541 research outputs found
Orientational correlations in confined DNA
We study how the orientational correlations of DNA confined to nanochannels
depend on the channel diameter D by means of Monte Carlo simulations and a
mean-field theory. This theory describes DNA conformations in the
experimentally relevant regime where the Flory-de Gennes theory does not apply.
We show how local correlations determine the dependence of the end-to-end
distance of the DNA molecule upon D. Tapered nanochannels provide the necessary
resolution in D to study experimentally how the extension of confined DNA
molecules depends upon D. Our experimental and theoretical results are in
qualitative agreement.Comment: Revised version including supplemental material, 7 pages, 8 figure
Extension of nano-confined DNA: quantitative comparison between experiment and theory
The extension of DNA confined to nanochannels has been studied intensively
and in detail. Yet quantitative comparisons between experiments and model
calculations are difficult because most theoretical predictions involve
undetermined prefactors, and because the model parameters (contour length, Kuhn
length, effective width) are difficult to compute reliably, leading to
substantial uncertainties. Here we use a recent asymptotically exact theory for
the DNA extension in the "extended de Gennes regime" that allows us to compare
experimental results with theory. For this purpose we performed new
experiments, measuring the mean DNA extension and its standard deviation while
varying the channel geometry, dye intercalation ratio, and ionic buffer
strength. The experimental results agree very well with theory at high ionic
strengths, indicating that the model parameters are reliable. At low ionic
strengths the agreement is less good. We discuss possible reasons. Our approach
allows, in principle, to measure the Kuhn length and effective width of a
single DNA molecule and more generally of semiflexible polymers in solution.Comment: Revised version, 6 pages, 2 figures, 1 table, supplementary materia
A dozen colliding wind X-ray binaries in the star cluster R136 in the 30Doradus region
We analyzed archival Chandra X-ray observations of the central portion of the
30 Doradus region in the Large Magellanic Cloud. The image contains 20 X-ray
point sources with luminosities between and erg s (0.2 -- 3.5 keV). A dozen sources have bright WN
Wolf-Rayet or spectral type O stars as optical counterparts. Nine of these are
within pc of R136, the central star cluster of NGC2070. We derive an
empirical relation between the X-ray luminosity and the parameters for the
stellar wind of the optical counterpart. The relation gives good agreement for
known colliding wind binaries in the Milky Way Galaxy and for the identified
X-ray sources in NGC2070. We conclude that probably all identified X-ray
sources in NGC2070 are colliding wind binaries and that they are not associated
with compact objects. This conclusion contradicts Wang (1995) who argued, using
ROSAT data, that two earlier discovered X-ray sources are accreting black-hole
binaries. Five of the eighteen brightest stars in R136 are not visible in our
X-ray observations. These stars are either single, have low mass companions or
very wide orbits. The resulting binary fraction among early type stars is then
unusually high (at least 70%).Comment: 23 pages, To appear in August in Ap
s-Process Nucleosynthesis in Carbon Stars
We present the first detailed and homogeneous analysis of the s-element
content in Galactic carbon stars of N-type. Abundances of Sr,Y, Zr (low-mass
s-elements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, hs) are
derived using the spectral synthesis technique from high-resolution spectra.
