Has Blending Compromised Cepheid-Based Determinations of the Extragalactic Distance Scale?

We examine the suggestion that half of the HST Key Project- and Sandage/Saha-observed galaxies have had their distances systematically underestimated, by 0.1-0.3 mag in the distance modulus, due to the underappreciated influence of stellar profile blending on the WFC chips. The signature of such an effect would be a systematic trend in (i) the Type Ia supernovae corrected peak luminosity and (ii) the Tully-Fisher residuals, with increasing calibrator distance, and (iii) a differential offset between PC and WFC distance moduli, within the same galaxy. The absence of a trend would be expected if blending were negligible (as has been inherently assumed in the analyses of the aforementioned teams). We adopt a functional form for the predicted influence of blending that is consistent with the models of Mochejska et al. and Stanek & Udalski, and demonstrate that the expected correlation with distance predicted by these studies is not supported by the data. We conclude that the Cepheid-based extragalactic distance scale has not been severely compromised by the neglect of blending.Comment: 14 pages, 2 figures, 1 table, LaTeX, accepted for publication in Astrophysical Journal Letters, also available at http://casa.colorado.edu/~bgibson/publications.htm

Constraining the Variability and Binary Fraction of Galactic Center Young Stars

We present constraints on the variability and binarity of young stars in the central 10 arcseconds (~0.4 pc) of the Milky Way Galactic Center (GC) using Keck Adaptive Optics data over a 12 year baseline. Given our experiment's photometric uncertainties, at least 36% of our sample's known early-type stars are variable. We identified eclipsing binary systems by searching for periodic variability. In our sample of spectroscopically confirmed and likely early-type stars, we detected the two previously discovered GC eclipsing binary systems. We derived the likely binary fraction of main sequence, early-type stars at the GC via Monte Carlo simulations of eclipsing binary systems, and find that it is at least 32% with 90% confidence.Comment: Accepted for publication in Proceedings of IAU Symposium 322: The Multi-Messenger Astrophysics of the Galactic Centre, 2 pages, 1 figur

An Adaptive Optics Survey of Stellar Variability at the Galactic Center

We present a $\approx 11.5$ year adaptive optics (AO) study of stellar variability and search for eclipsing binaries in the central $\sim 0.4$ pc ($\sim 10''$) of the Milky Way nuclear star cluster. We measure the photometry of 563 stars using the Keck II NIRC2 imager ($K'$-band, $\lambda_0 = 2.124 \text{ } \mu \text{m}$). We achieve a photometric uncertainty floor of $\Delta m_{K'} \sim 0.03$ ($\approx 3\%$), comparable to the highest precision achieved in other AO studies. Approximately half of our sample ($50 \pm 2 \%$) shows variability. $52 \pm 5\%$ of known early-type young stars and $43 \pm 4 \%$ of known late-type giants are variable. These variability fractions are higher than those of other young, massive star populations or late-type giants in globular clusters, and can be largely explained by two factors. First, our experiment time baseline is sensitive to long-term intrinsic stellar variability. Second, the proper motion of stars behind spatial inhomogeneities in the foreground extinction screen can lead to variability. We recover the two known Galactic center eclipsing binary systems: IRS 16SW and S4-258 (E60). We constrain the Galactic center eclipsing binary fraction of known early-type stars to be at least $2.4 \pm 1.7\%$. We find no evidence of an eclipsing binary among the young S-stars nor among the young stellar disk members. These results are consistent with the local OB eclipsing binary fraction. We identify a new periodic variable, S2-36, with a 39.43 day period. Further observations are necessary to determine the nature of this source.Comment: 69 pages, 28 figures, 12 tables. Accepted for publication in The Astrophysical Journa

Redshifts for 2410 Galaxies in the Century Survey Region

The `Century Survey' strip covers 102 square degrees within the limits 8.5h \leq \alpha_{1950} \leq 16.5h, 29.0 degrees \leq \delta_{1950} \leq 30.0 degrees. The strip passes through the Corona Borealis supercluster and the outer region of the Coma cluster. Within the Century Survey region, we have measured 2410 redshifts which constitute four overlapping complete redshift surveys: (1) 1728 galaxies with Kron-Cousins R_{phot} \leq 16.13 covering the entire strip, (2) 507 galaxies with R_{phot} \leq 16.4 in the right ascension range 8h 32m \leq \alpha_{1950} \leq 10h 45m, (3) 1251 galaxies with absorption- and K-corrected R_{CCD, corr} \leq 16.2 covering the right ascension range 8.5h \leq \alpha_{1950} \leq 13.5h and (4) 1255 galaxies with absorption- and K-corrected V_{CCD, corr} \leq 16.7 also covering the right ascension range 8.5h \leq \alpha_{1950} \leq 13.5h. All of these redshift samples are more than 98 % complete to the specified magnitude limit.Comment: 18 pages, 9 figures, 3 tables, 2 abbreviated tables. In press, to appear in Astronomical Journal, Dec. 2001 issu

