153 research outputs found
Vector speckle grid: instantaneous incoherent speckle grid for high-precision astrometry and photometry in high-contrast imaging
Photometric and astrometric monitoring of directly imaged exoplanets will
deliver unique insights into their rotational periods, the distribution of
cloud structures, weather, and orbital parameters. As the host star is occulted
by the coronagraph, a speckle grid (SG) is introduced to serve as astrometric
and photometric reference. Speckle grids are implemented as diffractive
pupil-plane optics that generate artificial speckles at known location and
brightness. Their performance is limited by the underlying speckle halo caused
by evolving uncorrected wavefront errors. The speckle halo will interfere with
the coherent SGs, affecting their photometric and astrometric precision. Our
aim is to show that by imposing opposite amplitude or phase modulation on the
opposite polarization states, a SG can be instantaneously incoherent with the
underlying halo, greatly increasing the precision. We refer to these as vector
speckle grids (VSGs). We derive analytically the mechanism by which the
incoherency arises and explore the performance gain in idealised simulations
under various atmospheric conditions. We show that the VSG is completely
incoherent for unpolarized light and that the fundamental limiting factor is
the cross-talk between the speckles in the grid. In simulation, we find that
for short-exposure images the VSG reaches a 0.3-0.8\% photometric error
and astrometric error, which is a
performance increase of a factor 20 and 5, respectively.
Furthermore, we outline how VSGs could be implemented using liquid-crystal
technology to impose the geometric phase on the circular polarization states.
The VSG is a promising new method for generating a photometric and astrometric
reference SG that has a greatly increased astrometric and photometric
precision.Comment: Accepted for publication in A&
The polarization-encoded self-coherent camera
The exploration of circumstellar environments by means of direct imaging to
search for Earth-like exoplanets is one of the challenges of modern astronomy.
One of the current limitations are evolving non-common path aberrations (NCPA)
that originate from optics downstream of the main wavefront sensor. The
self-coherent camera (SCC) is an integrated coronagraph and focal-plane
wavefront sensor that generates wavefront information-encoding Fizeau fringes
in the focal plane by adding a reference hole (RH) in the Lyot stop. Here, we
aim to show that by featuring a polarizer in the RH and adding a polarizing
beamsplitter downstream of the Lyot stop, the RH can be placed right next to
the pupil. We refer to this new variant of the SCC as the polarization-encoded
self-coherent camera (PESCC). We study the performance of the PESCC
analytically and numerically, and compare it, where relevant, to the SCC. We
show analytically that the PESCC relaxes the requirements on the focal-plane
sampling and spectral resolution with respect to the SCC by a factor of 2 and
3.5, respectively. Furthermore, we find via our numerical simulations that the
PESCC has effectively access to 16 times more photons, which improves the
sensitivity of the wavefront sensing by a factor of . We also show that
without additional measurements, the RH point-spread function (PSF) can be
calibrated using PESCC images, enabling coherent differential imaging (CDI) as
a contrast-enhancing post-processing technique for every observation. In
idealized simulations (clear aperture, charge two vortex coronagraph, perfect
DM, no noise sources other than phase and amplitude aberrations) and in
circumstances similar to those of space-based systems, we show that WFSC
combined with CDI can achieve a raw contrast of between 1 and 18 .Comment: Accepted for publication in A&A. Shortened abstract. 17 pages, 21
figure
Fully broadband vAPP coronagraphs enabling polarimetric high contrast imaging
We present designs for fully achromatic vector Apodizing Phase Plate (vAPP)
coronagraphs, that implement low polarization leakage solutions and achromatic
beam-splitting, enabling observations in broadband filters. The vAPP is a pupil
plane optic, inducing the phase through the inherently achromatic geometric
phase. We discuss various implementations of the broadband vAPP and set
requirements on all the components of the broadband vAPP coronagraph to ensure
that the leakage terms do not limit a raw contrast of 1E-5. Furthermore, we
discuss superachromatic QWPs based of liquid crystals or quartz/MgF2
combinations, and several polarizer choices. As the implementation of the
(broadband) vAPP coronagraph is fully based on polarization techniques, it can
easily be extended to furnish polarimetry by adding another QWP before the
coronagraph optic, which further enhances the contrast between the star and a
polarized companion in reflected light. We outline several polarimetric vAPP
system designs that could be easily implemented in existing instruments, e.g.
