An extremely low mid-infrared extinction law toward the Galactic Center
  and 4% distance precision to 55 classical Cepheids
  Distances and extinction values are usually degenerate. To refine the
distance to the general Galactic Center region, a carefully determined
extinction law (taking into account the prevailing systematic errors) is
urgently needed. We collected data for 55 classical Cepheids projected toward
the Galactic Center region to derive the near- to mid-infrared extinction law
using three different approaches. The relative extinction values obtained are
AJ/AKs = 3.005, AH/AKs = 1.717, A[3.6]/AKs = 0.478, A[4.5]/AKs = 0.341,
A[5.8]/AKs = 0.234, A[8.0]/AKs = 0.321, AW1/AKs = 0.506, and AW2/AKs = 0.340.
We also calculated the corresponding systematic errors. Compared with previous
work, we report an extremely low and steep mid-infrared extinction law. Using a
seven-passband 'optimal distance' method, we improve the mean distance
precision to our sample of 55 Cepheids to 4%. Based on four confirmed Galactic
Center Cepheids, a solar Galactocentric distance of R_0 = 8.10\pm0.19\pm0.22
kpc is determined, featuring an uncertainty that is close to the limiting
distance accuracy (2.8%) for Galactic Center Cepheids.
