105 research outputs found
A Strategy for Identifying the Grid Stars for the Space Interferometry Mission (SIM)
We present a strategy to identify several thousand stars that are
astrometrically stable at the micro-arcsecond level for use in the SIM (Space
Interferometry Mission) astrometric grid. The requirements on the grid stars
make this a rather challenging task. Taking a variety of considerations into
account we argue for K giants as the best type of stars for the grid, mainly
because they can be located at much larger distances than any other type of
star due to their intrinsic brightness. We show that it is possible to identify
suitable candidate grid K giants from existing astrometric catalogs. However,
double stars have to be eliminated from these candidate grid samples, since
they generally produce much larger astrometric jitter than tolerable for the
grid. The most efficient way to achieve this is probably by means of a radial
velocity survey. To demonstrate the feasibility of this approach, we repeatedly
measured the radial velocities for a pre-selected sample of 86 nearby Hipparcos
K giants with precisions of 5-8 m/s. The distribution of the intrinsic radial
velocity variations for the bona-fide single K giants shows a maximum around 20
m/s, which is small enough not to severely affect the identification of stellar
companions around other K giants. We use the results of our observations as
input parameters for Monte-Carlo simulations on the possible design of a radial
velocity survey of all grid stars. Our favored scenario would result in a grid
which consists to 68% of true single stars and to 32% of double or multiple
stars with periods mostly larger than 200 years, but only 3.6% of all grid
stars would display astrometric jitter larger than 1 microarcsecond. This
contamination level is probably tolerable.Comment: LaTeX, 21 pages, 8 figures, accepted by PASP (February 2001 issue).
Also available at http://beehive.ucsd.edu/ftp/pub/grid/kgiants.htm
Chemically peculiar stars in the Large Magellanic Cloud
The detection of magnetic chemically peculiar (CP2) stars in open clusters of
extragalactic systems can give observational answers to many unsolved
questions. The mean percentage of CP2 stars in the Milky Way is of the order of
5% for the spectral range from early B- to F-type, luminosity class V objects.
The origin of the CP2 phenomenon seems to be closely connected to the overall
metallicity and global magnetic field environment. The theoretical models are
still only tested by observations in the Milky Way. It is therefore essential
to provide high quality observations in rather different global environments.
The young clusters NGC 2136/7 were observed in the Delta a photometric system.
This intermediate band photometric system samples the depth of the 520nm flux
depression by comparing the flux at the center with the adjacent regions with
bandwidths of 11nm to 23nm. The Delta a photometric system is most suitable for
detecting CP2 stars with high efficiency, but is also capable of detecting a
small percentage of non-magnetic CP objects. We present high precision
photometric Delta a observations of 417 objects in NGC 2136/7 and its
surrounding field, of which five turned out to be bona fide magnetic CP stars.
In addition, we discovered two Be/Ae stars. From our investigations of NGC
1711, NGC 1866, NGC 2136/7, their surroundings, and one independent field of
the LMC population, we derive an occurrence of classical chemically peculiar
stars of 2.2(6)% in the LMC, which is only half the value found in the Milky
Way. The mass and age distribution of the photometrically detected CP stars is
not different from that of similar objects in galactic open clusters.Comment: 5 pages, 4 figures, accepted by A&
The Infrared Continuum Sizes of Be Star Disks
We present an analysis of the near-infrared continuum emission from the
circumstellar gas disks of Be stars using a radiative transfer code for a
parametrized version of the viscous decretion disk model. This isothermal gas
model creates predicted images that we use to estimate the HWHM emission radius
along the major axis of the projected disk and the spatially integrated flux
excess at wavelengths of 1.7, 2.1, 4.8, 9, and 18 ?m. We discuss in detail the
effect of the disk base density, inclination angle, stellar effective
temperature, and other physical parameters on the derived disk sizes and color
excesses. We calculate color excess estimates relative to the stellar V -band
flux for a sample of 130 Be stars using photometry from 2MASS and the AKARI
infrared camera all-sky survey. The color excess relations from our models make
a good match of the observed color excesses of Be stars. We also present our
results on the projected size of the disk as a function of wavelength for the
classical Be star ? Tauri, and we show that the model predictions are
consistent with interferometric observations in the H, K', and 12 \mu m bands
The Sludge Dewaterability in Advanced Wastewater Treatment: A Survey of Four Different Membrane BioReactor Pilot Plants
The wasted activated sludge dewaterability represents a major concern
for Wastewater Treatment Plants (WWTPs) managers. Indeed, whereas the
dewatered sludge could represents a re-usable matrix, the principal drawback
related to the wasted sludge dewaterability is the high water content due to the
presence of extracellular polymeric substances (EPS) that allow the trapping of
water molecules within the bio sludge flocs. In order to provide an outlook of
the dewaterability features of activated sludge derived from advanced WWTP,
the present research reports a long term survey (over two years) aimed at
assessing the principal dewaterability parameters of the sludge wasted from
different Membrane BioReactor pilot plants
Time resolved spectroscopy of BD+46 442: gas streams and jet creation in a newly discovered evolved binary with a disk
Previous studies have shown that many post-AGB stars with dusty disks are
associated with single-lined binary stars. To verify the binarity hypothesis on
a larger sample, we started a high-resolution spectral monitoring of about 40
field giants, whose binarity was suspected based on either a light curve, an
infrared excess, or a peculiar chemical composition. Here we report on the
discovery of the periodic RV variations in BD+46 442, a high-latitude F giant
with a disk. We interpret the variations due to the motion around a faint
companion, and deduce the following orbital parameters: Porb = 140.77 d, e =
0.083, asini=0.31 AU. We find it to be a moderately metal-poor star
([M/H]=-0.7) without a strong depletion pattern in the photospheric abundances.
