813 research outputs found
Geometric shape of invariant manifolds for a class of stochastic partial differential equations
Invariant manifolds play an important role in the study of the qualitative
dynamical behaviors for nonlinear stochastic partial differential equations.
However, the geometric shape of these manifolds is largely unclear. The purpose
of the present paper is to try to describe the geometric shape of invariant
manifolds for a class of stochastic partial differential equations with
multiplicative white noises. The local geometric shape of invariant manifolds
is approximated, which holds with significant likelihood. Furthermore, the
result is compared with that for the corresponding deterministic partial
differential equations
The Reddest DR3 SDSS/XMM Quasars
We have cross-correlated the SDSS DR3 Schneider et al. (2005) quasar catalog
with the XMM-Newton archive. Color and redshift selections (g - r > 0.5 and 0.9
z < 2.1) result in a sample of 17 red, moderate redshift quasars. The redshift
selection minimizes possible contamination due to host galaxy emission and
Lyalpha forest absorption. Both optical and X-ray information are required to
distinguish between the two likely remaining causes of the red colors: 1)
dust-reddening and 2) an intrinsically red continuum. We find that 7 of 17
quasars can be classified as probable `intrinsically red' objects. These 7
quasars have unusually broad MgII emission lines (=10,500 km s^{-1}),
moderately flat, but unabsorbed X-ray spectra =1.66+/-0.08), and low
accretion rates (mdot/mdot_{Edd}} ~ 0.01). We suggest low accretion rates as a
possible physical explanation for quasars with intrinsically red optical
continua. We find that 8 of 17 quasars can be classified as dust-reddened.
Three of these have upper-limits on the absorption column from X-ray spectral
fits of N_H = 3-13 x 10^{22} cm^2, while the other five quasars must be
absorbed by at least N_H = 10^{23} cm^2 in order to be consistent with a
comparably selected alpha_{ox}-l_{uv} distribution. Two objects in the sample
are unclassified.Comment: 20 pages, 12 figures, Accepted for publication in Ap
Solubilization in micellar systems. Analytical and environmental applications
Abstract: Surfactant-based separation techniques largely exploit the solubilization power of micelles and other amphiphilic aggregates towards a great variety of hydrophilic and hydrophobic solutes. Micellar extraction, based on the cloud-point phenomenon, and micellar ultrafiltration have been succesfilly applied for preconcentration and removal of organic and inorganic analytes of environmental concern from aqueous media. In particular, the recent development of chelating surfactant aggregates allows to perform very selective demetallation treatments. The factors which control the separation efficiencies of these methods are examined and discussed
Metallicities of 0.3<z<1.0 Galaxies in the GOODS-North Field
We measure nebular oxygen abundances for 204 emission-line galaxies with
redshifts 0.3<z<1.0 in the Great Observatories Origins Deep Survey North
(GOODS-N) field using spectra from the Team Keck Redshift Survey (TKRS). We
also provide an updated analytic prescription for estimating oxygen abundances
using the traditional strong emission line ratio, R_{23}, based on the
photoionization models of Kewley & Dopita (2003). We include an analytic
formula for very crude metallicity estimates using the [NII]6584/Halpha ratio.
Oxygen abundances for GOODS-N galaxies range from 8.2< 12+log(O/H)< 9.1
corresponding to metallicities between 0.3 and 2.5 times the solar value. This
sample of galaxies exhibits a correlation between rest-frame blue luminosity
and gas-phase metallicity (i.e., an L-Z relation), consistent with L-Z
correlations of previously-studied intermediate-redshift samples. The zero
point of the L-Z relation evolves with redshift in the sense that galaxies of a
given luminosity become more metal poor at higher redshift. Galaxies in
luminosity bins -18.5<M_B<-21.5 exhibit a decrease in average oxygen abundance
by 0.14\pm0.05 dex from z=0 to z=1. This rate of metal enrichment means that
28\pm0.07% of metals in local galaxies have been synthesized since z=1, in
reasonable agreement with the predictions based on published star formation
rate densities which show that ~38% of stars in the universe have formed during
the same interval. (Abridged)Comment: AASTeX, 49 pages, 16 figures, accepted for publication in The
Astrophysical Journa
The UCSD HIRES/KeckI Damped Lya Abundance Database: II. The Implications
We present a comprehensive analysis of the damped Lya abundance database
presented in the first paper of this series. This database provides a
homogeneous set of abundance measurements for many elements including Si, Cr,
Ni, Zn, Fe, Al, S, Co, O, and Ar from 38 damped Lya systems with z > 1.5. With
little exception, these damped Llya systems exhibit very similar relative
abundances. There is no significant correlation in X/Fe with [Fe/H] metallicity
and the dispersion in X/Fe is small at all metallicity.
