6,742 research outputs found
The shape of the initial cluster mass function: what it tells us about the local star formation efficiency
We explore how the expulsion of gas from star-cluster forming cloud-cores due
to supernova explosions affects the shape of the initial cluster mass function,
that is, the mass function of star clusters when effects of gas expulsion are
over. We demonstrate that if the radii of cluster-forming gas cores are roughly
constant over the core mass range, as supported by observations, then more
massive cores undergo slower gas expulsion. Therefore, for a given star
formation efficiency, more massive cores retain a larger fraction of stars
after gas expulsion. The initial cluster mass function may thus differ from the
core mass function substantially, with the final shape depending on the star
formation efficiency. A mass-independent star formation efficiency of about 20
per cent turns a power-law core mass function into a bell-shaped initial
cluster mass function, while mass-independent efficiencies of order 40 per cent
preserve the shape of the core mass function.Comment: accepted in Ap
Oxford Nanopore sequencing, hybrid error correction, and de novo assembly of a eukaryotic genome
Monitoring the progress of DNA molecules through a membrane pore has been postulated as a method for sequencing DNA for several decades. Recently, a nanopore-based sequencing instrument, the Oxford Nanopore MinION, has become available, and we used this for sequencing the Saccharomyces cerevisiae genome. To make use of these data, we developed a novel open-source hybrid error correction algorithm Nanocorr specifically for Oxford Nanopore reads, because existing packages were incapable of assembling the long read lengths (5-50 kbp) at such high error rates (between approximately 5% and 40% error). With this new method, we were able to perform a hybrid error correction of the nanopore reads using complementary MiSeq data and produce a de novo assembly that is highly contiguous and accurate: The contig N50 length is more than ten times greater than an Illumina-only assembly (678 kb versus 59.9 kbp) and has >99.88% consensus identity when compared to the reference. Furthermore, the assembly with the long nanopore reads presents a much more complete representation of the features of the genome and correctly assembles gene cassettes, rRNAs, transposable elements, and other genomic features that were almost entirely absent in the Illumina-only assembly
The Star Clusters in the Starburst Irregular Galaxy NGC 1569
We examine star clusters in the irregular, starburst galaxy NGC 1569 from HST
images. In addition to the two known super star clusters, we identify 45 other
clusters that are compact but resolved. Integrated UVI colors of the clusters
span a large range, and suggest that ages range from 3 Myrs to 1 Gyr. However,
most of the clusters were formed at the tail end of the recent starburst.
Numerous clusters in addition to the know super star clusters are similar in
luminosity to a small globular cluster. We examined the radial surface
brightness of four of the clusters. Their half-light radii and core radii are
in the range observed in present-day globular clusters. Therefore, conditions
that produced the recent starburst have also been those necessary for producing
compact, bright star clusters.
We examine resolved stars in the outer parts of the two super star clusters.
Cluster A is dominated by bright blue stars with a small population of red
supergiants. Sub-components A1 and A2 have similar colors and a two-dimensional
color map does not offer evidence that one component is dominated by red
supergiants and the other not. The contradiction of the presence of red super-
giants with Wolf-Rayet stars may instead not be a contradiction at all since
there coexistence in a coeval population is not inconsistent with the evolution
of massive stars. Cluster B is dominated by red supergiants, and this is
confirmed by the presence of the stellar CO absorption feature in an integrated
spectrum. The various age indicators are consistent with a picture in which
cluster B is of order 10--20 Myrs old, and cluster A is >4-5 Myrs old.Comment: To be published in AJ, November 200
Nonlinear software sensor for monitoring genetic regulation processes with noise and modeling errors
Nonlinear control techniques by means of a software sensor that are commonly
used in chemical engineering could be also applied to genetic regulation
processes. We provide here a realistic formulation of this procedure by
introducing an additive white Gaussian noise, which is usually found in
experimental data. Besides, we include model errors, meaning that we assume we
do not know the nonlinear regulation function of the process. In order to
illustrate this procedure, we employ the Goodwin dynamics of the concentrations
[B.C. Goodwin, Temporal Oscillations in Cells, (Academic Press, New York,
1963)] in the simple form recently applied to single gene systems and some
operon cases [H. De Jong, J. Comp. Biol. 9, 67 (2002)], which involves the
dynamics of the mRNA, given protein, and metabolite concentrations. Further, we
present results for a three gene case in co-regulated sets of transcription
units as they occur in prokaryotes. However, instead of considering their full
dynamics, we use only the data of the metabolites and a designed software
sensor. We also show, more generally, that it is possible to rebuild the
complete set of nonmeasured concentrations despite the uncertainties in the
regulation function or, even more, in the case of not knowing the mRNA
