Baade's Window Field Research Articles

Insights on the Milky Way bulge formation from the correlations between kinematics and metallicity

Two main scenarios for the formation of the Galactic bulge are invoked, the first one through gravitational collapse or hierarchical merging of subclumps, the second through secular evolution of the Galactic disc. We aim to constrain the formation of the Galactic bulge through studies of the correlation between kinematics and metallicities in Baade's Window (l=1, b=-4) and two other fields along the bulge minor axis (l=0, b=-6 and b=-12). We combine the radial velocity and the [Fe/H] measurements obtained with FLAMES/GIRAFFE at the VLT with a spectral resolution of R=20000, plus for the Baade's Window field the OGLE-II proper motions, and compare these with published N-body simulations of the Galactic bulge. We confirm the presence of two distinct populations in Baade's Window found in Hill et al. 2010: the metal-rich population presents bar-like kinematics while the metal-poor population shows kinematics corresponding to an old spheroid or a thick disc one. In this context the metallicity gradient along the bulge minor axis observed by Zoccali et al. (2008), visible also in the kinematics, can be related to a varying mix of these two populations as one moves away from the Galactic plane, alleviating the apparent contradiction between the kinematic evidence of a bar and the existence of a metallicity gradient. We show evidences that the two main scenarios for the bulge formation co-exist within the Milky Way bulge.

FABRY-PÉROT ABSORPTION LINE SPECTROSCOPY OF THE GALACTIC BAR. II. STELLAR METALLICITIES

We measure the Ca II 8542 line strength in 3360 stars along three lines-of-sight in the Galactic bar: (l,b) ~ (+/-5.0,-3.5) and Baade's Window, using Fabry-Perot (FP) absorption line spectroscopy. This is the first attempt to show that reliable absorption line strengths can be measured using FP spectroscopy. The Ca II 8542 line is a good indicator of metallicity and its calibration to [Fe/H] is determined for globular cluster red giants in previous investigations. We derive such a calibration for the bulge giants and use it to infer metallicities for our full red clump sample (2488 stars) at all three lines-of-sight. We present the stellar metallicity distributions along the major axis of the bar. We find the mean [Fe/H] = -0.09 +/- 0.04 dex in Baade's Window, and find the distribution in this field to agree well with previous works. We find gradients in the mean metallicity and its dispersion w.r.t Baade's WIndow of -0.45 and -0.20 dex respectively at l = +5.5, and of -0.10 dex and -0.20 dex at l = -5.0. We detect a signature of a possible tidal stream at l = +5.5, in both our velocity and metallicity distributions. Its radial velocity indicates that it is not associated with the Sagittarius stream. We also measure the metallicity of a bulge globular cluster NGC 6522 in our Baade's Window field to be -0.90 +/- 0.10 dex, in agreement with recent measurements of Zoccali et al. (2008). This agreement demonstrates the reliability of our metallicity measurements.

The shortest period M dwarf eclipsing system BW3 V38

The spectroscopic data for the short-period () eclipsing binary V38, discovered by the OGLE micro-lensing team in Baade's Window field BW3, are analyzed. Radial velocity curves are derived from mid-resolution spectra obtained with EMMI-NTT at ESO – La Silla, and a simultaneous solution of the existing light curve by OGLE and of the new radial velocity curves is obtained. The system is formed by almost twin M3e dwarf components that are very close, but not yet in contact. The spectra of both dwarfs show signatures of the presence of strong chromospheres. Spectroscopy definitely confirms, therefore, what was suggested on the basis of photometry: BW3 V38 is indeed a unique system, as no other similar binary with M components and in such a tight orbit is known. Within the limits posed by the relatively large errors, due to the combined effect of system faintness and of the constraints on exposure time, the derived physical parameters seem to agree with the relations obtained from the other few known eclipsing binaries with late type components (which indicate a discrepancy between the available evolutionary models and the data at ~10% level). A possible explanation is the presence of strong magnetic fields and fast rotation (that applies to the BW3 V38 case as well). A simple computation of the system secular evolution by angular momentum loss and spin-orbit synchronization shows that the evolution of a system with M dwarf components is rather slow, and indicates as well a possible reason why systems similar to BW3 V38 are so rare.

