Baade's Window Research Articles

The Color‐Magnitude Diagram in Baade's Window Revisited

We have reanalyzed the OGLE V and I photometry of ~500,000 stars in Baade's Window, and we confirm the extinction map published by Stanek. However, we find that the interpretation of the OGLE color-magnitude diagram for the disk stars proposed by Paczynski et al. was incorrect: the dominant disk population in Baade's Window is old, and we find no evidence for a large hole in the inner Galactic disk. We find evidence for a small systematic error in the OGLE photometry for stars below the clump: the (V-I) color indices of the faint stars with V > 18 are too red by ~0.1 mag. We find tentative evidence from the OGLE and HST photometry that the bulge main-sequence turnoff point is brighter by ~0.5 mag than it is in either 47 Tuc or NGC 6791, indicating that the dominant population of the Galactic bulge is considerably younger than those clusters.

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 Zero Point of Extinction toward Baade's Window from K Giants

We measure the zero point of the Stanek extinction map by comparing the observed (V - K) colors of 206 K giant stars with their intrinsic (V - K)0 colors as derived from their Hβ indices. We find that the zero point of the Stanek map should be changed by ΔAV = -0.10 ± 0.06 mag, obtaining as a bonus a threefold reduction of the previous statistical error. The most direct way to test for systematic errors in this determination would be to conduct a parallel measurement based on the (V - K) colors of type ab RR Lyrae stars.

Eclipsing Binaries in the OGLE Variable Star Catalog.II.Light Curves of the W UMA-Type Systems in Baade's Window

Light curves of the contact systems visible in the direction of Baade's Window have been analyzed using the first coefficients of the Fourier representation. The results confirm that the geometric contact between components is usually weak. Systems showing significant differences in the depths of eclipses are very rare in the volume-limited sample to 3 kpc: only 2 among 98 contact systems show the difference larger than 0.065 mag; for most systems the difference is <0.04 mag. If this relative frequency of 1/50 is representative, then one among 12,500 - 15,000 Main Sequence F-K spectral-type stars is either a semi-detached or poor-thermal-contact system. Below the orbital period of 0.37 day, no systems with appreciable differences in the eclipse depths have been discovered. Since large depth differences are expected to be associated with the broken-contact phase of the Thermal Relaxation Oscillations, this phase must be very short for orbital periods above 0.37 day and possibly entirely absent for shorter periods. In the full sample, which is dominated by intrinsically bright, distant, long-period systems, larger eclipse-depth differences are more common with about 9% of binaries showing this effect. Sizes of these differences correlate with the sense of light-curve asymmetries (differing heights of maxima) for systems with orbital periods longer than 0.4 day suggesting an admixture of semi-detached systems with accretion hot spots on cooler components. The light-curve amplitudes in the full sample as well as in its volume-limited sub-sample are surprisingly small and strongly suggest a mass-ratio distribution steeply rising toward more dissimilar components. Many low mass-ratio systems remain to be discovered in the sky field.

Eclipsing Binaries in the OGLE Variable Star Catalog.I.W Uma-Type Systems as Distance and Population Tracers in Baade's Window

An algorithm, based on Fourier decomposition of light curves, permitted to define a sample of 388 contact binaries with well observed light curves, periods shorter one day and with available V-I colors (the R-sample), from among 933 eclipsing binary systems in the OGLE variable-star catalog for 9 fields of Baade's Window. The sample of such systems which was visually classified by the OGLE project as EW-type binaries (the O-sample) is by 55% larger and consists of 604 stars. The algorithm prevents inclusion of RR Lyr and SX Phe stars which in visual classification might be mistakenly taken for contact binaries with periods equal to twice their pulsation periods. Determinations of distances for the contact systems, utilizing the M_I = M_I(log P, V-I) absolute-magnitude calibration and the map of reddening and extinction of Stanek (1996), indicate an approximately uniform distribution of contact binaries almost all the way to the Galactic Bulge, implying heights up to z = 420 - 450 pc. This distribution, as well as a tendency for the colors to be concentrated in the region normally occupied by old Turn-Off-Point stars, confirm the currently held opinion that contact binary systems belong to the old stellar population of the Galaxy. The apparent frequency estimated in the volume-limited sense, relative to nearby MS dwarfs to the distances of 2 and 3 kpc, is one contact system per about 250 - 300 stars: it is one contact system per 400 MS stars if M-dwarfs are included. The apparent density of contact systems is 7-10x10^-5 pc^-3.