We search the database for trends indicative of dust depletion and in a few
cases find strong evidence. Specifically, we identify a correlation between
[Si/Ti] and [Zn/Fe] which is unambiguous evidence for depletion.
We present a discussion on the nucleosynthetic history of the damped Lya
systems by focusing on abundance patterns which are minimally affected by dust
depletion. We find [Si/Fe] -> +0.25 dex as [Zn/Fe] -> 0 and that the [Si/Fe]
values exhibit a plateau of ~+0.3 dex at [Si/H] < -1.5 dex. Together these
trends indicate significant alpha-enrichment in the damped Lya systems at low
metallicity, an interpretation further supported by the observed O/Fe, S/Fe and
Ar/Fe ratios. We also discuss Fe-peak nucleosynthesis and the odd-even effect.
To assess the impact of dust obscuration, we present estimates of the
dust-to-gas ratios for the damped Lya sightlines and crudely calculate dust
extinction corrections. The distribution of extinction corrections suggests the
effects of dust obscuration are minimal and that the population of 'missing'
damped systems has physical characteristics similar to the observed sample.
We update our investigation on the chemical evolution of the early universe
in neutral gas. [significantly abridged]Comment: 29 pages, 26 figures. Uses emulateapj.sty. Accepted to ApJ: Oct 15,
200
Unmanned Aircraft System Traffic Management (UTM) Concept of Operations
Many applications of small Unmanned Aircraft System (sUAS) have been envisioned. These include surveillance of key assets such as pipelines, rail, or electric wires, deliveries, search and rescue, traffic monitoring, videography, and precision agriculture. These operations are likely to occur in the same airspace in presence of many static and dynamic constraints such as airports, and high wind areas. Therefore, small UAS, typically 55 pounds and below, operations need to be managed to ensure safety and efficiency of operations is maintained. This paper will describe the Concept of Operations (ConOps) for NASA's UAS Traffic Management (UTM) research initiative. The UTM ConOps is focused on safely enabling large-scale small UAS (sUAS) operations in low altitude airspace. The UTM construct supports large-scale visual line of sight and beyond visual line of sight operations. It is based on two primary mantras: (1) flexibility where possible and structure where necessary (2) a risk-based approach where geographical needs and use case indicate the airspace performance requirements. Preliminary stakeholder feedback and initial UTM tests conducted by NASA show promise of UTM to enable large-scale low altitude UAS operations safely
Continuum and Emission-Line Properties of Broad Absorption Line Quasars
We investigate the continuum and emission-line properties of 224 broad
absorption line quasars (BALQSOs) with 0.9<z<4.4 drawn from the Sloan Digital
Sky Survey (SDSS) Early Data Release (EDR), which contains 3814 bona fide
quasars. We find that low-ionization BALQSOs (LoBALs) are significantly
reddened as compared to normal quasars, in agreement with previous work.
High-ionization BALQSOs (HiBALs) are also more reddened than the average
nonBALQSO. Assuming SMC-like dust reddening at the quasar redshift, the amount
of reddening needed to explain HiBALs is E(B-V)~0.023 and LoBALs is
E(B-V)~0.077 (compared to the ensemble average of the entire quasar sample). We
find that there are differences in the emission-line properties between the
average HiBAL, LoBAL, and nonBAL quasar. These differences, along with
differences in the absorption line troughs, may be related to intrinsic quasar
properties such as the slope of the intrinsic (unreddened) continuum; more
extreme absorption properties are correlated with bluer intrinsic continua.
