Cosmic Ray Ionization Research Articles

Modeling chemistry during star formation: water deuteration in dynamic star-forming regions

Context. Recent observations of the HDO/H2O ratio toward protostars in isolated and clustered environments show an apparent dichotomy, where isolated sources show higher D/H ratios than clustered counterparts. Establishing which physical and chemical processes create this differentiation can provide new insights into the chemical evolution of water during star formation and the chemical diversity during the star formation process and in young planetary systems. Aims. We seek to determine to what degree the local cloud environment influences the D/H ratio of water in the hot corinos toward low-mass protostars and establish which physical and chemical conditions can reproduce the observed HDO/H2O and D2O/HDO ratios in hot corinos. Methods. The evolution of water during star formation is modeled using 3D physicochemical models of a dynamic star-forming environment. The physical evolution during the protostellar collapse is described by tracer particles from a 3D MHD simulation of a molecular cloud region. Each particle trajectory is post-processed using RADMC-3D to calculate the temperature and radiation field. The chemical evolution is simulated using a three-phase grain-surface chemistry model and the results are compared with interferometric observations of H2O, HDO, and D2O in hot corinos toward low-mass protostars. Results. The physicochemical model reproduces the observed HDO/H2O and D2O/HDO ratios in hot corinos, but shows no correlation with cloud environment when similar initial conditions are tested. The observed dichotomy in water D/H ratios requires variation in the initial conditions, for example the duration and temperature of the prestellar phase. Reproducing the observed D/H ratios in hot corinos requires a prestellar phase duration t ~ 1−3 Myr and temperatures in the range T ~ 10−20 K prior to collapse. Furthermore, high cosmic-ray ionization rates (ξH2 ~ 10−15 s−1) appear to be incompatible with the observed D/H ratios toward low-mass protostars. Conclusions. This work demonstrates that the observed differentiation between clustered and isolated protostars stems from differences in the molecular cloud or prestellar core conditions and does not arise during the protostellar collapse itself. The observed D/H ratios for water in hot corinos are consistent with chemical inheritance of water, and no resetting during the protostellar collapse, providing a direct link between the prestellar chemistry and the hot corino.

Fast methods for tracking grain coagulation and ionization

Dust grains play a major role in many astrophysical contexts. They affect the chemical, magnetic, dynamical, and optical properties of their environment, from galaxies down to the interstellar medium, star-forming regions, and protoplanetary disks. Their coagulation leads to shifts in their size distribution and ultimately to the formation of planets. However, although the coagulation process is reasonably uncomplicated to numerically implement by itself, it is difficult to couple it with multidimensional hydrodynamics numerical simulations because of its high computational cost. We propose here a simple method for tracking the coagulation of grains at far lower cost. Given an initial grain size distribution, the state of the distribution at time t is solely determined by the value of a single variable integrated along the trajectory, independently of the specific path taken by the grains. Although this method cannot account for processes other than coagulation, it is mathematically exact, fast, inexpensive, and can be used to evaluate the effect of grain coagulation in most astrophysical contexts. It is applicable to all coagulation kernels in which local physical conditions and grain properties can be separated. We also describe another method for calculating the average electric charge of grains and the density of ions and electrons in environments that are shielded from radiation fields, given the density and temperature of the gas, the cosmic-ray ionization rate, and the average mass of the ions. The equations we provide are fast to integrate numerically and can be used in multidimensional numerical simulations to self-consistently calculate on the fly the local resistivities that are required to model nonideal magnetohydrodynamics.

