Smith-Perez Charlotte, Hembruff Aidan, Peeters Els
et al.
Context. Polycyclic aromatic hydrocarbons (PAHs) constitute a significant fraction of the Universe’s carbon budget, playing a key role in the cosmic carbon cycle and dominating the mid-infrared spectra of astrophysical environments in which they reside. Although PAHs are known to form in the circumstellar envelopes of post-asymptotic giant branch stars, their formation and evolution are still not well understood.
Aims. We aim to understand how pristine complex hydrocarbons and PAHs in circumstellar environments transition to the PAHs observed in the interstellar medium.
Methods. The mid-infrared PAH spectra (5–18 μm) of the planetary nebula, NGC 7027, were investigated using spectral cubes from JWST MIRI-MRS.
Results. We report the first detection of spatially resolved variations of the PAH spectral profiles across class 𝒜, 𝒜ℬ, and ℬ in all major PAH bands (6.2, 7.7, 8.6, and 11.2 μm) within a single source, NGC 7027. These variations are linked to morphological structures within NGC 7027. Clear correlations are revealed between the 6.2, 7.7, and 8.6 μm features, where the red components (6.26, 7.8, and 8.65 μm) exhibit a strong correlation and the same is found for the blue components of the 6.2 and 7.7 μm features (6.205 and 7.6 μm). The blue component of the 8.6 μm feature (8.56 μm) appears to be independent of the other components. We link this behavior to differences in the molecular structure of their PAH subpopulations. Decomposition of the 11.2 μm band confirms two previously identified components, with the broader 11.25 μm component attributed to emission from very small grains or PAH clusters rather than PAH emission.
Conclusions. We show that PAH profile classes generally vary with proximity to the central star’s UV radiation field, suggesting class ℬ PAHs represent more processed species while class 𝒜 PAHs remain relatively pristine, challenging current notions on the spectral evolution of PAHs.
Cory M. Whitcomb, J.-D. T. Smith, Elizabeth Tarantino
et al.
We explore the physical origins of the observed deficit of polycyclic aromatic hydrocarbons (PAHs) at subsolar metallicity using JWST/NIRCam imaging of the nearby galaxy M101, covering regions from solar metallicity ( Z _⊙ ) down to 0.4 Z _⊙ . These maps are used to trace the radial evolution of the shortest-wavelength PAH feature at 3.3 μ m, which is emitted preferentially by the smallest PAHs (<100 carbon atoms). The fractional contribution of PAH 3.3 μ m to the total PAH luminosity (ΣPAH) increases by 3× as metallicity declines, rising from ∼1% to ∼3% over the observed range, consistent with prior predictions from the inhibited grain growth model based on Spitzer spectroscopy. We explore model refinements including photon effects and alternative size evolution prescriptions and find that a modest amount of small grain photodestruction remains possible, provided the grain size cutoff does not exceed ∼55 carbon atoms. The best-fit models predict 3.3 μ m/ΣPAH will rise to ∼5.6%–7.7% at 10% Z _⊙ . Surprisingly, even as ΣPAH drops significantly relative to the total infrared luminosity (TIR) as metallicity declines, 3.3 μ m/TIR alone rises , potentially indicating the mass fraction of the smallest PAH grains increases as the total dust content in galaxies drops. The current model cannot fully reproduce this trend even if the unusually strong effects of changing radiation field hardness on 3.3 μ m/TIR are included. This may be evidence that the smallest PAHs are uniquely robust against destruction and inhibited growth effects. These results highlight the pivotal role that short-wavelength PAH emission can play in studies of low-metallicity and high-redshift galaxies.
Context. Stellar flares are sudden brightenings caused by magnetic reconnection and are frequently observed on late-type stars. High-cadence photometry of flares provides valuable insights into the mechanisms of these events, yet such observations remain scarce.
Aims. We seek to explore the sub-second fine structure of stellar flares and assess the information content in high-speed photometry.
Methods. New 0.3 s-cadence photometry from a six-year-long observing campaign of the active M-dwarf AD Leo is presented. We use time–frequency analysis to detect quasi-periodic pulsations in the decay phase of flares. We explore statistical measures of time-series complexity of the detected flares to quantify the information gain achievable with high-cadence photometry.
Results. We detect 42 flares in 211 hours of observations. The flare frequency distribution is consistent with the previous literature. We find no quasi-periodic pulsations with periods below a few seconds, and identify two candidate signals with periods around 1 and 3 min. Using different measures of complexity on the binned flare light curves, we confirm the advantages of high observing cadence. However, we also find a plateau up to a binning of ≈4−5 s for a few complex flares, suggesting that an exposure time of a few seconds is usually enough to retain most of the information carried by a single-filter observation.