Stellar parameters in the bulge cluster NGC 6553

Globular clusters in the Galactic bulge form a flattened system, extending from the Galactic center to about 4.5 kpc from the Sun (Barbuy et al. 1997). A study of abundance ratios in these clusters is very important for a more complete understanding of the bulge formation. In this work we present a spectroscopic analysis of individual stars in NGC 6553. This cluster is a key one because it is located at d⊙ ≍ 5.1 kpc, therefore relatively close to us, and at the same time it is representative of the Galactic bulge stellar population: (a) Ortolani et al. (1995) showed that NGC 6553 and NGC 6528 show very similar Colour-Magnitude Diagrams (CMDs), and NGC 6528 is located at d⊙ ≍ 7.83 kpc, very close to the Galactic center; (b) the stellar populations of the Baade Window is also very similar to that of NGC 6553 and NGC 6528 as Ortolani et al. (1995) have shown by comparing their luminosity functions.

The microlensing optical depth of the COBEbulge

We examine the left-right asymmetry in the cleaned COBE/DIRBE near-infrared data of the inner Galaxy and show (i) that the Galactic bar is probably not seen very nearly end-on, and (ii) that even if it is, it is not highly elongated. The assumption of constant mass-to-light ratio is used to derive simulated terminal-velocity plots for the ISM from our model luminosity distributions. By comparing these plots with observed terminal velocities we determine the mass-to-light ratio of the near-IR bulge and disk. Assuming that all this mass contributes to gravitational microlensing we compute optical depths $\tau$ for microlensing in Galactic-centre fields. For three models with bar major axis between $10\deg-25\deg$ from the Sun-Galactic Center line, the resulting optical depths in Baade's window lie in the range $0.83\times10^{-6} \lta \tau \lta 0.89\times10^{-6}$ for main-sequence stars and $1.2\times10^{-6} \lta \tau \lta 1.3\times10^{-6}$ for red-clump giants. We discuss a number of uncertainties including possible variations of the near-infrared mass-to-light ratio. We conclude that, although the values predicted from analyzing the COBE and gas velocity data are inconsistent at the $2-2.5\sigma$ level with recent observational determinations of $\tau$, we believe they should be taken seriously.

Luminosity Function of the Perigalactocentric Region

We present H and K photometry of 42,000 stars in an area of 250 arcmin$^{2}$ centered on the Galactic center. We use the photometry to construct a dereddened K band luminosity function (LF) for this region, excluding the excessively crowded inner 2' of the Galaxy. This LF is intermediate between the LF of Baade's window and the LF of inner 2' of the Galactic center. We speculate that the bright stars in this region have an age which is intermediate between the starburst population in the Galactic center and the old bulge population. We present the coordinates and mags for 16 stars with K_{0} < 5 for spectroscopic follow up.

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.

