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We introduce TPCNet , a neural network predictor that combines convolutional and transformer architectures with positional encodings, for neutral atomic hydrogen (H i) spectral analysis. Trained on synthetic data sets, our models predict cold neutral gas fraction (⁠|$f_\mathrm{CNM}$|⁠) and H i opacity correction factor (⁠|$\mathcal {R_{\mathrm{H\,{\small I}}}}$|⁠) from emission spectra based on the learned relationships between the desired output parameters and observables (optically thin column density and peak brightness). As a follow-up to Murray et al.'s shallow Convolutional Neural Network (CNN), we construct deep CNN models and compare them to TPCNet models. TPCNet outperforms deep CNNs, achieving a 10 per cent average increase in testing accuracy, algorithmic (training) stability, and convergence speed. Our findings highlight the robustness of the proposed model with sinusoidal positional encoding applied directly to the spectral input, addressing perturbations in training data set shuffling and convolutional network weight initializations. Higher spectral resolutions with increased spectral channels offer advantages, albeit with increased training time. Diverse synthetic data sets enhance model performance and generalization, as demonstrated by producing |$f_\mathrm{CNM}$| and |$\mathcal {R_{\mathrm{H\,{\small I}}}}$| values consistent with evaluation ground truths. Applications of TPCNet to observed emission data reveal strong agreement between the predictions and Gaussian decomposition-based estimates (from emission and absorption surveys), emphasizing its potential in H i spectral analysis. [ABSTRACT FROM AUTHOR]

Over-ionized, recombining plasma is an emerging class of X-ray bright supernova remnants (SNRs). This unique thermal state, where the ionization temperature (⁠|$T_{\rm z}$|⁠) is significantly higher than the electron temperature (⁠|$T_{\rm e}$|⁠), is not expected from the standard evolution model that assumes a point explosion in a uniform interstellar medium, thus requiring a new scenario for the dynamical and thermal evolution. A recently proposed idea attributes the over-ionization state to additional ionization contribution from the low-energy tail of shock-accelerated protons. However, this new scenario has been left untested, especially from the atomic physics point of view. We report calculation results of the proton impact ionization rates of heavy-element ions in ejecta of SNRs. We conservatively estimate the requirement for accelerated protons, and find that their relative number density to thermal electrons needs to be higher than |$5\ (k T_{\rm e}/1\:\mbox{keV})\%$| in order to explain the observed over-ionization degree at |$T_{\rm z}/T_{\rm e} \ge 2$| for K-shell emission. We conclude that the proton ionization scenario is not feasible because such a high abundance of accelerated protons is prohibited by the injection fraction from thermal to non-thermal energies, which is expected to be |$\sim\! 1\%$| at most. [ABSTRACT FROM AUTHOR]

We present the largest Galactic neutral hydrogen H i absorption survey to date, utilizing the Australian SKA Pathfinder Telescope at an unprecedented spatial resolution of 30 arcsec. This survey, GASKAP-H i , unbiasedly targets 2714 continuum background sources over 250 square degrees in the direction of the Magellanic Clouds, a significant increase compared to a total of 373 sources observed by previous Galactic absorption surveys across the entire Milky Way. We aim to investigate the physical properties of cold (CNM) and warm (WNM) neutral atomic gas in the Milky Way foreground, characterized by two prominent filaments at high Galactic latitudes (between |$-45^{\circ }$| and |$-25^{\circ }$|⁠). We detected strong H i absorption along 462 lines of sight above the 3 |$\sigma$| threshold, achieving an absorption detection rate of 17 per cent. GASKAP-H i 's unprecedented angular resolution allows for simultaneous absorption and emission measurements to sample almost the same gas clouds along a line of sight. A joint Gaussian decomposition is then applied to absorption-emission spectra to provide direct estimates of H i optical depths, temperatures, and column densities for the CNM and WNM components. The thermal properties of CNM components are consistent with those previously observed along a wide range of Solar neighbourhood environments, indicating that cold H i properties are widely prevalent throughout the local interstellar medium. Across our region of interest, CNM accounts for |$\sim$| 30 per cent of the total H i gas, with the CNM fraction increasing with column density towards the two filaments. Our analysis reveals an anticorrelation between CNM temperature and its optical depth, which implies that CNM with lower optical depth leads to a higher temperature. [ABSTRACT FROM AUTHOR]

The galactic tidal interaction is a possible mechanism to trigger active star formation in galaxies. Recent analyses using H  i data in the Large Magellanic Cloud (LMC) proposed that the tidally driven H  i flow, the L-component, is colliding with the LMC disk, the D-component, and is triggering high-mass star formation toward the active star-forming regions R136 and N44. In order to explore the role of the collision over the entire LMC disk, we investigated the I-component, the collision-compressed gas between the L- and D-components, over the LMC disk, and found that |$74\%$| of the O/WR stars are located toward the I-component, suggesting their formation in the colliding gas. We compared four star-forming regions (R136, N44, N11, and the N77–N79–N83 complex). We found a positive correlation between the number of high-mass stars and the compressed gas pressure generated by collisions, suggesting that pressure may be a key parameter in star formation. [ABSTRACT FROM AUTHOR]

Our ability to trace the star-forming molecular gas is important to our understanding of the Universe. We can trace this gas using CO emission, converting the observed CO intensity into the H |$_2$| gas mass of the region using the CO-to-H |$_2$| conversion factor (⁠|$X_{\rm{{\small CO}}}$|⁠). In this paper, we use simulations to study the conversion factor and the molecular gas within galaxies. We analysed a suite of simulations of isolated disc galaxies, ranging from dwarfs to Milky Way-mass galaxies, that were run using the fire-2 subgrid models coupled to the chimes non-equilibrium chemistry solver. We use the non-equilibrium abundances from the simulations, and we also compare to results using abundances assuming equilibrium, which we calculate from the simulation in post-processing. Our non-equilibrium simulations are able to reproduce the relation between CO and H |$_2$| column densities, and the relation between |$X_{\rm{{\small CO}}}$| and metallicity, seen within observations of the Milky Way. We also compare to the xCOLD GASS survey, and find agreement with their data to our predicted CO luminosities at fixed star formation rate. We also find the multivariate function used by xCOLD GASS overpredicts the H |$_2$| mass for our simulations, motivating us to suggest an alternative multivariate function of our fitting, though we caution that this fitting is uncertain due to the limited range of galaxy conditions covered by our simulations. We also find that the non-equilibrium chemistry has little effect on the conversion factor (<5 per cent) for our high-mass galaxies, though still affects the H |$_2$| mass and |$L_{\rm{{\small CO}}}$| by |$\approx$| 25  per cent. [ABSTRACT FROM AUTHOR]

To investigate the gas-phase hydrogenation processes of large, astronomically relevant cationic polycyclic aromatic hydrocarbon (PAH) molecules under the interstellar environments, the ion–molecule collision reaction between six PAH cations and H-atoms is studied. The experimental results show that the hydrogenated PAH cations are efficiently formed, and no even–odd hydrogenated mass patterns are observed in the hydrogenation processes. The structure of newly formed hydrogenated PAH cations and the bonding energy for the hydrogenation reaction pathways are investigated with quantum theoretical calculations. The exothermic energy for each reaction pathway is relatively high, and the competition between hydrogenation and dehydrogenation is confirmed. From the theoretical calculation, the bonding ability plays an important role in the gas-phase hydrogenation processes. The factors that affect the hydrogenation chemical reactivity are discussed, including the effect of carbon skeleton structure, the side-edged structure, the molecular size, the five- and six-membered C-ring structure, the bay region structure, and the neighbouring hydrogenation. The infrared spectra of hydrogenated PAH cations are also calculated. These results we obtain once again validate the complexity of hydrogenated PAH molecules, and provide the direction for the simulations and observations under the co-evolution interstellar chemistry network. We infer that if we do not consider other chemical evolution processes (e.g. photoevolution), then the hydrogenation states and forms of PAH compounds are intricate and complex in the interstellar medium. [ABSTRACT FROM AUTHOR]