Conclusions. New photometric observations of AD Leo revealed sub-structures of flare light curves on a timescale of a few seconds, but we found no features on timescales below that.
Jonathan Squire, Eliot Quataert, Philip F. Hopkins
We show that there exist two qualitatively distinct turbulent states of the zero-net-vertical-flux shearing box. The first, which has been studied in detail previously, is characterized by a weakly magnetized ($\beta\sim50$) midplane with slow periodic reversals of the mean azimuthal field (dynamo cycles). The second, the 'low-$\beta$ state,' which is the main subject of this paper, is characterized by a strongly magnetized $\beta\sim 1$ midplane dominated by a coherent azimuthal field with much stronger turbulence and much larger accretion stress ($\alpha \sim 1$). The low-$\beta$ state emerges in simulations initialized with sufficiently strong azimuthal magnetic fields. The mean azimuthal field in the low-$\beta$ state is quasi steady (no cycles) and is sustained by a dynamo mechanism that compensates for the continued loss of magnetic flux through the vertical boundaries; we attribute the dynamo to the combination of differential rotation and the Parker instability, although many of its details remain unclear. Vertical force balance in the low-$\beta$ state is dominated by the mean magnetic pressure except at the midplane, where thermal pressure support is always important (this holds true even when simulations are initialized at $\beta \ll 1$, provided the thermal scale height of the disk is well resolved). The efficient angular momentum transport in the low-$\beta$ state may resolve long-standing tension between predictions of magnetorotational turbulence (at high $\beta$) and observations; likewise, the bifurcation in accretion states we identify may be important for understanding the state transitions observed in dwarf novae, X-ray binaries, and changing-look AGN. We discuss directions for future work, including the implications of our results for global accretion disk models and simulations.
Coronal (1–10 MK) X-rays display dramatic variability over the Sun’s iconic 11 yr magnetic dynamo cycle: already a factor of 4 in the soft 0.1–2.4 keV “ROSAT band,” soaring to more than 100 at harder energies (>10 keV). The high-energy variations impact heliospheric space weather (SW); presumably likewise for host-star analogs. In an effort to better document long-term coronal variability and X-ray cycles of other stars, measurements of 19 late-type F–M dwarfs and subgiants were obtained from archives of the three contemporary long-lived X-ray observatories: Chandra, XMM-Newton, and Swift. The X-ray event lists were time-filtered to suppress transients like flares and telemetry dropouts. A novel scheme, based on empirical coronal models, harmonized flux conversions across the different instruments. The Sun was included based on high-energy irradiance time series. Results generally confirmed previous findings: high-contrast, decadal-class X-ray modulations were found exclusively at low-to-medium L _X / L _BOL ; higher X-ray intensity stars displayed lower-amplitude, faster variations, if cycling at all; whereas the highest activity classes showed stable (“saturated”) long-term X-ray trends, but punctuated by persistent flaring. In addition, several variants of “dynamo diagrams” are presented to illustrate possible correlations among key parameters, such as rotation period and cycle duration. Early versions of such diagrams had displayed what appeared to be clear trends, although additional observations in recent years have tended to downplay the previous relationships. The diverse X-ray behaviors hold implications for stellar SW, as well as posing tough challenges for dynamo theory.
We conduct the first systematic survey of a total of 11 optical coronal lines (CLs) in the spectra of a large sample of low-redshift ( z < 0.8) Type 1 quasars observed by the Sloan Digital Sky Survey (SDSS). We find that strong CL emission is rare in SDSS even in Type 1 quasars; only 885 out of 19,508 (4.5%) galaxies show at least one CL, with higher ionization potential lines (>100 eV) being even rarer. The [Ne v ] λ 3426 line, which constitutes the majority of detections, is strongly correlated with the bolometric luminosity. These findings suggest that the optical CLs are significantly suppressed in the majority of local active galactic nuclei (AGNs), possibly as a result of the presence of dust in the emitting regions. We find that the incidence of ionized outflows is significantly higher in CL emitters compared with non-CL emitters, possibly suggesting that dust destruction in outflows enhances CL emission in AGNs. Many CLs show line profiles that are broader than those of narrow lines, and are blueshifted relative to the lower ionization potential lines, suggesting outflows in the highly ionized gas. Given the limited number of detections, we do not find any statistically significant trends in detection statistics or line ratios with black hole mass, Eddington ratio, or AGN bolometric luminosity. The catalog is publicly available and can provide a useful database of the CL properties of low-redshift quasars that can be compared to the growing number of high- z AGNs discovered by JWST.
