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Abstract We present a view of the stellar halo in the inner-central regions of the Milky Way (R≲ 10 kpc) mapped by RR Lyrae stars. The combined BRAVA-RR/APOGEE RR Lyrae catalog is used to obtain a sample of 281 RR Lyrae stars located in the bulge region of the Galaxy, but with orbits indicating they belong to the inner-central halo. The RR Lyrae stars in the halo are more metal-poor than the bulge RR Lyrae stars and have pulsation properties more consistent with an accreted population. We use the Milky Way-like zoom-in cosmological simulation Auriga to compare the properties of the RR Lyrae stars to those expected from the “Gaia-Enceladus-Sausage” (GES) merger. The integrals of motions and eccentricities of the RR Lyrae stars are consistent with a small fraction of 6–9% ± 2% of the inner-central halo RR Lyrae population having originated from GES. This fraction, lower than what is seen in the solar neighborhood, is consistent with trends seen in the Auriga simulation, where a GES-like merger would have a decreasing fraction of GES stars at small Galactocentric radii compared to other accreted populations. Very few of the Auriga inner Galaxy GES-18 particles have properties consistent with belonging to a bulge population with (z_max< 1.1 kpc), indicating that no (or very few) RR Lyrae stars with bulge orbits should have originated from GES. 

Abstract RR Lyrae stars (RRLs) are excellent tracers of stellar populations for old, metal-poor components in the the Milky Way and the Local Group. Their luminosities have a metallicity dependence, but determining spectroscopic [Fe/H] metallicities for RRLs, especially at distances outside the solar neighborhood, is challenging. Using 40 RRLs with metallicities derived from both Fe(ii) and Fe(i) abundances, we verify the calibration between the [Fe/H] of RRLs from the calcium triplet. Our calibration is applied to all RRLs with Gaia Radial Velocity Spectrometer (RVS) spectra in Gaia DR3 and to 80 stars in the inner Galaxy from the BRAVA-RR survey. The coadded Gaia RVS RRL spectra provide RRL metallicities with an uncertainty of 0.25 dex, which is a factor of two improvement over the Gaia photometric RRL metallicities. Within our Galactic bulge RRL sample, we find a dominant fraction with low energies without a prominent rotating component. Due to the large fraction of such stars, we interpret these stars as belonging to the in situ metal-poor Galactic bulge component, although we cannot rule out that a fraction of these belong to an ancient accretion event such as Kraken/Heracles. 

Abstract Patchick 99 is a candidate globular cluster located in the direction of the Galactic bulge, with a proper motion almost identical to the field and extreme field star contamination. A recent analysis suggests it is a low-luminosity globular cluster with a population of RR Lyrae stars. We present new spectra of stars in and around Patchick 99, targeting specifically the three RR Lyrae stars associated with the cluster as well as the other RR Lyrae stars in the field. A sample of 53 giant stars selected from proper motions and a position on the color–magnitude diagram are also observed. The three RR Lyrae stars associated with the cluster have similar radial velocities and distances, and two of the targeted giants also have radial velocities in this velocity regime and [Fe/H] metallicities that are slightly more metal-poor than the field. Therefore, if Patchick 99 is a bona fide globular cluster, it would have a radial velocity of −92 ± 10 km s⁻¹, a distance of 6.7 ± 0.4 kpc (as determined from the RR Lyrae stars), and an orbit that confines it to the inner bulge. 

Abstract The Milky Way Bulge extra-tidal star survey is a spectroscopic survey with the goal of identifying stripped globular cluster stars from inner Galaxy clusters. In this way, an indication of the fraction of metal-poor bulge stars that originated from globular clusters can be determined. We observed and analyzed stars in and around BH 261, an understudied globular cluster in the bulge. From seven giants within the tidal radius of the cluster, we measured an average heliocentric radial velocity of 〈RV〉 = −61 ± 2.6 km s⁻¹with a radial velocity dispersion of 〈σ〉 = 6.1 ± 1.9 km s⁻¹. The large velocity dispersion may have arisen from tidal heating in the cluster’s orbit about the Galactic center, or because BH 261 has a high dynamical mass as well as a high mass-to-light ratio. From spectra of five giants, we measure an average metallicity of 〈[Fe/H]〉 = −1.1 ± 0.2 dex. We also spectroscopically confirm an RR Lyrae star in BH 261, which yields a distance to the cluster of 7.1 ± 0.4 kpc. Stars with 3D velocities and metallicities consistent with BH 261 reaching to ∼0.°5 from the cluster are identified. A handful of these stars are also consistent with the spatial distribution of potential debris from models focusing on the most recent disruption of the cluster. 

