3,554 research outputs found
An assessment of chemical contaminants in the marine sediments of southwest Puerto Rico
This report summarizes the results of a characterization
of chemical contaminants in the sediments in southwest
Puerto Rico. The report is part of a project to integrate
various analytical specialties to assess linkages between
chemical contaminants and the condition of coral reefs. In this phase of the project, over 120 chemical contaminants were analyzed in sediments collected, including a number of organic (e.g., hydrocarbons), inorganic (e.g., metals), and biological (bacterial) compounds/analytes. The report also provides a preliminary analysis of the association between
sediment contaminants and coral species richness.
Overall, the levels of chemical contaminants in the study area between Guanica Bay and the town of La Parguera were fairly low. At most of the sites sampled, particularly
adjacent to the town of La Parguera, concentrations of
organic and inorganic contaminants were below the median values from NOAA’s National Status and Trends Program, which monitors the Nation’s coastal and estuarine waters for chemical contaminants. Elevated levels of a number of contaminant classes were seen at the two sites sampled within Guanica Bay.
An initial analysis of modeled PAH (hydrocarbon) data and coral species richness (reef building species) indicated a strong negative correlation between the presence of PAHs in the sediments and coral species richness. Additional work is needed to assess possible reasons for this observed pattern. (PDF contains 126 pages)
Chemical abundances of damped Lyman alpha systems in the XQ-100 survey
The XQ-100 survey has provided high signal-noise spectra of 100 redshift
3-4.5 quasars with the X-Shooter spectrograph. The metal abundances for 13
elements in the 41 damped Lyman alpha systems (DLAs) identified in the XQ-100
sample are presented, and an investigation into abundances of a variety of DLA
classes is conducted. The XQ-100 DLA sample contains five DLAs within 5000 km/s
of their host quasar (proximate DLAs; PDLAs) as well as three sightlines which
contain two DLAs within 10,000 km/s of each other along the same line-of-sight
(multiple DLAs; MDLAs). Combined with previous observations in the literature,
we demonstrate that PDLAs with logN(HI)<21.0 show lower [S/H] and [Fe/H]
(relative to intervening systems with similar redshift and N(HI)), whilst
higher [S/H] and [Si/H] are seen in PDLAs with logN(HI)>21.0. These abundance
discrepancies are independent of their line-of-sight velocity separation from
the host quasar, and the velocity width of the metal lines (v90). Contrary to
previous studies, MDLAs show no difference in [alpha/Fe] relative to single
DLAs matched in metallicity and redshift. In addition, we present follow-up
UVES data of J0034+1639, a sightline containing three DLAs, including a
metal-poor DLA with [Fe/H]=-2.82 (the third lowest [Fe/H] in DLAs identified to
date) at z=4.25. Lastly we study the dust-corrected [Zn/Fe], emphasizing that
near-IR coverage of X-Shooter provides unprecedented access to MgII, CaII and
TiII lines (at redshifts 3-4) to provide additional evidence for subsolar
[Zn/Fe] ratio in DLAs.Comment: Accepted to MNRAS. 19 pages plus Appendix material (102 pages total
Host Galaxies of Gamma-Ray Bursts and their Cosmological Evolution
We use numerical simulations of large scale structure formation to explore
the cosmological properties of Gamma-Ray Burst (GRB) host galaxies. Among the
different sub-populations found in the simulations, we identify the host
galaxies as the most efficient star-forming objects, i.e. galaxies with high
specific star formation rates. We find that the host candidates are low-mass,
young galaxies with low to moderate star formation rate. These properties are
consistent with those observed in GRB hosts, most of which are sub-luminous,
blue galaxies. Assuming that host candidates are galaxies with high star
formation rates would have given conclusions inconsistent with the
observations. The specific star formation rate, given a galaxy mass, is shown
to increase as the redshift increases. The low mass of the putative hosts makes
them difficult to detect with present day telescopes and the probability
density function of the specific star formation rate is predicted to change
depending on whether or not these galaxies are observed.Comment: 11 pages, 10 figures. Accepted for publication in MNRA
Approximation of L\"owdin Orthogonalization to a Spectrally Efficient Orthogonal Overlapping PPM Design for UWB Impulse Radio
In this paper we consider the design of spectrally efficient time-limited
pulses for ultrawideband (UWB) systems using an overlapping pulse position
modulation scheme. For this we investigate an orthogonalization method, which
was developed in 1950 by Per-Olov L\"owdin. Our objective is to obtain a set of
N orthogonal (L\"owdin) pulses, which remain time-limited and spectrally
efficient for UWB systems, from a set of N equidistant translates of a
time-limited optimal spectral designed UWB pulse. We derive an approximate
L\"owdin orthogonalization (ALO) by using circulant approximations for the Gram
matrix to obtain a practical filter implementation. We show that the centered