As one class of important carbon reservoirs in interstellar clouds, large polycyclic aromatic hydrocarbons (PAHs) and their derivative species play an important role in the formation and evolution of interstellar carbonaceous compounds. To understand these chemical routes, the gas-phase ion–molecular collision reaction between large, astronomically relevant PAH (dicoronylene, DC, C _48 H _20 ) cations and smaller neutral superhydrogenated PAHs (2, 3–benzofluorene, C _17 H _12 ) are investigated. Series of large DC/2, 3–benzofluorene cluster cations (e.g., [(C _17 H _12 ) _6 C _48 H _14 ] ^+ , 236 atoms, and [(C _17 H _12 ) _5 C _48 ] ^+ , 193 atoms) are efficiently formed by gas-phase condensation under laser irradiation conditions. With theoretical calculations, the structure of newly formed DC/2, 3-benzofluorene cluster cations and the bonding energy for these formation reactions are obtained. Moreover, the IR spectra of DC/2, 3-benzofluorene cluster cations are also calculated. The gas-phase reactions between large PAH species occur relatively easily, resulting in a very large number of reactions and very complex molecular clusters. The adduct processes and the formed molecular structure relatively depend on the carbon reaction sites. The carbon edge sites have different chemical reactivity, which may affect the abundance of these relevant interstellar substances. Furthermore, intermolecular hydrogen transfer plays an important role in cluster formation processes, which can lead the newly formed clusters to become more stable. We infer that small superhydrogenated PAH molecules (e.g., 2, 3-benzofluorene) can effectively aggregate on the large PAH molecules (e.g., dehydrogenated DC cations or carbon clusters) in the gas phase, which provides proposed chemical-evolution routes (ion–molecular reaction pathways) for the formation of the nanometer-sized dust grains in a bottom-up process (in building block pathways) in the interstellar medium.
Sheng-Han Xiong, Yong-Zhuang Li, Xiao-Mei Kuang
et al.
Abstract In this article, the shadow and the quasinormal modes (QNMs) of an exact asymptotically flat hairy electrically charged black hole solution with a dilaton potential are investigated. Using the constraint equation among the integration constant $$\eta $$ η of the gravitational field, the mass M, the electric charge Q and the coupling constant $$\nu $$ ν between the U(1) field and the dilaton field, we find that the shadow radii, the Lyapunov exponent $$\lambda $$ λ and the coordinate angular velocity $$\Omega _{c}$$ Ω c only significantly affected by $$\nu $$ ν if the Q is close to the extremal value. Especially, near the $$\nu =1$$ ν = 1 critical point the phenomenon exhibits singular enhancement. Furthermore, the QNMs are numerically computed by using the Hatsuda method and verified with the higher-order WKB approximations with the Padé summation. We find that the QNMs are close to that of the low energy limit of the string theory when $$\nu $$ ν is large enough. In the eikonal limit, the real and imaginary parts are proved to be approximated by $$\Omega _{c}$$ Ω c and $$\lambda $$ λ , respectively.
Astrophysics, Nuclear and particle physics. Atomic energy. Radioactivity
This paper summarizes the body of work that we have done over the years on the oscillation processes in sunspots, including their umbra, penumbra, and close vicinity. The study analyzes a number of aspects that impede adequate determining of some characteristics of propagating oscillations and lead to misinterpretation. Using running penumbral waves as an example, we show that their horizontal propagation with decreasing frequency is delusive. The effect is due to different oscillations propagating along magnetic field lines with gradually increasing inclination. This also applies to the three-minute oscillations in the sunspot umbral chromosphere. The change in the inclination of the strips in the half-tone space-time diagrams, which are employed to determine the oscillation propagation velocities along coronal loops, is caused by the projection effect as opposed to real changes in the velocity. We propose to use flare modulation of the natural oscillations of the medium to eliminate the uncertainties that arise while measuring the phase differences between signals of the same parameters, which is employed for estimating wave propagation velocities in the solar atmosphere.
Signals generated by dust impacting spacecraft can be detected by electric field instruments. These signals have been simulated by numerous models. However, few models can accurately characterize the expansion of the plasma cloud generated by dust impact. The COMSOL model presented in this paper provides a way to understand the expansion properties of ions and electrons. The model can also be used to analyze the various expected waveforms of dust impact signals as a function of different parameters, such as the spacecraft voltage and the ambient plasma temperature. The results show that close to 50% of ions and electrons in the impact plasma cloud are collected by spacecraft at weak spacecraft potentials and that a fraction of the ions is still collected rather than all of them streaming away from the impact location at V _SC > 0 V. The model also confirms that in the expanding plasma cloud, ions are in the form of plumes, while electrons diffuse in an isotropic manner.