Submillimeter imaging of the Galactic Center starburst Sgr B2

Context. Star-forming galaxies emit bright molecular and atomic lines in the submillimeter and far-infrared (FIR) domains. However, it is not always clear which gas heating mechanisms dominate and which feedback processes drive their excitation. Aims. The Sgr B2 complex is an excellent template to spatially resolve the main OB-type star-forming cores from the extended cloud environment and to study the properties of the warm molecular gas in conditions likely prevailing in distant extragalactic nuclei. Methods. We present 168 arcmin2 spectral images of Sgr B2 taken with Herschel/SPIRE-FTS in the complete ~450−1545 GHz band. We detect ubiquitous emission from mid-J CO (up to J = 12−11), H2O 21,1−20,2, [C I] 492, 809 GHz, and [N II] 205 μm lines. We also present velocity-resolved maps of the SiO (2−1), N2H+, HCN, and HCO+ (1−0) emission obtained with the IRAM 30 m telescope. Results. The cloud environment (~1000 pc2 around the main cores) dominates the emitted FIR (~80%), H2O 752 GHz (~60%) mid-J CO (~91%), [C I] (~93%), and [N II] 205 μm (~95%) luminosity. The region shows very extended [N II] 205 μm emission (spatially correlated with the 24 and 70 μm dust emission) that traces an extended component of diffuse ionized gas of low ionization parameter (U ≃ 10−3) and low LFIR / MH2 ≃ 4−11 L⊙M⊙−1 ratios (scaling as ∝Tdust6). The observed FIR luminosities imply a flux of nonionizing photons equivalent to G0 ≈ 103. All these diagnostics suggest that the complex is clumpy and this allows UV photons from young massive stars to escape from their natal molecular cores. The extended [C I] emission arises from a pervasive component of neutral gas with nH ≃ 103 cm−3. The high ionization rates in the region, produced by enhanced cosmic-ray (CR) fluxes, drive the gas heating in this component to Tk ≃ 40−60 K. The mid-J CO emission arises from a similarly extended but more pressurized gas component (Pth / k ≃ 107 K cm−3): spatially unresolved clumps, thin sheets, or filaments of UV-illuminated compressed gas (nH ≃ 106 cm−3). Specific regions of enhanced SiO emission and high CO-to-FIR intensity ratios (ICO / IFIR ≳ 10−3) show mid-J CO emission compatible with C-type shock models. A major difference compared to more quiescent star-forming clouds in the disk of our Galaxy is the extended nature of the SiO and N2H+ emission in Sgr B2. This can be explained by the presence of cloud-scale shocks, induced by cloud-cloud collisions and stellar feedback, and the much higher CR ionization rate (>10−15 s−1) leading to overabundant H3+ and N2H+. Conclusions. Sgr B2 hosts a more extreme environment than star-forming regions in the disk of the Galaxy. As a usual template for extragalactic comparisons, Sgr B2 shows more similarities to nearby ultra luminous infrared galaxies such as Arp 220, including a “deficit” in the [C I] / FIR and [N II] / FIR intensity ratios, than to pure starburst galaxies such as M 82. However, it is the extended cloud environment, rather than the cores, that serves as a useful template when telescopes do not resolve such extended regions in galaxies.

OH in the diffuse interstellar medium: physical modelling and prospects with upcoming SKA precursor/pathfinder surveys

ABSTRACT Hydroxyl ($\rm OH$) is known to form efficiently in cold gas (T ∼ 100 K) along with the molecule $\rm H_2$ and can be used as an efficient tracer of the diffuse molecular gas in the interstellar medium (ISM). Using a simple formalism describing the $\rm H\, I/H_2$ transition and a reduced network of major chemical reactions, we present a semi-analytical prescription to estimate the abundances of O-bearing molecules in the diffuse ISM. We show that predictions based on our prescription are in good agreement with the estimates obtained using the meudon pdr code which utilizes the full reaction network. We investigate the dependence of the relative abundances of $\rm OH/H\, I$ and $\rm OH/H_2$ on the variations of physical conditions i.e. the metallicity, number density (n), cosmic ray ionization rate (ζ), and strength of UV field (χ) in the medium. We find that the $\rm OH/H\, I$ abundances observed in the Galactic ISM can be reproduced by models with n ∼ 50 cm−3, χ ∼ 1 (Mathis field), and ζ ∼ 3 × 10−17 s−1, with a variation of about 1 dex allowed around these values. Using the constrained $\rm H_2$ column density distribution function at z ∼ 3, we estimate the $\rm OH$ column density distribution function and discuss future prospects with the upcoming large radio absorption line surveys.

Chemical modeling of the complex organic molecules in the extended region around Sagittarius B2