JHKL Photometry and the K-Band Luminosity Function at the Galactic Center

view Abstract Citations (123) References (66) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS JHKL Photometry and the K-Band Luminosity Function at the Galactic Center Blum, R. D. ; Sellgren, K. ; Depoy, D. L. Abstract J, H, K, and L photometry for the star's in the central ∼2' (∼ 5 pc) of the Galaxy are presented. Using the observed J - H, H - K, and K - L colors and assumed intrinsic colors, we determine the interstellar extinction at 2.2 μm (AK) for approximately 1100 individual stars. The mean AK(=3.3 mag) is similar to previous results, but we find that the reddening is highly variable, and some stars are likely to be seen through AK > 6 mag. The dereddened K-band luminosity function points to a significantly brighter component to the stellar population (>1.5 mag at K) than found in the stellar population in Baade's window, confirming previous work done at lower spatial resolution. The observed flux of all Galactic center stars with estimated K0 (dereddened magnitude) ≤7.0 mag is ∼25% of the total in the 2' x 2' field. Our observations confirm the recent finding that several bright M stars in the Galactic center are variable. Our photometry also establishes the near-infrared variability of the M1-2 supergiant, IRS 7. Publication: The Astrophysical Journal Pub Date: October 1996 DOI: 10.1086/177917 arXiv: arXiv:astro-ph/9604109 Bibcode: 1996ApJ...470..864B Keywords: GALAXY: CENTER; GALAXY: STELLAR CONTENT; INFRARED: STARS; ISM: DUST; EXTINCTION; STARS: LUMINOSITY FUNCTION; MASS FUNCTION; Astrophysics E-Print: Accepted by the ApJ. LaTex, 25 pages text. Uses aastex v4. A complete PostScript version including eight (embedded, eps) figures and four tables (separate planotables) is available ftp://degobah.colorado.edu/pub/rblum/RCS1/ full text sources arXiv | ADS | data products SIMBAD (163) CDS (1)

Proper-Motion Anisotropy, Rotation, and the Shape of the Galactic Bulge

view Abstract Citations (20) References (45) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Proper-Motion Anisotropy, Rotation, and the Shape of the Galactic Bulge Zhao, Hongsheng ; Rich, R. Michael ; Biello, Joseph Abstract We explain the apparent proper-motion anisotropy σl/σb = 1.15±0.06 at Baade's window (l, b) = (1°, -4°) observed by Spaenhauer, Jones, & Whitford by rotation broadening in the longitude direction. We find that the observed solid-body rotation rate of the bulge can account for the observed proper-motion anisotropy. We find also that the sample is not homogeneous in kinematics. The M giants and the late-type and/or metal-rich K giants in the sample have a bigger apparent anisotropy σl/σb and smaller intrinsic dispersion σb than the whole sample. Also, σr > σb for the metal-rich stars. We show that these anisotropies are consistent with a rapidly rotating and intrinsically anisotropical bar in the center. Publication: The Astrophysical Journal Pub Date: October 1996 DOI: 10.1086/177882 Bibcode: 1996ApJ...470..506Z Keywords: GALAXY: KINEMATICS AND DYNAMICS; GALAXY: STRUCTURE; STARS: LATE-TYPE full text sources ADS | data products SIMBAD (3)

THE COOL STELLAR POPULATIONS OF EARLY-TYPE GALAXIES AND THE GALACTIC BULGE

THE COOL STELLAR POPULATIONS OF EARLY-TYPE GALAXIES AND THE GALACTIC BULGE

K Giants in Baade's Window. II. The Abundance Distribution

This is the second in a series of papers in which we analyze spectra of over 400 K and M giants in Baade's Window, including most of the stars with proper motions measured by Spaenhauer et al. [AJ, 103, 297 (1992)]. In our first paper, we measured line--strength indices of Fe, Mg, CN and H$\beta$ and calibrated them on the system of Faber et al. [ApJS, 57, 711 (1985)]. Here, we use the $\langle{\rm Fe}\rangle$ index to derive an abundance distribution in [Fe/H] for 322 stars with effective temperatures between 3900 K and 5160 K. Our derived values of [Fe/H] agree well with those measured from high--resolution echelle spectra (e.g., McWilliam \& Rich [ApJS, 91, 749 (1994)]) for the small number of stars in common. We find a mean abundance $\langle{\rm [Fe/H]}\rangle = -0.11 \pm 0.04$ for our sample of Baade's Window K giants. More than half the sample lie in the range $-0.4 <$ \feh\ $<+0.3$. We estimate line--of--sight distances for individual stars in our sample and confirm that, in Baade's Window, most K giants with $V < 15.5$ are foreground disk stars, but the great majority (more than 80\%) with $V > 16$ belong to the bulge. We also compare the metallicities derived from the CN and Mg$_2$ indices to those from iron. Most of the metal--rich stars in our sample appear to be CN--weak, in contrast to the situation in metal--rich globular clusters and elliptical galaxies. The metal--poor half of our sample ([Fe/H] $< 0$) shows evidence for a mild Mg overenhancement ([Mg/Fe] $\sim +0.2$); but this is not seen in the more metal--rich stars ([Fe/H] $\geq$ 0). The K giants in Baade's Window therefore share some, but not all, of the characteristics of stars in elliptical galaxies as inferred from their integrated light.

