arXiv subjects

Jacob White

Publications and source records attributed to Jacob White.

16 recordsLinked to original sources

arXiv2024

Astronomy's climate emissions: Global travel to scientific meetings in 2019

Travel to academic conferences -- where international flights are the norm -- is responsible for a sizeable fraction of the greenhouse gas (GHG) emissions associated with academic work. In order to provide a benchmark for comparison with other fields, as well as for future reduction strategies and assessments, we estimate the CO2-equivalent emissions for conference travel in the field of astronomy for the prepandemic year 2019. The GHG emission of the international astronomical community's 362 conferences and schools in 2019 amounted to 42,500 tCO2e, assuming a radiative-forcing index factor of 1.95 for air travel. This equates to an average of 1.0 $\pm$ 0.6 tCO2e per participant per meeting. The total travel distance adds up to roughly 1.5 Astronomical Units, that is, 1.5 times the distance between the Earth and the Sun. We present scenarios for the reduction of this value, for instance with virtual conferencing or hub models, while still prioritizing the benefits conferences bring to the scientific community.

physics.soc-ph↗

arXiv2022

Latent Signal Models: Learning Compact Representations of Signal Evolution for Improved Time-Resolved, Multi-contrast MRI

Purpose: Training auto-encoders on simulated signal evolution and inserting the decoder into the forward model improves reconstructions through more compact, Bloch-equation-based representations of signal in comparison to linear subspaces. Methods: Building on model-based nonlinear and linear subspace techniques that enable reconstruction of signal dynamics, we train auto-encoders on dictionaries of simulated signal evolution to learn more compact, non-linear, latent representations. The proposed Latent Signal Model framework inserts the decoder portion of the auto-encoder into the forward model and directly reconstructs the latent representation. Latent Signal Models essentially serve as a proxy for fast and feasible differentiation through the Bloch-equations used to simulate signal. This work performs experiments in the context of T2-shuffling, gradient echo EPTI, and MPRAGE-shuffling. We compare how efficiently auto-encoders represent signal evolution in comparison to linear subspaces. Simulation and in-vivo experiments then evaluate if reducing degrees of freedom by inserting the decoder into the forward model improves reconstructions in comparison to subspace constraints. Results: An auto-encoder with one real latent variable represents FSE, EPTI, and MPRAGE signal evolution as well as linear subspaces characterized by four basis vectors. In simulated/in-vivo T2-shuffling and in-vivo EPTI experiments, the proposed framework achieves consistent quantitative NRMSE and qualitative improvement over linear approaches. From qualitative evaluation, the proposed approach yields images with reduced blurring and noise amplification in MPRAGE shuffling experiments. Conclusion: Directly solving for non-linear latent representations of signal evolution improves time-resolved MRI reconstructions through reduced degrees of freedom.

eess.SP↗

arXiv2022

Autofocusing+: Noise-Resilient Motion Correction in Magnetic Resonance Imaging

Image corruption by motion artifacts is an ingrained problem in Magnetic Resonance Imaging (MRI). In this work, we propose a neural network-based regularization term to enhance Autofocusing, a classic optimization-based method to remove motion artifacts. The method takes the best of both worlds: the optimization-based routine iteratively executes the blind demotion and deep learning-based prior penalizes for unrealistic restorations and speeds up the convergence. We validate the method on three models of motion trajectories, using synthetic and real noisy data. The method proves resilient to noise and anatomic structure variation, outperforming the state-of-the-art demotion methods.

eess.IV↗

arXiv2021

A Deep Search for Five Molecules in the 49 Ceti Debris Disk

Surprisingly strong CO emission has been observed from more than a dozen debris disks around nearby main-sequence stars. The origin of this CO is unclear, in particular whether it is left over from the protoplanetary disk phase or is second-generation material released from collisions between icy bodies like debris dust. The primary unexplored avenue for distinguishing the origin of the material is understanding its molecular composition. Here we present a deep search for five molecules (CN, HCN, HCO+, SiO, and CH3OH) in the debris disk around 49 Ceti. We take advantage of the high sensitivity of the Atacama Large Millimeter/submillimeter Array (ALMA) at Band 7 to integrate for 3.2 hours at modest spatial (1") and spectral (0.8 km/s) resolution. Our search yields stringent upper limits on the flux of all surveyed molecular lines, which imply abundances relative to CO that are orders of magnitude lower than those observed in protoplanetary disks and Solar System comets, and also those predicted in outgassing models of second-generation material. However, if CI shielding is responsible for extending the lifetime of any CO produced in second-generation collisions, as proposed by Kral et al. (2018), then the line ratios do not reflect true ice phase chemical abundances, but rather imply that CO is shielded by its own photodissociation product, CI, but other molecules are rapidly photodissociated by the stellar and interstellar radiation field.

