Deciphering the 3d Structure of the Old Galactic Bulge From the Ogle Rr Lyrae Stars

Abstract

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 an major axis located in the Galactic plane and inclined at an angle of i = 20 • ± 3 • 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 the 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.

Chemical abundances in a high-velocity RR Lyrae star near the bulge

“…Although it has a halo-like orbit, it may well be that the majority of bulge RR Lyrae stars reside in an inner halo (e.g., Minniti et al 1999;Kunder & Chaboyer 2008). This is also suggested by their spatial distribution (Dékány et al 2013; although see also Pietrukowicz et al 2015). A larger sample of spectroscopically well-studied bulge RR Lyrae stars and a larger sample of bulge RR Lyrae stars with space velocities is needed to address the uniqueness of this star.…”

“…In addition to this, the earlier study by Monaco et al (2007) showed that there is a southern and a northern stream that peak around Last, we consider that m176 is a normal RR Lyrae star located toward the direction of the bulge, which is on the tail of the RR Lyrae star velocity distribution. Its spectroscopic [Fe/H] is expected for a RR Lyrae stars located toward the bulge (e.g., Walker & Terndrup 1991;Pietrukowicz et al 2015), as is its distance from the Galactic center. Although it has a halo-like orbit, it may well be that the majority of bulge RR Lyrae stars reside in an inner halo (e.g., Minniti et al 1999;Kunder & Chaboyer 2008).…”

Common Origin of Two Rr Lyrae Populations and the Double Red Clump in the Milky Way Bulge

“…Jang & Lee (2015) and Joo & Lee (2013) for the details of our model construction. Recent metallicity measurements of bulge RR Lyrae stars show sharply peaked metallicity distribution with an average [Fe/H] in the range of -1.25 to -1.03, depending on the method and metallicity scale (Walker & Terndrup 1991;Kunder & Chaboyer 2008;Pietrukowicz et al 2015). In our modeling, we therefore adopt [Fe/H] = -1.1 for the RR ab stars in the bulge.…”

“…In the observed diagram, I band amplitude was used instead of log T eff , but since there is a good correlation between amplitude and temperature among RR Lyrae stars, our models can be compared with the observation. Four tick marks are added in panel (c) of Figure 3 to provide rough estimates of I-band amplitudes at different temperatures, which were calculated from the maximum points on the empirical relation between the I-band amplitude and log P of bulge RR ab stars ( Figure 15 of Pietrukowicz et al 2015), together with a small shift in amplitude (-0.11) to match the observed value at the fundamental blue edge.…”

“…These two sequences of RR ab stars are separated by ∆log P ≈ 0.015 (∆P ≈ 0.02 day) at low amplitudes, in the sense that "population B" has a longer period than "population A" at the given amplitude. Pietrukowicz et al (2015) have ascribed this to the metallicity difference between the two populations (∆[Fe/H] ≈ 0.17), but it is unlikely from the horizontal-branch (HB) models that this metallicity difference alone can produce the observed period-shift (Sweigart et al 1987;Lee et al 1990;Jang et al 2014, see Section 2 below). The difference in metallicity between the two populations is also not clear as it is estimated to be negligible (∆[Fe/H] < 0.02 dex) when the difference is obtained from the modal or median values instead of the average values (see Section 6 of Pietrukowicz et al 2015).…”