FORMATION OF THE GALAXY: CLUES FROM GLOBULAR CLUSTER AGES AND ABUNDANCES

FORMATION OF THE GALAXY: CLUES FROM GLOBULAR CLUSTER AGES AND ABUNDANCES

Echelle Spectroscopy of a Metal-Rich K Giant in Baade's Window Using the Keck High-Resolution Spectrograph

Echelle Spectroscopy of a Metal-Rich K Giant in Baade's Window Using the Keck High-Resolution Spectrograph

Extinction Map of Baade’s Window

Recently Wo\'zniak \& Stanek (1996) proposed a new method to investigate interstellar extinction, based on two band photometry, which uses red clump stars as a means to construct the reddening curve. I apply this method to the color-magnitude diagrams obtained by the Optical Gravitational Lensing Experiment (OGLE) to construct an extinction map of $(40')^2 $ region of Baade's Window, with resolution of $\sim30\;arcsec$. Such a map should be useful for studies of this frequently observed region of the Galactic bulge. The map and software useful for its applications are available via {\tt anonymous ftp}. The total extinction $A_V$ varies from $1.26\;mag$ to $2.79\;mag$ within the $(40')^2 $ field of view centered on $(\alpha_{2000}, \delta_{2000}) =$ (18:03:20.9,--30:02:06), i.e. $(l,b)=(1.001,-3.885)$. The ratio $ A_V/E(V-I) = 2.49\pm0.02 $ is determined with this new method.

Metallicity Indices For Multipopulation Models.I.The Galactic Bulge

view Abstract Citations (33) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Metallicity Indices For Multipopulation Models.I.The Galactic Bulge Idiart, T. P. ; de Freitas Pacheco, J. A. ; Costa, R. D. D. Abstract We report new spectroscopic observations on integrated light from the galactic bulge in Baade's window in the range 4700-6000 A. Metallicity indices Hβ, Mg2, Mgb, Fe5270, Fe5335, and Na D were measured. Foreground disk stars may certainly affect the hydrogen index, but their contribution to the magnesium and iron indices is probably small. The velocity dispersion derived from the Fourier quotient method is 112t 14 km/s, in very good agreement with kinematical data from bulge K giants, giving further support to the interpretation that the integrated light is dominated by those stars. Multipopulation models synthesizing the observed iron and magnesium indices indicate a mean metallicity [Fe/H]=-0.02, a mean abundance ratio [Mg/Fe]=+0.45, and a mean age of 14.0±1.2 Gyr for the bulk of the bulge stars. The derived metallicity and abundance of "α elements" (here represented by magnesium) are consistent with existing low-resolution spectra and photometric data. Publication: The Astronomical Journal Pub Date: March 1996 DOI: 10.1086/117862 Bibcode: 1996AJ....111.1169I Keywords: GALAXY: CENTER; GALAXY: ABUNDANCES full text sources ADS | data products SIMBAD (3) Related Materials (1) Part 2: 1996AJ....112.2541I

