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White dwarf stars have attracted considerable attention in the past 15 years as hosts for potentially habitable planets, but their low luminosity and continuous cooling are major challenges for habitability. Recently, astronomers have found that about 6% of massive white dwarfs seem to have "paused" their cooling for up to ~10 Gyr. The leading explanation for this cooling delay is the distillation of neutron-rich isotopes such as $^{22}$Ne in the white dwarf's interior, which releases a considerable amount of gravitational energy as the star's internal structure rearranges. Here, we consider the impact of $^{22}$Ne distillation on the evolution of white dwarf habitable zones. We find that $^{22}$Ne distillation in the white dwarf host dramatically increases the time that a planet can continuously reside within the habitable zone (giving more time for life to arise) and that long-lasting habitable zones are located farther from the star (decreasing the impact of tidal forces). These properties may make white dwarfs undergoing $^{22}$Ne distillation more promising locations for habitability than white dwarfs undergoing standard cooling., Comment: 6 pages, 2 figures, 2 tables. To be submitted to OJA

We increase the spectroscopic completeness of the 100 pc white dwarf sample in the SDSS footprint with 840 additional spectra. Our spectroscopy is 86% complete for white dwarfs hotter than $T_{\rm eff}= 5000$ K, where H$\alpha$ remains visible and provides reliable constraints on the atmospheric composition. We identify 2108 DA white dwarfs with pure hydrogen atmospheres, and show that ultramassive DA white dwarfs with $M\geq1.1~M_{\odot}$ are an order of magnitude less common below 10,000 K. This is consistent with a fraction of them getting stuck on the crystallization sequence due to $^{22}$Ne distillation. In addition, there are no ultramassive DA white dwarfs with $M\geq1.1~M_{\odot}$ and $T_{\rm eff}\leq6000$ K in our sample, likely because Debye cooling makes them rapidly fade away. We detect a significant trend in the fraction of He-atmosphere white dwarfs as a function of temperature; the fraction increases from 9% at 20,000 K to 32% at 6000 K. This provides direct evidence of convective mixing in cool DA white dwarfs. Finally, we detect a relatively tight sequence of low-mass DQ white dwarfs in color-magnitude diagrams for the first time. We discuss the implications of this tight DQ sequence, and conclude with a discussion of the future prospects from the upcoming ULTRASAT mission and the large-scale multi-fiber spectroscopic surveys., Comment: ApJ, in press

White dwarfs are stellar remnants devoid of a nuclear energy source, gradually cooling over billions of years and eventually freezing into a solid state from the inside out. Recently, it was discovered that a population of freezing white dwarfs maintains a constant luminosity for a duration comparable to the age of the universe, signaling the presence of a powerful yet unknown energy source that inhibits the cooling. For certain core compositions, the freezing process is predicted to trigger a solid-liquid distillation mechanism, due to the solid phase being depleted in heavy impurities. The crystals thus formed are buoyant and float up, thereby displacing heavier liquid downward and releasing gravitational energy. Here we show that distillation interrupts the cooling for billions of years and explains all the observational properties of the unusual delayed population. With a steady luminosity surpassing that of some main-sequence stars, these white dwarfs defy their conventional portrayal as dead stars. Our results highlight the existence of peculiar merger remnants and have profound implications for the use of white dwarfs in dating stellar populations., Comment: Published in Nature, 6 March 2024

We present the first discoveries of the double-lined double white dwarf (DBL) survey that targets over-luminous sources with respect to the canonical white dwarf cooling sequence according to a set of well-defined criteria. The primary goal of the DBL survey is to identify compact double white dwarf binary star systems from a unique spectral detection of both stars, which then enables a precise quantification of the atmospheric parameters and radial velocity variability of a system. Our search of 117 candidates that were randomly selected from a magnitude limited sample of 399 yielded a 29% detection efficiency with 34 systems exhibiting a double-lined signature. A further 38 systems show strong evidence of being single-lined or potentially-double-lined double white dwarf binaries and 7 single-lined sources from the full observed sample are radial velocity variable. The 45 remaining candidates appear as a single WD with no companion or a non-DA white dwarf, bringing the efficiency of detecting binaries to 62%. Atmospheric fitting of all double-lined systems reveals a large fraction that have two similar mass components that combine to a total mass of 1.0-1.3 solar masses - a class of double white dwarf binaries that may undergo a sub-Chandrasekhar mass type Ia detonation or merge to form a massive O/Ne WD, although orbital periods are required to infer on which timescales. One double-lined system located 49pc away, WDJ181058.67+311940.94, is super-Chandrasekhar mass, making it the second such double white dwarf binary to be discovered., Comment: Accepted for publication in MNRAS. 17 pages (plus 5 pages in the appendix). 12 figures

