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The Optical Aurorae of Europa, Ganymede, and Callisto
No full textThe tenuous atmospheres of the Galilean satellites are sourced from their surfaces and produced by a combination of plasma-surface interactions and thermal processes. Even though they are thin, these atmospheres can be studied via their auroral emissions, and most work to date has focused on their aurorae at UV wavelengths. Here we present the first detections of the optical aurorae of Ganymede and Callisto, as well as detections of new optical auroral lines at Europa, based on observations of the targets over 10 Jupiter eclipses from 1998 to 2021 with Keck/HIRES. We present measurements of O i emission at 6300/6364, 5577, 7774, and 8446 Å and place upper limits on hydrogen at 6563 Å. These constitute the first detections of emissions at 7774 and 8446 Å at a planetary body other than Earth. The simultaneous measurement of multiple emission lines provides robust constraints on atmospheric composition. We find that the eclipse atmospheres of Europa and Ganymede are composed predominantly of O₂, with average column densities of (4.1 ± 0.1) × 101₂ cm⁻² and (4.7 ± 0.1) × 10₂ cm⁻², respectively. We find weak evidence for H2O in Europa's bulk atmosphere at an H₂O/O₂ ratio of ∼0.25, and place only an upper limit on H₂O in Ganymede's bulk atmosphere, corresponding to H₂O/O₂ < 0.6. The column density of O₂ derived for Callisto is (4.0 ± 0.9) × 10¹⁵ cm⁻² for an assumed electron density of 0.15 cm⁻³, but electron properties at Callisto's orbit are very poorly constrained
Organizing for Collective Action: Olson Revisited
No full textWe study a standard collective action problem in which successful achievement of a group interest requires costly participation by some fraction of its members. How should we model the internal organization of these groups when there is asymmetric information about the preferences of their members? How effective should we expect it to be as we increase the group's size n? We model it as an optimal honest and obedient communication mechanism and we show that for large n it can be implemented with a very simple mechanism that we call the Voluntary Based Organization. Two new results emerge from this analysis. Independently of the assumptions on the underlying technology, the limit probability of success in the best honest and obedient mechanism is the same as in an unorganized group, a result that is not generally true if obedience is omitted. An optimal organization, however, provides a key advantage: when the probability of success converges to zero, it does so at a much slower rate than in an unorganized group. Because of this, significant probabilities of success are achievable with simple honest and obedient organizations even in very large groups
Enhanced superconductivity in spin–orbit proximitized bilayer graphene
No full textIn the presence of a large perpendicular electric field, Bernal-stacked bilayer graphene (BLG) features several broken-symmetry metallic phases as well as magnetic-field-induced superconductivity1. The superconducting state is quite fragile, however, appearing only in a narrow window of density and with a maximum critical temperature T꜀ ≈ 30 mK. Here we show that placing monolayer tungsten diselenide (WSe₂) on BLG promotes Cooper pairing to an extraordinary degree: superconductivity appears at zero magnetic field, exhibits an order of magnitude enhancement in T꜀ and occurs over a density range that is wider by a factor of eight. By mapping quantum oscillations in BLG–WSe₂ as a function of electric field and doping, we establish that superconductivity emerges throughout a region for which the normal state is polarized, with two out of four spin-valley flavours predominantly populated. In-plane magnetic field measurements further reveal that superconductivity in BLG–WSe₂ can exhibit striking dependence of the critical field on doping, with the Chandrasekhar–Clogston (Pauli) limit roughly obeyed on one end of the superconducting dome, yet sharply violated on the other. Moreover, the superconductivity arises only for perpendicular electric fields that push BLG hole wavefunctions towards WSe₂, indicating that proximity-induced (Ising) spin–orbit coupling plays a key role in stabilizing the pairing. Our results pave the way for engineering robust, highly tunable and ultra-clean graphene-based superconductors
Cosmic Evolution of Gas and Star Formation
No full textAtacama Large Millimeter/submillimeter Array (ALMA) observations of the long-wavelength dust continuum are used to estimate the gas masses in a sample of 708 star-forming galaxies at z = 0.3−4.5. We determine the dependence of gas masses and star formation efficiencies (SFEs; SFR per unit gas mass) on redshift (z), M