Context. The chemical differentiation of seven complex organic molecules (COMs) in the extended region around Sagittarius B2 (Sgr B2) has been previously observed: CH2OHCHO, CH3OCHO, t-HCOOH, C2H5OH, and CH3NH2 were detected both in the extended region and near the hot cores Sgr B2(N) and Sgr B2(M), while CH3OCH3 and C2H5CN were only detected near the hot cores. The density and temperature in the extended region are relatively low in comparison with Sgr B2(N) and Sgr B2(M). Different desorption mechanisms, including photodesorption, reactive desorption, and shock heating, and a few other mechanisms have been proposed to explain the observed COMs in the cold regions. However, they fail to explain the deficiency of CH3OCH3 and C2H5CN in the extended region around Sgr B2. Aims. Based on known physical properties of the extended region around Sgr B2, we explored under what physical conditions the chemical simulations can fit the observations and explain the different spatial distribution of these seven species in the extended region around Sgr B2. Methods. We used the macroscopic Monte Carlo method to perform a detailed parameter space study. A static physical model and an evolving physical model including a cold phase and a warm-up phase were used, respectively. The fiducial models adopt the observed physical parameters except for the local cosmic ray ionization rate ζCR. In addition to photodesorption that is included in all models, we investigated how chain reaction mechanism, shocks, an X-ray burst, enhanced reactive desorption and low diffusion barriers could affect the results of chemical modeling. Results. All gas-grain chemical models based on static physics cannot fit the observations, except for the high abundances of CH3NH2 and C2H5CN in some cases. The simulations based on evolving physical conditions can fit six COMs when T ~ 30−60 K in the warm-up phase, but the best-fit temperature is still higher than the observed dust temperature of 20 K. The best agreement between the simulations and all seven observed COMs at a lower temperature T ~ 27 K is achieved by considering a short-duration ≈102 yr X-ray burst with ζCR = 1.3 × 10−13 s−1 at the early stage of the warm-up phase when it still has a temperature of 20 K. The reactive desorption is the key mechanism for producing these COMs and inducing the low abundances of CH3OCH3 and C2H5CN. Conclusions. We conclude that the evolution of the extended region around Sgr B2 may have begun with a cold, T ≤ 10 K phase followed by a warm-up phase. When its temperature reached about T ~ 20 K, an X-ray flare from the Galactic black hole Sgr A* with a short duration of no more than 100 yr was acquired, affecting strongly the Sgr B2 chemistry. The observed COMs in Sgr B2 are able to retain their observed abundances only several hundred years after such a flare, which could imply that such short-term X-rays flares occur relatively often, likely associated with the accretion activity of the Sgr A* source.

Comparison between PHITS and GEANT4 Simulations of the Heavy Ion Beams at the BEVALAC at LBNL and the Booster Accelerator at BNL

Heavy charged particles have been discussed for clinical use due to their superior dose-depth distribution compared to conventional radiation such as X-rays. In addition, high-charge and energy (HZE) ions in galactic cosmic rays (GCR) present important health risks for crewed space missions to the Earth's moon or Mars. Experiments at heavy ion accelerators are used in radiobiology studies; however, numerical simulations of track segment or Bragg peak irradiations are complicated by the details of the beam-line and dosimetry systems. The goal of the present work is in support of biophysics modeling of historical radiobiology data at Lawrence Berkeley National Laboratory (LBNL) and more recent results from the Brookhaven National Lab (BNL) facility (NASA Space Radiation Lab (NSRL)). In this work, the Spread-Out Bragg Peak (SOBP) of 4He, 12C, and 20Ne particles, and a Bragg curve of 56Fe ion have been simulated numerically in the geometries of LBNL and BNL using the Monte-Carlo based PHITS and GEANT4 simulation toolkits. The dose contributions of primary particles and secondary particles, including neutrons and photons, in the target material are computed and discussed as well. Comparisons suggest more contributions of secondaries in GEANT4 simulations compared to PHITS simulations, and less statistical fluctuation and better prediction of neutrons in PHITS simulations. Neutrons and gamma-rays are estimated to make minor contributions to absorbed doses for these beams.

Rigorous Theory for Secondary Cosmic-Ray Ionization

Abstract The energy spectrum of electrons produced in molecular gas by interstellar cosmic rays (CRs) is rigorously calculated as a function of gas column density N traversed by the CRs. This allows us to accurately compute the local value of the secondary ionization rate of molecular hydrogen, , as a function of the local primary ionization rate, ζ p (N). The ratio increases monotonically with N, and can considerably exceed the value of ≈0.67 commonly adopted in the literature. For sufficiently soft interstellar spectra, the dependence versus N is practically insensitive to their particular shape and thus is a general characteristic of the secondary CR ionization in dense gas.