In the century-long history of quantum theory, Bell's theorem stands out among the most thought-provoking results of its foundations. It reveals a conflict between quantum theory's predictions and those allowed by a general framework aligning with locality and realism. Experimental vindications of the quantum predictions were of Nobel-Prize merit and contributed to an established conclusion that nature is non-local. In stark contrast with this orthodoxy, I show that within the Heisenberg picture of unitary quantum theory, Bell inequalities are violated with local elements of reality interacting locally. Here is how: Upon measuring her particle of the entangled pair, Alice smoothly and locally evolves into two non-interacting versions of herself, each of which witnesses and records a different outcome -- she \emph{foliates}. Everything that suitably interacts with the Alices foliates in turn, creating worlds which, for all practical purposes, are independent and autonomous. At spacelike separation, an analogous process occurs to Bob when he measures his particle, locally differentiating him and his surroundings into two non-interacting instances. To confirm the violation of Bell inequalities, Alice and Bob must further interact to produce a record of the joint outcomes. The record arises from the two local worlds of Alice, and those of Bob, and foliates into four instances that respectively indicate `$00$', `$01$', `$10$' and `$11$'. If at least one input of the CHSH test is `$0$', the total measure of records displaying the same outcome is $\cos^2(\pi/8)$; if not, this measure pertains to the records of different outcomes. This article formalizes and explains this local account of Bell `nonlocality'., Comment: 17 pages, 2 figures, 2 tables

Effective management of cooling tower systems requires thorough disinfection. While traditional chemical water treatment methods are currently the most prominent strategy, they are costly and yield limited results when relied upon as the sole approach. Cross-flow microsand filtration systems offer a promising alternative with the added benefit of potentially increasing evaporative cooling efficiency, thus saving energy. A comprehensive data-driven analysis over two cooling seasons evaluated the energetic performance of a system equipped with and without an operating filter. For similar environmental conditions, the coefficient of performance was on average 18% higher and was higher 63% of the time when the filter was operating, indicating superior heat transfer efficiency and significant energy savings. It was also 41% higher during periods of high cooling demand. Consequently, the filter and the system work more efficiently at high wet-bulb temperature and thermal load. Machine learning modeling suggested that operating the filter year-round could save between 5% and 13% of the energy bill, primarily during the cooling season. Continuous filter operation is essential as it mitigates biofouling, underscoring its long-term significance, even during periods of lower thermal loads. Integrating filtration systems into cooling tower management therefore fosters sustainable practices by decreasing energy consumption and biofouling.

White dwarfs are the dense, burnt-out remnants of the vast majority of stars, condemned to cool over billions of years as they steadily radiate away their residual thermal energy. To first order, their atmosphere is expected to be made purely of hydrogen due to the efficient gravitational settling of heavier elements. However, observations reveal a much more complex situation, as the surface of a white dwarf (1) can be dominated by helium rather than hydrogen, (2) can be polluted by trace chemical species, and (3) can undergo significant composition changes with time. This indicates that various mechanisms of element transport effectively compete against gravitational settling in the stellar envelope. This phenomenon is known as the spectral evolution of white dwarfs and has important implications for Galactic, stellar, and planetary astrophysics. This invited review provides a comprehensive picture of our current understanding of white dwarf spectral evolution. We first describe the latest observational constraints on the variations in atmospheric composition along the cooling sequence, covering both the dominant and trace constituents. We then summarise the predictions of state-of-the-art models of element transport in white dwarfs and assess their ability to explain the observed spectral evolution. Finally, we highlight remaining open questions and suggest avenues for future work., Comment: Invited review published in the '2023 Astronomy Prize Awardees Collection' of Astrophysics and Space Science; accepted version following peer review

Two years ago, the Heritage Foundation "Backgrounder" "Equity Elementary" (ED616087) first examined the extent to which the idea that educational institutions should have Chief Diversity Officers (CDO) had spread from higher education into public school districts. That "Backgrounder" also examined whether there was a relationship between the presence of a CDO within a district and the size and trend in achievement gaps between minority and white students. Despite CDOs ostensibly having the goal of reducing the extent to which black and Hispanic achievement, on average, lags behind average white achievement, the authors found that districts with CDOs had "larger" racial achievement gaps, which had been growing larger despite the adoption of CDOs during the preceding decade. This "Backgrounder" updates information on the number of school districts with CDOs and conducts a new analysis on how minority student achievement fared during the COVID-19 pandemic in districts with CDOs relative to districts without CDOs. Districts with a CDO suffered greater pandemic learning losses by black and Hispanic students. Not only did these black and Hispanic students experience significantly larger declines in math achievement, but those declines, on average, exceeded the rate of decline among white students. In fact, CDOs' energies appear to have been directed to politically contentious issues: Districts with CDOs were significantly more likely to have policies to keep parents in the dark about children's "gender" confusion or dysphoria. Chief Diversity Officers enforce the ideological orthodoxies opposed by majorities of parents instead of assisting with student learning or closing minority achievement gaps.