The N-stars analyzed are of nearly solar metallicity and show moderate
s-element enhancements, similar to those found in S stars, but smaller than
those found in the only previous similar study (Utsumi 1985), and also smaller
than those found in supergiant post-AGB stars. This is in agreement with the
present understanding of the envelope s-element enrichment in giant stars,
which is increasing along the spectral sequence M-->MS-->S-->SC-->C during the
AGB phase. We compare the observational data with recent -process
nucleosynthesis models for different metallicities and stellar masses. Good
agreement is obtained between low mass AGB star models (M < 3 M_o) and
s-elements observations. In low mass AGB stars, the 13C(alpha, n)16O reaction
is the main source of neutrons for the s-process; a moderate spread, however,
must exist in the abundance of 13C that is burnt in different stars. By
combining information deriving from the detection of Tc, the infrared colours
and the theoretical relations between stellar mass, metallicity and the final
C/O ratio, we conclude that most (or maybe all) of the N-stars studied in this
work are intrinsic, thermally-pulsing AGB stars; their abundances are the
consequence of the operation of third dredge-up and are not to be ascribed to
mass transfer in binary systems.Comment: 31 pages, 10 figures, 6 tables. Accepted in Ap
Analysis of colour-magnitude diagrams of rich LMC clusters: NGC 1831
We present the analysis of a deep colour-magnitude diagram (CMD) of NGC 1831,
a rich star cluster in the LMC. The data were obtained with HST/WFPC2 in the
F555W (~V) and F814W (~I) filters, reaching m_555 ~ 25. We discuss and apply a
method of correcting the CMD for sampling incompleteness and field star
contamination. Efficient use of the CMD data was made by means of direct
comparisons of the observed to model CMDs. The model CMDs are built by an
algorithm that generates artificial stars from a single stellar population,
characterized by an age, a metallicity, a distance, a reddening value, a
present day mass function and a fraction of unresolved binaries. Photometric
uncertainties are empirically determined from the data and incorporated into
the models as well. Statistical techniques are presented and applied as an
objective method to assess the compatibility between the model and data CMDs.
By modelling the CMD of the central region in NGC 1831 we infer a metallicity Z
= 0.012, 8.75 < log(tau) < 8.80, 18.54 < (m-M)_0 < 18.68 and 0.00 < E(B-V) <
0.03. For the position dependent PDMF slope (alpha = -dlog(Phi(M))/dlog(M)), we
clearly observe the effect of mass segregation in the system: for projected
distances R < 30 arcsec, alpha ~ 1.7, whereas 2.2 < alpha < 2.5 in the outer
regions of NGC 1831.Comment: 12 pages, 14 figure
Nanoconfined circular and linear DNA - equilibrium conformations and unfolding kinetics
Studies of circular DNA confined to nanofluidic channels are relevant both
from a fundamental polymer-physics perspective and due to the importance of
circular DNA molecules in vivo. We here observe the unfolding of DNA from the
circular to linear configuration as a light-induced double strand break occurs,
characterize the dynamics, and compare the equilibrium conformational
statistics of linear and circular configurations. This is important because it
allows us to determine to which extent existing statistical theories describe
the extension of confined circular DNA. We find that the ratio of the
extensions of confined linear and circular DNA configurations increases as the
buffer concentration decreases. The experimental results fall between
theoretical predictions for the extended de Gennes regime at weaker confinement
and the Odijk regime at stronger confinement. We show that it is possible to
directly distinguish between circular and linear DNA molecules by measuring the
emission intensity from the DNA. Finally, we determine the rate of unfolding
and show that this rate is larger for more confined DNA, possibly reflecting
the corresponding larger difference in entropy between the circular and linear
configurations.Comment: 21 pages, 7 figures, 1 tabl
A Survey of Local Group Galaxies Currently Forming Stars: III. A Search for Luminous Blue Variables and Other H-alpha Emission-Lined Stars
We describe a search for H-alpha emission-lined stars in M31, M33, and seven
dwarfs in or near the Local Group (IC 10, NGC 6822, WLM, Sextans B, Sextans A,
Pegasus and the Phoenix dwarf) using interference filter imaging with the KPNO
and CTIO 4-m telescope and Mosaic cameras. The survey is aimed primarily at
identifying new Luminous Blue Variables (LBVs) from their spectroscopic
similarity to known LBVs, avoiding the bias towards photometric variability,
which may require centuries to manifest itself if LBVs go through long
quiescent periods. Followup spectroscopy with WIYN confirms that our survey
