Baade's Window Research Articles

Harmonizing the RR Lyrae and clump distance scales -- stretching the short distance scale to intermediate ranges?

I explore the consequences of making the RR Lyrae and clump giant distance scales consistent in the solar neighborhood, Galactic bulge and Large Magellanic Cloud (LMC). I employ two major assumptions: (1) that the absolute magnitude -metallicity, M{sub V}(RR) - [Fe/H], relation for RR Lyrae stars is universal, and (2) that absolute I-magnitudes of clump giants, M{sub I}(RC), in Baade's Window can be inferred from the local Hipparcos calibration of clump giants' magnitudes. A comparison between the solar neighborhood and Baade's Window sets M{sub V}(RR) at [Fe/H] = -1.6 in the range (0.59 {+-} 0.05, 0.70 {+-} 0.05), somewhat brighter than the statistical parallax solution. A comparison between Baade's Window and the LMC sets the M{sub I}{sup LMC}(RC) in the range (-0.33 {+-} 0.09, -0.53 {+-} 0.09). The distance modulus to the LMC is {mu}{sup LMC} {element_of} (18.24 {+-} 0.08, 18.44 {+-} 0.07). I argue that the currently available information slightly favors the short distance scale but is insufficient to select the correct solutions with high confidence.

A Near‐Infrared Photometric Study of the Low‐Latitude Globular Clusters Liller 1, Djorgovski 1, HP 1, and NGC 6528

Images recorded through J,H,Ks,2.2 μm continuum, and 2.3 μm CO filters are used to investigate the stellar contents of the low Galactic latitude globular clusters NGC 6528, Liller 1, Djorgovski 1, and HP 1, as well as surrounding bulge fields. Metallicities are estimated for the latter three clusters by comparing the colors and CO indexes of giant branch stars with those in other clusters and the bulge, while reddenings are estimated from the colors of bright bulge stars in the surrounding fields. In some cases the metallicities and reddenings are significantly different from previous estimates. The horizontal branch (HB) in NGC 6528 occurs at K = 13.2, and the K luminosity function of HB stars in this cluster is not significantly different from those in the moderately metal-rich clusters NGC 6304 and NGC 6316. R', the ratio of HB to bright red giant branch (RGB) and asymptotic giant branch (AGB) stars, is 1.25 ± 0.25, in good agreement with its value in other globular clusters. The RGB bump in NGC 6528 is detected at K = 13.8, and the relative brightness of this feature with respect to the HB is consistent with the cluster being very metal rich. The HB in Liller 1 occurs at K = 14.4. Stars in Liller 1 have smaller J-K colors and CO indexes than bulge giants with the same brightness, and a comparison between the Liller 1 and NGC 6528 sequences on the (K, J-K), and (K,CO) color-magnitude diagrams (CMDs) indicates that Liller 1 has a metallicity comparable to, and perhaps even slightly lower than, that of NGC 6528. Stars in Liller 1 and NGC 6528 occupy the same region of the (J-H, H-K) two-color diagram as bulge giants, which is consistent with these clusters forming as part of the bulge. Stars in Djorgovski 1 and HP 1 have significantly smaller CO indexes than stars with the same brightness in the surrounding bulge fields, and CMDs for these clusters are constructed by selecting objects according to CO index. After adjusting for differences in distance and reddening, the Djorgovski 1 giant branch on the (K, J-K) CMD is well matched by that of the [Fe/H] = -2.2 cluster M92, while the giant branch of HP 1 follows that of the [Fe/H] = -1.6 globular cluster M13. The metallicities inferred for both clusters are consistent with integrated J-K colors and CO indexes measured through moderately large (~35''-45'') apertures. Based on the brightness of the RGB tip, it is concluded that Djorgovski 1 has a distance modulus μ0 = 15.4, while the distance modulus of HP 1 is μ0 = 14.7. The brightest stars in the fields surrounding Liller 1, Djorgovski 1, and HP 1 follow the same relation between MK and CO index as M giants in Baade's window. Stars as faint as the HB have been detected in the field surrounding HP 1, and the relative numbers of HB and upper giant branch stars indicate that R' = 1.14 ± 0.11. This relatively low value reinforces earlier studies, which found that R', and hence the helium abundance, likely does not vary with radius in the innermost regions of the Galactic bulge.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] NICMOS Color Transformations and Photometric Calibrations

This paper presents color transformations for Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (NICMOS) camera 2 observations to the ground-based Carnegie image tube photometric system at Cerro Tololo Inter-American Observatory (CIT/CTIO), using observations of 19 moderately bright, red stars in Baade's window in the color range 0.7 < (J-K) < 1.6. We emphasize that these transformations will be accurate only for cool giants of near-solar metallicity. We simulate both NICMOS and ground-based photometry by convolving near-IR spectra taken above the atmosphere with different filter transmission profiles. The results are consistent with our transformation.