Atomic Hydrogen-21 cm transition (H  i) is an excellent tracer to study and understand the properties of the atomic gas in the Galaxy. Using the Westerbork Synthesis Radio Telescope, we observed 12 quasar sightlines to detect galactic H  i in absorption. We achieve an optical depth rms of ∼1−2 × 10−3, essential to detect the warm neutral medium (WNM). We detect H  i absorption in all our sightlines except along 1006+349, for which we set a strict upper limit on the spin temperature as 〈 T s〉 > 570 K. We find around 50 per cent of our sightlines have 〈 T s〉 > 500 K, indicating a WNM dominance. Further, we calculate an upper limit of the cold neutral medium (CNM) fraction along our sightlines and find a median CNM fraction of ∼0.12. With our observations, we reconfirm the existence of a threshold column density of ∼2 × 1020 |$\rm atoms \, cm^{-2}$| to form CNM in the interstellar medium (ISM). Using a two-temperature model of the H  i disc, we explore the distribution of spin temperature in the Galactic ISM. We find that a simple fixed axisymmetric two-temperature model could not produce either the observed column density or the integral optical depth. This indicates the existence of a more complex distribution of spin temperatures in the Galaxy. [ABSTRACT FROM AUTHOR]

We derived the dust-to-H  i ratio of the intermediate-velocity clouds (IVCs), the high-velocity clouds (HVCs), and the local H  i gas, by carrying out a multiple-regression analysis of the 21 cm H  i emission combined with the sub-mm dust optical depth. The method covers over 80 per cent of the sky contiguously at a resolution of 47 arcmin and is distinguished from the absorption-line measurements toward bright galaxies and stars covering a tiny fraction of the sky. Major results include that the ratio of the IVCs is in a range of 0.1–1.5 with a mode at 0.6 (relative to the solar-neighbourhood value, likewise below) and that a significant fraction, ∼20 per cent, of the IVCs include dust-poor gas with a ratio of <0.5. It is confirmed that 50 per cent of the HVC Complex C has a ratio of <0.3, and that the Magellanic Stream has the lowest ratio with a mode at ∼0.1. The results prove that some IVCs have low metallicity gas, contrary to the previous absorption-line measurements. Considering that the recent works show that the IVCs are interacting and exchanging momentum with the high-metallicity Galactic halo gas, we argue that the high-metallicity gas contaminates a significant fraction of the IVCs. Accordingly, we argue that the IVCs include a significant fraction of the low-metallicity gas supplied from outside the Galaxy as an alternative to the Galactic-fountain model. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of the excitation of C  i fine-structure levels along 57 sight lines in the Large and Small Magellanic Clouds (LMC and SMC). The sightlines were selected by the detection of H2 in FUSE spectra. Using archival HST/COS and HST/STIS spectra, we detected absorption of C  i fine-structure levels and measured their populations for 29 and 28 sightlines in the LMC and SMC, respectively. The C  i column density ranges from 1013 to |$10^{14}\, {\rm cm}^{-2}$| for the LMC and 1013 to |$10^{15.4}\, {\rm cm}^{-2}$| for the SMC. We found excitation of C  i fine-structure levels in the LMC and SMC to be 2–3 times higher than typical values in local diffuse interstellar medium (ISM). Comparing excitation of both C  i fine-structure levels and H2 rotational levels with a grid of PDR Meudon models, we find that neutral cold gas in the LMC and SMC is illuminated by a stronger UV field than in local ISM (⁠|$\chi =5^{+7}_{-3}$| units of Mathis field for the LMC and |$2^{+4}_{-1}$| for the SMC) and has on average higher thermal pressure (log  p / k  = 4.2 ± 0.4 and 4.3 ± 0.5, respectively). Magellanic Clouds sight lines likely probe regions near star-formation sites, which also affects the thermal state and C  i /H2 relative abundances. At the same time, obtained enhanced UV field is consistent with some measurements at high redshifts. Together with low metallicities, this make Magellanic Clouds an interesting test case to study the central parts of high redshift galaxies. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of the excitation of C  i fine-structure levels along 57 sight lines in the Large and Small Magellanic Clouds (LMC and SMC). The sightlines were selected by the detection of H2 in FUSE spectra. Using archival HST/COS and HST/STIS spectra, we detected absorption of C  i fine-structure levels and measured their populations for 29 and 28 sightlines in the LMC and SMC, respectively. The C  i column density ranges from 1013 to |$10^{14}\, {\rm cm}^{-2}$| for the LMC and 1013 to |$10^{15.4}\, {\rm cm}^{-2}$| for the SMC. We found excitation of C  i fine-structure levels in the LMC and SMC to be 2–3 times higher than typical values in local diffuse interstellar medium (ISM). Comparing excitation of both C  i fine-structure levels and H2 rotational levels with a grid of PDR Meudon models, we find that neutral cold gas in the LMC and SMC is illuminated by a stronger UV field than in local ISM (⁠|$\chi =5^{+7}_{-3}$| units of Mathis field for the LMC and |$2^{+4}_{-1}$| for the SMC) and has on average higher thermal pressure (log  p / k  = 4.2 ± 0.4 and 4.3 ± 0.5, respectively). Magellanic Clouds sight lines likely probe regions near star-formation sites, which also affects the thermal state and C  i /H2 relative abundances. At the same time, obtained enhanced UV field is consistent with some measurements at high redshifts. Together with low metallicities, this make Magellanic Clouds an interesting test case to study the central parts of high redshift galaxies. [ABSTRACT FROM AUTHOR]

Shock boundary layers are regions bounded by a shock wave on the one side and tangential discontinuity on the other side. These boundary layers are commonly observed in astrophysics. For example, they exist in the regions of the interaction of the stellar winds with the surrounding interstellar medium. Additionally, the shock layers (SLs) are often penetrated by the flows of interstellar atoms, as, for instance, in the astrospheres of the stars embedded by the partially ionized interstellar clouds. This paper presents a simple toy model of an SL that aims to qualitatively describe the influence of charge exchange with interstellar hydrogen atoms on the plasma flow in astrospheric SLs. To clearly explore this effect, magnetic fields are neglected, and the geometry is kept as simple as possible. The model explains why the cooling of plasma due to charge exchange in the inner heliosheath leads to an increase in plasma density in front of the heliopause. It also demonstrates that the source of momentum causes changes in the pressure profile within the SL. The paper also discusses the decrease in plasma density near the astropause in the outer SL in the case of layer heating, which is particularly relevant in light of the Voyager measurements in the heliospheric SL. [ABSTRACT FROM AUTHOR]

We propose to employ emission line luminosities obtained via optical spectroscopy to estimate the content of neutral hydrogen (H  i) in galaxies. We use the optical spectroscopy data from the Mapping of Nearby Galaxies at APO (MaNGA) survey released in the frames of public DR17 of the Sloan Digital Sky Survey (SDSS). We compare the H  i mass estimated by us for a large sample of SDSS/MaNGA galaxies with direct H  i measurements from the Arecibo Legacy Fast ALFA survey and find a tight correlation between the masses with the correlation coefficient (CC) of 0.91 and the rms scatter of 0.15 dex for the logarithmic mass. The obtained relationship is verified via another sample of MaNGA galaxies with H  i masses measured with the Green Bank Telescope. Despite the coarser angular resolution of the radio data, the relation between the estimated and measured directly H  i mass is tight as well – in this case CC = 0.74 and the rms is 0.29 dex. The established relations allow us to estimate the total mass of neutral hydrogen as well as the spatial distribution of H  i surface density in galaxies from optical spectroscopic observations only in a simple and efficient way. [ABSTRACT FROM AUTHOR]

