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1. Evidence for a Global Slowdown in Seafloor Spreading Since 15 Ma

   https://doi.org/10.1029/2022GL097937  

   Dalton, Colleen A.; Wilson, Douglas S.; Herbert, Timothy D. (March 2022, Geophysical Research Letters)

   Abstract The rate of ocean‐crust production exerts control over sea level, mantle heat loss, and climate. Different strategies to account for incomplete seafloor preservation have led to differing conclusions about how much production rates have changed since the Cretaceous, if at all. We construct a new global synthesis of crust production along 18 mid‐ocean ridges for the past 19 Myr at high temporal resolution. We find that the global production rate during 6–5 Ma was only 69%–75% of the 16–15 Ma interval. The reduction in crust production is mostly due to slower seafloor spreading along almost all ridge systems. While the total ridge length has varied little since 19 Ma, some fast‐spreading ridges have grown shorter and slow‐spreading ridges grown longer, amplifying the spreading‐rate changes. Our production curves represent a new data set for investigating the forces driving plate motions and the role of tectonic degassing on climate. 

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2. Early Spectroscopy and Dense Circumstellar Medium Interaction in SN 2023ixf

   https://doi.org/10.3847/2041-8213/acf9a4  

   Bostroem, K. Azalee; Pearson, Jeniveve; Shrestha, Manisha; Sand, David J.; Valenti, Stefano; Jha, Saurabh W.; Andrews, Jennifer E.; Smith, Nathan; Terreran, Giacomo; Green, Elizabeth; et al (October 2023, The Astrophysical Journal Letters)

   Abstract We present the optical spectroscopic evolution of SN 2023ixf seen in subnight cadence spectra from 1.18 to 15 days after explosion. We identify high-ionization emission features, signatures of interaction with material surrounding the progenitor star, that fade over the first 7 days, with rapid evolution between spectra observed within the same night. We compare the emission lines present and their relative strength to those of other supernovae with early interaction, finding a close match to SN 2020pni and SN 2017ahn in the first spectrum and SN 2014G at later epochs. To physically interpret our observations, we compare them to CMFGEN models with confined, dense circumstellar material around a red supergiant (RSG) progenitor from the literature. We find that very few models reproduce the blended Niii(λλ4634.0,4640.6)/Ciii(λλ4647.5,4650.0) emission lines observed in the first few spectra and their rapid disappearance thereafter, making this a unique diagnostic. From the best models, we find a mass-loss rate of 10⁻³–10⁻²M_⊙yr⁻¹, which far exceeds the mass-loss rate for any steady wind, especially for an RSG in the initial mass range of the detected progenitor. These mass-loss rates are, however, similar to rates inferred for other supernovae with early circumstellar interaction. Using the phase when the narrow emission features disappear, we calculate an outer dense radius of circumstellar materialR_CSM,out≈ 5 × 10¹⁴cm, and a mean circumstellar material density ofρ= 5.6 × 10⁻¹⁴g cm⁻³. This is consistent with the lower limit on the outer radius of the circumstellar material we calculate from the peak Hαemission flux,R_CSM,out≳ 9 × 10¹³cm. 

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3. Exploring Changes in Quasar Spectral Energy Distributions across C iv Parameter Space

   https://doi.org/10.3847/1538-4357/ac6a5d  

   Rivera, Angelica B.; Richards, Gordon T.; Gallagher, Sarah C.; McCaffrey, Trevor V.; Rankine, Amy L.; Hewett, Paul C.; Shemmer, Ohad (June 2022, The Astrophysical Journal)

   Abstract We examine the UV/X-ray properties of 1378 quasars in order to link empirical correlations to theoretical models of the physical mechanisms dominating quasars as a function of mass and accretion rate. The clarity of these correlations is improved when (1) using Civbroad emission line equivalent width (EQW) and blueshift (relative to systemic) values calculated from high signal-to-noise ratio reconstructions of optical/UV spectra and (2) removing quasars expected to be absorbed based on their UV/X-ray spectral slopes. In addition to using the traditional Civparameter space measures of CivEQW and blueshift, we define a “Civ∥ distance” along a best-fit polynomial curve that incorporates information from both Civparameters. We find that the Civ∥ distance is linearly correlated with both the optical-to-X-ray slope,α_ox, and broad-line HeiiEQW, which are known spectral energy distribution indicators, but does not require X-ray or high spectral resolution UV observations to compute. The Civ∥ distance may be a better indicator of the mass-weighted accretion rate, parameterized byL/L_Edd, than the CivEQW or blueshift alone, as those relationships are known to break down at the extrema. Conversely, there is only a weak correlation with the X-ray energy index (Γ), an alternateL/L_Eddindicator. We find no X-ray or optical trends in the direction perpendicular to the Civdistance that could be used to reveal differences in accretion disk, wind, or corona structure that could be widening the CivEQW–blueshift distribution. A different parameter (such as metallicity) not traced by these data must come into play. 

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4. The phonology of A'ingae

   https://doi.org/10.1111/lnc3.12512  

   Dąbkowski, Maksymilian (April 2024, Language and Linguistics Compass)

   Abstract A'ingae (or Cofán,ISO639‐3: con) is an indigenous language isolate spoken in northeast Ecuador and southern Colombia. This paper presents the first comprehensive overview of the A'ingae phonology, including descriptions of (i) the language's phonemic inventory, (ii) phonotactics and a number of related phonological rules, (iii) nasality and nasal spreading, as well as (iv) stress, glottalisation, their morphophonology, and aspects of clause‐level prosody. 

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5. Electron kinetics in a positive column of AC discharges in a dynamic regime

   https://doi.org/10.1088/1361-6595/acee1c  

   Humphrey, Nathan A; Kolobov, Vladimir I (August 2023, Plasma Sources Science and Technology)

   Abstract We have performed hybrid kinetic-fluid simulations of a positive column in alternating current (AC) argon discharges over a range of driving frequenciesfand gas pressurepfor the conditions when the spatial nonlocality of the electron energy distribution function (EEDF) is substantial. Our simulations confirmed that the most efficient conditions of plasma maintenance are observed in the dynamic regime when time modulations of mean electron energy (temperature) are substantial. The minimal values of the root mean square electric field and the electron temperature have been observed atf/pvalues of about 3 kHz Torr⁻¹in a tube of radiusR= 1 cm. The ionization rate and plasma density reached maximal values under these conditions. The numerical solution of a kinetic equation allowed accounting for the kinetic effects associated with spatial and temporal nonlocality of the EEDF. Using thekineticenergy of electrons as an independent variable, we solved an anisotropic tensor diffusion equation in phase space. We clarified the role of different flux components during electron diffusion in phase space over surfaces of constanttotalenergy. We have shown that the kinetic theory uncovers a more exciting and rich physics than the classical ambipolar diffusion (Schottky) model. Non-monotonic radial distributions of excitation rates, metastable densities, and plasma density have been observed in our simulations atpR >6 Torr cm. The predicted off-axis plasma density peak in the dynamic regime has never been observed in experiments so far. We hope our results stimulate further experimental studies of the AC positive column. The kinetic analysis could help uncover new physics even for such a well-known plasma object as a positive column in noble gases. 

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