The distance to the Galactic centre based on Population II Cepheids and RR Lyrae stars

Context. The distance to the Galactic centre (GC) is important for determining the distance scale in the Universe. The value derived by Eisenhauer et al. (2005) of 7.62 +/- 0.32 kpc based on the orbit of one star around the central black hole is shorter than most other distance estimates based on a variety of different methods. Aims. To establish an independent distance to the GC with high accuracy, Population-II Cepheids were used that have been discovered in the ogle-II and ogle-III surveys. Methods. Thirty-nine Population-II Cepheids were monitored with the SOFI infrared camera on 4 nights spanning 14 days, typically obtaining between 5 and 11 epochs of data. Light curves were fitted using the known periods from the OGLE data to determine the mean K-band magnitude with an accuracy of 0.01-0.02 mag. It so happens that 37 RR Lyrae stars are in the field-of-view of the observations, and mean K-band magnitudes are derived for this sample as well. Results. After correction for reddening, the period-luminosity relation of Population-II Cepheids in the K-band is determined, and the derived slope of -2.24 +/- 0.14 is consistent with the value derived by Matsunaga et al. (2006, MNRAS, 370, 1979). Fixing the slope to their more accurate value results in a zero point and implies a distance modulus to the GC of 14.51 +/- 0.12, with an additional systematic uncertainty of 0.07 mag. Similarly, from the RR Lyrae K-band period-luminosity relation, we derive a value of 14.48 +/- 0.17(random) +/- 0.07 (syst.). The two independent determinations are averaged to find 14.50 +/- 0.10(random) +/- 0.07(syst.), or 7.94 +/- 0.37 +/- 0.26 kpc. The absolute magnitude scale of the adopted period-luminosity relations is tied to an LMC distance modulus of 18.50 +/- 0.07.