Kinematics of Bulge Giants in F588

Radial velocities for 331 stars in the F588 bulge field at 1, b = (80, 70) are obtained. The mean Galactocentric velocity for 319 stars with R &lt; 12.3 is V = 54.4 + 4.7 km S 1, and the velocity dispersion is Cr = 84.4 + 3.3 km 5-1 These stars have metal abundances based on a set of spectral indices calibrated against a grid of 400 standard stars. Dividing the sample according to metallicity, the halo and bulge components are kinematically distinct. The 65 K giants with [Fe/H] &lt; -1 (mostly halo giants) have = -7.0 + 13.6 km 1 and Cr = 109.4 + 9.6 km s 1 The 31 "pure" halo giants (with [Fe/ H] &lt; -1.5) have V = -5.8 + 20.4 km 5-1 and Cr = 113.5 + 14.4 km 5-1 The mean Galactocentric velocity and velocity dispersion for 194 bulge K giants (stars with [Fe/H] &gt; - 1.0) are = 65.9 + 5.1 km s 1 and q = 71.9 + 3.6 km 1 The difference between halo and bulge kinematics is of very high statistical significance. The Galactic bulge and halo are distinct components, defined by consistency among [Fe/H], V , and Cr. Bulge giants are also found to have their kinematics correlated with their metallicities. After discussing other possible causes, including disk and halo contamination, it is argued that this effect is real. The kinematics of 26 M giants in our sample are compared with those of other evolved bulge tracers (Mira variables, OH/IR stars, SiO masers, etc.). These M giants observed in the field have kinematics consistent with their being bulge members. The dependence of velocity dispersion on distance from the Galactic center is determined by combining the data with existing data from Baade's window. While the line-of-sight velocity dispersion of the bulge giants decreases steeply with distance from the Galactic center, that of halo giants decreases very slowly if at - all. The mass in the inner 1.5 kpc is determined to be M_{1 5 }= (1.38 + 0.26) x 1010 M_{0 }and M_{1 5 }= (1.74 + 0.28) x 1010 M_{0 }for the cases of isotropic and anisotropic inner halo, respectively. On the basis of kinematics and metallicities, we conclude that the bulge is a distinct galactic component and not the inner extension of the halo, thin disk, or thick disk.

Field Stars and Clusters of the Galactic Bulge: Implications for Galaxy Formation

The results of a kinematic study of the Galactic bulge based on spectra of red giants in fields at projected distances of 1.4-1.8 kpc from the Galactic center are presented. There is a marked trend of kinematics with metallicity, in the sense that the more metal-poor population has higher velocity dispersion and lower rotation velocity than the metal-rich population. The K giants more metal--poor than [Fe/H] = -1 have halo-like kinematics, with no significant rotation and sigma = 120 km/s independent of Galactocentric distance. The velocity dispersion of the giants with [Fe/H] > -1 decreases with increasing Galactocentric distance, and this population is rotating with V = 0.9 km/s/degree. The present observations, together with the observed metallicity gradient imply bulge formation through dissipational collapse. In such a picture, low angular momentum gas lost from the formation of the halo was deposited in the bulge. The bulge would then be younger than the halo. Observations of the Galactic globular cluster system are consistent with this picture, if we associate the metal-rich globular clusters within 3 kpc of the Galactic center with the bulge rather than with the thick disk or halo. Data on the RR Lyr in Baade's window are also consistent with this picture if these stars are considered part of the inner halo rather than the bulge.

A steady-state dynamical model for the COBE-detected Galactic bar

A 3D steady state stellar dynamical model for the Galactic bar is constructed with 485 orbit building blocks using an extension of Schwarzschild technique. The weights of the orbits are assigned using non-negative least square method. The model fits the density profile of the COBE light distribution, the observed solid body stellar rotation curve, the fall-off of minor axis velocity dispersion and the velocity ellipsoid at Baade's window. We show that the model is stable. Maps and tables of observable velocity moments are made for easy comparisons with observation. The model can also be used to set up equilibrium initial conditions for N-body simulations to study stability. The technique used here can be applied to interpret high quality velocity data of external bulges/bars and galactic nuclei.