65 research outputs found
Rheology of Ring Polymer Melts: From Linear Contaminants to Ring/Linear Blends
Ring polymers remain a major challenge to our current understanding of
polymer dynamics. Experimental results are difficult to interpret because of
the uncertainty in the purity and dispersity of the sample. Using both
equilibrium and non-equilibrium molecular dynamics simulations we have
systematically investigated the structure, dynamics and rheology of perfectly
controlled ring/linear polymer blends with chains of such length and
flexibility that the number of entanglements is up to about 14 per chain, which
is comparable to experimental systems examined in the literature. The smallest
concentration at which linear contaminants increase the zero-shear viscosity of
a ring polymer melt of these chain lengths by 10% is approximately one-fifth of
their overlap concentration. When the two architectures are present in equal
amounts the viscosity of the blend is approximately twice as large as that of
the pure linear melt. At this concentration the diffusion coefficient of the
rings is found to decrease dramatically, while the static and dynamic
properties of the linear polymers are mostly unaffected. Our results are
supported by a primitive path analysis.Comment: 5 pages, 4 figures, accepted by PR
Molecular Dynamics Simulation Study of Nonconcatenated Ring Polymers in a Melt: I. Statics
Molecular dynamics simulations were conducted to investigate the structural
properties of melts of nonconcatenated ring polymers and compared to melts of
linear polymers. The longest rings were composed of N=1600 monomers per chain
which corresponds to roughly 57 entanglement lengths for comparable linear
polymers. For the rings, the radius of gyration squared was found to scale as N
to the 4/5 power for an intermediate regime and N to the 2/3 power for the
larger rings indicating an overall conformation of a crumpled globule. However,
almost all beads of the rings are "surface beads" interacting with beads of
other rings, a result also in agreement with a primitive path analysis
performed in the following paper (DOI: 10.1063/1.3587138). Details of the
internal conformational properties of the ring and linear polymers as well as
their packing are analyzed and compared to current theoretical models.Comment: 15 pages, 14 figure
Persistent starspot signals on M dwarfs: multi-wavelength Doppler observations with the Habitable-zone Planet Finder and Keck/HIRES
Young, rapidly-rotating M dwarfs exhibit prominent starspots, which create
quasiperiodic signals in their photometric and Doppler spectroscopic
measurements. The periodic Doppler signals can mimic radial velocity (RV)
changes expected from orbiting exoplanets. Exoplanets can be distinguished from
activity-induced false positives by the chromaticity and long-term incoherence
of starspot signals, but these qualities are poorly constrained for
fully-convective M stars. Coherent photometric starspot signals on M dwarfs may
persist for hundreds of rotations, and the wavelength dependence of starspot RV
signals may not be consistent between stars due to differences in their
magnetic fields and active regions. We obtained precise multi-wavelength RVs of
four rapidly-rotating M dwarfs (AD Leo, G 227-22, GJ 1245B, GJ 3959) using the
near-infrared (NIR) Habitable-zone Planet Finder, and the optical Keck/HIRES
spectrometer. Our RVs are complemented by photometry from Kepler, TESS, and the
Las Cumbres Observatory (LCO) network of telescopes. We found that all four
stars exhibit large spot-induced Doppler signals at their rotation periods, and
investigated the longevity and optical-to-NIR chromaticity for these signals.
The phase curves remain coherent much longer than is typical for Sunlike stars.
Their chromaticity varies, and one star (GJ 3959) exhibits optical and NIR RV
modulation consistent in both phase and amplitude. In general, though, we find
that the NIR amplitudes are lower than their optical counterparts. We conclude
that starspot modulation for rapidly-rotating M stars frequently remains
coherent for hundreds of stellar rotations, and gives rise to Doppler signals
that, due to this coherence, may be mistaken for exoplanets.Comment: Accepted for publication in the Astrophysical Journa
First radial velocity results from the MINiature Exoplanet Radial Velocity Array (MINERVA)
The MINiature Exoplanet Radial Velocity Array (MINERVA) is a dedicated
observatory of four 0.7m robotic telescopes fiber-fed to a KiwiSpec
spectrograph. The MINERVA mission is to discover super-Earths in the habitable
zones of nearby stars. This can be accomplished with MINERVA's unique
combination of high precision and high cadence over long time periods. In this
work, we detail changes to the MINERVA facility that have occurred since our
previous paper. We then describe MINERVA's robotic control software, the
process by which we perform 1D spectral extraction, and our forward modeling
Doppler pipeline. In the process of improving our forward modeling procedure,
we found that our spectrograph's intrinsic instrumental profile is stable for
at least nine months. Because of that, we characterized our instrumental
profile with a time-independent, cubic spline function based on the profile in
the cross dispersion direction, with which we achieved a radial velocity
precision similar to using a conventional "sum-of-Gaussians" instrumental
profile: 1.8 m s over 1.5 months on the RV standard star HD 122064.
Therefore, we conclude that the instrumental profile need not be perfectly
accurate as long as it is stable. In addition, we observed 51 Peg and our
results are consistent with the literature, confirming our spectrograph and
Doppler pipeline are producing accurate and precise radial velocities.Comment: 22 pages, 9 figures, submitted to PASP, Peer-Reviewed and Accepte
Integrating transposable elements in the 3D genome
Chromosome organisation is increasingly recognised as an essential component of genome regulation, cell fate and cell health. Within the realm of transposable elements (TEs) however, the spatial information of how genomes are folded is still only rarely integrated in experimental studies or accounted for in modelling. Whilst polymer physics is recognised as an important tool to understand the mechanisms of genome folding, in this commentary we discuss its potential applicability to aspects of TE biology. Based on recent works on the relationship between genome organisation and TE integration, we argue that existing polymer models may be extended to create a predictive framework for the study of TE integration patterns. We suggest that these models may offer orthogonal and generic insights into the integration profiles (or "topography") of TEs across organisms. In addition, we provide simple polymer physics arguments and preliminary molecular dynamics simulations of TEs inserting into heterogeneously flexible polymers. By considering this simple model, we show how polymer folding and local flexibility may generically affect TE integration patterns. The preliminary discussion reported in this commentary is aimed to lay the foundations for a large-scale analysis of TE integration dynamics and topography as a function of the three-dimensional host genome
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