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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]

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]

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]

We present a method to determine the spin temperature TS and volume density n of H I gas simultaneously along the tangent-point circle of Galactic rotation in the Milky Way by using the χ² method. The best-fit TS is shown to range either in TS ~ 100-120 K or in 1000-3000 K, indicating that the gas is in the cold H I phase with high density and large optical depth, or in warm H I with low density and small optical depth. Averaged values at 3 ≤ R ≤ 8 kpc are obtained to be TS = 106.7 ± 16.0 K and n = 1.53 ± 0.86 H cm-3 for cold H I, and 1720 ± 1060 K and 0.38 ± 0.10 H cm-3 for warm H I, where R = 8 |sinl| kpc is the galacto-centric distance along the tangent-point circle. The cold H I appears in spiral arms and rings, whereas warm H I appears in the inter-arm regions. The cold H I is denser by a factor of ~4 than warm H I. The present analysis has revealed the hidden H I mass in the cold and optically thick phase in the Galactic disk. The total H I mass inside the solar circle is shown to be greater by a factor of 2-2.5 than the current estimation by the optically thin assumption. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) distribution of the volume-density molecular fraction, defined by fpmol = pH2/(pHI + pH2), is studied in the Milky Way Galaxy. The molecular front appears at galacto-centric distance of R ~ 8 kpc, where the galactic-scale phase transition from atomic to molecular hydrogen occurs with fpmol dropping from ~0.8 to 0.2 within a radial interval as narrow as ~0.5 kpc. The fpmol front is much sharper than that of the surface density molecular fraction. The fpmol front also appears in the direction vertical to the galactic plane with a full width of the high- fpmol disk to be ~100 pc. The radial and vertical fpmol profiles, particularly the front behavior, are fitted by theoretical curves calculated using the observed density profile and assumed radiation field andmetallicity with exponential gradients. The molecular fraction was found to be enhanced along spiral arms at radii R ~ 6 to 10 kpc, such as the Perseus arm. This implies that the molecular clouds are produced from HI in the arms and are dissociated in the interarm regions in the transition region around the molecular front. We also show that there is a threshold value of mean HI density, over which HI is transformed into molecular gas. [ABSTRACT FROM AUTHOR]

We compared theoretical and observational molecular mass fractions (fmol: ratio of molecular gas density to total gas density) using observational data of ten nearby spiral galaxies. For determination of fmol, the three parameters–interstellar pressure P, UV radiation U, and metallicity Z–were obtained from the spectral line data of 12CO(J = 1–0), H i, Hα, [O iii], and [O ii]. Interstellar pressure was calculated with the sum of the hydrogen gas densities and the stellar potential based on the Ks-band data. For most data other than metallicity, we used archived NRO CO Atlas, THINGS, SINGS, and 2MASS data. For comparison, we also investigated the dependence of the CO-to-H2 conversion factor XCO. It was found that the theoretical fmol agreed with the observational fmol only when the interstellar pressure is calculated with both the gas density and stellar disk potential. To fit observations more accurately, either the metallicity or the UV radiation needs to be adjusted. It was also found that, in UV radiation scaling, scaling factor γ has a correlation with the diffuse fraction of the Hα emission line data, fDIG. As for XCO, it was shown that the difference between both values of fmol becomes the least when XCO is 1.0 × 1020 cm−2 (K km s−1)−1. [ABSTRACT FROM AUTHOR]