astro-ph.SR↗

arXiv2020

On Cohen-Macaulay Hopf monoids in species

We study Cohen-Macaulay Hopf monoids in the category of species. The goal is to apply techniques from topological combinatorics to the study of polynomial invariants arising from combinatorial Hopf algebras. Given a polynomial invariant arising from a linearized Hopf monoid, we show that under certain conditions it is the Hilbert polynomial of a relative simplicial complex. If the Hopf monoid is Cohen-Macaulay, we give necessary and sufficient conditions for the corresponding relative simplicial complex to be relatively Cohen-Macaulay, which implies that the polynomial has a nonnegative $h$-vector. We apply our results to the weak and strong chromatic polynomials of acyclic mixed graphs, and the order polynomial of a double poset.

math.CO↗

arXiv2019

The Next Generation Very Large Array

The next generation Very Large Array (ngVLA) is a transformational radio observatory being designed by the U.S. National Radio Astronomy Observatory (NRAO). It will provide order of magnitude improvements in sensitivity, resolution, and uv coverage over the current Jansky Very Large Array (VLA) at ~1.2-50 GHz and extend the frequency range up to 70-115 GHz. This document is a white paper written by members of the Canadian community for the 2020 Long Range Plan panel, which will be making recommendations on Canada's future directions in astronomy. Since Canadians have been historically major users of the VLA and have been valued partners with NRAO for ALMA, Canada's participation in ngVLA is welcome. Canadians have been actually involved in ngVLA discussions for the past five years, and have played leadership roles in the ngVLA Science and Technical Advisory Councils. Canadian technologies are also very attractive for the ngVLA, in particular our designs for radio antennas, receivers, correlates, and data archives, and our industrial capacities to realize them. Indeed, the Canadian designs for the ngVLA antennas and correlator/beamformer are presently the baseline models for the project. Given the size of Canada's radio community and earlier use of the VLA (and ALMA), we recommend Canadian participation in the ngVLA at the 7% level. Such participation would be significant enough to allow Canadian leadership in gVLA's construction and usage. Canada's participation in ngVLA should not preclude its participation in SKA; access to both facilities is necessary to meet Canada's radio astronomy needs. Indeed, ngVLA will fill the gap between those radio frequencies observable with the SKA and ALMA at high sensitivities and resolutions. Canada's partnership in ngVLA will give it access to cutting-edge facilities together covering approximately three orders of magnitude in frequency.

astro-ph.IM↗

arXiv2019

Constraining the Radio Emission of TRAPPIST-1

TRAPPIST-1 is an ultracool dwarf (UCD) with a system of 7 terrestrial planets, at least three of which orbit in the habitable zone. The radio emission of such low-mass stars is poorly understood; few UCDs have been detected at radio frequencies at all, and the likelihood of detection is only loosely correlated with stellar properties. Relative to other low-mass stars, UCDs with slow rotation such as TRAPPIST-1 tend to be radio dim, whereas rapidly rotating UCDs tend to have strong radio emission - although this is not always the case. We present radio observations of TRAPPIST-1 using ALMA at 97.5 GHz and the VLA at 44 GHz. TRAPPIST-1 was not detected at either frequency and we place $3 \sigma$ upper flux limits of 10.6 and 16.2 $\mu$Jy, respectively. We use our results to constrain the magnetic properties and possible outgoing high energy particle radiation from the star. The presence of radio emission from UCDs is indicative of a stellar environment that could pose a threat to life on surrounding planets. Gyrosynchrotron emission, discernible at frequencies between 20 and 100 GHz, is one of the only processes that can be used to infer the presence of high energy particles released during magnetic reconnection events. Since M dwarfs are frequent hosts of terrestrial planets, characterizing their stellar emission is a crucial part of assessing habitability. Exposure to outgoing high energy particle radiation - traceable by radio flux - can erode planetary atmospheres. While our results do not imply that the TRAPPIST-1 planets are suitable for life, we find no evidence that they are overtly unsuitable due to proton fluxes.