Water represents the main component of the icy mantles on dust grains, it is of extreme importance for the formation of new species and it represents the main component for life. Water is observed both in the gas-phase and frozen in the interstellar medium (ISM), where the solid-phase formation route has been proven essential to explain abundances in molecular clouds. So far, experiments have focused on very low temperatures (around 10 K). We present the experimental evidence of solid water formation on coronene, PAH-like surface, for a higher range of temperatures. Water is efficiently formed up to 85 K through the interaction of oxygen and hydrogen atomic beams with a carbonaceous grain analogue. The beams are aimed towards the surface connected to a cryostat exploring temperatures from 10 to 100 K. The results are obtained with a QMS and analysed through a temperature-programmed desorption technique. We observe an efficient water formation on coronene from 10 up to 85 K mimicking the temperature conditions from the dense ISM to translucent regions, where the ice mantle onset is supposed to start. The results show the catalytic nature of coronene and the role of chemisorption processes. The formation of the icy mantles could be happening in less dense and warmer environments, helping explaining oxygen depletion in the ISM. The findings have several applications such as the disappearance of PAHs in translucent regions and the snowlines of protoplanetary discs. We stress on how JWST projects characterizing PAHs can be combined with H2O observations to study water formation at warm temperatures. [ABSTRACT FROM AUTHOR]

Sodium-bearing species such as NaCl in the gas phase have been observed in an assortment of carbon-rich and oxygen-rich stellar atmospheres and interstellar environments such as the high-mass protostellar disc surrounding Orion Src1 and the proto-binary system, IRAS 16547−4247. Their detection in relatively low-temperature regions is yet to be made. In this paper, we consider the synthesis of sodium-bearing species with an emphasis on NaCl, via both gas-phase and grain-surface chemistry under assorted interstellar conditions. We also consider the chemistry leading to the gas-phase species NaH and NaOH. Two classes of numerical simulations were run: models under isothermal conditions at temperatures from 10 to 800 K with varied intervals, and three-phase warm-up models that consist of an initial isothermal collapse at 10 K, followed by a warm-up phase in which temperature rises linearly to 200 K, and finally a hot core phase. We have included reactive desorption for both models to produce gaseous NaCl, NaH, and NaOH. We found that for isothermal models over a broad parameter space, the fractional abundances of gaseous NaCl and NaOH can reach above 2 × 10−10 and approx. 1 × 10−10, respectively, are in the detection range of observational facilities such as Atacama Large Millimeter/Submillimeter Array and JWST. For warm-up models, we found that if we consider molecules to be co-desorbed with water, gaseous NaCl can have a sufficiently large abundance for detection. We then conclude that both gaseous NaCl and NaOH can be detected; however, more experiments and quantum mechanical calculations are needed to constrain the relevant reaction rates better. [ABSTRACT FROM AUTHOR]

We present a study S K -edge using high-resolution HETGS Chandra spectra of 36 low-mass X-ray binaries. For each source, we have estimated column densities for S  i , S  ii , S  iii , S  xiv , S  xv , and S  xvi ionic species, which trace the neutral, warm, and hot phases of the Galactic interstellar medium. We also estimated column densities for a sample of interstellar dust analogues. We measured their distribution as a function of Galactic latitude, longitude, and distances to the sources. While the cold-warm column densities tend to decrease with the Galactic latitude, we found no correlation with distances or Galactic longitude. This is the first detailed analysis of the sulphur K -edge absorption due to ISM using high-resolution X-ray spectra. [ABSTRACT FROM AUTHOR]

We use the Breit–Pauli R -matrix method to calculate accurate energies and radiative data for states in C  i up to n = 30 and with l ≤ 3. We provide the full data set of decays to the five 2s2 2p2 ground configuration states 3P0,1,2, 1D2, and 1S0. This is the first complete set of data for transitions from n ≥ 5. We compare oscillator strengths and transition probabilities with the few previously calculated values for such transitions, finding generally good agreement (within 10 per cent) with the exception of values recently recommended by National Institute of Standards and Technology, where significant discrepancies are found. We then calculate spectral line intensities originating from the Rydberg states using typical chromospheric conditions and assuming local thermal equilibrium, and compare them with well-calibrated Solar and Heliospheric Observatory Solar Ultraviolet Measurements of Emitted Radiation ultraviolet (UV) spectra of the quiet Sun. The relative intensities of the Rydberg series are in excellent agreement with observation, which provides firm evidence for the identifications and blends of nearly 200 UV lines. Such comparison also resulted in a large number of new identifications of C  i lines in the spectra. We also estimate optical depth effects and find that these can account for much of the absorption noted in the observations. The atomic data can be applied to model a wide range of solar and astrophysical observations. [ABSTRACT FROM AUTHOR]

The goal of this study is tracking of possible changes in the spectrum of the interstellar cloud seen along the sightline towards a runaway star AE Aur. The study considers variations in the radial velocities of the interstellar molecules and atoms, dwelling in the cloud, for the period ≈17 yr. The most recent results of 6 yr monitoring of equivalent widths variability for the CH and CH+ absorption lines are also presented. The study was done using the CH B-X(0,0) 3886 Å, A-X (0,0) 4300 Å and the CH+ A-X (1-0) 3958 Å, A-X (0,0) 4232 Å lines as well as the interstellar potassium K  i 7699 Å line. Moreover, a short discussion concerning the distance to the star is included. The present study is a continuation of our previous works and works of other researchers dedicated to the problem of small-scale structure in diffuse interstellar clouds. High resolution optical spectra from different spectrographs have been used in this work. The results of the study do not demonstrate significant changes in the radial velocities of the spectral features mentioned. The intensities of the CH and CH+ lines show slight tendency to vary at 1–3  per cent level. [ABSTRACT FROM AUTHOR]

The hydrogen atoms penetrate the heliosphere from the local interstellar medium, and while being ionized, they form the population of pickup protons. The distribution of pickup protons is modified by the adiabatic heating (cooling) induced by the solar wind plasma compression (expansion). In this study, we emphasize the importance of the adiabatic energy change in the inner heliosheath that is usually either neglected or considered improperly. The effect of this process on the energy and spatial distributions of pickup protons and energetic neutral atoms (ENAs), which originate in the charge exchange of pickup protons, has been investigated and quantified using a kinetic model. The model employs the global distributions of plasma and hydrogen atoms in the heliosphere from the simulations of a kinetic-magnetohydrodynamic model of solar wind interaction with the local interstellar medium. The findings indicate that the adiabatic energy change is responsible for the broadening of the pickup proton velocity distribution and the significant enhancement of ENA fluxes (up to ∼5 and ∼20 times in the upwind and downwind directions at energies ∼1–2 keV for an observer at 1 au). It sheds light on the role of adiabatic energy change in explaining the discrepancies between the ENA flux observations and the results of numerical simulations. [ABSTRACT FROM AUTHOR]