The Hubble flow around the CenA / M83 galaxy complex

We present HST/ACS images and color-magnitude diagrams for 24 nearby galaxies in and near the constellation of Centaurus with radial velocities V_LG < 550 km/s. Distances are determined based on the luminosities of stars at the tip of the red giant branch that range from 3.0 Mpc to 6.5 Mpc. The galaxies are concentrated in two spatially separated groups around Cen A (NGC 5128) and M 83 (NGC 5236). The Cen A group itself has a mean distance of 3.76 +/-0.05 Mpc, a velocity dispersion of 136 km/s, a mean harmonic radius of 192 kpc, and an estimated orbital/virial mass of (6.4 - 8.1) x 10^12 M_sun. This elliptical dominated group is found to have a relatively high mass-to-light ratio: M/L_B = 125 M_sun/L_sun. For the M 83 group we derived a mean distance of 4.79 +/-0.10 Mpc, a velocity dispersion of 61 km/s, a mean harmonic radius of 89 kpc, and estimated orbital/virial mass of (0.8 - 0.9) x 10^12 M_sun. This spiral dominated group is found to have a relatively low M/L_B = 34 M_sun/L_sun. The radius of the zero-velocity surface around Cen A lies at R_0 = 1.40 +/-0.11 Mpc, implying a total mass within R_0 of M_T = (6.0 +/-1.4) x 10^12 M_sun. This value is in good agreement with the Cen A virial/orbital mass estimates and provides confirmation of the relatively high M/L_B of this elliptical-dominated group. The centroids of both the groups, as well as surrounding field galaxies, have very small peculiar velocities, < 25 km/s, with respect to the local Hubble flow with H_0 = 68 km/s/Mpc.Comment: 31 pages including 9 figures and 3 tables. Accepted for publication in Astronomical Journal, 133, N0. 2 (February), 200

The Tip of the Red Giant Branch Distance to the Large Magellanic Cloud

We present the I-band luminosity function of the red giant branch stars in the Large Magellanic Cloud (LMC) using the data from the Magellanic Clouds Photometric Survey (Zaritsky, Harris & Thompson, 1997). Selecting stars in uncrowded, low-extinction regions, a discontinuity in the luminosity function is observed at I_0 = 14.54 mag. Identifying this feature with the tip of the red giant branch (TRGB), and adopting an absolute TRGB magnitude of -4.05 +- 0.04 mag based on the calibration of Lee, Freedman & Madore (1993), we obtain a distance modulus of 18.59 +- 0.09 (random) +- 0.16 (systematic) mag. If the theoretical TRGB calibration provided by Cassisi & Salaris (1997) is adopted instead, the derived distance would be 4% greater. The LMC distance modulus reported here, 18.59 +- 0.09, is larger by 0.09 mag (1-sigma) than the value that is most commonly used in the extragalactic distance scale calibrated by the period-luminosity relation of the Cepheid variable stars. Our TRGB distance modulus agrees with several RR Lyrae distances to the LMC based on HIPPARCOS parallaxes. Finally, we note that using the same MCPS data, we obtain a distance modulus of 18.29 +- 0.03 mag using the red clump method, which is shorter by 0.3 mag compared to the TRGB estimate.Comment: 19 pages, 5 figure

Mixed valency in cerium oxide crystallographic phases: Determination of valence of the different cerium sites by the bond valence method

We have applied the bond valence method to cerium oxides to determine the oxidation states of the Ce ion at the various site symmetries of the crystals. The crystals studied include cerium dioxide and the two sesquioxides along with some selected intermediate phases which are crystallographically well characterized. Our results indicate that cerium dioxide has a mixed-valence ground state with an f-electron population on the Ce site of 0.27 while both the A- and C-sesquioxides have a nearly pure f^1 configuration. The Ce sites in most of the intermediate oxides have non-integral valences. Furthermore, many of these valences are different from the values predicted from a naive consideration of the stoichiometric valence of the compound

Morphological and molecular characterisation of Streptomyces spp. which suppress pathogenic fungi