SPHERE and SCExAO.Comment: 11 pages, 5 figures, presented at SPIE Astronomical Telescopes and
Instrumentation 201
The Envelope Kinematics and a Possible Disk Around the Class 0 Protostar within BHR7
We present a characterization of the protostar embedded within the BHR7 dark
cloud, based on both photometric measurements from the near-infrared to
millimeter and interferometric continuum and molecular line observations at
millimeter wavelengths. We find that this protostar is a Class 0 system, the
youngest class of protostars, measuring its bolometric temperature to be
50.5~K, with a bolometric luminosity of 9.3~L. The near-infrared and
\textit{Spitzer} imaging show a prominent dark lane from dust extinction
separating clear bipolar outflow cavities. Observations of CO
(), CO (), and other molecular lines
with the Submillimeter Array (SMA) exhibit a clear rotation signature on scales
1300~AU. The rotation can be traced to an inner radius of 170~AU and
the rotation curve is consistent with an R profile, implying that
angular momentum is being conserved. Observations of the 1.3~mm dust continuum
with the SMA reveal a resolved continuum source, extended in the direction of
the dark lane, orthogonal to the outflow. The deconvolved size of the continuum
indicates a radius of 100~AU for the continuum source at the assumed
distance of 400~pc. The visibility amplitude profile of the continuum emission
cannot be reproduced by an envelope alone and needs a compact component. Thus,
we posit that the resolved continuum source could be tracing a Keplerian disk
in this very young system. If we assume that the continuum radius traces a
Keplerian disk (R120~AU) the observed rotation profile is consistent with
a protostar mass of 1.0~.Comment: 36 pages; 11 Figures, 2 Tables; Accepted to Ap
The Lie Algebraic Significance of Symmetric Informationally Complete Measurements
Examples of symmetric informationally complete positive operator valued
measures (SIC-POVMs) have been constructed in every dimension less than or
equal to 67. However, it remains an open question whether they exist in all
finite dimensions. A SIC-POVM is usually thought of as a highly symmetric
structure in quantum state space. However, its elements can equally well be
regarded as a basis for the Lie algebra gl(d,C). In this paper we examine the
resulting structure constants, which are calculated from the traces of the
triple products of the SIC-POVM elements and which, it turns out, characterize
the SIC-POVM up to unitary equivalence. We show that the structure constants
have numerous remarkable properties. In particular we show that the existence
of a SIC-POVM in dimension d is equivalent to the existence of a certain
structure in the adjoint representation of gl(d,C). We hope that transforming
the problem in this way, from a question about quantum state space to a
question about Lie algebras, may help to make the existence problem tractable.Comment: 56 page
Bacterial porin disrupts mitochondrial membrane potential and sensitizes host cells to apoptosis
The bacterial PorB porin, an ATP-binding beta-barrel protein of pathogenic Neisseria gonorrhoeae, triggers host cell apoptosis by an unknown mechanism. PorB is targeted to and imported by host cell mitochondria, causing the breakdown of the mitochondrial membrane potential (delta psi m). Here, we show that PorB induces the condensation of the mitochondrial matrix and the loss of cristae structures, sensitizing cells to the induction of apoptosis via signaling pathways activated by BH3-only proteins. PorB is imported into mitochondria through the general translocase TOM but, unexpectedly, is not recognized by the SAM sorting machinery, usually required for the assembly of beta-barrel proteins in the mitochondrial outer membrane. PorB integrates into the mitochondrial inner membrane, leading to the breakdown of delta psi m. The PorB channel is regulated by nucleotides and an isogenic PorB mutant defective in ATP-binding failed to induce delta psi m loss and apoptosis, demonstrating that dissipation of delta psi m is a requirement for cell death caused by neisserial infection
An Outer Membrane Receptor of Neisseria meningitidis Involved in Zinc Acquisition with Vaccine Potential
Since the concentration of free iron in the human host is low, efficient iron-acquisition mechanisms constitute important virulence factors for pathogenic bacteria. In Gram-negative bacteria, TonB-dependent outer membrane receptors are implicated in iron acquisition. It is far less clear how other metals that are also scarce in the human host are transported across the bacterial outer membrane. With the aim of identifying novel vaccine candidates, we characterized in this study a hitherto unknown receptor in Neisseria meningitidis. We demonstrate that this receptor, designated ZnuD, is produced under zinc limitation and that it is involved in the uptake of zinc. Upon immunization of mice, it was capable of inducing bactericidal antibodies and we could detect ZnuD-specific antibodies in human convalescent patient sera. ZnuD is highly conserved among N. meningitidis isolates and homologues of the protein are found in many other Gram-negative pathogens, particularly in those residing in the respiratory tract. We conclude that ZnuD constitutes a promising candidate for the development of a vaccine against meningococcal disease for which no effective universal vaccine is available. Furthermore, the results suggest that receptor-mediated zinc uptake represents a novel virulence mechanism that is particularly important for bacterial survival in the respiratory tract
Calibration of the instrumental polarization effects of SCExAO-CHARIS’ spectropolarimetric mode
SCExAO at the Subaru telescope is a visible and near-infrared high-contrast imaging instrument employing extreme adaptive optics and coronagraphy. The instrument feeds the near-infrared light (JHK) to the integral field spectrograph CHARIS. Recently, a Wollaston prism was added to CHARIS’ optical path, giving CHARIS a spectropolarimetric capability that is unique among high-contrast imaging instruments. We present a comprehensive and detailed Mueller matrix model describing the instrumental polarization effects of the complete optical path, thus the telescope and instrument, using measurements with the internal source and observations of standard stars. The 22 wavelength bins of CHARIS provide a unique opportunity to investigate in detail the wavelength dependence of the instrumental polarization effects. We find that the image derotator (K-mirror) produces strongly wavelength-dependent crosstalk, in the worst case converting ~95% of the incident linear polarization to circularly polarized light that cannot be measured. We fit the crosstalk of the half-wave plate (HWP) for all wavelengths with a simple two-parameter model of an achromatic HWP consisting of a layer of quartz and a layer of MgF2. While the magnitude of the telescope-induced polarization varies with wavelength, its angle varies solely with the altitude angle of the telescope. We show initial steps toward correcting on-sky data for the instrumental polarization effects, with which we aim to achieve a polarimetric accuracy <0.1% in the degree of linear polarization. Our calibrations of CHARIS’ spectropolarimetric mode enable unique quantitative polarimetric studies of circumstellar disks and planetary and brown dwarf companions
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