We examine spatial variations of the C0 |$/$| CO abundance ratio (X C/CO) in the vicinity of the γ-ray supernova remnant W 51 C, based on [C  i ] (3 P 1–3 P 0), 12CO(1–0), and 13CO(1–0) observations with the ASTE and Nobeyama 45 m telescopes. We find that X C/CO varies in a range of 0.02–0.16 (0.05 in median) inside the molecular clouds of A V > 100 mag, where photodissociation of CO by the interstellar UV is negligible. Furthermore, X C/CO is locally enhanced by a factor of up to four near the W 51 C center, depending on the projected distance from the W 51 C center. In high- A V molecular clouds, X C/CO is determined by the ratio of the cosmic-ray (CR) ionization rate to the H2 density, and we find no clear spatial variation of the H2 density against the projected distance. Hence, the high CR ionization rate may locally enhance X C/CO near the W 51 C center. We also find that the observed spatial extent of the enhanced X C/CO (∼17 pc) is consistent with the diffusion distance of CRs with an energy of 100 MeV. This fact suggests that the low-energy CRs accelerated in W 51 C enhance X C/CO. The CR ionization rate in the X C/CO-enhanced cloud is estimated to be 3 × 10−16 s−1 on the basis of time-dependent photodissociation region simulations of X C/CO, the value of which is 30 times higher than that in the standard Galactic environment. These results demonstrate that [C  i ] is a powerful probe to investigate the interaction between CRs and the interstellar medium for a wide area in the vicinity of supernova remnants. [ABSTRACT FROM AUTHOR]

Using a quantum-mechanical close-coupling method, we calculate cross-sections for fine-structure excitation and relaxation of Si and S atoms in collisions with atomic hydrogen. Rate coefficients are calculated over a range of temperatures for astrophysical applications. We determine the temperature-dependent critical densities for the relaxation of Si and S in collisions with H and compare these to the critical densities for collisions with electrons. The present calculations should be useful in modelling environments exhibiting the [S  i ] 25 μm and [S  i ] 57 μm far-infrared emission lines or where cooling of S and Si by collisions with H is of interest. [ABSTRACT FROM AUTHOR]

The problem of missing baryons in the local universe remains an open question. One proposed alternative is that at low redshifts missing baryons are in the form of the warm-hot intergalactic medium (WHIM). In order to test this idea, we present a detailed analysis of X-ray high-resolution spectra of six extragalactic sources, Mrk 421, 1ES 1028+511, 1ES 1553+113, H2356−309, PKS 0558−504, and PG 1116+215, obtained with the XMM–Newton Reflection Grating Spectrometer to search for signals of WHIM and/or circumgalactic medium X-ray-absorbing gas. We fit the X-ray absorption with the IONeq model, allowing us to take into account the presence of X-ray spectral features due to the multiphase component of the local interstellar medium (ISM). An additional IONeq component is included to model the WHIM absorption, instead of the traditional Gaussian absorption line modelling. We found no statistical improvement in the fits when including such component in any of the sources, concluding that we can safely reject a successful detection of WHIM absorbers towards these lines of sights. Our simulation shows that the presence of the multiphase ISM absorption features prevents the detection of low-redshift WHIM absorption features in the >17 Å spectral region for moderate exposures using high-resolution spectra. [ABSTRACT FROM AUTHOR]

Being the backbone element of deoxyribonucleic acid, phosphorus is a key component in the search for life in the Universe. To aid in its detection, we present line emissivity ratios for the five lowest-lying forbidden [P  ii ] transitions, namely those among the levels 3s23p2(3P0, 3P1, 3P2, 1D2, and 1S0). The wavelengths range between 0.44 and 70 µm, and several lie within the spectroscopic domain observable with the JWST. These line ratios have been calculated using a new collisional-radiative-cascade (CRR) model combining calculated collision strengths and level-specific recombination rate coefficients, with both data sets computed using the accurate Breit–Pauli R-matrix method. The CRR model includes a new scheme for (e + ion) recombination to emission-line formation. We compare its effect to models incorporating only electron-impact excitation and spontaneous radiative decay. We find that (e + ion) recombination has a significant impact on all line ratios, and represents a major improvement in physical accuracy of emission-line models. [ABSTRACT FROM AUTHOR]

The ubiquity of very thin and lengthy cold neutral medium (CNM) has been reported by multiple authors in the H  i community. Yet, the reason of how the CNM can be so long and lengthy is still in debate. In this paper, we recognize a new type of instability due to the attractive nature of the pressure force in the unstable phase. We provide a new estimation of the average CNM filament aspect ratio with the consideration of force balances at the phase boundary, which is roughly 5–20 in common CNM environment. We show that most of the cold filaments are less filamentary than what usually predicted via MHD turbulence theory or inferred from observations: The average length of CNM filament is roughly 1/2 of that in isothermal MHD turbulence with similar turbulence conditions. This suggests that the 'cold filaments' that are identified in observations might not be in pressure equilibrium or generated via other mechanisms. [ABSTRACT FROM AUTHOR]

The recent development of the velocity gradient technique shows the capability of the technique for tracing magnetic field morphology in diffuse interstellar gas and molecular clouds. In this paper, we perform a systematic numerical study of the performance of the velocity and synchrotron gradient for a wide range of magnetization in the subsonic environment. Addressing the studies of magnetic fields in atomic hydrogen, we also study the formation of velocity caustics in spectroscopic channel maps in the presence of thermal broadening. We show that the velocity caustics can be recovered when applied to the cold neutral medium and the gradient technique (GT) can reliably trace magnetic fields there. Finally, we discuss the changes in the anisotropy of observed structure functions when we apply to the analysis the procedures developed within the framework of GT studies. [ABSTRACT FROM AUTHOR]

A recent publication discovered one of the largest filamentary neutral hydrogen features dubbed Cattail from high-resolution Five-hundred-meter Aperture Spherical radio Telescope observations that might be a new galactic arm of the Milky Way. We evaluate the turbulent and phase properties of Cattail via the newly developed Velocity Decomposition Algorithm and Force Balancing Model. We discover that if there exists a phase transition, then Cattail is unlikely in the cold neutral media phase. We also show that the Cattail is two disjoint features in caustics space, suggesting that the Cattail has two different turbulent systems. We check the spectra of the individual system separated via VDA to confirm this argument. We do not exclude the existence of smaller scale cold media being embedded within this structure. [ABSTRACT FROM AUTHOR]

We investigate the extended-Schmidt (ES) law in volume densities (ρSFR ∝  |$(\rho _{\rm gas}\rho _{\rm star}^{0.5})^{\alpha ^{\rm VES}}$|⁠) for spatially resolved regions in spiral, dwarf, and ultra-diffuse galaxies (UDGs), and compare to the volumetric Kennicutt–Schmidt (KS) law (ρSFR ∝  |$\rho _{\rm gas}^{\alpha ^{\rm VKS}}$|⁠). We first characterize these star formation laws in individual galaxies using a sample of 11 spirals, finding median slopes αVES = 0.98 and αVKS = 1.42, with a galaxy-to-galaxy rms fluctuation that is substantially smaller for the volumetric ES law (0.18 versus 0.41). By combining all regions in spirals with those in additional 13 dwarfs and one UDG into one single data set, it is found that the rms scatter of the volumetric ES law at given x -axis is 0.25 dex, which is also smaller than that of the volumetric KS law (0.34 dex). At the extremely low gas density regime as offered by the UDG, the volumetric KS law breaks down but the volumetric ES law still holds. On the other hand, as compared to the surface density ES law, the volumetric ES law instead has a slightly larger rms scatter, consistent with the scenario that the ES law has an intrinsic slope of αVES ≡1 but the additional observational error of the scale height increases the uncertainty of the volume density. The unity slope of the ES law implies that the star formation efficiency (= ρSFR/ρgas) is regulated by the quantity that is related to the |$\rho _{\rm star}^{0.5}$|⁠. [ABSTRACT FROM AUTHOR]