Streptomyces species are aerobes and chemoorganotrophic bacteria. These microorganisms produce a wide range of industrially significant compounds, specifically antibiotics and anti fungal substances. The objective of this study was to characterise soil-borne Streptomyces isolates using morphological and molecular traits in order to identify them to species level, and leverage from their potential to suppress the growth of Aspergillus flavus , Fusarium oxysporum and Penicillium italicum . Twenty-seven soil-borne putative Streptomyces, which elicited comprehensive\ua0antimicrobial activity against Aspergillus flavus, Fusarium oxysporum and Penicillium italicum, in a previous study, were evaluated. On the basis of morphology, the bacteria resembled the genus Streptomyces. Initially, colonies phenotypically appeared to have a relatively smooth surface but as growth progressed the bacteria developed a weft of aerial mycelium granular, powdery or velvety in appearance. Bacteria produced a wide variety of pigments which in turn were responsible for the colour of the vegetative and aerial mycelia, colour ranged from white to cream or buff shades and yellow to orange or brown. Microscopic analyses and morphological characteristics generated sub-groups of the isolates and clustered them according to their similarities. One bacterial strain was randomly selected from each cluster and investigated using molecular characteristics. Partial 16S rDNAs from the selected representative isolates from each subgroup, were sequenced and phylogenetic analysis performed. The 16S\ua0rDNA\ua0sequences of the isolates indicated that they were related to\ua0 Streptomyces species: S. bungoensis , S. thermocarboxydus , S. corchorusii and S. lasaliensis, that are known secondary metabolite producers possessing antimicrobial activity against plant pathogens.Les esp\ue8ces de Streptomyces sont des bact\ue9ries a\ue9robies et chimio-organotrophes. Ces micro-organismes produisent une large gamme de compos\ue9s d\u2019importance industrielle, en particulier des antibiotiques et des substances antifongiques. L\u2019objectif de cette \ue9tude \ue9tait de caract\ue9riser les isolats de Streptomyces transmis par le sol \ue0 l\u2019aide de traits morphologiques et mol\ue9culaires afin de les identifier au niveau de l\u2019esp\ue8ce, et de tirer parti de leur potentiel \ue0 supprimer la croissance d\u2019 Aspergillus flavus , Fusarium oxysporum et Penicillium italicum . Vingt-sept Streptomyces putatifs transmis par le sol, qui ont suscit\ue9 une activit\ue9 antimicrobienne compl\ue8te contre Aspergillus flavus, Fusarium oxysporum et Penicillium italicum, dans une \ue9tude pr\ue9c\ue9dente, ont \ue9t\ue9 \ue9valu\ue9s. Sur la base de la morphologie, les bact\ue9ries ressemblaient au genre Streptomyces. Au d\ue9part, les colonies semblaient ph\ue9notypiquement avoir une surface relativement lisse, mais au fur et \ue0 mesure que la croissance progressait, les bact\ue9ries d\ue9veloppaient une trame de myc\ue9lium a\ue9rien d\u2019aspect granuleux, poudreux ou velout\ue9. Les bact\ue9ries produisaient une grande vari\ue9t\ue9 de pigments qui \ue0 leur tour \ue9taient responsables de la couleur des myc\ue9liums v\ue9g\ue9tatifs et a\ue9riens, la couleur variait du blanc au cr\ue8me ou au chamois et du jaune \ue0 l\u2019orange ou au brun. Des analyses microscopiques et des caract\ue9ristiques morphologiques ont g\ue9n\ue9r\ue9 des sous-groupes d\u2019isolats et les ont regroup\ue9s en fonction de leurs similitudes. Une souche bact\ue9rienne a \ue9t\ue9 s\ue9lectionn\ue9e au hasard dans chaque groupe et \ue9tudi\ue9e en utilisant des caract\ue9ristiques mol\ue9culaires. Des ADNr 16S partiels provenant des isolats repr\ue9sentatifs s\ue9lectionn\ue9s de chaque sous-groupe ont \ue9t\ue9 s\ue9quenc\ue9s et une analyse phylog\ue9n\ue9tique a \ue9t\ue9 effectu\ue9e. Les s\ue9quences d\u2019ADNr 16S des isolats ont indiqu\ue9 qu\u2019ils \ue9taient apparent\ue9s aux esp\ue8ces de Streptomyces: S. bungoensis , S. thermocarboxydus , S. corchorusii et S. lasaliensis, qui sont des producteurs de m\ue9tabolites secondaires connus poss\ue9dant une activit\ue9 antimicrobienne contre les phytopathog\ue8nes

In vitro suppression of pathogenic fungi by Streptomyces spp.

