26,278 research outputs found
The complex stellar populations in the lines of sight to open clusters in the third Galactic quadrant
Multi-color photometry of the stellar populations in five fields in the third
Galactic quadrant centred on the clusters NGC 2215, NGC 2354, Haffner 22,
Ruprecht 11, and ESO489SC01 is interpreted in terms of a warped and flared
Galactic disk, without resort to an external entity such as the popular
Monoceros or Canis Major overdensities. Except for NGC 2215, the clusters are
poorly or unstudied previously. The data generate basic parameters for each
cluster, including the distribution of stars along the line of sight. We use
star counts and photometric analysis, without recourse to Galactic-model-based
predictions or interpretations, and confirms earlier results for NGC 2215 and
NGC 2354. ESO489SC01 is not a real cluster, while Haffner~22 is an overlooked
cluster aged about 2.5 Gyr. Conclusions for Ruprecht~11 are preliminary,
evidence for a cluster being marginal. Fields surrounding the clusters show
signatures of young and intermediate-age stellar populations. The young
population background to NGC~2354 and Ruprecht~11 lies 8-9 kpc from the Sun and
1 kpc below the formal Galactic plane, tracing a portion of the
Norma-Cygnus arm, challenging Galactic models that adopt a sharp cut-off of the
disk 12-14 kpc from the Galactic center. The old population is metal poor with
an age of 2-3 Gyr, resembling star clusters like Tombaugh 2 or NGC 2158. It has
a large color spread and is difficult to locate precisely. Young and old
populations follow a pattern that depends critically on the vertical location
of the thin and/or thick disk, and whether or not a particular line of sight
intersects one, both, or none.Comment: 16 pages, 9 eps figures, in press in MNRA
The PL calibration for Milky Way Cepheids and its implications for the distance scale
The rationale behind recent calibrations of the Cepheid PL relation using the
Wesenheit formulation is reviewed and reanalyzed, and it is shown that recent
conclusions regarding a possible change in slope of the PL relation for
short-period and long-period Cepheids are tied to a pathological distribution
of HST calibrators within the instability strip. A recalibration of the
period-luminosity relation is obtained using Galactic Cepheids in open clusters
and groups, the resulting relationship, described by log L/L_sun =
2.415(+-0.035) + 1.148(+-0.044)log P, exhibiting only the moderate scatter
expected from color spread within the instability strip. The relationship is
confirmed by Cepheids with HST parallaxes, although without the need for
Lutz-Kelker corrections, and in general by Cepheids with revised Hipparcos
parallaxes, albeit with concerns about the cited precisions of the latter. A
Wesenheit formulation of Wv = -2.259(+-0.083) - 4.185(+-0.103)log P for
Galactic Cepheids is tested successfully using Cepheids in the inner regions of
the galaxy NGC 4258, confirming the independent geometrical distance
established for the galaxy from OH masers. Differences between the extinction
properties of interstellar and extragalactic dust may yet play an important
role in the further calibration of the Cepheid PL relation and its application
to the extragalactic distance scale.Comment: Accepted for Publication (Astrophysics & Space Science
The common nodulation genes of Astragalus sinicus rhizobia are conserved despite chromosomal diversity
The nodulation genes of Mesorhizobium sp. (Astragalus sinicus) strain 7653R were cloned by functional complementation of Sinorhizobium meliloti nod mutants. The common nod genes, nodD, nodA, and nodBC, were identified by heterologous hybridization and sequence analysis. The nodA gene was found to be separated from nodBC by approximately 22 kb and was divergently transcribed. The 2.0-kb nodDBC region was amplified by PCR from 24 rhizobial strains nodulating A. sinicus, which represented different chromosomal genotypes and geographic origins. No polymorphism was found in the size of PCR products, suggesting that the separation of nodA from nodBC is a common feature of A. sinicus rhizobia. Sequence analysis of the PCR-amplified nodA gene indicated that seven strains representing different 16S and 23S ribosomal DNA genotypes had identical nodA sequences. These data indicate that, whereas microsymbionts of A. sinicus exhibit chromosomal diversity, their nodulation genes are conserved, supporting the hypothesis of horizontal transfer of nod genes among diverse recipient bacteria
Reddenings of FGK supergiants and classical Cepheids from spectroscopic data
Accurate and homogeneous atmospheric parameters (Teff, log (g), Vt, [Fe/H])
are derived for 74 FGK non-variable supergiants from high-resolution, high
signal-to-noise ratio, echelle spectra. Extremely high precision for the
inferred effective temperatures (10-40 K) is achieved by using the line-depth
ratio method. The new data are combined with atmospheric values for 164
classical Cepheids, observed at 675 different pulsation phases, taken from our
previously published studies. The derived values are correlated with unreddened
B-V colours compiled from the literature for the investigated stars in order to
obtain an empirical relationship of the form: (B-V)o = 57.984 - 10.3587(log
Teff)^2 + 1.67572(log Teff)^3 - 3.356(log (g)) + 0.0321(Vt) + 0.2615[Fe/H] +
0.8833((log (g))(log Teff)). The expression is used to estimate colour excesses
E(B-V) for individual supergiants and classical Cepheids, with a precision of
+-0.05 mag. for supergiants and Cepheids with n=1-2 spectra, reaching +-0.025
mag. for Cepheids with n>2 spectra, matching uncertainties for the most
sophisticated photometric techniques. The reddening scale is also a close match
to the system of space reddenings for Cepheids. The application range is for
spectral types F0--K0 and luminosity classes I and II.Comment: accepted for publication (MNRAS
The Stability of Polar Oxide Surfaces
The structures of the polar surfaces of ZnO are studied using ab initio calculations and surface x-ray diffraction. The experimental and theoretical relaxations are in good agreement. The polar surfaces are shown to be very stable; the cleavage energy for the (0001)-Zn and (0001̅ )-O surfaces is 4.0J/m2 comparable to 2.32J/m2 for the most stable nonpolar (1010) surface. The surfaces are stabilized by an electronic mechanism involving the transfer of 0.17 electrons between them. This leads to 2D metallic surface states, which has implications for the use of the material in gas sensing and catalytic applications
Long Range Forces from Pseudoscalar Exchange
Using dispersion theoretic techniques, we consider coherent long range forces
arising from double pseudoscalar exchange among fermions. We find that Yukawa
type coupling leads to spin independent attractive potentials whereas
derivative coupling renders spin independent repulsive potentials.Comment: 27 pages, REVTeX, 3 figures included using epsfi
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