Context.Bulge globular clusters (BGCs) are exceptional tracers of the formation and chemodynamical evolution of this oldest Galactic component. Until now, observational difficulties have prevented us from taking full advantage of these powerful Galactic archeological tools. Aims.The bulge Cluster APOgee Survey (CAPOS) addresses this key topic by observing a large number of BGCs, most of which have been poorly studied until now. We aim to obtain accurate mean values for metallicity, [α/Fe], and radial velocity, as well as abundances for eleven other elements. Here, we present final parameters based on the APOGEE Stellar Parameter and Chemical Abundances Pipeline (ASPCAP) for all 18 CAPOS BGCs. Methods.We used atmospheric parameters, abundances, and velocities from ASPCAP in DR17. Results.First, we carried out a stringent selection of cluster members, finding a total of 303 with a spectral signal-to-noise value of S/N>70 and an additional 125 with a lower S/N. We confirmed the result of prior ASPCAP multiple population studies, namely, that stars with high [N/Fe] abundances show higher [Fe/H] than their lower [N/Fe] counterparts. Furthermore, the Mg, Ca, and globalαabundances exhibit similar trends, while Si is well-behaved. The [Fe/H] value of these second-population stars was corrected to derive the mean metallicity. Mean metallicities were determined to a precision of 0.05 dex, [α/Fe] to 0.06 dex, and radial velocity to 3.4 km/s. No clusters displayed any strong evidence of internal metallicity variations, including M22. Abundances for eleven other elements using only first-population stars were calculated. Our values are shown to be in good general agreement with the literature. We developed a new chemodynamical GC classification scheme, synthesizing the results of several recent studies. We also compiled a set of up-to-date metallicities. The BGC metallicity distribution is bimodal, with peaks near [Fe/H] = −0.45, and −1.1, with the metal-poor peak displaying a strong dominance. The entire in situ sample, including disk and BGCs, displays the same bimodality, while ex situ GCs are unimodal, with a peak around −1.6. Surprisingly, we see only a small and statistically insignificant difference in the mean [Si/Fe] of in situ and ex situ GCs. The four GCs with the lowest [Si/Fe] values are all ex situ and relatively young, with three belonging to Sagittarius; no other correlations are evident. 

In this work, we derive systemic velocities for 8456 RR Lyrae stars. This is the largest dataset of these variables in the Galactic bulge to date. In combination withGaiaproper motions, we computed their orbits using an analytical gravitational potential similar to that of the Milky Way (MW) and identified interlopers from other MW structures, which amount to 22% of the total sample. Our analysis revealed that most interlopers are associated with the halo, and the remainder are linked to the Galactic disk. We confirm the previously reported lag in the rotation curve of bulge RR Lyrae stars, regardless of the removal of interlopers. The rotation patterns of metal-rich RR Lyrae stars are consistent with the pattern of nonvariable metal-rich giants, following the MW bar, while metal-poor stars rotate more slowly. The analysis of the orbital parameter space was used to distinguish bulge stars that in the bar reference frame have prograde orbits from those on retrograde orbits. We classified the prograde stars into orbital families and estimated the chaoticity (in the form of the frequency drift, log ΔΩ) of their orbits. RR Lyrae stars with banana-like orbits have a bimodal distance distribution, similar to the distance distribution seen in metal-rich red clump stars. The fraction of stars with banana-like orbits decreases linearly with metallicity, as does the fraction of stars on prograde orbits (in the bar reference frame). The retrograde-moving stars (in the bar reference frame) form a centrally concentrated nearly spherical distribution. From analyzing anN-body+SPH simulation, we found that some stellar particles in the central parts oscillate between retrograde and prograde orbits and that only a minority stays prograde over a long period of time. Based on the simulation, the ratio of prograde and retrograde stellar particles seems to stabilize within some gigayears after the bar formation. The nonchaoticity of retrograde orbits and their high numbers can explain some of the spatial and kinematical features of the MW bulge that have been often associated with a classical bulge. 