*, and star formation rate (SFR) relative to the main sequence (MS). We find that 70% of the increase in SFRs of the MS is due to the increased gas masses at earlier epochs, while 30% is due to increased efficiency of star formation (SF). For galaxies above the MS this is reversed—with 70% of the increased SFR relative to the MS being due to elevated SFEs. Thus, the major evolution of star formation activity at early epochs is driven by increased gas masses, while the starburst activity taking galaxies above the MS is due to enhanced triggering of star formation (likely due to galactic merging). The interstellar gas peaks at z = 2 and dominates the stellar mass down to z = 1.2. Accretion rates needed to maintain continuity of the MS evolution reach >100 M_⊙ yr⁻¹ at z > 2. The galactic gas contents are likely the driving determinant for both the rise in SF and AGN activity from z = 5 to their peak at z = 2 and subsequent fall at lower z. We suggest that for self-gravitating clouds with supersonic turbulence, cloud collisions and the filamentary structure of the clouds regulate the star formation activity
The Kinematics and Excitation of Infrared Water Vapor Emission from Planet-forming Disks: Results from Spectrally Resolved Surveys and Guidelines for JWST Spectra
No full textThis work presents ground-based spectrally resolved water emission at R = 30,000–100,000 over infrared wavelengths covered by the JWST (2.9–12.8 μm). Two new surveys with iSHELL and the VISIR are combined with previous spectra from the CRIRES to cover parts of multiple rovibrational and rotational bands observable within telluric transmission bands, for a total of ≈160 spectra and 85 disks (30 of which are JWST targets in Cycle 1). The general expectation of a range of regions and excitation conditions traced by infrared water spectra is for the first time supported by the combined kinematics and excitation as spectrally resolved at multiple wavelengths. The main findings from this analysis are: (1) water lines are progressively narrower from the rovibrational bands at 2–9 μm to the rotational lines at 12 μm, and partly match broad and narrow emission components, respectively, as extracted from rovibrational CO spectra; (2) rotation diagrams of resolved water lines from upper-level energies of 4000–9500 K show vertical spread and curvatures indicative of optically thick emission (≈10¹⁸ cm⁻²) from a range of excitation temperatures (≈800–1100 K); and (3) the new 5 μm spectra demonstrate that slab model fits to the rotational lines at >10 μm strongly overpredict the rovibrational emission bands at μm, implying vibrational excitation not in thermodynamic equilibrium. We discuss these findings in the context of emission from a disk surface and a molecular inner disk wind, and provide a list of guidelines to support the analysis of spectrally unresolved JWST spectra
Coupling homogeneous and heterogeneous catalysis for enhancement of HCOOH electrooxidation via the dehydrogenation pathway
No full textThe homogeneous/heterogeneous catalyst combination of VO²⁺ in anolyte with Pd/C at the anode is first introduced in a formic acid fuel cell to enhance HCOOH electrooxidation. The VO²⁺/Pd catalyst combination establishes a stepwise reaction pathway involving HCOOH dehydrogenation to form V²⁺ from VO²⁺ reduction and subsequent V²⁺ electrooxidation to regain VO²⁺. The fuel cell with the VO²⁺/Pd combination presents a peak power density of 341.3 mW cm⁻² and stable power density higher than 30 mW cm⁻²
Non-equilibrium spectral phase transitions in coupled nonlinear optical resonators
No full textCoupled systems with multiple interacting degrees of freedom provide a fertile ground for emergent dynamics, which is otherwise inaccessible in their solitary counterparts. Here we show that coupled nonlinear optical resonators can undergo self-organization in their spectrum leading to a first-order phase transition. We experimentally demonstrate such a spectral phase transition in time-multiplexed coupled optical parametric oscillators. We switch the nature of mutual coupling from dispersive to dissipative and access distinct spectral regimes of the parametric oscillator dimer. We observe abrupt spectral discontinuity at the first-order transition point. Furthermore, we show how non-equilibrium phase transitions can lead to enhanced sensing, where the applied perturbation is not resolvable by the underlying linear system. Our approach could be exploited for sensing applications that use nonlinear driven-dissipative systems, leading to performance enhancements without sacrificing sensitivity
Pivot Hamiltonians as generators of symmetry and entanglement