ALO and L\"owdin pulses converge pointwise to the same Nyquist pulse as N tends
to infinity. The set of translates of the Nyquist pulse forms an orthonormal
basis or the shift-invariant space generated by the initial spectral optimal
pulse. The ALO transform provides a closed-form approximation of the L\"owdin
transform, which can be implemented in an analog fashion without the need of
analog to digital conversions. Furthermore, we investigate the interplay
between the optimization and the orthogonalization procedure by using methods
from the theory of shift-invariant spaces. Finally we develop a connection
between our results and wavelet and frame theory.Comment: 33 pages, 11 figures. Accepted for publication 9 Sep 201
The effect of sampling effort on estimates of methane ebullition from peat
We investigated the effect of sample size and sampling duration on methane bubble flux (ebullition) estimates from peat using a computer model. A field scale (10 m), seasonal (> 100 days) simulation of ebullition from a two-dimensional structurally-varying peat profile was modelled at fine spatial resolution (1 mm × 1 mm). The spatial and temporal scale of this simulation was possible because of the computational efficiency of the reduced complexity approach that was implemented, and patterns of simulated ebullition were consistent with those found in the field and laboratory. The simulated ebullition from the peat profile suggested that decreases in peat porosity – which cause increases in gas storage – produce ebullition that becomes increasingly patchy in space and erratic in time. By applying different amounts of spatial and temporal sampling effort it was possible to determine the uncertainty in ebullition estimates from the peatland. The results suggest that traditional methods to measure ebullition can equally overestimate and underestimate flux by 20% and large ebullition events can lead to large overestimations of flux when sampling effort is low. Our findings support those of field studies, and we recommend that ebullition should be measured frequently (hourly to daily) and at many locations (n > 14)
Administrators in higher education: organizational expansion in a transforming institution
Recent European research has revealed growth in the number of administrators and professionals across different sections of universities—a long established trend in US universities. We build on this research by investigating the factors associated with variation in the proportion of administrators across 761 Higher Education Institutions (HEIs) in 11 European countries. We argue that the enactment of expanded and diversified missions of HE is one of the main factors nurturing universities’ profesional and administrative bodies. Our findings support such an assertion; regardless of geographical and institutional differences, HEIs with high levels of “entrepreneurialism” (e.g. in service provision and external engagement) are characterized by a larger proportion of administrative staff. However, we find no empirical support for arguments citing structural pressures and demands on HEIs due to higher student enrolments, budget cuts or deregulation as engines driving such change. Instead, our results point towards, as argued by neo-institutionalists, the diffusion of formal organization as a model of institutional identity and purpose, which is especially prevalent at high levels of external connectedness
High-Ion Absorption in Seven GRB Host Galaxies at z=2-4: Evidence for both Circumburst Plasma and Outflowing Interstellar Gas
(Abridged) We use VLT/UVES high-resolution optical spectroscopy of seven GRB
afterglows at z_GRB>2 to investigate circumburst and interstellar plasma in the
host galaxies. Our sample consists of GRBs 021004, 050730, 050820, 050922C,
060607, 071031, and 080310. Four of these spectra were taken in rapid-response
mode, within 30 minutes of the Swift GRB detection. We identify several
distinct categories of high-ion absorption at velocities close to z_GRB: (i)
Strong high-ion components at z_GRB itself are always seen in OVI, CIV, and
SiIV, and usually (in 6 of 7 cases) in NV. We discuss circumburst and
interstellar models for the origin of this absorption. Using the non-detection
of SIV* toward GRB 050730 together with a UV photo-excitation model, we place a
lower limit of 400 pc on the distance of the SIV-bearing gas from the GRB. (ii)
Complex, multi-component CIV and SiIV profiles extending over 100-400 km/s
around z_GRB are observed in each spectrum; these velocity fields are similar
to those measured in damped Lyman-alpha systems at similar redshifts,
suggesting a galactic origin. (iii) Asymmetric, blueshifted, absorption-line
wings covering 65-140 km/s are seen in the CIV, SiIV, and OVI profiles in 4 of
the 7 spectra. The wing kinematics together with the observation that two wings
show "Galactic" CIV/SiIV ratios suggest these features trace outflowing ISM gas
in the GRB host galaxies. (iv) High-velocity (HV; 500-5000 km/s) components are
detected in 6 of the 7 spectra. The HV components show diverse properties. In
the cases of GRBs 071031 and 080310, both the ionization level (very high
CIV/SiIV ratios and absence of neutral-phase absorption) and the kinematics of
the HV components can be explained by Wolf-Rayet winds from the GRB
