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 A_J AK_s = 3.005, A_H AK_s = 1.717, A_[3.6]/ AK_s = 0.478, A_[4.5]/ AK_s = 0.341 , A_[5.8] /AK_s = 0.234,_A [8.0]/AK_s = 0.321, A_W1/AK_s = 0.506 = , and A_W2/AK_s = 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 R₀ = 8.10 ± 0.19 ± 0.22 kpc is determined, featuring an uncertainty that is close to the limiting distance accuracy (2.8%) for Galactic Center Cepheids.