14,192 research outputs found
Osteology of the southwestern darters, Ethostoma (Oligocephalus) (Teleostei, Percidae) - with comparison to other North American percid fishes
http://deepblue.lib.umich.edu/bitstream/2027.42/57169/1/OP733.pd
Comment on "Self-Purification in Semiconductor Nanocrystals"
In a recent Letter [PRL 96, 226802 (2006)], Dalpian and Chelikowsky claimed
that formation energies of Mn impurities in CdSe nanocrystals increase as the
size of the nanocrystal decreases, and argued that this size dependence leads
to "self-purification" of small nanocrystals. They presented
density-functional-theory (DFT) calculations showing a strong size dependence
for Mn impurity formation energies, and proposed a general explanation. In this
Comment we show that several different DFT codes, pseudopotentials, and
exchange-correlation functionals give a markedly different result: We find no
such size dependence. More generally, we argue that formation energies are not
relevant to substitutional doping in most colloidally grown nanocrystals.Comment: 1 page, 1 figur
Symbiont 'bleaching' in planktic foraminifera during the Middle Eocene Climatic Optimum
Many genera of modern planktic foraminifera are adapted to nutrient-poor (oligotrophic) surface waters by hosting photosynthetic symbionts, but it is unknown how they will respond to future changes in ocean temperature and acidity. Here we show that ca. 40 Ma, some fossil photosymbiont-bearing planktic foraminifera were temporarily 'bleached' of their symbionts coincident with transient global warming during the Middle Eocene Climatic Optimum (MECO). At Ocean Drilling Program (ODP) Sites 748 and 1051 (Southern Ocean and mid-latitude North Atlantic, respectively), the typically positive relationship between the size of photosymbiont-bearing planktic foraminifer tests and their carbon isotope ratios (δ13C) was temporarily reduced for ∼100 k.y. during the peak of the MECO. At the same time, the typically photosymbiont-bearing planktic foraminifera Acarinina suffered transient reductions in test size and relative abundance, indicating ecological stress. The coincidence of minimum δ18O values and reduction in test size–δ13C gradients suggests a link between increased sea-surface temperatures and bleaching during the MECO, although changes in pH and nutrient availability may also have played a role. Our findings show that host-photosymbiont interactions are not constant through geological time, with implications for both the evolution of trophic strategies in marine plankton and the reliability of geochemical proxy records generated from symbiont-bearing planktic foraminifera
The lag and duration-luminosity relations of gamma-ray burst pulses
Relations linking the temporal or/and spectral properties of the prompt
emission of gamma-ray bursts (hereafter GRBs) to the absolute luminosity are of
great importance as they both constrain the radiation mechanisms and represent
potential distance indicators. Here we discuss two such relations: the
lag-luminosity relation and the newly discovered duration-luminosity relation
of GRB pulses. We aim to extend our previous work on the origin of spectral
lags, using the duration-luminosity relation recently discovered by Hakkila et
al. to connect lags and luminosity. We also present a way to test this relation
which has originally been established with a limited sample of only 12 pulses.
We relate lags to the spectral evolution and shape of the pulses with a linear
expansion of the pulse properties around maximum. We then couple this first
result to the duration-luminosity relation to obtain the lag-luminosity and
lag-duration relations. We finally use a Monte-Carlo method to generate a
population of synthetic GRB pulses which is then used to check the validity of
the duration-luminosity relation. Our theoretical results for the lag and
duration-luminosity relations are in good agreement with the data. They are
rather insensitive to the assumptions regarding the burst spectral parameters.
Our Monte Carlo analysis of a population of synthetic pulses confirms that the
duration-luminosity relation must be satisfied to reproduce the observational
duration-peak flux diagram of BATSE GRB pulses. The newly discovered
duration-luminosity relation offers the possibility to link all three
quantities: lag, duration and luminosity of GRB pulses in a consistent way.
