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Search Results: 1 - 10 of 146477 matches for " Barry F. Madore "
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Sigma One
Barry F. Madore
Physics , 2010, DOI: 10.1088/0004-6256/139/5/2052
Abstract: We demonstrate that it is possible to calculate not only the mean of an underlying population but also its dispersion, given only a single observation and physically reasonable constraints (i.e., that the quantities under consideration are non-negative and bounded). We suggest that this counter-intuitive conclusion is in fact at the heart of most modeling of astronomical data.
Star Formation Timescales and the Schmidt Law
Barry F. Madore
Physics , 2010, DOI: 10.1088/2041-8205/716/2/L131
Abstract: We offer a simple parameterization of the rate of star formation in galaxies. In this new approach, we make explicit and decouple the timescales associated (a) with disruptive effects the star formation event itself, from (b) the timescales associated with the cloud assembly and collapse mechanisms leading up to star formation. The star formation law in near-by galaxies, as measured on sub-kiloparsec scales, has recently been shown by Bigiel et al. to be distinctly non-linear in its dependence on total gas density. Our parameterization of the spatially resolved Schmidt-Sanduleak relation naturally accommodates that dependence. The parameterized form of the relation is rho_* ~ epsilon x rho_g/(tau_s + rho_g ^{-n}), where rho_g is the gas density, epsilon is the efficiency of converting gas into stars, and rho_g^{-n} captures the physics of cloud collapse. Accordingly at high gas densities quiescent star formation is predicted to progress as rho_* ~ rho_g, while at low gas densities rho_* ~ rho_g^{1+n}, as is now generally observed. A variable efficiency in locally converting gas into stars as well as the unknown plane thickness variations from galaxy to galaxy, and radially within a given galaxy, can readily account for the empirical scatter in the observed (surface density rather than volume density) relations, and also plausibly account for the noted upturn in the relation at very high apparent projected column densities.
Atlas and Catalog of Collisional Ring Galaxies
Barry F. Madore,Erica Nelson,Kristen Petrillo
Physics , 2009, DOI: 10.1088/0067-0049/181/2/572
Abstract: We present a catalog and imaging atlas of classical (collisional) RING galaxies distilled from the Arp-Madore Atlas of Southern Peculiar Galaxies and Associations and supplemented with other known RING galaxies from the published literature. The catalog lists the original host object, compiles available redshifts and presents newly determined positions for the central (target) galaxy and its nearest companion(s). 127 collisional RING systems are illustrated and their components identified. All of the RINGS have plausible colliders identified; many are radial-velocity confirmed companions. Finally, we make note of the existence of a rare sub-class of RING galaxies exemplified by AM 2136-492, double/concentric RING galaxies. These objects are predicted by numerical simulations, but they appear to be quite rare and/or short-lived in nature.
Concerning the Slope of the Cepheid Period-Luminosity Relation
Barry F. Madore,Wendy L. Freedman
Physics , 2009, DOI: 10.1088/0004-637X/696/2/1498
Abstract: We discuss the impact of possible differences in the slope of the Cepheid Period-Luminosity relation on the determination of extragalactic distances in the context of recent studies that suggest changes in this slope. We show that the Wesenheit function W = V - R x ((V-I), widely used for the determination of Cepheid distances, is expected to be highly insensitive to changes in the slope of the underlying (monochromatic) Period-Luminosity (PL) relations. This occurs because the reddening trajectories in the color-magnitude plane are closely parallel to lines of constant period. As a result W-based Period-Luminosity relations have extremely low residual dispersion, which is because differential (and total line-of-sight) reddening is eliminated in the definition of W and the residual scatter due to a star's intrinsic color/position within the Cepheid is also largely insensitive to W. Basic equations are presented and graphically illustrated, showing the insensitivity of W to changes in the monochromatic PL relations.
The Hubble Constant
Wendy L. Freedman,Barry F. Madore
Physics , 2010, DOI: 10.1146/annurev-astro-082708-101829
Abstract: Considerable progress has been made in determining the Hubble constant over the past two decades. We discuss the cosmological context and importance of an accurate measurement of the Hubble constant, and focus on six high-precision distance-determination methods: Cepheids, tip of the red giant branch, maser galaxies, surface brightness fluctuations, the Tully-Fisher relation and Type Ia supernovae. We discuss in detail known systematic errors in the measurement of galaxy distances and how to minimize them. Our best current estimate of the Hubble constant is 73 +/-2 (random) +/-4 (systematic) km/s/Mpc. The importance of improved accuracy in the Hubble constant will increase over the next decade with new missions and experiments designed to increase the precision in other cosmological parameters. We outline the steps that will be required to deliver a value of the Hubble constant to 2% systematic uncertainty and discuss the constraints on other cosmological parameters that will then be possible with such accuracy.
A 3D Treatment of Radiative Transfer: Including Multi-wavelength Scattering; Absorption and Far Infrared Emission; and Arbitrary Geometry
Matthew Trewhella,Barry F. Madore,Leslie Kuchinski
Physics , 1998,
Abstract: We have developed a new model that uses a cellular approach to calculate radiative transfer of starlight through dusty media. The model is designed to be user friendly enough to be distributed as a tool for use by the general astronomical community. Its features include treatment of scattering, absorption and far infrared emission; propagating many wavelengths simultaneously; giving 26 views of the output from different direction; and the ability to cope with arbitrary geometry. As an example of the model's use, we have simulated the propagation of starlight through the edge-on galaxy NGC 891. As well as recovering the familiar edge-on view, we have a prediction for what the galaxy may look like if seen at other viewing angles.