Oleg Illiashenko, Vyacheslav Kharchenko, Ievgen Babeshko
et al.
The entropy-oriented approach called security- or cybersecurity-informed safety (SIS or CSIS, respectively) is discussed and developed in order to analyse and evaluate the safety and dependability of autonomous transport systems (ATSs) such as unmanned aerial vehicles (UAVs), unmanned maritime vehicles (UMVs), and satellites. This approach allows for extending and integrating the known techniques FMECA (Failure Modes, Effects, and Criticality Analysis) and IMECA (Intrusion MECA), as well as developing the new SISMECA (SIS-based Intrusion Modes, Effects, and Criticality Analysis) technique. The ontology model and templates for SISMECA implementation are suggested. The methodology of safety assessment is based on (i) the application and enhancement of SISMECA considering the particularities of various ATSs and roles of actors (regulators, developers, operators, customers); (ii) the development of a set of scenarios describing the operation of ATS in conditions of cyberattacks and physical influences; (iii) AI contribution to system protection for the analysed domains; (iv) scenario-based development and analysis of user stories related to different cyber-attacks, as well as ways to protect ATSs from them via AI means/platforms; (v) profiling of AI platform requirements by use of characteristics based on AI quality model, risk-based assessment of cyberattack criticality, and efficiency of countermeasures which actors can implement. Examples of the application of SISMECA assessment are presented and discussed.
Abstract In principle, the local cosmic void can be simply modeled by the spherically symmetric Lemaitre–Tolman–Bondi (LTB) metric. In practice, the real local cosmic void is probably not spherically symmetric. In this paper, to reconstruct a more realistic profile of the local cosmic void, we divide it into several segments. Each segment with certain solid angle is modeled by its own LTB metric. Meanwhile, we divide the 1048 type Ia supernovae (SNIa) of the Pantheon Survey into corresponding subsets according to their distribution in the galactic coordinate system. Obviously, each SNIa subset can only be used to reconstruct the profile of one segment. Finally, we can patch together an irregular profile for the local cosmic void with the whole Pantheon sample. Note that, the paucity of each data subset lead us to focus on the inner part of each void segment and assume that the half radii of the void segments are sufficient to constrain the whole segment. We find that, despite $$2\sigma $$ 2 σ signals of anisotropy limited to the depth of the void segments, the constraints on every void segment are consistent with $$\Lambda $$ Λ CDM model at 95% CL. Moreover, our constraints are too weak to challenge the cosmic homogeneity and isotropy.
Astrophysics, Nuclear and particle physics. Atomic energy. Radioactivity
Markus J. Bonse, Emily O. Garvin, Timothy D. Gebhard
et al.
Over the past decade, hundreds of nights have been spent on the world’s largest telescopes to search for and directly detect new exoplanets using high-contrast imaging (HCI). Thereby, two scientific goals are of central interest: first, to study the characteristics of the underlying planet population and distinguish between different planet formation and evolution theories. Second, to find and characterize planets in our immediate solar neighborhood. Both goals heavily rely on the metric used to quantify planet detections and nondetections. Current standards often rely on several explicit or implicit assumptions about noise. For example, it is often assumed that the residual noise after data postprocessing is Gaussian. While being an inseparable part of the metric, these assumptions are rarely verified. This is problematic as any violation of these assumptions can lead to systematic biases. This makes it hard, if not impossible, to compare results across data sets or instruments with different noise characteristics. We revisit the fundamental question of how to quantify detection limits in HCI. We focus our analysis on the error budget resulting from violated assumptions. To this end, we propose a new metric based on bootstrapping that generalizes current standards to non-Gaussian noise. We apply our method to archival HCI data from the NACO instrument at the Very Large Telescope and derive detection limits for different types of noise. Our analysis shows that current standards tend to give detection limits that are about one magnitude too optimistic in the speckle-dominated regime. That is, HCI surveys may have excluded planets that can still exist.
The existence of universal quantum computers has been theoretically well established. However, building up a real quantum computer system not only relies on the theory of universality, but also needs methods to satisfy requirements on other features, such as programmability, modularity, scalability, etc. To this end, here we study the recently proposed model of quantum von Neumann architecture by putting it in a practical and broader setting, namely, the hierarchical design of a computer system. We analyze the structures of quantum CPU and quantum control units and draw their connections with computational advantages. We also point out that a recent demonstration of our model would require less than 20 qubits.