This review highlights the role of the Gaia space mission in transforming white dwarf research. These stellar remnants constitute 5-7% of the local stellar population in volume, yet before Gaia the lack of trigonometric parallaxes hindered their identification. The mission's Data Release 2 in 2018 provided the first unbiased colour-absolute magnitude diagram of the local stellar population, identifying 260,000 white dwarfs, with the number later increasing to over 355,000 in Data Release 3. Since then, more than 400 white dwarf studies have made critical use of Gaia data, establishing it as a fundamental resource for white dwarf identification, fundamental parameter determination and more recently spectral type characterisation. The review underscores the routine reliance on Gaia parallaxes and extensive use of its photometry in white dwarf surveys. We also discuss recent discoveries firmly grounded in Gaia data, including white dwarf mergers, exotic compact binaries and evolved planetary systems., Comment: Accepted to New Astronomy Reviews (Special Issue "Gaia, the first crop of discoveries")

Poorly managed postoperative acute pain can have long-lasting negative impacts and pose a major healthcare issue. There is limited investigation to understand and address the unique needs of patients experiencing acute pain. In this paper, we tackle this gap through an interview study with 14 patients who recently underwent postoperative acute pain to understand their challenges in pain self-management and their need for supportive tools. Our analysis identified various factors associated with the major aspects of acute pain self-management. Together, our findings indicated that tools for supporting these patients need to carefully consider information and support delivery to adapt to rapid changes in pain experiences, offer personalized and dynamic assistance that adapts to individual situations in context, and monitor emotion when promoting motivation. Overall, our work provided valuable knowledge to address the less-investigated but highly-needed problem of designing technology for the self-management of acute pain and similar health conditions., Comment: 11 pages, CHI '24

Boltzmann's constant reflects a historical misunderstanding of the concept of entropy, whose informational nature is obfuscated when expressed in J/K. We suggest that the development of temperature and energy, historically prior to that of entropy, does not amount to their logical priority: Temperature should be defined in terms of entropy, not vice versa. Following the precepts of information theory, entropy is measured in bits, and coincides with information capacity at thermodynamic equilibrium. Consequently, not only is the temperature of an equilibrated system expressed in J/bit, but it acquires an operational meaning: It is the cost in energy to increase its information capacity by 1 bit. Our proposal also supports the notion of available capacity, analogous to free energy. Finally, it simplifies Landauer's cost and clarifies that it is a cost of displacement, not of erasure., Comment: 12 pages, 2 figures

We present a comprehensive overview of a volume-complete sample of white dwarfs located within 40 pc of the Sun, a significant proportion of which were detected in Gaia Data Release 3 (DR3). Our DR3 sample contains 1076 spectroscopically confirmed white dwarfs, with just five candidates within the volume remaining unconfirmed (more than 99 per cent spectroscopic completeness). Additionally, 28 white dwarfs were not in our initial selection from Gaia DR3, most of which are in unresolved binaries. We use Gaia DR3 photometry and astrometry to determine a uniform set of white dwarf parameters, including mass, effective temperature, and cooling age. We assess the demographics of the 40 pc sample, specifically magnetic fields, binarity, space density and mass distributions., Comment: Accepted for publication in MNRAS. 24 pages, 25 figures