Multiwavelength Characteristics of Period-Luminosity Relations
Barry F. Madore,Wendy L. Freedman
Physics , 2011, DOI: 10.1088/0004-637X/744/2/132
Abstract: We present a physically motivated explanation for the observed, monotonic increase in slope, and the simultaneous (and also monotonic) decrease in the width/scatter of the Leavitt Law (the Cepheid Period-Luminosity (PL) relation) as one systematically moves from the blue and visual into the near and mid-infared. We calibrate the wavelength-dependent, surface-brightness sensitivities to temperature using the observed slopes of PL relations from the optical through the mid-infrared, and test the calibration by comparing the theoretical predictions with direct observations of the wavelength dependence of the scatter in the Large Magellanic Cloud Cepheid PL relation. In doing so we find the slope of the Period-Radius (PR) relation is c = 0.724 +/- 0.006. Investigating the effect of differential reddening suggests that this value may be overestimated by as much as 10%; however the same slope of the PR relation fits the (very much unreddened) Cepheids in IC1613, albeit with lower precision. The discussion given is general, and also applies to RR Lyrae stars, which also show similarly increasing PL slopes and decreasing scatter with increasing wavelength.
Hipparcos Parallaxes and the Cepheid Distance Scale
Barry F. Madore,Wendy L. Freedman
Physics , 1997, DOI: 10.1086/305041
Abstract: Hipparcos parallaxes have recently become available for a sample of Galactic Cepheids, and we have used these new distances to calibrate the Cepheid period-luminosity (PL) relation at six wavelengths (BVIJHK). Comparing these calibrations with previously published multiwavelength PL relations we find agreement to within 0.07 +/- 0.14 mag, or 4 +/- 7% in distance. Unfortunately, the current parallax errors for the fundamental pulsators (ranging in signal-to-noise = pi /sigma_pi from 0.3 to 5.3, at best) preclude an unambiguous interpretation of the observed differences, which may arise from a combination of true distance modulus, reddening and/or metallicity effects. We explore these effects and discuss their implications for the distance to the Large Magellanic Cloud (LMC) and the Cepheid-based extragalactic distance scale. These results suggest a range of LMC moduli between 18.44 +/-0.35 and 18.57 +/-0.11 mag; however, other effects on the Cepheid PL relation (e.g., extinction, metallicity, statistical errors) are still as significant as any such reassessment of its zero point.
A Physically-Based Method for Scaling Cepheid Light Curves for Future Distance Determinations
Wendy L. Freedman,Barry F. Madore
Physics , 2010, DOI: 10.1088/0004-637X/719/1/335
Abstract: We present a technique for decomposing Cepheid light curves into their fundamental constituent parts; that is, their radius and temperature variations. We demonstrate that any given pair of optical luminosity and color curves can be used to predict the shape, amplitude and phase of a Cepheid's light variation at any other wavelength. With such predictions in hand, a single new observation at any given new wavelength can be used to normalize the properties of the predicted light curve, and in specific, derive a precise value of the time-averaged mean. We suggest that this method will be of great advantage in efficiently observing and precisely obtaining the mean properties of known Cepheids scheduled to be observed at new wavelengths, specifically in the mid-infrared where JWST will be operating.
Two New Tests of the Metallicity Sensitivity of the Cepheid Period-Luminosity Relation (The Leavitt Law)
Wendy L. Freedman,Barry F. Madore
Physics , 2011, DOI: 10.1088/0004-637X/734/1/46
Abstract: We undertake a new test of the metallicity sensitivity of the Leavitt Law for Classical Cepheids. We derive an empirical calibration of the apparent luminosities of Cepheids as measured from the optical through the mid-infrared (0.45-8.0um) as a function of spectroscopic [Fe/H] abundances of individual Cepheids in the Large Magellanic Cloud from Romaniello et al. (2008). The cumulative trend over the entire wavelength range shows a nearly monotonic behavior. The sense of the trend is consistent with differential line-blanketing in the optical, leading to stars of high metallicity being fainter in the optical. This is followed by a reversal in the trend at longer wavelengths, with the cross-over occurring near the K band at about 2.2um, consistent with a subsequent redistribution of energy resulting in a mild brightening of Cepheids (with increased metallicity) at mid-infrared wavelengths. This conclusion agrees with that of Romaniello et al. based on a differential comparison of the mean V- and K-band Leavitt Laws for the Galaxy, SMC and LMC, but is opposite in sign to most other empirical tests of the sensitivity of Cepheid distances to mean [O/H] HII region abundances. We also search for a correlation of Cepheid host-galaxy metallicity with deviations of the galaxy's Cepheid distance from that predicted from a pure Hubble flow. Based on Cepheid distances to 26 nearby galaxies in the local flow, only a very weak signal is detected giving Dmu_o = -0.17 (+/- 0.31) ([O/H] - 8.80) - 0.21 (+/-0.10). This is in agreement with previous determinations, but statistically inconclusive.
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