Interstellar chemistry is important for galaxy formation, as it determines the rate at which gas can cool, and enables us to make predictions for observable spectroscopic lines from ions and molecules. We explore two central aspects of modelling the chemistry of the interstellar medium (ISM): (1) the effects of local stellar radiation, which ionizes and heats the gas, and (2) the depletion of metals on to dust grains, which reduces the abundance of metals in the gas phase. We run high-resolution (400 M⊙ per baryonic particle) simulations of isolated disc galaxies, from dwarfs to Milky Way-mass, using the fire galaxy formation models together with the chimes non-equilibrium chemistry and cooling module. In our fiducial model, we couple the chemistry to the stellar fluxes calculated from star particles using an approximate radiative transfer scheme; and we implement an empirical density-dependent prescription for metal depletion. For comparison, we also run simulations with a spatially uniform radiation field, and without metal depletion. Our fiducial model broadly reproduces observed trends in H  i and H2 mass with stellar mass, and in line luminosity versus star formation rate for [C  ii ] |$_{158 \rm {\mu m}}$|⁠ , [O  i ] |$_{63 \rm {\mu m}}$|⁠ , [O  iii ] |$_{88 \rm {\mu m}}$|⁠ , [N  ii ] |$_{122 \rm {\mu m}}$|⁠ , and H α6563Å. Our simulations with a uniform radiation field predict fainter luminosities, by up to an order of magnitude for [O  iii ] |$_{88 \rm {\mu m}}$| and H α6563Å, while ignoring metal depletion increases the luminosity of carbon and oxygen lines by a factor ≈ 2. However, the overall evolution of the galaxy is not strongly affected by local stellar fluxes or metal depletion, except in dwarf galaxies where the inclusion of local fluxes leads to weaker outflows and hence higher gas fractions. [ABSTRACT FROM AUTHOR]

By mapping the molecular fraction of the Galactic Centre (GC), we quantitatively address the question of how much molecular and central the CMZ (central molecular zone) is. For this purpose, we analyse the CO and H  i -line archival data, and determine the column- (surface-) and volume-molecular fractions, |$f_{\rm mol}^\Sigma$| and |$f_{\rm mol}^\rho$|⁠ , which are the ratio of column-mass density of H2 projected on the sky to that of total gas (H2 + H  i) from the line intensities, and the ratio of volume-mass densities of H2 to total gas from the brightness temperature, respectively. It is shown that |$f_{\rm mol}^\Sigma$| is as high as ∼0.9–0.95 in the CMZ, and |$f_{\rm mol}^\rho$| is 0.93–0.98 in the GC Arms I and II attaining the highest value of ∼0.98 toward Sgr B2. The expanding molecular ring (EMR, or the parallelogram) has a slightly smaller |$f_{\rm mol}^\rho$| as ∼0.9–0.93. We define the CMZ as the region with |$f_{\rm mol}^\Sigma \ge 0.8\rm {-\!\!-}0.9$| between the shoulders of plateau-like distribution of H2 column density from l  = −1 |${_{.}^{\circ}}$| 1 to +1 |${_{.}^{\circ}}$| 8 having Gaussian vertical distribution with a half thickness of ±0 |${_{.}^{\circ}}$| 2. The CMZ is embedded in the Central H  i Zone (CHZ), which is defined as an H  i disc between l ∼ −2° and +2 |${_{.}^{\circ}}$| 5, b  = −0 |${_{.}^{\circ}}$| 5 and +0 |${_{.}^{\circ}}$| 5. Based on the analysis, we discuss the origin of CMZ and interstellar physics such as the volume filling factors of molecular and H  i gases inferred from the difference between |$f_{\rm mol}^\Sigma$| and |$f_{\rm mol}^\rho$|⁠. [ABSTRACT FROM AUTHOR]

A detailed comparison of H  i and CO line cube data of the Galactic Centre (GC) region from the archives is obtained. The central molecular zone (CMZ) is shown to be embedded in the H  i disc (central H  i zone, CHZ) of radius ∼320 pc and vertical scale height ∼70 pc. A radio continuum belt is shown to run parallel to molecular Arms I and II. The belt draws a double infinity (∞∞) on the sky, connecting Sgr E (l ∼ −1 |${_{.}^{\circ}}$| 2), C, B1, B2, and Sgr D (+1 |${_{.}^{\circ}}$| 2), and is interpreted as a warping star-forming ring. The molecular Arms are closely associated with the H  i arms on the longitude-velocity diagram (LVD), showing coherent rigid-body ridges. Due to the close relationship between H  i and CO, the H  i line absorption can be used to determine the Arms' position relative to Sgr A, B1, B2, and C. Combining the trigonometric data of proper motions of Sgr A* and maser sources of Sgr B2 as well as radial velocities, the 3D velocity vector of Sgr B2 is determined. From these analyses, the molecular Arm I with Sgr B2 is shown to be located in the near side of Sgr A*, and Arm II with Sgr C in the other side, both composing a pair of symmetrical Arms around the GC. We present a possible 3D view of Sgr A through E and Arms I and II along with a parameter list. [ABSTRACT FROM AUTHOR]

We present the results of several collisional-radiative models describing optically thin emissivities of the main lines in neutral helium formed by recombination, for a grid of electron temperatures and densities, typical of H  ii regions and planetary nebulae. Accurate emissivities are required for example to measure the helium abundance in nebulae and as a consequence its primordial value. We compare our results with those obtained by previous models, finding significant differences, well above the target accuracy of 1 per cent. We discuss in some detail our chosen set of atomic rates and the differences with those adopted by previous models. The main differences lie in the treatment of electron and proton collision rates and we discuss which transitions are least sensitive to the choice of these rates and therefore best suited to high-precision abundance determinations. We have focused our comparisons on the case B approximation where only He and He+ are considered, but also present results of full models including the bare nuclei, photoexcitation, and photoionization, and either black-body or observed illuminating spectrum in the case of the Orion nebula, to indicate which spectral lines are affected by opacity. For those transitions, accurate radiative transfer calculations should be performed. We provide tables of emissivities for all transitions within n ≤ 5 and all those between the n ≤ 5 and n ′ ≤ 25 states, in the log T e (K) = 103.0(0.1)4.6 and log  N e (cm−3) = 102(0.5)6 ranges, and a fortran code to interpolate to any T e, N e within these ranges. [ABSTRACT FROM AUTHOR]

We present a study of the cold atomic hydrogen (H  i) content of molecular clouds simulated within the SILCC-Zoom project for solar neighbourhood conditions. We produce synthetic observations of H  i at 21 cm, including H  i self-absorption (HISA) and observational effects. We find that H  i column densities, |$N_{\rm H\, \small {\rm I}}$|⁠ , of ≳1022 cm−2 are frequently reached in molecular clouds with H  i temperatures as low as ∼10 K. Hence, HISA observations assuming a fixed H  i temperature tend to underestimate the amount of cold H  i in molecular clouds by a factor of 3–10 and produce an artificial upper limit of |$N_{\rm H\, \small {\rm I}}$| around 1021 cm−2. We thus argue that the cold H  i mass in molecular clouds could be a factor of a few higher than previously estimated. Also, |$N_{\rm H\, \small {\rm I}}$| PDFs obtained from HISA observations might be subject to observational biases and should be considered with caution. The underestimation of cold H  i in HISA observations is due to both the large H  i temperature variations and the effect of noise in regions of high optical depth. We find optical depths of cold H  i around 1–10, making optical depth corrections essential. We show that the high H  i column densities (≳1022 cm−2) can in parts be attributed to the occurrence of up to 10 individual H  i –H2 transitions along the line of sight. This is also reflected in the spectra, necessitating Gaussian decomposition algorithms for their in-depth analysis. However, also for a single H  i –H2 transition, |$N_{\rm H\, \small {\rm I}}$| frequently exceeds 1021 cm−2, challenging one-dimensional, semi-analytical models. This is due to non-equilibrium chemistry effects and the fact that H  i –H2 transition regions usually do not possess a one-dimensional geometry. Finally, we show that the H  i gas is moderately supersonic with Mach numbers of a few. The corresponding non-thermal velocity dispersion can be determined via HISA observations within a factor of ∼2. [ABSTRACT FROM AUTHOR]