We perform nonlocal thermodynamic equilibrium (NLTE) model atmosphere analyses of the three hottest hypervelocity stars (space velocities between $\approx$ 1500-2800 km s$^{-1}$) known to date, which were recently discovered spectroscopically and identified as runaways from Type Ia supernovae. The hottest of the three (J0546$+$0836, effective temperature $T_\mathrm{eff}$ = 95,000 $\pm$ 15,000 K, surface gravity log g = $5.5 \pm 0.5$) has an oxygen-dominated atmosphere with a significant amount of carbon (C = $0.10 \pm 0.05$, O = $0.90 \pm 0.05$, mass fractions). Its mixed absorption+emission line spectrum exhibits photospheric absorption lines from O V and O VI as well as O III and O IV emission lines that are formed in a radiation-driven wind with a mass-loss rate of the order of $10^{-8}$ $M_\odot$ yr$^{-1}$. Spectroscopically, J0546$+$0836 is a [WC]-PG1159 transition-type pre-white dwarf. The second object (J0927$-$6335) is a PG1159-type white dwarf with a pure absorption-line spectrum dominated by C III/C IV and O III/O IV. We find $T_\mathrm{eff}$ = 60,000 $\pm$ 5000 K, log g = $7.0 \pm 0.5$, and a carbon- and oxygen-dominated atmosphere with C = $0.47 \pm 0.25$, O = $0.48 \pm 0.25$, and possibly a minute amount of helium (He = $0.05 \pm 0.05$). Comparison with post-AGB evolutionary tracks suggests a mass of $M\approx0.5$ $M_\odot$ for both objects, if such tracks can safely be applied to these stars. We find the third object (J1332$-$3541) to be a relatively massive ($M=0.89 M_\odot$) hydrogen-rich (DAO) white dwarf with $T_\mathrm{eff}$ = 65,657 $\pm$ 2390 K, log g = $8.38 \pm 0.08$, and abundances H = $0.65 \pm 0.04$ and He = $0.35 \pm 0.04$. We discuss our results in the context of the "dynamically driven double-degenerate double-detonation" (D$^6$) scenario proposed for the origin of these stars., Comment: Accepted for publication in A&A

Since 2018, the potential for a high-energy neutrino telescope, named the Pacific Ocean Neutrino Experiment (P-ONE), has been thoroughly examined by two pathfinder missions, STRAW and STRAW-b, short for short for Strings for Absorption Length in Water. The P-ONE project seeks to install a neutrino detector with a one cubic kilometer volume in the Cascadia Basin's deep marine surroundings, situated near the western shores of Vancouver Island, Canada. To assess the environmental conditions and feasibility of constructing a neutrino detector of that scale, the pathfinder missions, STRAW and STRAW-b, have been deployed at a depth of 2.7 km within the designated site for P-ONE and were connected to the NEPTUNE observatory, operated by Ocean Networks Canada (ONC). While STRAW focused on analyzing the optical properties of water in the Cascadia Basin, \ac{strawb} employed cameras and spectrometers to investigate the characteristics of bioluminescence in the deep-sea environment. This report introduces the STRAW-b concept, covering its scientific objectives and the instrumentation used. Furthermore, it discusses the design considerations implemented to guarantee a secure and dependable deployment process of STRAW-b. Additionally, it showcases the data collected by battery-powered loggers, which monitored the mechanical stress on the equipment throughout the deployment. The report also offers an overview of STRAW-b's operation, with a specific emphasis on the notable advancements achieved in the data acquisition (DAQ) system and its successful integration with the server infrastructure of ONC., Comment: 20 pages, 11 figures, 2 tables

Spinal cord segmentation is clinically relevant and is notably used to compute spinal cord cross-sectional area (CSA) for the diagnosis and monitoring of cord compression or neurodegenerative diseases such as multiple sclerosis. While several semi and automatic methods exist, one key limitation remains: the segmentation depends on the MRI contrast, resulting in different CSA across contrasts. This is partly due to the varying appearance of the boundary between the spinal cord and the cerebrospinal fluid that depends on the sequence and acquisition parameters. This contrast-sensitive CSA adds variability in multi-center studies where protocols can vary, reducing the sensitivity to detect subtle atrophies. Moreover, existing methods enhance the CSA variability by training one model per contrast, while also producing binary masks that do not account for partial volume effects. In this work, we present a deep learning-based method that produces soft segmentations of the spinal cord. Using the Spine Generic Public Database of healthy participants ($\text{n}=267$; $\text{contrasts}=6$), we first generated participant-wise soft ground truth (GT) by averaging the binary segmentations across all 6 contrasts. These soft GT, along with aggressive data augmentation and a regression-based loss function, were used to train a U-Net model for spinal cord segmentation. We evaluated our model against state-of-the-art methods and performed ablation studies involving different loss functions and domain generalization methods. Our results show that using the soft segmentations along with a regression loss function reduces CSA variability ($p < 0.05$, Wilcoxon signed-rank test). The proposed spinal cord segmentation model generalizes better than the state-of-the-art methods amongst unseen datasets, vendors, contrasts, and pathologies (compression, lesions), while accounting for partial volume effects., Comment: Revision Submitted to Medical Image Analysis

We introduce a definition of multivariate majorization that is new to the economics literature. Our majorization technique allows us to generalize Mussa and Rosen's (1978) "ironing" to a broad class of multivariate principal-agents problems. Specifically, we consider adverse selection problems in which agents' types are one dimensional but informational externalities create a multidimensional ironing problem. Our majorization technique applies to discrete and continuous type spaces alike and we demonstrate its usefulness for contract theory and mechanism design. We further show that multivariate majorization yields a natural extension of second-order stochastic dominance to multiple dimensions and derive its implications for decision making under multivariate risk.