Eclipsing Binaries in the OGLE Variable Star Catalog. IV. The Precontact, Equal-Mass Systems

We used the database of eclipsing binaries detected by the OGLE microlensing project in the pencil-beam search volume toward Baade's Window to define a sample of 74 detached, equal-mass, main-sequence binary stars with short orbital periods in the range 0.19 < P < 8 days. The logarithmic slope of the period distribution log N ∝ (-0.8 ± 0.2) was used to infer the angular-momentum–loss (AML) efficiency for the late, rapidly rotating members of close binaries. It is very likely that the main cause of the negative slope is a discovery selection bias that progressively increases with the orbital period length. Assuming a power-law dependence for the correction for the bias ∝ -C log P (with C ≥ 0) the AML braking-efficiency exponent α in dH/dt = P-α can take any value α = -1.1 (±0.2) + C. Very simple considerations of discovery biases suggest C 4/3, which would give an AML braking law very close to the saturated one, with no dependence on the period. However, except for plausibility arguments, we have no firm data to support this estimate of C, so that α remains poorly constrained. The results signal the utmost importance of the detection bias evaluation for variable star databases used in analyses similar to the one presented in this study.

ISOGAL survey of Baade's Windows in the mid-infrared

The ISOGAL mid-infrared survey of areas close to the Galactic Plane aims to determine their stellar content and its possible bearing on the history of the Galaxy. The NGC6522 and Sgr I Baade's Windows of low obscuration towards the inner parts of the Bulge represent ideal places in which to calibrate and understand the ISOGAL colour-magnitude diagrams. The survey observations were made with the ISOCAM instrument of the ISO satellite. The filter bands chosen were LW2(~7 microns) and LW3(~15 microns). The results presented here show that most of the detected objects are late M-type giants on the AGB, with a cut-off for those earlier than M3-M4. The most luminous members of these two Bulge fields at 7 microns are the Mira variables. However, it is evident that they represent the end of a sequence of increasing 15 micron dust emission which commences with M giants of earlier sub-type. In observations of late-type giants the ISOCAM 15 micron band is mainly sensitive to the cool silicate or aluminate dust shells which overwhelm the photospheric emission. However, in ordinary M-giant stars, the 7 micron band is not strongly affected by dust emission and may be influenced instead by absorption. The nu2 band of water at 6.25 microns and the SiO fundamental at 7.9 microns are likely contribitors to this effect. A group of late M stars has been found which vary little or not at all but have infrared colours typical of well-developed dust shells. Their luminosities are similar to those of 200-300 day Miras but they have slightly redder [7]-[15] colours which form an extension of the ordinary M giant sequence. The Mira dust shells show a mid-infrared [7]-[15] colour-period relation. ca 700 days.

Anomalous RR Lyrae (V−I)0Colors in Baade’s Window

We compare (V-I)0-(V-K)0 color-color and (V-I)0-log P period-color diagrams for Baade's window and local RRab Lyrae stars. We find that for a fixed log P, the Baade's window RR Lyrae stars are ~0.17 mag redder in (V-I)0 than the local RR Lyrae stars. We also show that there is no such effect observed in (V-K)0. We argue that an extinction misestimate toward Baade's window is not a plausible explanation of the discrepancy. Unlike Baade's window RR Lyrae stars, the local ones follow a blackbody color-color relation and are well approximated by theoretical models. We test two parameters, iron metallicity and surface gravity, and find that their effects are too small to explain the (V-I)0 discrepancy between the two groups of stars. We suggest that the anomalous (V-I)0 behavior of the Baade's window RR Lyrae stars may be caused by an enhancement in α-element abundance. We note that a similar color discrepancy for clump giant stars has been recently reported by Paczyn´ski, and we caution that RR Lyrae stars and clump giants, often used as standard candles, can be subject to the same type of systematics.

