Volume 481, Number 2, April II 2008
|Page(s)||441 - 448|
|Section||Stellar structure and evolution|
|Published online||04 February 2008|
The distance to the Galactic centre based on Population II Cepheids and RR Lyrae stars*
Instituut voor Sterrenkunde, Celestijnenlaan 200 D, 3001 Leuven, Belgium e-mail: email@example.com
2 Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478, Warsaw, Poland
3 INAF Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
4 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany
Accepted: 11 January 2008
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.
Key words: stars: distances / stars: variables: Cepheids / stars: variables: RR Lyrae / Galaxy: bulge / Galaxy: center
© ESO, 2008
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