K‐Band Calibration of the Red Clump Luminosity

The average near-infrared (K-band) luminosity of 238 Hipparcos red clump giants is derived and then used to measure the distance to the Galactic center. These Hipparcos red clump giants have been previously employed as I-band standard candles. The advantage of the K-band is a decreased sensitivity to reddening and perhaps a reduced systematic dependence on metallicity. In order to investigate the latter, and also to refer our calibration to a known metallicity zero point, we restrict our sample of red clump calibrators to those with abundances derived from high-resolution spectroscopic data. The mean metallicity of the sample is [Fe/H] = -0.18 dex (σ = 0.17 dex). The data are consistent with no correlation between MK and [Fe/H] and only weakly constrain the slope of this relation. The luminosity function of the sample peaks at MK = -1.61 ± 0.03 mag. Next, we assemble published optical and near-infrared photometry for ~20 red clump giants in a Baade's window field with a mean metallicity of [Fe/H] = -0.17 ± 0.09 dex, which is nearly identical to that of the Hipparcos red clump. Assuming that the average (V-I)0 and (V-K)0 colors of these two red clumps are the same, the extinctions in the Baade's window field are found to be AV = 1.56, AI = 0.87, and AK = 0.15, in agreement with previous estimates. We derive the distance to the Galactic center: (m - M)0 = 14.58 ± 0.11 mag, or R = 8.24 ± 0.42 kpc. The uncertainty in this distance measurement is dominated by the small number of Baade's window red clump giants examined here.

Luminous Long-Period Variables in Globular Clusters and the Galactic Bulge: Their Dependence on Metallicity

We derive the frequency of occurrence of luminous long-period variables (LLPVs) in globular clusters and in the Baade's window field of the Galactic bulge. LLPVs occur only in clusters with [Fe/H] ≳ -1.0. In these clusters their frequency of occurrence relative to the number of giant stars appears to be independent of metallicity. Integrated over all metallicities, Baade's window appears to be deficient in LLPVs. We estimate [Fe/H] values for Baade's window LLPVs from their period and a log P versus [Fe/H] relation, derived from the cluster variables, and find that this deficiency is due primarily to the absence of LLPVs with [Fe/H] ≳ 0.0. We propose that this is due to enhanced mass-loss rates in these LLPVs with a consequently abbreviated lifetime compared with lower metallicity LLPVs. A typical lifetime for cluster LLPVs is about 3 × 105 yr. Finally, we emphasize the need for a much more complete survey for LLPVs in globular clusters.

The Proper Motion of NGC 6522 in Baade's Window

We have detected seven stars with a common proper motion that are located within 25 of the globular cluster NGC 6522 in the Baade's window field of the Galactic bulge. We argue that these stars are members of the cluster, and derive a weighted mean proper motion and heliocentric radial velocity of l = 1.4 ± 0.2 mas yr-1, b = -6.2 ± 0.2 mas yr-1 and = -28.5 ± 6.5 km s-1. We rederive the distance to NGC 6522 [(0.91 ± 0.04)R0, where R0 is the Galactocentric distance] and metallicity ([Fe/H] = -1.28 ± 0.12), making use of recent revisions in the foreground extinction toward the cluster (AV = 1.42 ± 0.05). We find the spatial velocity of the cluster and conclude that the cluster stays close to the Galactic center, and may have experienced significant bulge/disk shocking during its lifetime.

THE SHORTEST-PERIOD M-DWARF ECLIPSING SYSTEM BW3 V38

ABSTRACT The photometric data for a short-period (0.1984 day) eclipsing binary V38 discovered by the OGLE microlensing team in Baade's Window field BW3 have been analyzed. The light-curve synthesis solution of the I-filter light curve and de-reddened color (V-IC)0=2.3 suggest a pair of strongly-distorted M-dwarfs, with parameters between those of YY~Gem and CM~Dra, revolving on the tightest known orbit among binaries consisting of main sequence stars. The primary, more massive and hotter, component maybe filling its Roche lobe. The very small amount of angular momentum in the orbital motion makes the system particularly important for studies of angular momentum loss at the faint end of the main sequence. Spectroscopic observations of the orbital radial velocity variations as well as of activity indicators are urgently needed for a better understanding of the angular-momentum and internal-structure evolutionary state of the system.