Observations of the cosmic evolution of different gas phases across time indicate a marked increase in the molecular gas mass density towards z ∼ 2–3. Such a transformation implies an accompanied change in the global distribution of molecular hydrogen column densities (⁠|$N_{\rm {H_2}}$|⁠). Using observations by PHANGS-ALMA/SDSS and simulations by GRIFFIN/IllustrisTNG we explore the evolution of this H2 column density distribution function [ |$f(N_{\rm {H}_2})$| ]. The H2 (and H  i) column density maps for TNG50 and TNG100 are derived in post-processing and are made available through the IllustrisTNG online API. The shape and normalization of |$f(N_{\rm {H}_2})$| of individual main-sequence star-forming galaxies are correlated with the star formation rate (SFR), stellar mass (M *), and H2 mass (⁠|$M_{\rm {H}_2}$|⁠) in both observations and simulations. TNG100, combined with H2 post-processing models, broadly reproduces observations, albeit with differences in slope and normalization. Also, an analytically modelled f (N), based on exponential gas discs, matches well with the simulations. The GRIFFIN simulation gives first indications that the slope of |$f(N_{\rm {H}_2})$| might not majorly differ when including non-equilibrium chemistry in simulations. The |$f(N_{\rm {H}_2})$| by TNG100 implies that higher molecular gas column densities are reached at z  = 3 than at z  = 0. Further, denser regions contribute more to the molecular mass density at z  = 3. Finally, H2 starts dominating compared to H  i only at column densities above log(⁠|$N_{\rm {H}_2} / \rm {cm}^{-2}) \sim 21.8\!-\!22$| at both redshifts. These results imply that neutral atomic gas is an important contributor to the overall cold gas mass found in the ISM of galaxies including at densities typical for molecular clouds at z  = 0 and 3. [ABSTRACT FROM AUTHOR]

We examine a sample of 106 galaxies for which the total luminosities of the two fine structure lines 3 P 1 → 3 P 0, and 3 P 2 → 3 P 1 of neutral atomic carbon (C) are available, and find their average excitation conditions to be strongly subthermal. This is deduced from the CI(2-1)/(1-0) ratios (⁠|$\rm R^{(ci)}_{21/10}$|⁠) modeled by the exact solutions of the corresponding three-level system, without any special assumptions about the kinematic state of the concomitant |$\rm H_2$| gas (and thus the corresponding line formation mechanism). This non-LTE excitation of the CI lines can induce the curious clustering of (CI,LTE)-derived gas temperatures near ∼25 K reported recently by Valentino et al. (2020), which is uncorrelated to the actual gas temperatures. The non-LTE CI line excitation in the interstellar medium of galaxies deprives us from a simple method for estimating molecular gas temperatures, and adds uncertainty in CI-based molecular gas mass estimates especially when the J = 2–1 line is used. However the |$\rm R^{(ci)}_{21/10}$|  =  |$\rm F(n, T_{k})$| ratio is now more valuable for joint CO/CI SLED and dust SED models of galaxies, and independent of the assumptions used in the CO radiative transfer models (e.g. the large velocity gradient approximation). Finally we speculate that the combination of low ratios |$\rm R^{(ci)}_{21/10} \lesssim 1$|   and high |$\rm T_{dust}$| values found in some extreme starbursts indicates massive low-density molecular wind and/or circumgalactic gas reservoirs. If verified by imaging observations this can be a useful indicator of the presence of such reservoirs in galaxies. [ABSTRACT FROM AUTHOR]

The paper describes profile broadening and peak wavelength variation of diffuse interstellar bands (DIBs) measured for 46 lines of sight, probably caused by the physical properties of intervening clouds. The full width at half-maximum of four studied DIBs (5780, 5797, 6196 and 6614 Å) demonstrates strong variability, sometimes doubling the widths of the features. Despite the high magnitude of the effect, our current analysis is restricted to the strongest DIBs because the weaker ones require a much higher signal-to-noise ratio. The profile broadening in the studied DIBs moves the profile's centres towards longer wavelengths, probably due to the excitation of higher levels of the P branch of the unknown molecular carrier. Moreover, DIBs are broader in clouds with abundantly populated vibrationally excited states of hydrogen molecules; that is, the broadening of DIBs correlates with the rotational temperature estimated on the H2 ν = 2 vibrational level. However, objects that demonstrate extremely broadened profiles of DIBs are scarce. The extreme peculiarity of the DIB profiles was detected in Herschel 36. Here we show the gradual growths of the widths of DIBs, confirmed in spectra from different instruments. [ABSTRACT FROM AUTHOR]

We use the 21-cm emission-line data from the Deep Investigation of Neutral Gas Origin-Very Large Array (DINGO-VLA) project to study the atomic hydrogen gas H  i of the Universe at redshifts z < 0.1. Results are obtained using a stacking analysis, combining the H  i signals from 3622 galaxies extracted from 267 VLA pointings in the G09 field of the Galaxy and Mass Assembly Survey (GAMA). Rather than using a traditional one-dimensional spectral stacking method, a three-dimensional cubelet stacking method is used to enable deconvolution and the accurate recovery of average galaxy fluxes from this high-resolution interferometric data set. By probing down to galactic scales, this experiment also overcomes confusion corrections that have been necessary to include in previous single-dish studies. After stacking and deconvolution, we obtain a 30σ H  i mass measurement from the stacked spectrum, indicating an average H  i mass of |${\rm{M_{\rm{{H}\,\small{I}}}}}=(1.67\pm 0.18)\times 10^{9}~{\rm{{\rm M}_{\odot }}}$|⁠. The corresponding cosmic density of neutral atomic hydrogen is |${\rm{\Omega _{\rm{{H}\,\small{I}}}}}=(0.38\pm 0.04)\times 10^{-3}$| at redshift of z = 0.051. These values are in good agreement with earlier results, implying there is no significant evolution of |$\Omega _{\rm{{H}\,\small{I}}}$| at lower redshifts. [ABSTRACT FROM AUTHOR]

In this paper, we search for the possible imprints of the secondary interstellar hydrogen atoms created at the heliospheric boundary in the full-sky maps of the hydrogen fluxes at the Earth orbit. By using our three-dimensional time-dependent kinetic model, the maps of the hydrogen fluxes are calculated for different phases of the solar cycle and different energy ranges. It is shown that the flux maps obtained during the solar minimum conditions for the energy range 1–20 eV have specific features such as blobs and tails, which are pronounced due to a signal of the secondary component of the interstellar atoms. We investigate how these features depend on parameters of the secondary atoms far away from the Sun and found that the geometry and shape of the tails depend on the averaged velocity and kinetic temperatures of the secondary population. The results of the paper provide a strategy where and when we need to look in order to detect the secondary component of the interstellar hydrogen separately from the primary component at 1  au. This can be important for future space missions devoted to the exploration of the heliospheric boundary. [ABSTRACT FROM AUTHOR]

