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1. Laboratory Study of the Cameron Bands and UV Doublet in the Middle Ultraviolet 180–300 nm by Electron Impact upon CO 2 with Application to Mars

   https://doi.org/10.3847/1538-4357/ac88c8  

   Lee, Rena A. ; Ajello, Joseph M. ; Malone, Charles P. ; Evans, J. Scott ; Veibell, Victoir ; Holsclaw, Gregory M. ; McClintock, William E. ; Hoskins, Alan C. ; Jain, Sonal K. ; Gérard, Jean-Claude ; et al ( October 2022 , The Astrophysical Journal)

   We have observed electron impact fluorescence from CO₂to excite the Cameron bands (CBs), CO (a³Π →X¹Σ⁺; 180–280 nm), the first-negative group (1NG) bands, CO⁺(B²Σ⁺→X²Σ⁺; 180–320 nm), the fourth-positive group (4PG) bands, CO (A¹Π →X¹Σ⁺; 111–280 nm), and the UV doublet, CO₂⁺($\tilde{B}{}^2{\mathrm{\Sigma }}_u^{+}\to \tilde{X}{}^2{\mathrm{\Pi }}_g;$288.3 and 289.6 nm) in the ultraviolet (UV). This wavelength range matches the spectral region of past and present spacecraft equipped to observe UV dayglow and aurora emissions from the thermospheres (100–300 km) of Mars and Venus. Our large vacuum system apparatus is able to measure the emission cross sections of the strongest optically forbidden UV transitions found in planetary spectra. Based on our cross-sectional measurements, previous CB emission cross-sectional errors exceed a factor of 3. The UV doublet lifetime is perturbed through$\tilde{B}{}^2{\mathrm{\Sigma }}_u^{+}-\tilde{A}{}^2{\mathrm{\Pi }}_u$spin–orbit coupling. Forward modeling codes of the Mars dayglow have not been accurate in the mid-UV due to systematic errors in these two emission cross sections. We furnish absolute emission cross sections for several band systems over electron energies 20–100 eV for CO₂. We present a CB lifetime, which together with emission cross sections, furnish a set of fundamental physical constants for electron transport codes such as AURIC (Atmospheric Ultraviolet Radiance Integrated Code). AURIC and Trans-Mars are used in the analysis of UV spectra from the Martian dayglow and aurora.

    

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2. Microphysics of Relativistic Collisionless Electron-ion-positron Shocks

   https://doi.org/10.3847/1538-4357/ac713e  

   Grošelj, Daniel ; Sironi, Lorenzo ; Beloborodov, Andrei M. ( July 2022 , The Astrophysical Journal)

   We perform particle-in-cell simulations to elucidate the microphysics of relativistic weakly magnetized shocks loaded with electron-positron pairs. Various external magnetizationsσ≲ 10⁻⁴and pair-loading factorsZ_±≲ 10 are studied, whereZ_±is the number of loaded electrons and positrons per ion. We find the following: (1) The shock becomes mediated by the ion Larmor gyration in the mean field whenσexceeds a critical valueσ_Lthat decreases withZ_±. Atσ≲σ_Lthe shock is mediated by particle scattering in the self-generated microturbulent fields, the strength and scale of which decrease withZ_±, leading to lowerσ_L. (2) The energy fraction carried by the post-shock pairs is robustly in the range between 20% and 50% of the upstream ion energy. The mean energy per post-shock electron scales as${\overline{E}}_{\mathrm{e}}\propto {\left(Z_{\pm }+1\right)}^{-1}$. (3) Pair loading suppresses nonthermal ion acceleration at magnetizations as low asσ≈ 5 × 10⁻⁶. The ions then become essentially thermal with mean energy${\overline{E}}_{\mathrm{i}}$, while electrons form a nonthermal tail, extending from$E\sim {\left(Z_{\pm }+1\right)}^{-1}{\overline{E}}_{\mathrm{i}}$to${\overline{E}}_{\mathrm{i}}$. Whenσ= 0, particle acceleration is enhanced by the formation of intense magnetic cavities that populate the precursor during the late stages of shock evolution. Here, the maximum energy of the nonthermal ions and electrons keeps growing over the duration of the simulation. Alongside the simulations, we develop theoretical estimates consistent with the numerical results. Our findings have important implications for models of early gamma-ray burst afterglows.

    

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3. Electronic structure in a transition metal dipnictide TaAs 2

   https://doi.org/10.1088/1361-648X/ad04fc  

   Regmi, Sabin ; Huang, Cheng-Yi ; Khan, Mojammel A. ; Wang, Baokai ; Pradhan Sakhya, Anup ; Hosen, M. Mofazzel ; Thompson, Jesse ; Singh, Bahadur ; Denlinger, Jonathan D. ; Ishigami, Masahiro ; et al ( November 2023 , Journal of Physics: Condensed Matter)

   The family of transition-metal dipnictides has been of theoretical and experimental interest because this family hosts topological states and extremely large magnetoresistance (MR). Recently,${\mathrm{T}\mathrm{a}\mathrm{A}\mathrm{s}}_2$, a member of this family, has been predicted to support a topological crystalline insulating state. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES), we reveal both closed and open pockets in the metallic Fermi surface (FS) and linearly dispersive bands on the ($\bar{2}01$) surface, along with the presence of extreme MR observed from magneto-transport measurements. A comparison of the ARPES results with first-principles computations shows that the linearly dispersive bands on the measured surface of${\mathrm{T}\mathrm{a}\mathrm{A}\mathrm{s}}_2$are trivial bulk bands. The absence of symmetry-protected surface state on the ($\bar{2}01$) surface indicates its topologically dark nature. The presence of open FS features suggests that the open-orbit fermiology could contribute to the extremely large MR of${\mathrm{T}\mathrm{a}\mathrm{A}\mathrm{s}}_2$.