We establish the Subgradient Theorem for monotone correspondences -- a monotone correspondence is equal to the subdifferential of a potential if and only if it is conservative, i.e. its integral along a closed path vanishes irrespective of the selection from the correspondence along the path. We prove two attendant results: the Potential Theorem, whereby a conservative monotone correspondence can be integrated up to a potential, and the Duality Theorem, whereby the potential has a Fenchel dual whose subdifferential is another conservative monotone correspondence. We use these results to reinterpret and extend Baldwin and Klemperer's (2019) characterization of demand in economies with indivisible goods. We introduce a simple test for existence of Walrasian equilibrium in quasi-linear economies. Fenchel's Duality Theorem implies this test is met when the aggregate utility is concave, which is not necessarily the case with indivisible goods even if all consumers have concave utilities.

The convective dredge-up of carbon from the interiors of hydrogen-deficient white dwarfs has long been invoked to explain the presence of carbon absorption features in the spectra of cool DQ stars ($T_{\rm eff} < 10{,}000\,{\rm K}$). It has been hypothesized that this transport process is not limited to DQ white dwarfs and also operates, albeit less efficiently, in non-DQ hydrogen-deficient white dwarfs within the same temperature range. This non-DQ population is predominantly composed of DC white dwarfs, which exhibit featureless optical spectra. However, no direct observational evidence of ubiquitous carbon pollution in DC stars has thus far been uncovered. In this Letter, we analyze data from the Galaxy Evolution Explorer (GALEX) to reveal the photometric signature of ultraviolet carbon lines in most DC white dwarfs in the $8500\,{\rm K} \leq T_{\rm eff} \leq 10{,}500\,{\rm K}$ temperature range. Our results show that the vast majority of hydrogen-deficient white dwarfs experience carbon dredge-up at some point in their evolution., Comment: Accepted for publication in MNRAS Letters

Two recently discovered white dwarfs, WDJ041246.84$+$754942.26 and WDJ165335.21$-$100116.33, exhibit H$\alpha$ and H$\beta$ Balmer line emission similar to stars in the emerging DAHe class, yet intriguingly have not been found to have detectable magnetic fields. These white dwarfs are assigned the spectral type DAe. We present detailed follow-up of the two known DAe stars using new time-domain spectroscopic observations and analysis of the latest photometric time-series data from TESS and ZTF. We measure the upper magnetic field strength limit of both stars as $B < 0.05$ MG. The DAe white dwarfs exhibit photometric and spectroscopic variability, where in the case of WDJ041246.84$+$754942.26 the strength of the H$\alpha$ and H$\beta$ emission cores varies in anti-phase with its photometric variability over the spin period, which is the same phase relationship seen in DAHe stars. The DAe white dwarfs closely cluster in one region of the Gaia Hertzsprung-Russell diagram together with the DAHe stars. We discuss current theories on non-magnetic and magnetic mechanisms which could explain the characteristics observed in DAe white dwarfs, but additional data are required to unambiguously determine the origin of these stars., Comment: 20 pages, 16 figures. Published by MNRAS

The observational signature of core crystallization of white dwarfs has recently been discovered. However, the magnitude of the crystallization-powered cooling delay required to match observed white dwarfs is larger than predicted by conventional models, requiring additional mechanisms of energy release in white dwarf interiors. The most ideal benchmarks for understanding this discrepancy would be bright and nearby crystallizing white dwarfs with total ages that can be externally constrained. In this work we report that a recently discovered white dwarf is a bound companion to the triple star HD 190412, forming a new Sirius-like system in the solar neighbourhood. The location of HD 190412 C on the $T_{\text{eff}}-\text{mass}$ diagram implies it is undergoing crystallization, making this the first confirmed crystallizing white dwarf whose total age can be externally constrained. Motivated by the possibility that a cooling delay caused by crystallization can be directly detected for this white dwarf we employ a variety of methods to constrain the age of the system; however, our empirical age anomaly of $+3.1\pm1.9$ Gyr is ultimately too imprecise to reach statistical significance, preventing us from making strong constraints to models of white dwarf crystallization. Our results are nonetheless compatible with the recent hypothesis that $^{22}$Ne phase separation is responsible for the excess cooling delay of crystallizing white dwarfs. The discovery of this system at only 32 parsecs suggests that similar benchmark systems are likely to be common; future discoveries may therefore provide powerful tests for models of white dwarf crystallization., Comment: 19 pages, 8 figures, 8 tables. Published in MNRAS. Minor revisions included