dynamics. In addition, the rebuilding of concentrations is not affected by the
perturbation due to the additive white Gaussian noise and also we managed to
filter the noisy output of the biological systemComment: 21 pages, 7 figures; also selected in vjbio of August 2005; this
version corrects a misorder in the last three references of the published
versio
Physiological and Growth Responses of Midrotation Loblolly Pine to Treatments of Fire, Herbicide, and Fertilizer
The objectives of this study were to examine the effects of fertilizer and understory vegetation control (herbicide and prescribed fire) on mature tree physiology and to link observed physiological responses with tree growth. Photosynthetic rate (photosynthesis), transpiration, stomatal conductance, stem diameter, and crown area were measured in two midrotation loblolly pine plantations in East Texas. Rates of photosynthesis, E and gs for the midsummer measurements were significantly reduced by fertilizer treatments on both sites. Drought in east Texas during the last 3 years may have influenced this result. Trees receiving fire at the site with a comparatively more dense live pre-treatment understory exhibited higher rates of photosynthesis; however, prescribed fire had no positive effect on growth. Herbicide treatment significantly increased the change in diameter growth of unfertilized trees at both sites between 1999-2001 by an average of 5 percent, but had no effect on growth of fertilized trees. Increase in diameter growth per unit crown area was significantly greater in unfertilized trees receiving herbicide than in controls
Hubble Space Telescope Observations of the Oldest Star Clusters in the LMC
We present V, V-I color-magnitude diagrams (CMDs) for three old star clusters
in the Large Magellanic Cloud (LMC): NGC 1466, NGC 2257 and Hodge 11. Our data
extend about 3 magnitudes below the main-sequence turnoff, allowing us to
determine accurate relative ages and the blue straggler frequencies. Based on a
differential comparison of the CMDs, any age difference between the three LMC
clusters is less than 1.5 Gyr. Comparing their CMDs to those of M 92 and M 3,
the LMC clusters, unless their published metallicities are significantly in
error, are the same age as the old Galactic globulars. The similar ages to
Galactic globulars are shown to be consistent with hierarchial clustering
models of galaxy formation. The blue straggler frequencies are also similar to
those of Galactic globular clusters. We derive a true distance modulus to the
LMC of (m-M)=18.46 +/- 0.09 (assuming (m-M)=14.61 for M 92) using these three
LMC clusters.Comment: 22 pages; to be published in Ap
Experimental Evidence of a Haldane Gap in an S = 2 Quasi-linear Chain Antiferromagnet
The magnetic susceptibility of the quasi-linear chain Heisenberg
antiferromagnet (2,-bipyridine)trichloromanganese(III), MnCl_{3}(bipy), has
been measured from 1.8 to 300 K with the magnetic field, H, parallel and
perpendicular to the chains. The analyzed data yield and K. The magnetization, M, has been studied at 30 mK and 1.4 K in H up to 16
T. No evidence of long-range order is observed. Depending on crystal
orientation, at 30 mK until a critical field is achieved ( and $H_{c\bot} = 1.8\pm 0.2 T), where M increases continuously
as H is increased. These results are interpreted as evidence of a Haldane gap.Comment: 11 pages, 4 figure
The SAMI Galaxy Survey: Gas Streaming and Dynamical M/L in Rotationally Supported Systems
Line-of-sight velocities of gas and stars can constrain dark matter (DM)
within rotationally supported galaxies if they trace circular orbits
extensively. Photometric asymmetries may signify non-circular motions,
requiring spectra with dense spatial coverage. Our integral-field spectroscopy
of 178 galaxies spanned the mass range of the SAMI Galaxy Survey. We derived
circular speed curves (CSCs) of gas and stars from non-parametric Diskfit fits
out to . For 12/14 with measured H I profiles, ionized gas and H I
maximum velocities agreed. We fitted mass-follows-light models to 163 galaxies
by approximating the radial starlight profile as nested, very flattened mass
homeoids viewed as a S\'ersic form. Fitting broad-band SEDs to SDSS images gave
median stellar mass/light 1.7 assuming a Kroupa IMF vs. 2.6 dynamically.
Two-thirds of the dynamical mass/light measures were consistent with
star+remnant IMFs. One-fifth required upscaled starlight to fit, hence
comparable mass of unobserved baryons and/or DM distributed similarly across
the SAMI aperture that came to dominate motions as the starlight CSC declined
rapidly. The rest had mass distributed differently from starlight. Subtracting
fits of S\'ersic profiles to 13 VIKING Z-band images revealed residual weak
bars. Near the bar PA, we assessed m = 2 streaming velocities, and found
deviations usually <30 km/s from the CSC; three showed no deviation. Thus,
asymmetries rarely influenced our CSCs despite co-located shock-indicating,
emission-line flux ratios in more than 2/3.Comment: 21 pages, 15 figures. Accepted to MNRA
Gesture analysis for physics education researchers
Systematic observations of student gestures can not only fill in gaps in
students' verbal expressions, but can also offer valuable information about
student ideas, including their source, their novelty to the speaker, and their
construction in real time. This paper provides a review of the research in
gesture analysis that is most relevant to physics education researchers and
illustrates gesture analysis for the purpose of better understanding student
thinking about physics.Comment: 14 page
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