Using the atomic carbon [C  i ] (⁠|$^{3} \rm P_{1} \rightarrow {\rm ^3 P}_{0}$|⁠) and [C  i ] (⁠|$^{3} \rm P_{2} \rightarrow {\rm ^3 P}_{1}$|⁠) emission {hereafter [C  i ] (1 − 0) and [C  i ] (2 − 1), respectively} maps observed with the Herschel   Space   Observatory , and CO (1 − 0), H  i , infrared and submm maps from literatures, we estimate the [C  i ]-to-H2 and CO-to-H2 conversion factors of α[C  i ](1 − 0), α[C  i ](2 − 1), and αCO at a linear resolution |$\sim 1\,$| kpc scale for six nearby galaxies of M 51, M 83, NGC 3627, NGC 4736, NGC 5055, and NGC 6946. This is perhaps the first effort, to our knowledge, in calibrating both [C  i ]-to-H2 conversion factors across the spiral disks at spatially resolved |$\sim 1\,$| kpc scale though such studies have been discussed globally in galaxies near and far. In order to derive the conversion factors and achieve these calibrations, we adopt three different dust-to-gas ratio (DGR) assumptions that scale approximately with metallicity taken from precursory results. We find that for all DGR assumptions, the α[C  i ](1 − 0), α[C  i ](2 − 1), and αCO are mostly flat with galactocentric radii, whereas both α[C  i ](2 − 1) and αCO show decrease in the inner regions of galaxies. And the central αCO and α[C  i ](2 − 1) values are on average ∼2.2 and 1.8 times lower than its galaxy averages. The obtained carbon abundances from different DGR assumptions show flat profiles with galactocentric radii, and the average carbon abundance of the galaxies is comparable to the usually adopted value of 3 × 10−5. We find that both metallicity and infrared luminosity correlate moderately with the αCO, whereas only weakly with either the α[C  i ](1 − 0) or carbon abundance, and not at all with the α[C  i ](2 − 1). [ABSTRACT FROM AUTHOR]

In this paper, we introduce a method for stacking data cubelets extracted from interferometric surveys of galaxies in the redshifted 21-cm H}, i line. Unlike the traditional spectral stacking technique, which stacks one-dimensional spectra extracted from data cubes, we examine a method based on image domain stacks that makes deconvolution possible. To test the validity of this assumption, we mock a sample of 3622 equatorial galaxies extracted from the GAMA survey, recently imaged as part of a DINGO-VLA project. We first examine the accuracy of the method using a noise-free simulation and note that the stacked image and flux estimation are dramatically improved compared to traditional stacking. The extracted H  i mass from the deconvolved image agrees with the average input mass to within 3 per cent. However, with traditional spectral stacking, the derived H  i is incorrect by greater than a factor of 2. For a more realistic case of a stack with finite S/N, we also produced 20 different noise realizations to closely mimic the properties of the DINGO-VLA interferometric survey. We recovered the predicted average H  i mass to within ∼4 per cent. Compared with traditional spectral stacking, this technique extends the range of science applications where stacking can be used, and is especially useful for characterizing the emission from extended sources with interferometers. [ABSTRACT FROM AUTHOR]

The Interstellar Boundary Explorer (IBEX) has been measuring interstellar hydrogen fluxes at 1 au since 2009. In this paper, we analysed all available data obtained with the IBEX-Lo instrument at energies 11–41 eV using our numerical kinetic model of the interstellar hydrogen distribution in the heliosphere. We performed a fitting of the data to find independently the model parameters: the ratio of the solar radiation pressure to the solar gravitation (μ0), ionization rate of hydrogen atoms at 1 au (β0), parameters of the secondary interstellar atoms at 70 au from the Sun, which provide the best agreement with the data by minimization of metric χ2. We also analysed temporal variations of the ratio of the fluxes measured in a fixed direction at energy bin 1 and energy bin 2. It is found that in 2009–2011 and 2017–2016 the ratio provided by the model is smaller than in the IBEX-Lo data, while in 2012–2015, oppositely, the model ratio is larger compared to the data. This might be caused by the incorrect separation of the measured fluxes between energy channels in the data, or by some additional physical factors that are omitted in the model. Understanding this issue may be important for the preparation of future Interstellar Mapping and Acceleration Probe mission. At this stage, we relied on the sum of the fluxes measured in energy bins 1 and 2 for comparison to model predictions. [ABSTRACT FROM AUTHOR]

The interstellar boundary explorer (IBEX) has been measuring fluxes of the energetic neutral atoms (ENAs) using the IBEX-Hi (0.3–6 keV) instrument since 2008. We have developed a numerical time-depended code to calculate globally distributed flux (GDF) of hydrogen ENAs employing both (1) 3D kinetic-MHD model of the global heliosphere and (2) reconstruction of atom trajectories from 1 au, where they are observed by IBEX , to the point of their origin in the inner heliosheath (IHS). The key factor in the simulation is a detailed kinetic consideration of the pickup ions (PUIs), the supra-thermal component of protons in the heliosphere, which is 'parental' to the ENAs and originates in the region of the supersonic solar wind being picked by the heliospheric magnetic field. As a result of our study, we have concluded that (1) the developed model is able to reproduce the geometry of the multilobe structure seen in the IBEX-Hi GDF maps, (2) the GDF is extremely sensitive to the form of the velocity distribution function of PUIs in the IHS, and the accounting for the existence of an additional energetic population of PUIs is essential to explain the data, (3) despite a relatively good agreement, there are some quantitative differences between the model calculations and IBEX-Hi data. Possible reasons for these differences are discussed. [ABSTRACT FROM AUTHOR]

The need for a method to construct multidimensional distribution function is increasing recently, in the era of huge multiwavelength surveys. We have proposed a systematic method to build a bivariate luminosity or mass function of galaxies by using a copula. It allows us to construct a distribution function when only its marginal distributions are known, and we have to estimate the dependence structure from data. A typical example is the situation that we have univariate luminosity functions at some wavelengths for a survey, but the joint distribution is unknown. Main limitation of the copula method is that it is not easy to extend a joint function to higher dimensions (d > 2), except some special cases like multidimensional Gaussian. Even if we find such a multivariate analytic function in some fortunate case, it would often be inflexible and impractical. In this work, we show a systematic method to extend the copula method to unlimitedly higher dimensions by a vine copula. This is based on the pair-copula decomposition of a general multivariate distribution. We show how the vine copula construction is flexible and extendable. We also present an example of the construction of a stellar mass–atomic gas–molecular gas three-dimensional mass function. We demonstrate the maximum likelihood estimation of the best functional form for this function, as well as a proper model selection via vine copula. [ABSTRACT FROM AUTHOR]

We present a detailed analysis of the gaseous component of the Si K edge using high-resolution Chandra spectra of low-mass X-ray binaries. We fit the spectra with a modified version of the ISMabs model, including new photoabsorption cross-sections computed for all Si ionic species. We estimate column densities for Si  i , Si  ii , Si  iii , Si  xii , and Si  xiii , which trace the warm, intermediate temperature, and hot phases of the Galactic interstellar medium. We find that the ionic fractions of the first two phases are similar. This may be due to the physical state of the plasma determined by the temperature or due to the presence of absorber material in the close vicinity of the sources. Our findings highlight the need for accurate modelling of the gaseous component before attempting to address the solid component. [ABSTRACT FROM AUTHOR]

We present an analysis of moderately high-resolution optical spectra obtained for the sightline to CD−23 13777, an O9 supergiant that probes high-velocity interstellar gas associated with the supernova remnant W28. Absorption components at both high positive and high negative velocity are seen in the interstellar Na i D and Ca ii H and K lines towards CD−23 13777. The high-velocity components exhibit low Na i/Ca ii ratios, suggesting efficient grain destruction by shock sputtering. High column densities of CH+, and high CH+/CH ratios, for the components seen at lower velocity may be indicative of enhanced turbulence in the clouds interacting with W28. The highest positive and negative velocities of the components seen in Na i and Ca ii absorption towards CD−23 13777 imply that the velocity of the blast wave associated with W28 is at least 150 km s−1, a value that is significantly higher than most previous estimates. The line of sight to CD−23 13777 passes very close to a well-known site of interaction between the supernova remnant and a molecular cloud to the north-east. The north-east molecular cloud exhibits broad molecular line emission, OH maser emission from numerous locations, and bright extended GeV and TeV γ-ray emission. The sightline to CD−23 13777 is thus a unique and valuable probe of the interaction between W28 and dense molecular gas in its environs. Future observations at ultraviolet and visible wavelengths will help to better constrain the abundances, kinematics, and physical conditions in the shocked and quiescent gas along this line of sight. [ABSTRACT FROM AUTHOR]