A dynamical model of the inner Galaxy

An extension of Schwarzschild's galaxy-building technique is presented that, for the first time, enables one to build Schwarzschild models with known distribution functions (DFs). The new extension makes it possible to combine a DF that depends only on classical integrals with orbits that respect non-classical integrals. With such a combination, Schwarzschild's orbits are used only to represent the difference between the true galaxy DF and an approximating classical DF. The new method is used to construct a dynamical model of the inner Galaxy. The model is based on an orbit library that contains 22168 regular orbits. The model aims to reproduce the three-dimensional mass density of Binney, Gerhard & Spergel (1997), which was obtained through deprojection of the COBE surface photometry, and to reproduce the observed kinematics in three windows - namely Baade's Window and two off-axis fields. The model fits essentially all the available data within the innermost 3 kpc. The axis ratio and the morphology of the projected density contours of the COBE bar are recovered to good accuracy within corotation. The kinematic quantities - the line-of-sight streaming velocity and velocity dispersion, as well as the proper motions when available - are recovered, not merely for the fitted fields, but also for three new fields. The dynamical model deviates most from the input density close to the Galactic plane just outside corotation, where the deprojection of the surface photometry is suspect. The dynamical model does not reproduce the kinematics at the most distant window, where disk contamination may be severe.

Microlensing in the Galactic Bulge: Effects of the Disk behind the Bulge

A large number of microlensing events have been observed in the direction of the Galactic bulge, with a measured optical depth in the range 2 - 3 x 10^{-6}. It has been shown that most of these events are due to bulge stars being lensed by other bulge stars or by foreground disk stars. Among the stars observed in the bulge fields, there should also be disk stars located behind the bulge; here, we consider their effect on the microlensing rates. The optical depth of background disk stars is much higher than that of typical bulge stars, reaching 10^{-5} at 6 Kpc behind the bulge. Thus, although background disk stars are a very small fraction of the stars in Baade's window, we find that ~ 5 to 10 % of the optical depth should be due to disk stars more than 3 kpc behind the bulge. This fraction is sensitive to the luminosity function of disk stars at large scale-height, to the magnitude cutoff of the survey, and to the amplification bias effect causing large numbers of ``blended'' events. We consider also the effect of a warp and flare in the disk at large distances behind the bulge; this could increase the optical depth from the background disk to ~ 20 % of the total. Events on background disk stars should on average be longer than other events and could be distinguished also by measuring the proper motion or distance of the stars that have been microlensed. The number of these events could be an interesting probe to the structure and stellar population of the far-side of the Galactic disk.

Contact Binaries of the Galactic Disk: Comparison of the Baade's Window and Open Cluster Samples

The paper attempts to integrate the available data for contact binaries of the disk population in a deep Galactic field and in old open clusters. The two basic data sets consist of 98 systems in the volume-limited 3 kpc subsample of contact binaries detected by the OGLE microlensing project toward Baade's window (BW3) and of 63 members of 11 old open clusters (CL). Supplementary data on the intrinsically bright, but spatially rare, long-period binaries are provided by 238 systems in the BW sample to the distance of 5 kpc (BW5). The basic BW3 sample and the CL sample are remarkably similar in the period, color, luminosity, and variability-amplitude distributions, in spite of very different selections, for BW3—as a volume-limited subsample of all contact systems discovered by the OGLE project, and for CL—as a collection of contact systems discovered in open clusters that had been subject to searches differing in limiting magnitudes, cluster area coverage, and photometric errors. The contact systems are found in the color interval 0.3 5.5, but the available data are consistent with a continuation of the high apparent frequency beyond MV = 7.5, i.e., past the current short-period, low-luminosity end, delineated by the shortest period field system CC Com at MV = 6.7. The current data indicate that the sky-field sample of contact binaries starts showing discovery-selection effects at a level as high as V 10–11.

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 Near-Infrared Photometric Properties of Bright Giants in the Central Regions of the Galactic Bulge

Images recorded through broad (J, H, K), and narrow (CO, and 2.2micron continuum) band filters are used to investigate the photometric properties of bright (K < 13.5) stars in a 6 x 6 arcmin field centered on the SgrA complex. The giant branch ridgelines in the (K, J-K) and (K, H-K) color-magnitude diagrams are well matched by the Baade's Window (BW) M giant sequence if the mean extinction is A_K ~ 2.8 mag. Extinction measurements for individual stars are estimated using the M_K versus infrared color relations defined by M giants in BW, and the majority of stars have A_K between 2.0 and 3.5 mag. The extinction is locally high in the SgrA complex, where A_K ~ 3.1 mag. Reddening-corrected CO indices, CO_o, are derived for over 1300 stars with J, H, and K brightnesses, and over 5300 stars with H and K brightnesses. The distribution of CO_o values for stars with K_o between 11.25 and 7.25 can be reproduced using the M_K versus CO_o relation defined by M giants in BW. The data thus suggest that the most metal-rich giants in the central regions of the bulge and in BW have similar photometric properties and 2.3micron CO strengths. Hence, it appears that the central region of the bulge does not contain a population of stars that are significantly more metal-rich than what is seen in BW.