RR Lyrae stars toward the Galactic bulge are used to investigate whether this old stellar population traces the Galactic bar. Although the bar is known to dominate the mass in the inner Galaxy, there is no consensus on whether the RR Lyrae star population, which constitutes some of the most ancient stars in the bulge and thus traces the earliest epochs of star formation, contributes to the barred bulge. We create new reddening maps and derive new extinction laws from visual to near-infrared passbands using improved RR Lyrae period-absolute magnitude-metallicity relations, enabling distance estimates for individual bulge RR Lyrae variables. The extinction law is most uniform inR_IKsandR_JKsand the distances to individual RR Lyrae based on these colors are determined with an accuracy of 6 and 4%, respectively. Using only the near-infrared passbands for distance estimation, we infer the distance to the Galactic center equal tod^cen_JKs= 8217 ± 1(stat) ± 528(sys) pc after geometrical correction. We show that variations in the extinction law toward the Galactic bulge can mimic a barred spatial distribution in the bulge RR Lyrae star population in visual passbands. This arises from a gradient in extinction differences along Galactic longitudes and latitudes, which can create the perception of the Galactic bar, particularly when using visual passband-based distances. A barred angle in the RR Lyrae spatial distribution disappears when near-infrared passband-based distances are used, as well as when reddening law variations are incorporated in visual passband-based distances. The prominence of the bar, traced by RR Lyrae stars, depends on their metallicity, with metal-poor RR Lyrae stars ([Fe/H] < −1.0 dex) showing little to no tilt with respect to the bar. Metal-rich ([Fe/H] > −1.0 dex) RR Lyrae stars do show a barred bulge signature in spatial properties derived using near-infrared distances, with an angle ofι= 18 ± 5 deg, consistent with previous bar measurements from the literature. This also hints at a younger age for this RR Lyrae subgroup. The 5D kinematic analysis, primarily based on transverse velocities, indicates a rotational lag in RR Lyrae stars compared to red clump giants. Despite variations in the extinction law, our kinematic conclusions are robust across different distance estimation methods. 

We present a new set of tools to derive systemic velocities for single-mode RR Lyrae stars from visual and near-infrared spectra. We derived scaling relations and line-of-sight velocity templates using both APOGEE andGaiaspectroscopic products combined with photometricG-band amplitudes. We provide a means to estimate systemic velocities for the RR Lyrae subclasses, RRab and RRc. Our analysis indicates that the scaling relation between the photometric and line-of-sight velocity amplitudes is nonlinear, with a break in a linear relation occurring around ≈0.4 mag in both theV-band andG-band amplitudes. We did not observe such a break in the relation for the first-overtone pulsators. Using stellar pulsation models, we further confirm and examine the nonlinearity in scaling relation for the RRab subclass. We observed little to no variation with stellar parameters (mass, metallicity, and luminosity) in the scaling relation between the photometric and line-of-sight velocity amplitudes for fundamental-mode pulsators. We observed an offset in the scaling relation between the observations and stellar pulsation models, mainly in the low-amplitude RR Lyrae regime. This offset disappears when different sets of convective parameters are used. Thus, the Fourier amplitudes obtained from the photometry and line-of-sight velocity measurements can be utilized to constrain convective parameters of stellar pulsation models. The scaling relations and templates for APOGEE andGaiadata accurately predict systemic velocities compared to literature values. In addition, our tools derived from theGaiaspectra improve the precision of the derived systemic velocities by approximately 50 percent and provide a better description of the uncertainty distribution in comparison with previous studies. Our newly derived tools will be used for RR Lyrae variables observed toward the Galactic bulge. 