astro-ph.SR↗

arXiv2019

Probing Unseen Planet Populations with Resolved Debris Disk Structures

Thousands of exoplanets have been found with many widely different from the ones in our own system. Despite the success, systems with planets in wide orbits analogous to those of Jupiter and Saturn, in the critical first several hundred million years of evolution, are virtually unexplored. Where are the low-mass planets that are hidden from our exoplanet detection techniques? Is our Solar System's planetary architecture unique? High-fidelity debris disk images offer an effective method to answer these questions. We can use them to study the formation and evolution of low-mass planets from youth to the age of the Solar System, providing snapshots of the complex processes and valuable insights into the formation and migration history of giant planets at wide orbits. This white paper focuses on resolving debris structures in thermal emission that is applicable to a large unbiased sample. We summarize the properties of the known debris disks and assess the feasibility of resolving them within our current and future infrared and millimeter facilities by adopting uniform criteria. JWST and the 9-m Origins Space Telescope are the most promising missions in the coming decades to resolve almost half of the known disks at high fidelity. Resolved debris structures at multiple wavelengths and at all stages of evolution would reveal the properties of unseen planet populations, enabling a unique demographic study of overall planet formation and evolution.

astro-ph.EP↗

arXiv2019

Variability in the Assembly of Protostellar Systems

Understanding the collapse of clouds and the formation of protoplanetary disks is essential to understanding the formation of stars and planets. Infall and accretion, the mass-aggregation processes that occur at envelope and disk scales, drive the dynamical evolution of protostars. While the observations of protostars at different stages constrain their evolutionary tracks, the impact of variability due to accretion bursts on dynamical and chemical evolution of the source is largely unknown. The lasting effects on protostellar envelopes and disks are tracked through multi-wavelength and time domain observational campaigns, requiring deep X-ray, infrared, and radio imaging and spectroscopy, at a sufficient level of spatial detail to distinguish contributions from the various substructures (i.e., envelope from disk from star from outflow). Protostellar models derived from these campaigns will illuminate the initial chemical state of protoplanetary disks during the epoch of giant planet formation. Insight from individual star formation in the Milky Way is also necessary to understand star formation rates in extragalactic sources. This cannot be achieved with ground-based observatories and is not covered by currently approved instrumentation. Requirements: High (v < 10 km/s for survey; v < 1 km/s for followup) spectral resolution capabilities with relatively rapid response times in the IR (3-500 um), X-ray (0.1-10 keV), and radio (cm) are critical to follow the course of accretion and outflow during an outburst. Complementary, AU-scale radio observations are needed to probe the disk accretion zone, and 10 AU-scale to probe chemical and kinematic structures of the disk-forming regions, and track changes in the dust, ice, and gas within protostellar envelopes.

astro-ph.SR↗

arXiv2019

Planet formation: The case for large efforts on the computational side

Modern astronomy has finally been able to observe protoplanetary disks in reasonable resolution and detail, unveiling the processes happening during planet formation. These observed processes are understood under the framework of disk-planet interaction, a process studied analytically and modeled numerically for over 40 years. Long a theoreticians' game, the wealth of observational data has been allowing for increasingly stringent tests of the theoretical models. Modeling efforts are crucial to support the interpretation of direct imaging analyses, not just for potential detections but also to put meaningful upper limits on mass accretion rates and other physical quantities in current and future large-scale surveys. This white paper addresses the questions of what efforts on the computational side are required in the next decade to advance our theoretical understanding, explain the observational data, and guide new observations. We identified the nature of accretion, ab initio planet formation, early evolution, and circumplanetary disks as major fields of interest in computational planet formation. We recommend that modelers relax the approximations of alpha-viscosity and isothermal equations of state, on the grounds that these models use flawed assumptions, even if they give good visual qualitative agreement with observations. We similarly recommend that population synthesis move away from 1D hydrodynamics. The computational resources to reach these goals should be developed during the next decade, through improvements in algorithms and the hardware for hybrid CPU/GPU clusters. Coupled with high angular resolution and great line sensitivity in ground based interferometers, ELTs and JWST, these advances in computational efforts should allow for large strides in the field in the next decade.