The Luminosity Function and Initial Mass Function in the Galactic Bulge

We present deep photometry obtained with the Hubble Space Telescope in a field in Baade's window in the Galactic bulge. We derive a luminosity function down to I ~ 24.3, or V ~ 27.5, corresponding to M ~ 0.3 M_☉. The luminosity function from the turnoff down to this level appears remarkably similar to that observed in the solar neighborhood. We derive a mass function using both an empirical local mass-luminosity relation and a mass-luminosity relation from recent stellar model calculations, allowing for the presence of binaries and photometric errors. The mass function has a power-law form with dN/dM ∝ M^(-2.2) for M ≳ 0.7 M_☉. However, we find strong evidence for a break in the mass function slope around 0.5–0.7 M_☉, with a significantly shallower slope at lower masses. The value of the slope for the low masses depends on the assumed binary fraction and the accuracy of our completeness correction. This mass function should directly reflect the initial mass function.

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.

Eclipsing Binaries in the OGLE Variable Star Catalog. III. Long-Period Contact Systems

A sample of contact binaries discovered by the OGLE project in Baade's Window, with orbital periods longer than one day and with available color and light-curve data, has been analyzed. It consists of only 32 systems, in contrast to 388 WUMa-type systems with shorter periods which were analyzed before. Most systems are very distant and are probably located close to or in the galactic Bulge. Two groups of contact binaries are seen in the sample: (1) a continuation of the WUMa-type sequence, extending up to the orbital periods of 1.3 - 1.5 day, but rather sharply ending in this period range; (2) an inhomogeneous group of rare systems with long periods up to 26 days, all with red colors and relatively shallow eclipses. While the systems of the first group share most of the characteristics of the typical WUMa-type systems (except that they are on the average brighter and more distant, hence more reddened), the long-period systems do not seem to form an early-type extension of contact binaries, but may consist of a mixture of late-type objects, including tidally distorted red giants with invisible companions.

Galactocentric Distance with the OGLE and [ITAL]Hipparcos[/ITAL] Red Clump Stars

We compare red clump stars with parallaxes known to better than 10% in the Hipparcos catalog and corrected for the interstellar extinction, with the OGLE red clump stars in Baade's Window also corrected for the interstellar extinction. There are $\sim 600$ and $\sim 10,000$ such stars in the two data sets, respectively. We find empirically that the average I-band magnitude of red clump stars does not depend on their intrinsic color in the range $ 0.8 < (V-I)_0 < 1.4 $. The red clump luminosity function is well represented by a gaussian with the peak at $ M_{I_0,m} = -0.26 $, and the dispersion $ \sigma _{RC} \approx 0.2 $ mag. This allows a single step determination of the distance to the galactic center and gives $ R_0 = 8.4 \pm 0.4 $ kpc. The number of red clump stars is so large that a formal statistical error is only $\sim 1%$. The local stars are relatively blue and have a small color dispersion: $ = 1.01, \sigma_{(V-I)} = 0.08$, while for the bulge stars $ = 1.22, \sigma_{(V-I)_0} = 0.14$. Presumably, the bulge population has a broader range and a higher average metallicity than the local disk population.

The Zero Point of Extinction toward Baade's Window from RR Lyrae Stars

We measure the zero point of the Stanek extinction map by comparing the observed (V-K) colors of 20 RR Lyrae stars (type ab) found in the MACHO survey with their intrinsic (V-K)0 colors as a function of period as determined from nearby RR Lyrae stars. We find that the zero point of the Stanek map should be changed by ΔAV = -0.11 ± 0.05 mag, in excellent agreement with the recent measurement of Gould, Popowski, & Terndrup using K giants.

1.6. Are the Galactic bulge and bar the same?

An overview is given of the results obtained about Galactic Structure studies towards the Galactic Centre from star counts. The results indicate the presence of an old metal-rich population (Z = 0.005–0.08; t = 13–15 Gyr), presumably the old Galactic Bulge. In addition, a younger, less metal-rich population was found (Z = 0.005-0.03; t = 8–9 Gyr), likely related to the tri-axial bar found in other studies. In Baade's Window a ratio bulge/bar stars of approximately 1/2 was obtained from star counts.

