuming binary star statistics as measured in the field. Due to low stellar crowding, we estimate that chance blends have a negligible effect in comparison to physically associated binaries ( Kiss & Bedding 2005). Finding charts in Figure 6 demonstrate the stellar crowding conditions of the survey.…”

Application of the trend filtering algorithm to the MACHO database

“…This problem has been highlighted by the search for extrasolar planets by transit methods (Brown & Latham 2008) and problems concerning the measured PL relations in distant galaxies (Vilardell et al 2007). The problem is quite common in all photometric works (Kiss & Bedding 2005) and can be tackled at a certain degree by various data reduction methods (i.e., image subtraction and profile analysis, see Hoekstra et al 2005), but in difficult cases, the combined efforts by experts in large telescope photometry, spectroscopy, and stellar modeling are required (see, e.g., Mandushev et al 2005). Appendix A: Unbiased estimation of the standard deviation of the residuals of least squares fits to sparsely sampled target functions…”

Nainital Microlensing Survey – detection of short period Cepheids in the disk of M 31

“…Macri et al (2006) points out that fainter and low-amplitude Cepheids like those detected in our pixel survey are more affected by this blending problem. In general, different estimates put a blending of 0.1 to 0.3 mag in B and V bands in M 31 Cepheids (Mochejska et al 2000;Kiss & Bedding 2005;Vilardell et al 2007, and references therein). In the present case, it could be even larger due to choice of our filters (R and I bands) since large numbers of red stars are present in M 31.…”