detected a wealth of stars whose spectra are similar to the known LBVs. We
"classify" the spectra of known LBVs, and compare these to the spectra of the
new LBV candidates. We demonstrate spectacular spectral variability for several
of the new LBV candidates, such as AM2, previously classified as a Wolf-Rayet
star, which now shows FeI, FeII and Balmer emission lines but neither the NIII
4634,42 nor HeII 4686 emission that it did in 1982. Profound spectral changes
are also noted for other suspected and known LBVs. Several of the LBV
candidates also show >0.5 mag changes in V over the past 10-20 years. The
number of known or suspected LBVs is now 24 in M31, 37 in M33, 1 in NGC 6822,
and 3 in IC 10. We estimate that the total number of LBVs in M31 and M33 may be
several hundred, in contrast to the 8 known historically through large-scale
photometric variability. This has significant implications for the time scale
of the LBV phase. We also identify a few new WRs and peculiar emission-lined
objects.Comment: Accepted by the Astronomical Journal. Version with higher quality
figures may be downloaded from http://www.lowell.edu/users/massey/has.pdf.g
Hubble Space Telescope Photometry of Hodge 301: An "Old" Star Cluster in 30 Doradus
We present Hubble Space Telescope Planetary Camera UVI data for the
little-studied cluster Hodge 301 3' northwest of 30 Doradus' central ionizing
cluster R136. The average reddening of Hodge 301 is found to be =
(0.28+-0.05) mag from published infrared and ultraviolet photometry. Using two
different sets of evolutionary models, we derive an age of ~ 20-25 Myr for
Hodge 301, which makes it roughly 10 times as old as R136. Hodge 301 is the
most prominent representative of the oldest population in the 30 Dor starburst
region; a region that has undergone multiple star formation events. This range
of ages is an important consideration for the modelling of starburst regions.
Hodge 301 shows a widened upper main sequence largely caused by Be stars. We
present a list of Be star candidates. The slope of the initial mass function
for intermediate-mass main sequence stars ranging from 10 to 1.3 solar masses
is found to be -1.4+-0.1 in good agreement with a Salpeter law. There is no
indication for a truncation or change of slope of the IMF within this mass
range. In accordance with the age of Hodge 301 no obvious pre-main-sequence
stars are seen down to 1 solar mass. We estimate that up to 41+-7 stars with
more than 12 solar masses may have turned into supernovae since the formation
of the cluster. Multiple supernova explosions are the most likely origin of the
extremely violent gas motions and the diffuse X-ray emission observed in the
cluster surroundings.Comment: To appear in the Astronomical Journal (Feb 2000 issue). 16 pages in
two-column style. 9 separate figures, in part in significantly reduced
resolution for space reasons (bitmapped postscript or jpg
Evolution, nucleosynthesis and yields of low mass AGB stars at different metallicities (II): the FRUITY database
By using updated stellar low mass stars models, we can systematically
investigate the nucleosynthesis processes occurring in AGB stars, when these
objects experience recurrent thermal pulses and third dredge-up episodes. In
this paper we present the database dedicated to the nucleosynthesis of AGB
stars: the FRUITY (FRANEC Repository of Updated Isotopic Tables & Yields)
database. An interactive web-based interface allows users to freely download
the full (from H to Bi) isotopic composition, as it changes after each third
dredge-up episode and the stellar yields the models produce. A first set of AGB
models, having masses in the range 1.5 < M/Msun < 3.0 and metallicities 1e-3 <
Z < 2e-2, is discussed here. For each model, a detailed description of the
physical and the chemical evolution is provided. In particular, we illustrate
the details of the s-process and we evaluate the theoretical uncertainties due
to the parametrization adopted to model convection and mass loss. The resulting
nucleosynthesis scenario is checked by comparing the theoretical [hs/ls] and
[Pb/hs] ratios to those obtained from the available abundance analysis of
s-enhanced stars. On the average, the variation with the metallicity of these
spectroscopic indexes is well reproduced by theoretical models, although the
predicted spread at a given metallicity is substantially smaller than the
observed one. Possible explanations for such a difference are briefly
discussed. An independent check of the third dredge-up efficiency is provided
by the C-stars luminosity function. Consequently, theoretical C-stars
luminosity functions for the Galactic disk and the Magellanic Clouds have been
derived. We generally find a good agreement with observations.Comment: Accepted for Publication on The Astrophysical Journal Supplement
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