astro-ph.EP↗

arXiv2016

Coloring Complexes and Combinatorial Hopf Monoids

We generalize the notion of a coloring complex of a graph to linearized combinatorial Hopf monoids. We determine when a linearized combinatorial Hopf monoid has such a construction, and discover some inequalities that are satisfied by the quasisymmetric function invariants associated to the combinatorial Hopf monoid. We show that the collection of all such coloring complexes forms a linearized combinatorial Hopf monoid, which is the terminal object in the category of combinatorial Hopf monoids with convex characters. We also study several examples of combinatorial Hopf monoids.

math.CO↗

arXiv2016

Quasisymmetric Functions from Combinatorial Hopf Monoids and Ehrhart Theory

We investigate quasisymmetric functions coming from combinatorial Hopf monoids. We show that these invariants arise naturally in Ehrhart theory, and that some of their specializations are Hilbert functions for relative simplicial complexes. This class of complexes, called forbidden composition complexes, also forms a Hopf monoid, thus demonstrating a link between Hopf algebras, Ehrhart theory, and commutative algebra. We also study various specializations of quasisymmetric functions.

math.CO↗

arXiv2013

Double transitivity of Galois Groups in Schubert Calculus of Grassmannians

We investigate double transitivity of Galois groups in the classical Schubert calculus on Grassmannians. We show that all Schubert problems on Grassmannians of 2- and 3-planes have doubly transitive Galois groups, as do all Schubert problems involving only special Schubert conditions. We use these results to give a new proof that Schubert problems on Grassmannians of 2-planes have Galois groups that contain the alternating group. We also investigate the Galois group of every Schubert problem on Gr(4,8), finding that each Galois group either contains the alternating group or is an imprimitive permutation group and therefore fails to be doubly transitive. These imprimitive examples show that our results are the best possible general results on double transitivity of Schubert problems.

math.AG↗

arXiv2012

Fluctuating Surface Currents: A New Algorithm for Efficient Prediction of Casimir Interactions among Arbitrary Materials in Arbitrary Geometries. I. Theory

This paper presents a new method for the efficient numerical computation of Casimir interactions between objects of arbitrary geometries, composed of materials with arbitrary frequency-dependent electrical properties. Our method formulates the Casimir effect as an interaction between effective electric and magnetic current distributions on the surfaces of material bodies, and obtains Casimir energies, forces, and torques from the spectral properties of a matrix that quantifies the interactions of these surface currents. The method can be formulated and understood in two distinct ways: \textbf{(1)} as a consequence of the familiar \textit{stress-tensor} approach to Casimir physics, or, alternatively, \textbf{(2)} as a particular case of the \textit{path-integral} approach to Casimir physics, and we present both formulations in full detail. In addition to providing an algorithm for computing Casimir interactions in geometries that could not be efficiently handled by any other method, the framework proposed here thus achieves an explicit unification of two seemingly disparate approaches to computational Casimir physics.

quant-ph↗

arXiv2010

Computation of Casimir Interactions between Arbitrary 3D Objects with Arbitrary Material Properties

We extend a recently introduced method for computing Casimir forces between arbitrarily--shaped metallic objects [M. T. H. Reid et al., Phys. Rev. Lett._103_ 040401 (2009)] to allow treatment of objects with arbitrary material properties, including imperfect conductors, dielectrics, and magnetic materials. Our original method considered electric currents on the surfaces of the interacting objects; the extended method considers both electric and magnetic surface current distributions, and obtains the Casimir energy of a configuration of objects in terms of the interactions of these effective surface currents. Using this new technique, we present the first predictions of Casimir interactions in several experimentally relevant geometries that would be difficult to treat with any existing method. In particular, we investigate Casimir interactions between dielectric nanodisks embedded in a dielectric fluid; we identify the threshold surface--surface separation at which finite--size effects become relevant, and we map the rotational energy landscape of bound nanoparticle diclusters.

quant-ph↗

arXiv2009

Efficient Computation of Casimir Interactions between Arbitrary 3D Objects

We introduce an efficient technique for computing Casimir energies and forces between objects of arbitrarily complex 3D geometries. In contrast to other recently developed methods, our technique easily handles non-spheroidal, non-axisymmetric objects and objects with sharp corners. Using our new technique, we obtain the first predictions of Casimir interactions in a number of experimentally relevant geometries, including crossed cylinders and tetrahedral nanoparticles.

quant-ph↗