2,123 research outputs found
The Young Substellar Companion ROXs 12 B: Near-Infrared Spectrum, System Architecture, and Spin-Orbit Misalignment
ROXs 12 (2MASS J16262803-2526477) is a young star hosting a directly imaged
companion near the deuterium-burning limit. We present a suite of
spectroscopic, imaging, and time-series observations to characterize the
physical and environmental properties of this system. Moderate-resolution
near-infrared spectroscopy of ROXs 12 B from Gemini-North/NIFS and Keck/OSIRIS
reveals signatures of low surface gravity including weak alkali absorption
lines and a triangular -band pseudo-continuum shape. No signs of Pa
emission are evident. As a population, however, we find that about half (46
14\%) of young (15 Myr) companions with masses 20
possess actively accreting subdisks detected via Pa
line emission, which represents a lower limit on the prevalence of
circumplanetary disks in general as some are expected to be in a quiescent
phase of accretion. The bolometric luminosity of the companion and age of the
host star (6 Myr) imply a mass of 17.5 1.5
for ROXs 12 B based on hot-start evolutionary models. We identify a wide (5100
AU) tertiary companion to this system, 2MASS J16262774-2527247, which is
heavily accreting and exhibits stochastic variability in its light curve.
By combining sin measurements with rotation periods from , we
constrain the line-of-sight inclinations of ROXs 12 A and 2MASS
J16262774-2527247 and find that they are misaligned by
60. In addition, the orbital axis of ROXs 12 B is likely
misaligned from the spin axis of its host star ROXs 12 A, suggesting that ROXs
12 B formed akin to fragmenting binary stars or in an equatorial disk that was
torqued by the wide stellar tertiary.Comment: AJ, accepte
On Lagrangian and vortex-surface fields for flows with Taylor–Green and Kida–Pelz initial conditions
For a strictly inviscid barotropic flow with conservative body forces, the Helmholtz vorticity theorem shows that material or Lagrangian surfaces which are vortex surfaces at time t = 0 remain so for t > 0. In this study, a systematic methodology is developed for constructing smooth scalar fields φ(x, y, z, t = 0) for Taylor–Green and Kida–Pelz velocity fields, which, at t = 0, satisfy ω·∇φ = 0. We refer to such fields as vortex-surface fields. Then, for some constant C, iso-surfaces φ = C define vortex surfaces. It is shown that, given the vorticity, our definition of a vortex-surface field admits non-uniqueness, and this is presently resolved numerically using an optimization approach. Additionally, relations between vortex-surface fields and the classical Clebsch representation are discussed for flows with zero helicity. Equations describing the evolution of vortex-surface fields are then obtained for both inviscid and viscous incompressible flows. Both uniqueness and the distinction separating the evolution of vortex-surface fields and Lagrangian fields are discussed. By tracking φ as a Lagrangian field in slightly viscous flows, we show that the well-defined evolution of Lagrangian surfaces that are initially vortex surfaces can be a good approximation to vortex surfaces at later times prior to vortex reconnection. In the evolution of such Lagrangian fields, we observe that initially blob-like vortex surfaces are progressively stretched to sheet-like shapes so that neighbouring portions approach each other, with subsequent rolling up of structures near the interface, which reveals more information on dynamics than the iso-surfaces of vorticity magnitude. The non-local geometry in the evolution is quantified by two differential geometry properties. Rolled-up local shapes are found in the Lagrangian structures that were initially vortex surfaces close to the time of vortex reconnection. It is hypothesized that this is related to the formation of the very high vorticity regions
Phase Stabilization of a Frequency Comb using Multipulse Quantum Interferometry
From the interaction between a frequency comb and an atomic qubit, we derive
quantum protocols for the determination of the carrier-envelope offset phase,
using the qubit coherence as a reference, and without the need of frequency
doubling or an octave spanning comb. Compared with a trivial interference
protocol, the multipulse protocol results in a polynomial enhancement of the
sensitivity O(N^{-2}) with the number N of laser pulses involved. We present
specializations of the protocols using optical or hyperfine qubits,
Lambda-schemes and Raman transitions, and introduce methods where the reference
is another phase-stable cw-laser or frequency comb
Excitation spectra and rf-response near the polaron-to-molecule transition from the functional renormalization group
A light impurity in a Fermi sea undergoes a transition from a polaron to a
molecule for increasing interaction. We develop a new method to compute the
spectral functions of the polaron and molecule in a unified framework based on
the functional renormalization group with full self-energy feedback. We discuss
the energy spectra and decay widths of the attractive and repulsive polaron
branches as well as the molecular bound state and confirm the scaling of the
excited state decay rate near the transition. The quasi-particle weight of the
polaron shifts from the attractive to the repulsive branch across the
transition, while the molecular bound state has a very small residue
characteristic for a composite particle. We propose an experimental procedure
to measure the repulsive branch in a Li6 Fermi gas using rf-spectroscopy and
calculate the corresponding spectra.Comment: 15 pages, 13 figures; v2: version published in Phys. Rev.
Multiprocessing techniques for unmanned multifunctional satellites Final report,
Simulation of on-board multiprocessor for long lived unmanned space satellite contro
Stochastic analysis of nonlinear dynamics and feedback control for gene regulatory networks with applications to synthetic biology
The focus of the thesis is the investigation of the generalized repressilator model
(repressing genes ordered in a ring structure). Using nonlinear bifurcation analysis
stable and quasi-stable periodic orbits in this genetic network are characterized
and a design for a switchable and controllable genetic oscillator is proposed. The
oscillator operates around a quasi-stable periodic orbit using the classical engineering
idea of read-out based control. Previous genetic oscillators have been
designed around stable periodic orbits, however we explore the possibility of
quasi-stable periodic orbit expecting better controllability.
The ring topology of the generalized repressilator model has spatio-temporal
symmetries that can be understood as propagating perturbations in discrete lattices.
Network topology is a universal cross-discipline transferable concept and
based on it analytical conditions for the emergence of stable and quasi-stable
periodic orbits are derived. Also the length and distribution of quasi-stable oscillations
are obtained. The findings suggest that long-lived transient dynamics
due to feedback loops can dominate gene network dynamics.
Taking the stochastic nature of gene expression into account a master equation
for the generalized repressilator is derived. The stochasticity is shown to influence
the onset of bifurcations and quality of oscillations. Internal noise is shown to
have an overall stabilizing effect on the oscillating transients emerging from the
quasi-stable periodic orbits.
The insights from the read-out based control scheme for the genetic oscillator
lead us to the idea to implement an algorithmic controller, which would direct
any genetic circuit to a desired state. The algorithm operates model-free, i.e. in
principle it is applicable to any genetic network and the input information is a
data matrix of measured time series from the network dynamics. The application
areas for readout-based control in genetic networks range from classical tissue
engineering to stem cells specification, whenever a quantitatively and temporarily
targeted intervention is required
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