Atomic hydrogen masers occur in recombination plasmas in sufficiently dense H  ii regions. These hydrogen recombination line (HRL) masers have been observed in a handful of objects to date and the analysis of the atomic physics involved has been rudimentary. In this work, a new model of HRL masers is presented which uses an nl -model to describe the atomic populations interacting with free–free radiation from the plasma, and an escape probability framework to deal with radiative transfer effects. The importance of including the collisions between angular momentum quantum states and the free–free emission in models of HRL masers is demonstrated. The model is used to describe the general behaviour of radiative transfer of HRLs and to investigate the conditions under which HRL masers form. The model results show good agreement with observations collected over a broad range of frequencies. Theoretical predictions are made regarding the ratio of recombination lines from the same upper quantum level for these objects. [ABSTRACT FROM AUTHOR]

We follow-up on our systematic study of axisymmetric hydrodynamic simulations of protoplanetary nebula. The aim of this work is to generate the atomic analogues of the H2 near-infrared models of Paper I with the ZEUS code modified to include molecular and atomic cooling routines. We investigate stages associated with strong |$\mathrm{[Fe\, \small {II}] \, 1.64\, \mathrm{\mu m} }$| and |$\mathrm{ [S\, \small {II}] \, 6716}$| Å forbidden lines, the |$\mathrm{[O\, \small {I}]\, 6300}$| Å airglow line, and H |$\mathrm{\alpha }\, 6563$| Å emission. We simulate (⁠|$\mathrm{80-200\, km\, s^{-1}}$|⁠) dense (⁠|$\mathrm{10^{5}\, cm^{-3}}$|⁠) outflows expanding into a stationary ambient medium. In the case of an atomic wind interacting with an atomic medium, a decelerating advancing turbulent shell thickens with time. This contrasts with all other cases where a shell fragments into a multitude of cometary-shaped protrusions with weak oblique shocks as the main source of gas excitation. We find that the atomic wind-ambient simulation leads to considerably higher excitation, stronger peak, and integrated atomic emission as the nebula expands. The weaker emission when one component is molecular is due to the shell fragmentation into fingers so that the shock surface area is increased and oblique shocks are prevalent. Position–velocity diagrams indicate that the atomic-wind model may be most easy to distinguish with more emission at higher radial velocities. With post-AGB winds and shells often highly obscured and the multitude of configurations that are observed, this study suggests and motivates selection criteria for new surveys. [ABSTRACT FROM AUTHOR]

We present the results from the Suzaku X-ray Imaging Spectrometer observation of the mixed-morphology supernova remnant (SNR) HB9 (G160.9+2.6). We discovered recombining plasma (RP) in the western Suzaku observation region and the spectra here are well described by a model having collisional ionization equilibrium (CIE) and RP components. On the other hand, the X-ray spectra from the eastern Suzaku observation region are best reproduced by the CIE and non-equilibrium ionization model. We discuss possible scenarios to explain the origin of the RP emission based on the observational properties and concluded that the rarefaction scenario is a possible explanation for the existence of RP. In addition, the gamma-ray emission morphology and spectrum within the energy range of 0.2–300 GeV are investigated using 10 yr of data from the Fermi Large Area Telescope (LAT). The gamma-ray morphology of HB9 is best described by the spatial template of radio continuum emission. The spectrum is well fit to a log-parabola function and its detection significance was found to be 25σ. Moreover, a new gamma-ray point source located just outside the south-east region of the SNR's shell was detected with a significance of 6σ. We also investigated the archival H  i and CO data and detected an expanding shell structure in the velocity range of |$-10.5$| and |$+1.8$| km s−1 that is coinciding with a region of gamma-ray enhancement at the southern rim of the HB9 shell. [ABSTRACT FROM AUTHOR]

Small-scale fluctuations of different tracers of the interstellar medium can be used to study the nature of turbulence in astrophysical scales. Of these, the 'continuum' emission traces the fluctuations integrated along the line of sight whereas, the spectral line tracers give the information along different velocity channels as well. Recently, Miville-Deschênes et al. have measured the intensity fluctuation power spectrum of the continuum dust emission, and found a power-law behaviour with a power-law index of −2.9 ± 0.1 for a region of our Galaxy. Here, we study the same region using high-velocity resolution 21-cm emission from the diffuse neutral medium, and estimate the power spectrum at different spectral channels. The measured 21-cm power spectrum also follows a power law, however, we see a significant variation in the power-law index with velocity. The value of the power-law index estimated from the integrated map for different components are quite different which is indicative of the different nature of turbulence depending on temperature, density, and ionization fraction. We also measure the power spectra after smoothing the 21 cm emission to velocity resolution ranging from 1.03 to |$13.39\, {\rm km\, s^{-1}}$|⁠, but the power spectrum remains unchanged within the error bar. This indicates that the observed fluctuations are dominantly due to density fluctuations, and we can only constrain the power-law index of velocity structure function of 0.0 ± 1.1 which is consistent with the predicted Kolmogorov turbulence (γ = 2/3) and also with a shock-dominated medium (γ = 1.0). [ABSTRACT FROM AUTHOR]

The rate coefficients and the cross-sections for the formation of imidogen (NH) molecule (and its isotopologues: 15NH and ND) through radiative association are determined by employing quantum mechanical perturbation theory, classical Larmor formula, and Breit–Wigner theory. We suggest the radiative association process as possible route for NH production in diffuse interstellar clouds. [ABSTRACT FROM AUTHOR]

The neutral hydrogen 21-cm transition is a useful tracer of the neutral interstellar medium. However, it is often not straightforward to infer physical conditions from the observed 21-cm absorption and/or emission spectra. One complication when estimating the temperature of the atomic gas is that the linewidth can have a significant contribution from non-thermal broadening. Here, we propose a formalism to separate the thermal and non-thermal broadening using a self-consistent model of turbulence broadening of the H  i 21-cm absorption components. By applying this novel method, we have estimated the spin and the kinetic temperature of diffuse Galactic neutral hydrogen, and we have found that a large fraction of gas has a temperature in the unstable range. The turbulence is found to be subsonic or transonic in nature, and the clouds seem to have a bimodal size distribution. Assuming that the turbulence is magnetohydrodynamic in nature, the estimated magnetic field strength is of μG order, and is found to be uncorrelated with the H  i number density. [ABSTRACT FROM AUTHOR]

The total amount of O  vi present in the interstellar medium (ISM) obtained via absorption measurements in UV and X-ray spectra is currently in disagreement, with the latter being significantly larger (by a factor of 10 or more) than the former. Previous works have proposed that the blend of the O  vi Kα line (22.032 Å) with the O  ii Kβ-L12 line (22.04 Å) could account for the stronger absorption observed in the X-ray spectra. Here, we present a detailed study of the oxygen absorption in the local ISM, implementing our new model IGMabs which includes photoabsorption cross-sections of highly ionized species of abundant elements as well as turbulence broadening. By analysing high-resolution Chandra spectra of 13 low-mass X-ray binaries (LMXBs) and 29 extragalactic sources, we have estimated the column densities of O  i – O  iii and from O  vi – O  viii along multiple line-of-sights. We find that in most cases the O  ii Kβ-L12 line accounts for <30 |${{\ \rm per\ cent}}$| of the total O  vi K α + O  ii K β. We conclude that the amount of O  ii predicted by our model is still insufficient to explain the discrepancy between X-ray and UV measurements of O  vi column densities. [ABSTRACT FROM AUTHOR]