The use of living organisms or natural enemies of pathogens to control their populations is called biological disease control. It involves harnessing and introduction of exotic species of microorganism in a natural form, with the intention of controlling pathogens that may exist naturally in the same ecosystem. Prospects for biological control of Aspergillus flavus , Fusarium oxysporum and Penicillium italicum were investigated using Streptomyces spp. isolated from Chinhoyi University of Technology Farm soils in Mashonaland West, Zimbabwe. Twenty seven Streptomyces spp were obtained from the soil, and screened for antimicrobial activity and antagonism in in vitro pathogen inhibition assays, replicated thrice. Although majority of the isolates tested elicited no effect on test pathogens, 22% of the Streptomyces isolates were able to effectively suppress A. flavus, F. oxysporum and P. italicum by at least 55%. There was a significant interaction between Streptomyces isolates and pathogen (A. flavus, F. oxysporum and P. italicum) (P&lt;0.05) on fungal radial growth at days 7, 10 and 14 after pathogen-Streptomyces incubation. Antimicrobial potential against individual and multiple test pathogens was observed, with CUT-Streptomyces 4, CUT-Streptomyces 10, CUT-Streptomyces 11, CUT-Streptomyces 20 and CUT-Streptomyces 23 showing the greatest antimicrobial activity. CUT-Streptomyces isolates have the potential to suppress A. flavus, F. oxysporum and P. italicum in vitro.L\u2019utilisation d\u2019organismes vivants ou d\u2019ennemis naturels d\u2019agents pathog\ue8nes pour contr\uf4ler leurs populations est appel\ue9e contr\uf4le biologique des maladies. Il s\u2019agit d\u2019exploiter et d\u2019introduire des esp\ue8ces exotiques de microorganismes sous une forme naturelle, dans le but de contr\uf4ler les agents pathog\ue8nes pouvant exister naturellement dans le m\ueame \ue9cosyst\ue8me. Les perspectives de lutte biologique contre Aspergillus flavus , Fusarium oxysporum et Penicillium italicum ont \ue9t\ue9 \ue9tudi\ue9es \ue0 l\u2019aide de Streptomyces spp. isol\ue9es des sols agricoles de l\u2019Universit\ue9 de Technologie de Chinhoyi \ue0 Mashonaland West, Zimbabwe. Vingt-sept Streptomyces spp ont \ue9t\ue9 obtenus du sol et cribl\ue9s pour l\u2019activit\ue9 antimicrobienne et l\u2019antagonisme dans des tests d\u2019inhibition des agents pathog\ue8nes in vitro, r\ue9pliqu\ue9s trois fois. Bien que la majorit\ue9 des isolats test\ue9s ne provoquent aucun effet sur les agents pathog\ue8nes test\ue9s, 22% des isolats de Streptomyces sont capables de supprimer efficacement au moins 55% des A. flavus, F. oxysporum et P. italicum. Il y avait une interaction significative entre les isolats de Streptomyces et l\u2019agent pathog\ue8ne (A. flavus, F. oxysporum et P. italicum) (P&lt; 0.05) lors de la croissance radiale des champignons aux 7\ue8me, 10\ue8me et 14\ue8me jours apr\ue8s l\u2019incubation de l\u2019agent pathog\ue8ne-Streptomyces. Un potentiel antimicrobien contre des agents pathog\ue8nes individuels et multiples a \ue9t\ue9 observ\ue9. CUT-Streptomyces 4, CUT-Streptomyces 10, CUT-Streptomyces 11, CUT-Streptomyces 20 et CUT-Streptomyces 23 ont montr\ue9 l\u2019activit\ue9 antimicrobienne la plus \ue9lev\ue9e. Les isolats de CUT-Streptomyces ont le potentiel de supprimer A. flavus, F. oxysporum et P. italicum in vitro

A Database of Cepheid Distance Moduli and TRGB, GCLF, PNLF and SBF Data Useful for Distance Determinations

We present a compilation of Cepheid distance moduli and data for four secondary distance indicators that employ stars in the old stellar populations: the planetary nebula luminosity function (PNLF), the globular cluster luminosity function (GCLF), the tip of the red giant branch (TRGB), and the surface brightness fluctuation (SBF) method. The database includes all data published as of July 15, 1999. The main strength of this compilation resides in all data being on a consistent and homogeneous system: all Cepheid distances are derived using the same calibration of the period-luminosity relation, the treatment of errors is consistent for all indicators, measurements which are not considered reliable are excluded. As such, the database is ideal for inter-comparing any of the distance indicators considered, or for deriving a Cepheid calibration to any secondary distance indicator. Specifically, the database includes: 1) Cepheid distances, extinctions and metallicities; 2) apparent magnitudes of the PNLF cutoff; 3) apparent magnitudes and colors of the turnover of the GCLF (both in the V- and B-bands); 4) apparent magnitudes of the TRGB (in the I-band) and V-I colors at and 0.5 magnitudes fainter than the TRGB; 5) apparent surface brightness fluctuation magnitudes I, K', K_short, and using the F814W filter with the HST/WFPC2. In addition, for every galaxy in the database we give reddening estimates from DIRBE/IRAS as well as HI maps, J2000 coordinates, Hubble and T-type morphological classification, apparent total magnitude in B, and systemic velocity. (Abridged)Comment: Accepted for publication in the Astrophysical Journal Supplement Series. Because of space limitations, the figures included are low resolution bitmap images. Original figures can be found at http://www.astro.ucla.edu/~laura/pub.ht