期刊
ASTROPHYSICAL JOURNAL
卷 811, 期 2, 页码 -出版社
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/811/2/113
关键词
Galaxy: bulge; Galaxy: structure; stars: variables: RR Lyrae
资金
- National Science Centre, Poland [MAESTRO 2014/14/A/ST9/00121]
- Polish Ministry of Sciences and Higher Education [IP2012 005672, IdP2012 000162]
We have analyzed a sample of 27,258 fundamental-mode RR Lyrae variable stars (type RRab) detected recently toward the Galactic bulge by the Optical Gravitational Lensing Experiment (OGLE) survey. The data support our earlier claim that these metal-poor stars trace closely the barred structure formed of intermediate-age red clump giants. The distance to the Galactic center (GC) inferred from the bulge RR Lyrae stars is R-0 = 8.27 +/- 0.01(stat) +/- 0.40(sys) kpc. We show that their spatial distribution has the shape of a triaxial ellipsoid with a major axis located in the Galactic plane and inclined at an angle of i = 20 degrees +/- 3 degrees. to the Sun-GC line of sight. The obtained scale-length ratio of the major axis to the minor axis in the Galactic plane and to the axis vertical to the plane is 1:0.49(2):0.39(2). We do not see evidence for the bulge RR Lyrae stars forming an X-shaped structure. Based on the light curve parameters, we derive metallicities of the RRab variables and show that there is a very mild but statistically significant radial metallicity gradient. About 60% of the bulge RRab stars form two very close sequences on the period-amplitude (or Bailey) diagram, which we interpret as two major old bulge populations: A and B. Their metallicities likely differ. Population A is about four times less abundant than the slightly more metal-poor population B. Most of the remaining stars seem to represent other, even more metal-poor populations of the bulge. The presence of multiple old populations indicates that the Milky Way bulge was initially formed through mergers.
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