No full textIt is well-known that symmetry-protected topological (SPT) phases can be obtained from the trivial phase by an entangler, a finite-depth unitary operator U. Here, we consider obtaining the entangler from a local 'pivot' Hamiltonian H_(piv) such that U = e^(iπH_(piv)). This perspective of Hamiltonians pivoting between the trivial and SPT phase opens up two new directions which we explore here. (i) Since SPT Hamiltonians and entanglers are now on the same footing, can we iterate this process to create other interesting states? (ii) Since entanglers are known to arise as discrete symmetries at SPT transitions, under what conditions can this be enhanced to U(1) 'pivot' symmetry generated by H_(piv)? In this work we explore both of these questions. With regard to the first, we give examples of a rich web of dualities obtained by iteratively using an SPT model as a pivot to generate the next one. For the second question, we derive a simple criterion guaranteeing that the direct interpolation between the trivial and SPT Hamiltonian has a U(1) pivot symmetry. We illustrate this in a variety of examples, assuming various forms for H_(piv), including the Ising chain, and the toric code Hamiltonian. A remarkable property of such a U(1) pivot symmetry is that it shares a mutual anomaly with the symmetry protecting the nearby SPT phase. We discuss how such anomalous and non-onsite U(1) symmetries explain the exotic phase diagrams that can appear, including an SPT multicritical point where the gapless ground state is given by the fixed-point toric code state
Physics-informed dynamic mode decomposition
No full textIn this work, we demonstrate how physical principles—such as symmetries, invariances and conservation laws—can be integrated into the dynamic mode decomposition (DMD). DMD is a widely used data analysis technique that extracts low-rank modal structures and dynamics from high-dimensional measurements. However, DMD can produce models that are sensitive to noise, fail to generalize outside the training data and violate basic physical laws. Our physics-informed DMD (piDMD) optimization, which may be formulated as a Procrustes problem, restricts the family of admissible models to a matrix manifold that respects the physical structure of the system. We focus on five fundamental physical principles—conservation, self-adjointness, localization, causality and shift-equivariance—and derive several closed-form solutions and efficient algorithms for the corresponding piDMD optimizations. With fewer degrees of freedom, piDMD models are less prone to overfitting, require less training data, and are often less computationally expensive to build than standard DMD models. We demonstrate piDMD on a range of problems, including energy-preserving fluid flow, the Schrödinger equation, solute advection-diffusion and three-dimensional transitional channel flow. In each case, piDMD outperforms standard DMD algorithms in metrics such as spectral identification, state prediction and estimation of optimal forcings and responses
The interaction of supernova 2018evt with a substantial amount of circumstellar matter - An SN 1997cy-like event
No full textA rare class of supernovae (SNe) is characterized by strong interaction between the ejecta and several solar masses of circumstellar matter (CSM) as evidenced by strong Balmer-line emission. Within the first few weeks after the explosion, they may display spectral features similar to overluminous Type Ia SNe, while at later phase their observation properties exhibit remarkable similarities with some extreme case of Type IIn SNe that show strong Balmer lines years after the explosion. We present polarimetric observations of SN 2018evt obtained by the ESO Very Large Telescope from 172 to 219 d after the estimated time of peak luminosity to study the geometry of the CSM. The non-zero continuum polarization decreases over time, suggesting that the mass-loss of the progenitor star is aspherical. The prominent H α emission can be decomposed into a broad, time-evolving component and an intermediate-width, static component. The former shows polarized signals, and it is likely to arise from a cold dense shell (CDS) within the region between the forward and reverse shocks. The latter is significantly unpolarized, and it is likely to arise from shocked, fragmented gas clouds in the H-rich CSM. We infer that SN 2018evt exploded inside a massive and aspherical circumstellar cloud. The symmetry axes of the CSM and the SN appear to be similar. SN 2018evt shows observational properties common to events that display strong interaction between the ejecta and CSM, implying that they share similar circumstellar configurations. Our preliminary estimate also suggests that the circumstellar environment of SN 2018evt has been significantly enriched at a rate of ∼0.1 M_⊙ yr₋₁ over a period of >100 yr