The weak ferromagnet FeBO$_3$ is well known for being a unique system for modelling and testing magnetic dynamics primarily due to relatively simple and localized magnetic structure and its interesting spin wave dynamics. At room temperature, it has slightly canted iron moments lying in the a-b plane that result in a strong antiferromagnetic moment and a weak ferromagnetic moment, which results in pronounced ferromagnetic and antiferromagnetic spin modes. However, some previous studies have shown unusual low-temperature behavior that suggests a phase transition. By performing low-temperature magnetization measurements, both in bulk and on the mesoscale, we have observed a low temperature magnetic texture in this material in which a large c-axis magnetization occurs. Magnetic fields along the c-axis as high as 1300 Oe were observed close to the sample surface. This presents evidence for the onset of a Morin transition or another type of spin-reorientation phase transition wherein the Fe3+ moments would acquire a c-axis component to their canting below a critical temperature. The observation of this c-axis magnetization suggests that there is a different ground state in this material than has been previously expected and could be due to as yet unexplored intricacies of the Dzyaloshinskii-Moriya interaction.

We present the results from our ongoing spectroscopic survey targeting low mass white dwarf binaries, focusing on the southern sky. We used a Gaia DR2 and eDR3 based selection and identified 28 new binaries, including 19 new extremely low mass white dwarfs, one short period, likely eclipsing, DABZ, and two potential LISA binaries. We present orbital and atmospheric parameters for each new binary based on our spectroscopic follow-up. Four of our new binaries show periodic photometric variability in the TESS 2-minute cadence data, including one new eclipsing double-lined spectroscopic binary. Three others show periodic photometric variability in ZTF, including one new eclipsing binary. We provide estimates for the inclinations and scaled component radii for these ZTF variables, based on light curve modeling to our high-speed photometric follow-up observations. Our observations have increased the sample of ELM Survey binaries identified in the southern sky to 41, an increase of 64%. Future time domain surveys, such as BlackGEM and the Vera C. Rubin Observatory Legacy Survey of Space and Time, will efficiently identify the photometric variables in the southern sky and significantly increase the population of southern sky low mass white dwarf binaries, leading to a more complete all-sky population of these systems., Comment: 28 pages. 13 Figures, 5 Tables. Accepted for publication in The Astrophysical Journal

The Gaia colour--magnitude diagram reveals a striking separation between hydrogen-atmosphere white dwarfs and their helium-atmosphere counterparts throughout a significant portion of the white dwarf cooling track. However, pure-helium atmospheres have Gaia magnitudes that are too close to the pure-hydrogen case to explain this bifurcation. To reproduce the observed split in the cooling sequence, it has been shown that trace amounts of hydrogen and/or metals must be present in the helium-dominated atmospheres of hydrogen-deficient white dwarfs. Yet, a complete explanation of the Gaia bifurcation that takes into account known constraints on the spectral evolution of white dwarfs has thus far not been proposed. In this work, we attempt to provide such a holistic explanation by performing population synthesis simulations coupled with state-of-the-art model atmospheres and evolutionary calculations that account for element transport in the envelopes of white dwarfs. By relying on empirically grounded assumptions, these simulations successfully reproduce the bifurcation. We show that the convective dredge-up of optically undetectable traces of carbon from the deep interior is crucial to account for the observations. Neither the convective dilution/mixing of residual hydrogen nor the accretion of hydrogen or metals can be the dominant drivers of the bifurcation. Finally, we emphasize the importance of improving theoretical models for the average ionization level of carbon in warm dense helium, which governs the shape of the diffusive tail of carbon and in turn the predicted amount of dredged-up carbon., Comment: Accepted for publication in MNRAS, minor changes following reports from reviewers

Quantum teleportation is the name of a problem: how can the real-valued parameters encoding the state at Alice's location make their way to Bob's location via shared entanglement and only two bits of classical communication? Without an explanation, teleportation appears to be a conjuring trick. Investigating the phenomenon with Schr\"odinger states and reduced density matrices shall always leave loose ends because they are not local and complete descriptions of quantum systems. Upon demonstrating that the Heisenberg picture admits a local and complete description, Deutsch and Hayden rendered its explanatory power manifest by revealing the trick behind teleportation, namely, by providing an entirely local account. Their analysis is re-exposed and further developed., Comment: 22 pages, 4 figures

