508 research outputs found
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 year adaptive optics (AO) study of stellar
variability and search for eclipsing binaries in the central pc
() of the Milky Way nuclear star cluster. We measure the photometry
of 563 stars using the Keck II NIRC2 imager (-band, ). We achieve a photometric uncertainty floor of (), comparable to the highest precision achieved
in other AO studies. Approximately half of our sample () shows
variability. of known early-type young stars and 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 . 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<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< 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
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