Interestingly, many lines show periodic changes with the orbital phase: Halpha
switches between a double-peak emission and a PCyg-like profiles, while strong
metal lines appear split during the maximum redshift. Similar effects are
likely visible in the spectra of other post-AGB binaries, but their regularity
is not always realized due to sporadic observations. We propose that these
features result from an ongoing mass transfer from the evolved giant to the
companion. In particular, the blue-shifted absorption in Halpha, which occurs
only at superior conjunction, may result from a jet originating in the
accretion disk around the companion and seen in absorption towards the luminous
primary.Comment: 16 pages, accepted in A&
Nitrogen-limited mangrove ecosystems conserve N through dissimilatory nitrate reduction to ammonium
Earlier observations in mangrove sediments of Goa, India have shown denitrification to be a major pathway for N loss1. However, percentage of total nitrate transformed through complete denitrification accounted for <0–72% of the pore water nitrate reduced. Here, we show that up to 99% of nitrate removal in mangrove sediments is routed through dissimilatory nitrate reduction to ammonium (DNRA). The DNRA process was 2x higher at the relatively pristine site Tuvem compared to the anthropogenically-influenced Divar mangrove ecosystem. In systems receiving low extraneous nutrient inputs, this mechanism effectively conserves and re-circulates N minimizing nutrient loss that would otherwise occur through denitrification. In a global context, the occurrence of DNRA in mangroves has important implications for maintaining N levels and sustaining ecosystem productivity. For the first time, this study also highlights the significance of DNRA in buffering the climate by modulating the production of the greenhouse gas nitrous oxide
Enrichment and characterization of ammonia-oxidizing archaea from the open ocean : phylogeny, physiology and stable isotope fractionation
Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in The ISME Journal 5 (2011): 1796–1808, doi:10.1038/ismej.2011.58.Archaeal genes for ammonia oxidation are widespread in the marine environment, but
direct physiological evidence for ammonia oxidation by marine archaea is limited. We
report the enrichment and characterization of three strains of pelagic ammonia-oxidizing
archaea (AOA) from the north Pacific Ocean that have been maintained in laboratory
culture for over three years. Phylogenetic analyses indicate the three strains belong to a
previously identified clade of water column-associated AOA and possess 16S rRNA
genes and ammonia monooxygenase subunit a (amoA) genes highly similar (98-99%
identity) to those recovered in DNA and cDNA clone libraries from the open ocean. The
strains grow in natural seawater-based liquid medium while stoichiometrically converting
ammonium (NH4
+) to nitrite (NO2
-). Ammonia oxidation by the enrichments is only
partially inhibited by allylthiourea at concentrations known to inhibit cultivated
ammonia-oxidizing bacteria. The three strains were used to determine the nitrogen stable
isotope effect (15εNH3) during archaeal ammonia oxidation, an important parameter for
interpreting stable isotope ratios in the environment. Archaeal 15εNH3 ranged from 13-
41‰, within the range of that previously reported for ammonia-oxidizing bacteria.
Despite low amino acid identity between the archaeal and bacterial Amo proteins, their
functional diversity as captured by 15εNH3 is similar.This work was supported by a Woods Hole Oceanographic Institution (WHOI)
Postdoctoral Scholar fellowship to AES and the WHOI Ocean Life Institute
Altered Small-World Brain Networks in Schizophrenia Patients during Working Memory Performance
Impairment of working memory (WM) performance in schizophrenia patients (SZ) is well-established. Compared to healthy controls (HC), SZ patients show aberrant blood oxygen level dependent (BOLD) activations and disrupted functional connectivity during WM performance. In this study, we examined the small-world network metrics computed from functional magnetic resonance imaging (fMRI) data collected as 35 HC and 35 SZ performed a Sternberg Item Recognition Paradigm (SIRP) at three WM load levels. Functional connectivity networks were built by calculating the partial correlation on preprocessed time courses of BOLD signal between task-related brain regions of interest (ROIs) defined by group independent component analysis (ICA). The networks were then thresholded within the small-world regime, resulting in undirected binarized small-world networks at different working memory loads. Our results showed: 1) at the medium WM load level, the networks in SZ showed a lower clustering coefficient and less local efficiency compared with HC; 2) in SZ, most network measures altered significantly as the WM load level increased from low to medium and from medium to high, while the network metrics were relatively stable in HC at different WM loads; and 3) the altered structure at medium WM load in SZ was related to their performance during the task, with longer reaction time related to lower clustering coefficient and lower local efficiency. These findings suggest brain connectivity in patients with SZ was more diffuse and less strongly linked locally in functional network at intermediate level of WM when compared to HC. SZ show distinctly inefficient and variable network structures in response to WM load increase, comparing to stable highly clustered network topologies in HC
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