Despite the differences among BALQSO sub-types and nonBALQSOs, BALQSOs appear
to be drawn from the same parent population as nonBALQSOs when both are
selected by their UV/optical properties. We find that the overall fraction of
traditionally defined BALQSOs, after correcting for color-dependent selection
effects due to different SEDs of BALQSO and nonBALQSOs, is 13.4+/-1.2% and
shows no significant redshift dependence for 1.7<z<3.45. After a rough
completeness correction for the effects of dust extinction, we find that
approximately one in every six quasars is a BALQSO.Comment: 35 pages, 11 figures (1 color), 1 table; accepted by A
XSHOOTER spectroscopy of the enigmatic planetary nebula Lin49 in the Small Magellanic Cloud
We performed a detailed spectroscopic analysis of the fullerene C60-containing planetary nebula (PN) Lin49 in the Small Magellanic Cloud (SMC) using XSHOOTER at the European Southern Observatory Very Large Telescope and the Spitzer/Infrared Spectrograph instruments. We derived nebular abundances for nine elements. We used TLUSTY to derive photospheric parameters for the central star. Lin49 is C-rich and metal-deficient PN (Z ∼ 0.0006). The nebular abundances are in good agreement with asymptotic giant branch nucleosynthesis models for stars with initial mass 1.25 M⊙ and metallicity Z = 0.001. Using the TLUSTY synthetic spectrum of the central star to define the heating and ionizing source, we constructed the photoionization model with CLOUDY that matches the observed spectral energy distribution (SED) and the line fluxes in the UV to far-IR wavelength ranges simultaneously. We could not fit the ∼1–5 μm SED using a model with 0.005–0.1-μm-sized graphite grains and a constant hydrogen density shell owing to the prominent near-IR excess, while at other wavelengths the model fits the observed values reasonably well. We argue that the near-IR excess might indicate either (1) the presence of very small particles in the form of small carbon clusters, small graphite sheets, or fullerene precursors, or (2) the presence of a high-density structure surrounding the central star. We found that SMC C60 PNe show a near-IR excess component to lesser or greater degree. This suggests that these C60 PNe might maintain a structure nearby their central star
Evidence for TP-AGB stars in high redshift galaxies, and their effect on deriving stellar population parameters
We explore the effects of stellar population models on estimating star
formation histories, ages and masses of high redshift galaxies. The focus is on
the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) phase of stellar
evolution, whose treatment is a source of major discrepancy among different
evolutionary population synthesis. In particular, besides the models usually
adopted in the literature, we use models (by Maraston 2005), in which the
contribution of the TP-AGB phase is observationally calibrated and is the
dominant source of near-IR energy for stellar populations in the age range 0.2
to 2 Gyr. We use a sample of high-z galaxies in the HUDF, with spectroscopic
redshifts, and Spitzer IRAC and MIPS photometry from GOODS. We find that the
TP-AGB phase plays a key role in the interpretation of Spitzer data for high-z
galaxies, when the rest-frame near-IR is sampled. When fitting without dust
reddening, the models with the empirically-calibrated TP-AGB phase always
reproduce better the observed spectral energy distributions (SEDs). Allowing
for dust reddening improves the fits with literature models in some cases. In
both cases, the results from Maraston models imply younger ages by factors up
to 6 and lower stellar masses (by ~60 % on average). The observed strengths of
the MgUV spectral feature compare better to the predicted ones in the case of
the Maraston models, implying a better overall consistency of SED fitting.
Finally, we find that photometric redshifts improve significantly using these
models on the SEDs extending over the IRAC bands. This work provides the first
direct evidence of TP-AGB stars in the primeval Universe.Comment: 14 pages, 10 figures, 3 tables, submitted to the Astrophysical
Journa
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