We present a new set of period–absolute magnitude–metallicity (PMZ) relations for single-mode RR Lyrae stars calibrated for the opticalG_BP,V,G,G_RP, near-infraredI,J,H, andK_spassbands. We compiled a large dataset (over 100 objects) of fundamental and first-overtone RR Lyrae pulsators consisting of mean intensity magnitudes, reddenings, pulsation properties, iron abundances, and parallaxes measured by theGaiaastrometric satellite in its third data release. Our newly calibrated PMZ relations encapsulate the most up-to-date ingredients in terms of both data and methodology. They are intended to be used in conjunction with large photometric surveys targeting the Galactic bulge, including the Optical Gravitational Lensing Experiment (OGLE), the Vista Variables in the Vía Láctea Survey (VVV), and theGaiacatalog. In addition, our Bayesian probabilistic approach provides accurate uncertainty estimates of the predicted absolute magnitudes of individual RR Lyrae stars. Our derived PMZ relations provide consistent results when compared to benchmark distances to globular clusters NGC 6121 (also known as M 4), NGC 5139 (also known as omega Cen), and Large and Small Magellanic Clouds, which are stellar systems rich in RR Lyrae stars. Lastly, ourK_s-band PMZ relations match well with the previously published PMZ relations based onGaiadata and accurately predict the distance toward the prototype of this class of variables, the eponymic RR Lyr itself. 

Context.The inner Galaxy is a complex environment, and the relative contributions of different formation scenarios to its observed morphology and stellar properties are still debated. The different components are expected to have different spatial, kinematic, and metallicity distributions, and a combination of photometric, spectroscopic, and astrometric large-scale surveys is needed to study the formation and evolution of the Galactic bulge. Aims.The Blanco DECam Bulge Survey (BDBS) provides near-ultraviolet to near-infrared photometry for approximately 250 million unique stars over more than 200 square degrees of the southern Galactic bulge. By combining BDBS photometry with the latestGaiaastrometry, we aim to characterize the chemodynamics of red clump stars across the BDBS footprint using an unprecedented sample size and sky coverage. Methods.Our field of view of interest is |ℓ| ≤ 10°, −10° ≤b ≤ −3°. We constructed a sample of approximately 2.3 million red clump giants in the bulge with photometric metallicities, BDBS photometric distances, and proper motions. Photometric metallicities are derived from a (u − i)₀versus [Fe/H] relation; astrometry, including precise proper motions, is from the third data release (DR3) of the ESA satelliteGaia. We studied the kinematics of the red clump stars as a function of sky position and metallicity by investigating proper-motion rotation curves, velocity dispersions, and proper-motion correlations across the southern Galactic bulge. Results.By binning our sample into eight metallicity bins in the range of −1.5 dex < [Fe/H] < +1 dex, we find that metal-poor red clump stars exhibit lower rotation amplitudes, at ∼29 km s⁻¹kpc⁻¹. The peak of the angular velocity is ∼39 km s⁻¹kpc⁻¹for [Fe/H] ∼ −0.2 dex, exhibiting declining rotation at higher [Fe/H]. The velocity dispersion is higher for metal-poor stars, while metal-rich stars show a steeper gradient with Galactic latitude, with a maximum dispersion at low latitudes along the bulge minor axis. Only metal-rich stars ([Fe/H] ≳ −0.5 dex) show clear signatures of the bar in their kinematics, while the metal-poor population exhibits isotropic motions with an axisymmetric pattern around Galactic longitudeℓ = 0. Conclusions.This work describes the largest sample of bulge stars with distance, metallicity, and astrometry reported to date, and shows clear kinematic differences with metallicity. The global kinematics over the bulge agrees with earlier studies. However, we see striking changes with increasing metallicity, and, for the first time, kinematic differences for stars with [Fe/H]>  − 0.5, suggesting that the bar itself may have kinematics that depends on metallicity.