Determination of resonance locations in NGC 613 from morphological
  arguments
  In this paper, we present BVRI imaging data of NGC 613. We use these data to
determine the corotation radius of the bar, using the photometric phase
crossing method. This method uses the phase angle of the spiral structure in
several wavebands, and looks for a crossing between the blue (B) light and the
redder wavebands (e.g., R or I). For NGC 613, we find two phase crossings, an
outer phase crossing at 136 +/- 8 arcsec and an inner phase crossing at 16 +/-
8 arcsec. We argue that the outer phase crossing is due to the bar corotation
radius, and from the bar length of $R_{\rm bar}=90.0\pm4.0$ arcsec we go on to
calculate a relative bar pattern speed of R = 1.5 +/- 0.1, which is consistent
with the results of previous methods described in the literature. For a better
understanding of the inner phase crossing, we have created structure maps in
all four wavebands and a B-R color map. All of our structure maps and our color
map highlight a nuclear ring of star formation at a radius of ~4 arcsec, which
had also been observed recently using ALMA. Furthermore, the radius of our
inner phase crossing appears to be consistent with the size of a nuclear disk
of star formation that has been recently detected and described in the
literature. We therefore suggest that the phase crossing method can be used to
detect the size of nuclear star formation regions as well as the location of
corotation resonances in spiral galaxies.