1.12. Structure of the inner galactic disk and the bulge - first results

Is there an age and/or a metallicity gradient in the Bulge? This is a notoriously difficult question because of the well known age-metallicity-distance degeneracy in colour magnitude diagrams (CMD) as well as the severe crowding and large reddening towards the Galactic Bulge. The current observational data on the bulge in our galaxy and bulges in other spiral galaxies point in disparate directions, that is evidence for both early (e.g. existence of very old halo and bulge globular clusters) and late formation (e.g. Sgr dSph and bar instabilities) can be found as well as the existence and non-existence of metallicity gradients (for a review of the observational status see Wyse, Gilmore &amp; Franx 1997). We here present the CMDs for two fields, Baade's window and SGR-I, Fig.1c and d. Both these regions have low extinction. To determine the age and metallicity for these stars we compare the CMDs with CMDs of globular clusters, also observed with WFPC2, of known metallicity and age, Fig.1a and b. This method enables us to work entirely in the in-flight magnitude system of WFPC2 and there is no need for transformations to standard colours and magnitudes, something which is not straight forward for WFPC2 passbands.

The Stellar Content Near the Galactic Center

High angular resolution J,H,K, and L images are used to investigate the stellar content within 6 arcsec of SgrA*. The data, which are complete to K ∼ 16, are the deepest multicolor observations of this region published to date. The mean locus of the (K,H −K) CMD varies across the field, a result that is attributed to differential reddening with amplitude ∆AV ∼ 15 mag. The reddening variations within 3 arcsec of SgrA* are significantly smaller than this, and the resolved members of the compact star cluster immediately surrounding SgrA* have photometric properties that are not significantly different from objects at larger radii. We find that sources in our field with published 2μm spectra showing either line emission or CO absorption occupy different sequences on the (K, J − K) CMD. The emission line stars, which fall along the most richly populated sequence, have redder J−K colors than stars in the Magellanic Clouds with similar spectroscopic characteristics, and evidence is presented that this is due to excess infrared emission in the spectrum of the GC sources. The photometric properties of the giant branch, which is defined by stars showing CO absorption, are similar to those of the giant branch in Baade’s Window (BW). The mean J −K color and peak K brightness of the red giant branch are both consistent with a metal-rich population having an age ∼ 10 Gyr, while the width of the giant branch on the (K, J −K) CMD is indicative of an age spread ∆log(t) ≤ 1 dex. Therefore, if the inner bulge contains an underlying population of stars with ages in excess of ∼ 10 Gyr, as is the case in BW, then the region within a few arcsec of SgrA* cannot contain a large population of giant branch stars younger than 1 Gyr. We also report the detection of a modest population of faint, blue sources with K ≥ 14. We speculate that these are bright main sequence stars at the distance of the GC, although spectra will be required to confirm this interpretation. Nevertheless, the photometric properties of the largely unresolved compact knot of stars immediately surrounding SgrA* provide indirect evidence to support the presence of a large population of faint blue stars. In particular, we confirm previous studies that measure a relatively blue color for this cluster.

A Rigorous Reanalysis of theIRASVariable Population: Scale Lengths, Asymmetries, and Microlensing

Previous work reported a bar signature in color-selected IRAS variable stars. Here, we estimate the source density of these variables while consistently accounting for spatial incompleteness in the data using a likelihood approach. The existence of the bar is confirmed with a shoulder at a ≈ 3 kpc, an axis ratio of a:b = 2.2-2.7, and a position angle of 19° ± 1°. The ratio of nonaxisymmetric to axisymmetric components gives a similar estimate for the bar size of a = 3.3 ± 0.1 kpc and a position angle of 0 = 24° ± 2°. We estimate a scale length of 4.00 ± 0.55 kpc for the IRAS variable population, suggesting that these stars represent the old disk population. We use this density reconstruction to estimate the optical depth to microlensing for the large-scale bar in the Galactic disk. We find an enhancement over an equivalent axisymmetric disk by up to 40%, but this still too small to account for the MACHO result. In addition, we note a possibility for a significant asymmetry at positive and negative longitudes with optical depths comparable to those in Baade's window. Independent of our reconstruction, an infrared microlensing survey may be a sensitive tool for detecting or constraining structural asymmetries. More generally, this is a pilot study for Bayesian star count analyses. The Bayesian approach allows the assessment of prior probabilities to the unknown parameters of the model; the resulting likelihood function is straightforwardly modified to incorporate all available data.