Using 7 yr of MACHO survey data, we present a new determination of the optical depth to microlensing toward the Galactic bulge. We select the sample of 62 microlensing events (60 unique) on clump ...giant sources and perform a detailed efficiency analysis. We use only the clump giant sources because these are bright bulge stars and are not as strongly affected by blending as other events. Using a subsample of 42 clump events concentrated in an area of 4.5 deg super(2) with 739,000 clump giant stars, we find t = 2.17 super(+) sub(-) super(0) sub(0) super(.) sub(.) super(4) sub(3) super(7) sub(8) x 10 super(-6) at (l,b) = (1.50, - 2.68), somewhat smaller than found in most previous MACHO studies but in excellent agreement with recent theoretical predictions. We also present the optical depth in each of the 19 fields in which we detected events and find limits on optical depth for fields with no events. The errors in optical depth in individual fields are dominated by Poisson noise. We measure optical depth gradients of (1.06 c 0.71) x 10 super(-6) deg super(-1) and (0.29 c 0.43) x 10 super(-6) deg super(-1) in the Galactic latitude b and longitude l directions, respectively. Finally, we discuss the possibility of anomalous duration distribution of events in the field 104 centered on (l,b) = (3.11, - 3.01), as well as investigate spatial clustering of events in all fields.
I explore the consequences of making the RR Lyrae and clump giant distance scales consistent in the solar neighbourhood, Galactic bulge and Large Magellanic Cloud (LMC). I employ two major ...assumptions: (i) that the absolute magnitude—metallicity, MV(RR) — Fe/H, relation for RR Lyrae stars is universal, and (ii) that absolute I magnitudes of clump giants, MI(RC), in Baade's Window are known (e.g. can be inferred from the local Hipparcos-based calibration or theoretical modelling). A comparison between the solar neighbourhood and Baade's Window sets MV(RR) at Fe/H=−1.6 in the range (0.59±0.05,0.70±0.05), somewhat brighter than the statistical parallax solution. More luminous RR Lyrae stars imply younger globular clusters, which would be in better agreement with the conclusions from the currently favoured stellar evolution and cosmological models. A comparison between Baade's Window and the LMC sets MLMCI(RC) in the range (−0.33±0.09, −0.53±0.09). The distance modulus to the LMC is μLMC∈(18.24±0.08,18.44±0.07). UnlikeMLMCI(RC), this range in μLMC does not depend on the adopted value of the dereddened LMC clump magnitude, ILMC0(RC). I argue that the currently available information is insufficient to select the correct distance scale with high confidence.
Calibration of the MACHO Photometry Database Alcock, C.; Allsman, R. A.; Alves, D. R. ...
Publications of the Astronomical Society of the Pacific,
12/1999, Letnik:
111, Številka:
766
Journal Article
Recenzirano
Odprti dostop
The MACHO Project is a microlensing survey that monitors the brightnesses of ∼60 million stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud, and Galactic bulge. Our database presently ...contains about 80 billion photometric measurements, a significant fraction of all astronomical photometry. We describe the calibration of MACHO two‐color photometry and transformation to the standard Kron‐CousinsVandRsystem. Calibrated MACHO photometry may be properly compared with all other observations on the Kron‐Cousins standard system, enhancing the astrophysical value of these data. For ∼9 million stars in the LMC bar, independent photometric measurements of ∼20,000 stars with
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mag in field‐overlap regions demonstrate an internal precision
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,
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackageOT2,OT1{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\sigma _{R}=0.019$ \end{document}
,
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackageOT2,OT1{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\sigma _{V-R}=0.028$ \end{document}
mag. The accuracy of the zero point in this calibration is estimated to be ±0.035 mag for stars with colors in the range −
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−
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mag. A comparison of calibrated MACHO photometry with published photometric sequences and newHubble Space Telescopeobservations shows agreement. The current calibration zero‐point uncertainty for the remainder of the MACHO photometry database is estimated to be ±0.10 mag inVorRand ±0.04 mag inV−R. We describe the first application of calibrated MACHO data: the construction of a color‐magnitude diagram used to calculate our experimental sensitivity for detecting microlensing in the LMC.
We present a 9 million star color-magnitude diagram (9M CMD) of the Large Magellanic Cloud (LMC) bar. The 9M CMD reveals a complex superposition of different-age and -metallicity stellar populations, ...with important stellar evolutionary phases occurring over 3 orders of magnitude in number density. First, we count the nonvariable red and blue supergiants and the associated Cepheid variables and measure the stellar effective temperatures defining the Cepheid instability strip. Lifetime predictions of stellar evolution theory are tested, with implications for the origin of low-luminosity Cepheids. The highly evolved asymptotic giant branch (AGB) stars in the 9M CMD have a bimodal distribution in brightness, which we interpret as discrete old populations ((greater-or-similar sign)1 Gyr). The faint AGB sequence may be metal-poor and very old. Comparing the mean properties of giant branch and horizontal-branch (HB) stars in the 9M CMD with those of clusters, we identify NGC 411 and M3 as templates for the admixture of old stellar populations in the bar. However, there are several indications that the old and metal-poor field population has a red HB morphology: the RR Lyrae variables lie preferentially on the red edge of the instability strip, the AGB bump is very red, and the ratio of AGB bump stars to RR Lyrae variables is quite large. If the HB second parameter is age, the old and metal-poor field population in the bar likely formed after the oldest LMC clusters. Lifetime predictions of stellar evolution theory lead us to associate a significant fraction of the {approx}1 million red HB clump giants in the 9M CMD with the same old and metal-poor population producing the RR Lyrae stars and the AGB bump. In this case, compared with the age-dependent luminosity predictions of stellar evolution theory, the red HB clump is too bright relative to the RR Lyrae stars and AGB bump. Last, we show that the surface density profile of RR Lyrae variables is fitted by an exponential, favoring a disklike rather than a spheroidal distribution. We conclude that the age of the LMC disk is probably similar to the age of the Galactic disk. (c) 2000 The American Astronomical Society.
I demonstrate that the two unexpected results in the local universe-(1) anomalous intrinsic (V-I 0 colors of the clump giants and RR Lyrae stars in the Galactic center, and (2) very short distances ...to the Magellanic Clouds (LMC, SMC) as inferred from clump giants-are connected with each other. The (V-I 0 anomaly is partially resolved by using the photometry from phase II of the Optical Gravitational Lensing Experiment (OGLE) rather than phase I. The need for V- or I-magnitude-based change in the bulge (V-I 0 is one option to explain the remaining color discrepancy. Such a change may originate in a coefficient of selective extinction AV E V-I smaller than typically assumed. Application of the (V-I 0 correction (independent of its source) doubles the slope of the absolute magnitude-metallicity relation for clump giants, so that MI RC =-0.23+0.19 Fe/H. Consequently, the estimates of the clump distances to the LMC and SMC are affected. Udalski's distance modulus of µLMC=18.18+/-0.06 increases to 18.27+/-0.07. The distance modulus to the SMC increases by 0.12 to µSMC=18.77+/-0.08. I argue that a more comprehensive assessment of the metallicity effect on MI RC is needed.
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