Light curve and color evolution of SN 1572 revisited - can we determine the sub-type?

The value of historical supernovae lies partly in the fact that they happened in our Galaxy (i.e. nearby, so that possible previous companions/donors should be detectable) and that their exact ages are known. Also, some of our knowledge on extragalactic transients (and their distances) is based on Galactic examples. For the two bright pre-telescopic type Ia supernovae in the Galaxy, SN 1572 and SN 1604, it may be feasible to constrain the sub-types from critically revised historical transmissions on light curve and color evolution. Here, we will consider SN 1572. Both the composition of its supernova remnant and a light-echo spectrum show that it was of type Ia (thermonuclear). Regarding the sub-type, neither a fast Ia like SN 1991bg nor a normal Ia like SN 1996X was consistent with the reconstructed light curve as obtained from historical text transmissions by various observers; the color evolution based on just a few observations seemed to exclude a 1991bg-like SN (Ruiz-Lapuente 2003); we will revisit these issues here. We have compiled, translated, and analyzed well-dated historical records on brightness and color from many more sources then before by Baade (1945) and others, namely 37 brightness and 28 color reports (with 18 new magnitudes and 21 new color records) plus a few upper and lower limits. We could then quantify all records homogeneously in terms of magnitude and color index. Here, we take into account the color perception of the naked eye, e.g. too much admixture of white at bright magnitudes and hypersaturation at faint levels. Next, after correction for interstellar extinction, we compare the light curve and color evolution of SN 1572 with recently observed typical supernovae of different subtypes. Finally, we try to determine the sub-type of SN 1572. (Afterwards, we will also study SN 1604 in a similar way.) The combined knowledge of sub-type, peak brightness, and exact age of these Galactic supernovae will then further enlighten our understanding of their physics.