We revisit the fundamental principles of thermodynamic equilibrium in relation to heat transfer processes within the Earth’s atmosphere. A knowledge of equilibrium states at ambient temperatures (T) and pressures (p) and deviations for these p-T states due to various transport ‘forces’ and flux events give rise to gradients (dT/dz) and (dp/dz) of height z throughout the atmosphere. Fluctuations about these troposphere averages determine weather and climates. Concentric and time-span average values <T> (z, Δt)) and its gradients known as the lapse rate = d < T(z) >/dz have hitherto been assumed in climate models to be determined by a closed, reversible, and adiabatic expansion process against the constant gravitational force of acceleration (g). Thermodynamics tells us nothing about the process mechanisms, but adiabatic-expansion hypothesis is deemed in climate computer models to be convection rather than conduction or radiation. This prevailing climate modelling hypothesis violates the 2nd law of thermodynamics. This idealized hypothetical process cannot be the causal explanation of the experimentally observed mean lapse rate (approx.−6.5 K/km) in the troposphere. Rather, the troposphere lapse rate is primarily determined by the radiation heat-transfer processes between black-body or IR emissivity and IR and sunlight absorption. When the effect of transducer gases (H<sub>2</sub>O and CO<sub>2</sub>) is added to the Earth’s emission radiation balance in a 1D-2level primitive model, a linear lapse rate is obtained. This rigorous result for a perturbing cooling effect of transducer (‘greenhouse’) gases on an otherwise sunlight-transducer gas-free troposphere has profound implications. One corollary is the conclusion that increasing the concentration of an existing weak transducer, i.e., CO<sub>2</sub>, could only have a net cooling effect, if any, on the concentric average <T> (z = 0) at sea level and lower troposphere (z < 1 km). A more plausible explanation of global warming is the enthalpy emission ’footprint’ of all fuels, including nuclear.
Abstract Photon region (PR) in the strong gravitational field is defined as a compact region where photons can travel endlessly without going to infinity or disappearing at the event horizon. In Schwarzschild metric PR degenerates to the two-dimensional photon sphere $$r=3r_g/2$$ r=3rg/2 where closed circular photon orbits are located. The photon sphere as a three-dimensional hypersurface in spacetime is umbilic (its second quadratic form is pure trace). In Kerr metric the equatorial circular orbits have different radii for prograde, $$r_p$$ rp , and retrograde, $$r_r$$ rr , motion (where r is Boyer–Lindquist radial variable), while for $$r_p<r<r_r$$ rp<r<rr the spherical orbits with constant r exist which are no more planar, but filling some spheres. These spheres, however, do not correspond to umbilic hypersurfaces. In more general stationary axisymmetric spacetimes not allowing for complete integration of geodesic equations, the numerical integration show the existence of PR as well, but the underlying geometric structure was not fully identified so far. Here we suggest geometric description of PR in generic stationary axisymmetric spacetimes, showing that PR can be foliated by partially umbilic hypersurfaces, such that the umbilic condition holds for classes of orbits defined by the foliation parameter. New formalism opens a way of analytic description of PR in stationary axisymmetric spacetimes with non-separable geodesic equations.
Astrophysics, Nuclear and particle physics. Atomic energy. Radioactivity
The aim of experimental nuclear astrophysics is to provide information on the nuclear processes involved in astrophysical scenarios at the relevant energy range. However, the measurement of the cross section of nuclear reactions at low energies present formidable difficulties due to the very low reaction rates often overwhelmed by the background. Several approaches have been proposed and exploited to overcome such severe obstacles: in such frame, the idea to install a low energy - high intensity ion accelerator deep underground, to gain high luminosity while reducing the cosmic ray background, brought more than 25 years ago, to the pilot LUNA experiment. LUNA stands for Laboratory for Underground Nuclear Astrophysics: in the cave under the Gran Sasso mountain (in Italy) first a 50 kV and then a 400 kV single-ended accelerator for protons and alphas were deployed and produced plenty of data mainly on reactions of the H-burning phase in stars. Recently, similar facilities have been installed and/or proposed in other underground laboratories in US and China. LUNA as well is going to make a big step forward, with a new machine in the MV range which will be able to provide intense beams of protons, alphas and carbon ions. The rationale of underground nuclear astrophysics will be presented together with the last updates on the ongoing research programs.