Postmortem MRI allows brain anatomy to be examined at high resolution and to link pathology measures with morphometric measurements. However, automated segmentation methods for brain mapping in postmortem MRI are not well developed, primarily due to limited availability of labeled datasets, and heterogeneity in scanner hardware and acquisition protocols. In this work, we present a high resolution of 135 postmortem human brain tissue specimens imaged at 0.3 mm$^{3}$ isotropic using a T2w sequence on a 7T whole-body MRI scanner. We developed a deep learning pipeline to segment the cortical mantle by benchmarking the performance of nine deep neural architectures, followed by post-hoc topological correction. We then segment four subcortical structures (caudate, putamen, globus pallidus, and thalamus), white matter hyperintensities, and the normal appearing white matter. We show generalizing capabilities across whole brain hemispheres in different specimens, and also on unseen images acquired at 0.28 mm^3 and 0.16 mm^3 isotropic T2*w FLASH sequence at 7T. We then compute localized cortical thickness and volumetric measurements across key regions, and link them with semi-quantitative neuropathological ratings. Our code, Jupyter notebooks, and the containerized executables are publicly available at: https://pulkit-khandelwal.github.io/exvivo-brain-upenn, Comment: Preprint submitted to NeuroImage Project website: https://pulkit-khandelwal.github.io/exvivo-brain-upenn

When developing tools for automated cortical segmentation, the ability to produce topologically correct segmentations is important in order to compute geometrically valid morphometry measures. In practice, accurate cortical segmentation is challenged by image artifacts and the highly convoluted anatomy of the cortex itself. To address this, we propose a novel deep learning-based cortical segmentation method in which prior knowledge about the geometry of the cortex is incorporated into the network during the training process. We design a loss function which uses the theory of Laplace's equation applied to the cortex to locally penalize unresolved boundaries between tightly folded sulci. Using an ex vivo MRI dataset of human medial temporal lobe specimens, we demonstrate that our approach outperforms baseline segmentation networks, both quantitatively and qualitatively., Comment: Accepted at the 28th biennial international conference on Information Processing in Medical Imaging (IPMI 2023)

Closed cycle $He^{3}-He^{4}$ dilution cryostats became the platform of choice in quantum sciences in the era of helium shortage. However, in many experiments, the mechanical vibrations induced by the pulsed cryocoolers present a significant drawback reflected both in electronic and mechanical noises. Here, we present a hybrid dilution cryostat platform; we have automated a commercial closed-cycle system to operate on a cryocooler or on a liquid helium battery. We implemented a scanning SQUID microscope in the hybrid dilution refrigerator. In this work we show the design of the hybrid setup and how its operation eliminates vibration artefacts in magnetic imaging.

In the framework of our extensive modeling study of the spectral evolution of white dwarfs, we present here a new set of detailed calculations of the transport of residual hydrogen in helium-rich white dwarfs. First, we investigate the so-called float-up process at high effective temperature, whereby the upward diffusion of trace hydrogen leads to the formation of a hydrogen atmosphere. We examine the dependence of this phenomenon on the initial hydrogen abundance and on the strength of the radiative wind that opposes gravitational settling. Combined with our empirical knowledge of spectral evolution, our simulations provide new quantitative constraints on the hydrogen content of the hot helium-dominated white dwarf population. Then, we study the outcome of the so-called convective dilution process at low effective temperature, whereby the superficial hydrogen layer is mixed within the underlying helium-rich envelope. In stark contrast with previous works on convective dilution, we demonstrate that, under reasonable assumptions, our models successfully reproduce the observed atmospheric composition of cool DBA stars, thereby solving one of the most important problems of spectral evolution theory. This major improvement is due to our self-consistent modeling of the earlier float-up process, which predicts the existence of a massive hydrogen reservoir underneath the thin superficial layer. We argue that the trace hydrogen detected at the surface of DBA white dwarfs is, in most cases, of primordial origin rather than the result of external accretion., Comment: 21 pages, 9 figures, accepted for publication in The Astrophysical Journal

Van Roosbroeck's equations constitute a versatile tool to determine the dynamics of electrons under time- and space-dependent perturbations. Extensively utilized in ordinary semiconductors, their potential to model devices made from topological materials remains untapped. Here, we adapt van Roosbroeck's equations to theoretically study the bulk response of a Weyl semimetal to an ultrafast and spatially localized light pulse in the presence of a quantizing magnetic field. We predict a transient oscillatory photovoltage that originates from the chiral anomaly. The oscillations take place at the plasma frequency (THz range) and are damped by intervalley scattering and dielectric relaxation. Our results illustrate the ability of van Roosbroeck's equations to unveil the interplay between electronic band topology and fast carrier dynamics in microelectronic devices., Comment: 5 pages + appendix. Version accepted for publication