    

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4. Now You See It, Now You Don’t: Star Formation Truncation Precedes the Loss of Molecular Gas by ∼100 Myr in Massive Poststarburst Galaxies at z ∼ 0.6

   https://doi.org/10.3847/1538-4357/ac3dfa  

   Bezanson, Rachel ; Spilker, Justin S. ; Suess, Katherine A. ; Setton, David J. ; Feldmann, Robert ; Greene, Jenny E. ; Kriek, Mariska ; Narayanan, Desika ; Verrico, Margaret ( February 2022 , The Astrophysical Journal)

   We use ALMA observations of CO(2–1) in 13 massive (M_*≳ 10¹¹M_⊙) poststarburst galaxies atz∼ 0.6 to constrain the molecular gas content in galaxies shortly after they quench their major star-forming episode. The poststarburst galaxies in this study are selected from the Sloan Digital Sky Survey spectroscopic samples (Data Release 14) based on their spectral shapes, as part of the Studying QUenching at Intermediate-z Galaxies: Gas, angu$$\overrightarrow{L}\mathrm{ar}$$momentum, and Evolution ($\mathit{SQuIGG}\vec{L}E$) program. Early results showed that two poststarburst galaxies host large H₂reservoirs despite their low inferred star formation rates (SFRs). Here we expand this analysis to a larger statistical sample of 13 galaxies. Six of the primary targets (45%) are detected, with$M_{{\mathrm{H}}_2}\gtrsim {10}^9$M_⊙. Given their high stellar masses, this mass limit corresponds to an average gas fraction of$〈f_{{\mathrm{H}}_2}\equiv M_{{\mathrm{H}}_2}/M_{*}〉\sim 7\%$or ∼14% using lower stellar masses estimates derived from analytic, exponentially declining star formation histories. The gas fraction correlates with theD_n4000 spectral index, suggesting that the cold gas reservoirs decrease with time since burst, as found in local K+A galaxies. Star formation histories derived from flexible stellar population synthesis modeling support this empirical finding: galaxies that quenched ≲150 Myr prior to observation host detectable CO(2–1) emission, while older poststarburst galaxies are undetected. The large H₂reservoirs and low SFRs in the sample imply that the quenching of star formation precedes the disappearance of the cold gas reservoirs. However, within the following 100–200 Myr, the$\mathit{SQuIGG}\vec{L}E$galaxies require the additional and efficient heating or removal of cold gas to bring their low SFRs in line with standard H₂scaling relations.

    

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5. RR Lyrae Mid-infrared Period–Luminosity–Metallicity and Period–Wesenheit–Metallicity Relations Based on Gaia DR3 Parallaxes

   https://doi.org/10.3847/1538-4357/acb20a  

   Mullen, Joseph P. ; Marengo, Massimo ; Martínez-Vázquez, Clara E. ; Chaboyer, Brian ; Bono, Giuseppe ; Braga, Vittorio F. ; Dall’Ora, Massimo ; D’Orazi, Valentina ; Fabrizio, Michele ; Monelli, Matteo ; et al ( March 2023 , The Astrophysical Journal)

   We present new empirical infrared period–luminosity–metallicity (PLZ) and period–Wesenheit–metallicity (PWZ) relations for RR Lyae based on the latest Gaia Early Data Release 3 (EDR3) parallaxes. The relations are provided in the Wide-field Infrared Survey Explorer (WISE) W1 and W2 bands, as well as in the W(W1,V− W1) and W(W2,V− W2) Wesenheit magnitudes. The relations are calibrated using a very large sample of Galactic halo field RR Lyrae stars with homogeneous spectroscopic [Fe/H] abundances (over 1000 stars in the W1 band), covering a broad range of metallicities (−2.5 ≲ [Fe/H] ≲ 0.0). We test the performance of our PLZ and PWZ relations by determining the distance moduli of both galactic and extragalactic stellar associations: the Sculptor dwarf spheroidal galaxy in the Local Group (finding${\overline{\mu }}_0=19.47\pm 0.06$), the Galactic globular clusters M4 (${\overline{\mu }}_0=11.16\pm 0.05$), and the Reticulum globular cluster in the Large Magellanic Cloud (${\overline{\mu }}_0=18.23\pm 0.06$). The distance moduli determined through all our relations are internally self-consistent (within ≲0.05 mag) but are systematically smaller (by ∼2–3σ) than previous literature measurements taken from a variety of methods/anchors. However, a comparison with similar recent RR Lyrae empirical relations anchored with EDR3 likewise shows, to varying extents, a systematically smaller distance modulus for PLZ/PWZ RR Lyrae relations.

    

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