The First Spectroscopic Observations of Caustic Crossing in a Binary Microlensing Event

We present the first spectroscopic observations of a binary microlensing event when it was undergoing a caustic crossing with a high magnification of A ≈ 25. The event 96-BLG-3 was identified in real time by the MACHO collaboration, in the Baade's window field toward the Galactic bulge. Three spectra were taken consecutively, spanning the light-curve peak of the caustic crossing, each integration lasting 30 minutes. The spectrograms covered the wavelength range 3985-6665 A and are almost identical, the third one differing only in having an amplitude ≈ 6% lower than the others. By comparison with reference star spectra and by using spectrum synthesis techniques, we infer that the source star is a G0 IV-V star, with an effective temperature of Teff ± 150 K, a metallicity in the range [M/H] = +0.3 to +0.6, and a logarithmic surface gravity of log g = 4.25 ± 0.25. Using theoretical evolutionary tracks, we derive a radius of ≈ 1.4−0.4+ 0.6R☉ and hence a distance of 6.9−1.7+ 3.1 kpc, consistent with the source residing in the Galactic bulge. We also determine its heliocentric radial velocity to be vr ± 3 km s-1. Caustic-crossing microlensing events such as 96-BLG-3, if they are observed with 8-10 m class telescopes, can resolve the stellar surface of distant sources with a resolution of 1010 cm or better. This permits a detailed study of the center-to-limb variation of the stellar surface and the intrinsic properties of the lensed source.

Evidence for an old Galactic bulge from RR Lyrae stars in Baade's window - Implications for the formation of the Galaxy and the age of the universe

view Abstract Citations (111) References (70) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evidence for an Old Galactic Bulge From RR Lyrae Stars in Baade's Window: Implications for the Formation of the Galaxy and the Age of the Universe Lee, Young-Wook Abstract Recent abundance measurements of RR Lyrae variables in the Baade's window field of the Galactic nuclear bulge are examined, and the observed metallicity distributions of the RR Lyraes in different Galactic radial zones are compared with those predicted for the HB population models. It is shown that the observed systematic variation with Galactocentric distance is only explained if the age of the stellar population increases, in the mean, with decreasing R(G). Thus, the oldest stellar population, the RR Lyraes in the Galactic nuclear bulge, is indeed older than that in the halo. Publication: The Astronomical Journal Pub Date: November 1992 DOI: 10.1086/116358 Bibcode: 1992AJ....104.1780L Keywords: Galactic Bulge; Galactic Evolution; Lyra Constellation; Metallicity; Galactic Halos; Milky Way Galaxy; Universe; Astrophysics; STARS: VARIABLES: RR LYRAE; GALAXY: FORMATION; EARLY UNIVERSE full text sources ADS | data products SIMBAD (7)

The metallicity of RR Lyrae stars in Baade's window

We derive metallicities via the ΔS method for 59 RR Lyrae variables in the Baade's window field of the Galactic nuclear bulge. Both RRab and RRc variables have = −1.0. The metallicity distribution for the whole sample is sharply peaked, with a dispersion in [Fe/H] of only 0.16 dex, far smaller than suggested by previous studies. The narrow metallicity distribution does not permit the various current estimates of the magnitude of a luminosity/metallicity relation for RR Lyrae stars to be distinguished.

Luminous M giants in the bulge of M31

Spectroscopy of luminous red stars in the central bulge of M31 is reported. A number of these are shown to be late-type M giants similar to those in the Baade's window field of the bulge of the Milky Way. Among the M31 stars, an exceptionally luminous M0 Ie red supergiant was serendipitously discovered which has brightened by more than 5 mag in the last 2 yr. At peak brightness, this star was the most luminous red supergiant in the local group, with M(bol) = - 10. 26 refs.