A recent analysis of the 100 pc white dwarf sample in the SDSS footprint demonstrated for the first time the existence of a well defined ultracool -- or IR-faint -- white dwarf sequence in the Hertzsprung-Russell diagram. Here we take advantage of this discovery to enlarge the IR-faint white dwarf sample threefold. We expand our selection to the entire Pan-STARRS survey footprint as well as the Montreal White Dwarf Database 100 pc sample, and identify 37 candidates with strong flux deficits in the optical. We present follow-up Gemini optical spectroscopy of 30 of these systems, and confirm all of them as IR-faint white dwarfs. We identify an additional set of 33 objects as candidates based on their colors and magnitudes. We present a detailed model atmosphere analysis of all 70 newly identified IR-faint white dwarfs together with 35 previously known objects reported in the literature. We discuss the physics of model atmospheres and show that the key physical ingredient missing in our previous generation of model atmospheres was the high-density correction to the He-minus free-free absorption coefficient. With new model atmospheres calculated for the purpose of this analysis, we now obtain significantly higher effective temperatures and larger stellar masses for these IR-faint white dwarfs than the Teff and M values reported in previous analyses, thus solving a two decade old problem. In particular, we identify in our sample a group of ultramassive white dwarfs in the Debye cooling phase with stellar parameters never measured before., Comment: Accepted for publication in The Astrophysical Journal (33 pages, 21 figures)

We suggest a quantitative and objective notion of emergence. Our proposal uses algorithmic information theory as a basis for an objective framework in which a bit string encodes observational data. A plurality of drops in the Kolmogorov structure function of such a string is seen as the hallmark of emergence. Our definition offers some theoretical results, in addition to extending the notions of coarse-graining and boundary conditions. Finally, we confront our proposal with applications to dynamical systems and thermodynamics., Comment: 46 pages, 11 figures

Perception and mapping systems are among the most computationally, memory, and bandwidth intensive software components in robotics. Therefore, analysis, debugging, and optimization are crucial to improve perception systems performance in real-time applications. However, standard approaches often depict a partial picture of the actual performance. Fortunately, instrumentation and tracing offer a great opportunity for detailed performance analysis of real-time systems. In this paper, we show how our novel open-source tracing tools and techniques for ROS 2 enable us to identify delays, bottlenecks and critical paths inside centralized, or distributed, perception and mapping systems.

Distributed robotic systems rely heavily on the publish-subscribe communication paradigm and middleware frameworks that support it, such as the Robot Operating System (ROS), to efficiently implement modular computation graphs. The ROS 2 executor, a high-level task scheduler which handles ROS 2 messages, is a performance bottleneck. We extend ros2_tracing, a framework with instrumentation and tools for real-time tracing of ROS 2, with the analysis and visualization of the flow of messages across distributed ROS 2 systems. Our method detects one-to-many and many-to-many causal links between input and output messages, including indirect causal links through simple user-level annotations. We validate our method on both synthetic and real robotic systems, and demonstrate its low runtime overhead. Moreover, the underlying intermediate execution representation database can be further leveraged to extract additional metrics and high-level results. This can provide valuable timing and scheduling information to further study and improve the ROS 2 executor as well as optimize any ROS 2 system. The source code is available at: https://github.com/christophebedard/ros2-message-flow-analysis., Comment: 14 pages, 12 figures

We continue our comprehensive theoretical investigation of the spectral evolution of white dwarfs based on sophisticated simulations of element transport. In this paper, we focus on the transformation of PG 1159 stars into DO/DB white dwarfs due to the gravitational settling of heavy elements, and then into DQ white dwarfs through the convective dredge-up of carbon. We study the impact of several physical parameters on the evolution of the surface carbon abundance over a wide range of effective temperature. In the hot PG 1159 and DO phases, our calculations confirm that the temperature of the PG 1159-to-DO transition depends sensitively on the stellar mass and the wind mass-loss rate. We show that measured carbon abundances of DOZ white dwarfs are mostly accounted for by our models, with the notable exception of the coolest DOZ stars. In the cooler DB and DQ phases, the predicted atmospheric composition is strongly influenced by the stellar mass, the thickness of the envelope, the initial carbon content, the efficiency of convective overshoot, and the presence of residual hydrogen. We demonstrate that, under reasonable assumptions, our simulations reproduce very well the observed carbon abundance pattern of DQ stars, which thus allows us to constrain the extent of the overshoot region in cool helium-rich white dwarfs. We also argue that our calculations naturally explain a number of recent empirical results, such as the relative excess of low-mass DQ stars and the presence of trace hydrogen and/or carbon at the surface of most DC and DZ stars., Comment: 23 pages, 15 figures, accepted for publication in The Astrophysical Journal