progenitors.Comment: 20 pages, 9 figures (7 in color), accepted by A&A, updated with proof
corrections including changes to Table
On the consistency of the peculiar GRBs 060218 and 060614 with the Ep,i - Eiso correlation
We analyze and discuss the position of GRB 060218 and GRB 060614 in the Ep,i
- Eiso plane. GRB 060218 is important because of its similarity with GRB
980425, the proto-type event of the GRB-SN connection. While GRB 980425 is an
outlier of the Ep,i - Eiso correlation, we find that GRB 060218 is fully
consistent with it. This evidence, combined with the `chromatic' behavior of
the afterglow light curves, is at odds with the hypothesis that GRB 060218 was
a `standard' GRB seen off-axis and supports the existence of a class of truly
sub--energetic GRBs. GRB 060614 is a peculiar event not accompanied by a bright
Supernova. Based on published spectral information, we find that also this
event is consistent with the Ep,i - Eiso correlation. We discuss the
implications of our results for the rate of sub--energetic GRBs, the GRB/SN
connection and the properties of the newly discovered sub-class of long GRBs
not associated with bright Supernovae. We have included in our analysis other
recent GRBs with clear evidence (or clear no evidence) of associated SNe.Comment: 9 pages, 4 figures, FINAL REVISED VERSION (added Figure 4 and short
GRBs in Figure 1; minor changes and style corrections applied; references
updated), submitted to A&A on November 25, 200
The late jet in gamma-ray bursts and its interactions with a supernova ejecta and a cocoon
Late X-ray flares observed in X-ray afterglows of gamma-ray bursts (GRBs)
suggest late central engine activities at a few minuets to hours after the
burst. A few unambiguously confirmed cases of supernova associations with
nearby long GRBs imply that an accompanying supernova-like component might be a
common feature in all long GRB events. These motivate us to study the
interactions of a late jet, responsible for a x-ray flare, with various
components in a stellar explosion, responsible for a GRB. These components
include a supernova shell-like ejecta, and a cocoon that was produced when the
main jet producing the GRB itself was propagating through the progenitor star.
We find that the interaction between the late jet and the supernova ejecta may
produce a luminous (up to 10^49 erg s^-1) thermal X-ray transient lasting for ~
10 s. The interaction between the late jet and the cocoon produces synchrotron
self-absorbed non-thermal emission, with the observed peak X-ray flux density
from 0.001 micro Jy to 1 mJy at 1 keV and a peak optical flux density from 0.01
micro Jy to 0.1 Jy (for a redshift z= 2). The light curve due to the late jet -
cocoon interaction has very small pulse-width-to-time ratio, \Delta t / t
\approx 0.01 - 0.5, where t is the pulse peak time since the burst trigger.
Identifying these features in current and future observations would open a new
frontier in the study of GRB progenitor stars.Comment: 17 pages; MNRAS accepted; typos in the references are corrected
following the journal proofreadin
Intraspecific variation in M1 enamel development in modern humans: implications for human evolution
The timing and sequence of enamel development, as well as enamel thickness, was documented for individual cusps (protoconid, hypoconid,metaconid, entoconid) in 15 unworn permanent lower first molars (M1s) from a sample of modern human juveniles. These data were compared with previously published data for modern and fossil species reported in the literature.
Crown formation in all teeth was initiated in the protoconid and completed in the hypoconid. These cusps had significantly longer formation times (2.91 and 2.96 yrs, respectively) than the metaconid and entoconid (2.52 and 2.38 yrs, respectively), as well as thicker enamel, and each represented between 92e95% of the total crown formation time. Rates of enamel secretion in all cusps increased significantly from 2.97 mm in the inner enamel to 4.47 mm in the outer enamel. Two cusps of one individual were studied in more detail and did not follow this typical trajectory. Rather, there was a sharp decrease in the middle of enamel formation and then a slow recovery of secretion rates from the mid to outer enamel. This anomalous trajectory of enamel formation is discussed in the context of other nondental tissue responses to illness. Neither secretion rates nor periodicity differed significantly when compared between the cusps of each molar. Differences in cusp formation times, initiation, and completion suggest a relationship between the rates of enamel formation and enamel thickness. This fits with expectations about the mechanics of the chewing cycle and general lower molar morphology. A comparison with similar
data for some nonhuman primates and fossil hominoids suggests this relationship may hold true across several primate taxa. Other aspects of enamel growth differed between this human sample and certain fossil species. The lower molars formed slowly over a longer period of time,
which may reflect the extended growth period of modern humans. The methodological approach adopted in this study is discussed in the context of that used in other studies
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