Some evidence for its validity have been presented but its origin is not easy
to explain in the context of the internal shock model.Comment: 8 pages, 5 figures, 1 tabl
Comparison of outcomes of active surveillance in intermediate-risk versus low-risk localised prostate cancer patients: A systematic review and meta-analysis
Currently, there is no clear consensus regarding the role of active surveillance (AS) in the management of intermediate-risk prostate cancer (IRPC) patients. We aim to analyse data from the available literature on the outcomes of AS in the management of IRPC patients and compare them with low-risk prostate cancer (LRPC) patients. A comprehensive literature search was performed, and relevant data were extracted. Our primary outcome was treatment-free survival, and secondary outcomes were metastasis-free survival, cancer-specific survival, and overall survival. The DerSimonian–Laird random-effects method was used for the meta-analysis. Out of 712 studies identified following an initial search, 25 studies were included in the systematic review. We found that both IRPC and LRPC patients had nearly similar 5, 10, and 15 year treatment-free survival rate, 5 and 10 year metastasis-free survival rate, and 5 year overall survival rate. However, cancer-specific survival rates at 5, 10, and 15 years were significantly lower in IRPC compared to LRPC group. Furthermore, IRPC patients had significantly inferior long-term overall survival rate (10 and 15 year) and metastasis-free survival rate (15 year) compared to LRPC patients. Both the clinicians and the patients can consider this information during the informed decision-making process before choosing AS
A Survey of the Galactic Plane for 6.7-GHz Methanol Masers I: l = 325.0 - 335.0 ; b = -0.53 - 0.53
We report the results of the first complete survey of an area of the Galactic
Plane for maser emission from the 6.7-GHz transition of methanol. The survey
covers a 10.6-square-degree region of the Galactic Plane in the longitude range
325-335 degrees and latitude range -0.53-0.53 degrees. The survey is sensitive
to masers with a peak flux density greater than approximately 2.6 Jy. The
weakest maser detected has a peak flux density of 2.3 Jy and the strongest a
peak flux density of 425 Jy. We detected a total of 50 distinct masers, 26 of
which are new detections. We show that many 6.7-GHz methanol masers are not
associated with IRAS sources, and that some are associated with sources that
have colours differing from those of a typical ultra-compact HII region
(UCHII). We estimate that the number of UCHII regions in the Galaxy is
significantly more than suggested by IRAS-based estimates, possibly by more
than a factor of two.Comment: 19 pages including 4 figures, using LaTeX formatted with mn.sty,
accepted for publication in MNRA
The Spectroscopic Age of 47 Tuc
High signal-to-noise integrated spectra of the metal-rich globular cluster 47
Tuc, spanning the H-gamma(HR) and Fe4668 line indices, have been obtained. The
combination of these indices has been suggested (Jones & Worthey 1995, ApJ,
446, L31) as the best available mechanism for cleanly separating the
age-metallicity degeneracy which hampers the dating of distant, unresolved,
elliptical galaxies. For the first time, we apply this technique to a nearby
spheroidal system, 47 Tuc, for which independent ages, based upon more
established methods, exist. Such an independent test of the technique's
suitability has not been attempted before, but is an essential one before its
application to more distant, unresolved, stellar populations can be considered
valid. Because of its weak series of Balmer lines, relative to model spectra,
our results imply a spectroscopic ``age'' for 47 Tuc well in excess of 20 Gyr,
at odds with the colour-magnitude diagram age of 14+/-1 Gyr. The derived metal
abundance, however, is consistent with the known value. Emission ``fill-in'' of
the H-gamma line as the source of the discrepancy cannot be entirely excluded
by existing data, although the observational constraints are restrictive.Comment: 17 pages, 4 figures, LaTeX, accepted for publication in The
Astronomical Journal, also available at
http://casa.colorado.edu/~bgibson/publications.htm
The Most Metal-Poor Stars. II. Chemical Abundances of 190 Metal-Poor Stars Including 10 New Stars With [Fe/H] < -3.5
We present a homogeneous chemical abundance analysis of 16 elements in 190
metal-poor Galactic halo stars (38 program and 152 literature objects). The
sample includes 171 stars with [Fe/H] < -2.5, of which 86 are extremely metal
poor, [Fe/H] < -3.0. Our program stars include ten new objects with [Fe/H] <
-3.5. We identify a sample of "normal" metal-poor stars and measure the trends
between [X/Fe] and [Fe/H], as well as the dispersion about the mean trend for
this sample. Using this mean trend, we identify objects that are chemically
peculiar relative to "normal" stars at the same metallicity. These chemically
unusual stars include CEMP-no objects, one star with high [Si/Fe], another with
high [Ba/Sr], and one with unusually low [X/Fe] for all elements heavier than
Na. The Sr and Ba abundances indicate that there may be two nucleosynthetic
processes at lowest metallicity that are distinct from the main r-process.
Finally, for many elements, we find a significant trend between [X/Fe] versus
Teff which likely reflects non-LTE and/or 3D effects. Such trends demonstrate
that care must be exercised when using abundance measurements in metal-poor
stars to constrain chemical evolution and/or nucleosynthesis predictions.Comment: Accepted for publication in Ap
- …