Role of Polycyclic Aromatic Hydrocarbons on the Cosmic-Ray Ionization Rate in the Galaxy

Abstract The cosmic-ray (CR) ionization rate (ζ, s−1) plays an important role in the interstellar medium. It controls ion–molecular chemistry and provides a source of heating. Here we perform a grid of calculations using the spectral synthesis code CLOUDY along nine sightlines toward, HD 169454, HD 110432, HD 204827, λ Cep, X Per, HD 73882, HD 154368, Cyg OB2 5, and Cyg OB2 12. The value of ζ is determined by matching the observed column densities of H 3 + and H2. The presence of polycyclic aromatic hydrocarbons (PAHs) affects the free electron density, which changes the H 3 + density and the derived ionization rate. PAHs are ubiquitous in the Galaxy, but there are also regions where PAHs do not exist. Hence, we consider clouds within a range of PAH abundances and show their effects on the H 3 + abundance. We predict an average CR ionization rate for H2 (ζ(H2)) = (7.88 ± 2.89) × 10−16 s−1 for models with average Galactic PAH abundances, (PAH/H = 10−6.52), except Cyg OB2 5 and Cyg OB2 12. The value of ζ is nearly 1 dex smaller for sightlines toward Cyg OB2 12. We estimate the average value of ζ(H2) = (95.69 ± 46.56) × 10−16 s−1 for models without PAHs.

Gas phase Elemental abundances in Molecular cloudS (GEMS)

Context. Carbon monosulphide (CS) is among the most abundant gas-phase S-bearing molecules in cold dark molecular clouds. It is easily observable with several transitions in the millimeter wavelength range, and has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. However, chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur. Aims. The CS+O → CO + S reaction has been proposed as a relevant CS destruction mechanism at low temperatures, and could explain the discrepancy between models and observations. Its reaction rate has been experimentally measured at temperatures of 150−400 K, but the extrapolation to lower temperatures is doubtful. Our goal is to calculate the CS+O reaction rate at temperatures <150 K which are prevailing in the interstellar medium. Methods. We performed ab initio calculations to obtain the three lowest potential energy surfaces (PES) of the CS+O system. These PESs are used to study the reaction dynamics, using several methods (classical, quantum, and semiclassical) to eventually calculate the CS + O thermal reaction rates. In order to check the accuracy of our calculations, we compare the results of our theoretical calculations for T ~ 150−400 K with those obtained in the laboratory. Results. Our detailed theoretical study on the CS+O reaction, which is in agreement with the experimental data obtained at 150–400 K, demonstrates the reliability of our approach. After a careful analysis at lower temperatures, we find that the rate constant at 10 K is negligible, below 10−15 cm3 s−1, which is consistent with the extrapolation of experimental data using the Arrhenius expression. Conclusions. We use the updated chemical network to model the sulfur chemistry in Taurus Molecular Cloud 1 (TMC 1) based on molecular abundances determined from Gas phase Elemental abundances in Molecular CloudS (GEMS) project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate, ζH2, along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundance.

Photodissociation region diagnostics across galactic environments

ABSTRACT We present three-dimensional astrochemical simulations and synthetic observations of magnetized, turbulent, self-gravitating molecular clouds. We explore various galactic interstellar medium environments, including cosmic ray ionization rates in the range of ζCR = 10−17–$10^{-14}\, {\rm s}^{-1}$, far-UV intensities in the range of G0 = 1–103 and metallicities in the range of Z = 0.1–$2\, {\rm Z}_{\odot }$. The simulations also probe a range of densities and levels of turbulence, including cases where the gas has undergone recent compression due to cloud–cloud collisions. We examine: (i) the column densities of carbon species across the cycle of C ii, C i, and CO, along with O i, in relation to the H i-to-H2 transition; (ii) the velocity-integrated emission of [C ii] 158 μm, [13C ii] 158 μm, [C i] 609 μm and 370 μm, [O i] 63 μm and 146 μm, and of the first ten 12CO rotational transitions; (iii) the corresponding Spectral Line Energy Distributions; (iv) the usage of [C ii] and [O i] 63 μm to describe the dynamical state of the clouds; (v) the behaviour of the most commonly used ratios between transitions of CO and [C i]; and (vi) the conversion factors for using CO and C i as H2-gas tracers. We find that enhanced cosmic ray energy densities enhance all aforementioned line intensities. At low metallicities, the emission of [C ii] is well connected with the H2 column, making it a promising new H2 tracer in metal-poor environments. The conversion factors of XCO and XC i depend on metallicity and the cosmic ray ionization rate, but not on FUV intensity. In the era of ALMA, SOFIA, and the forthcoming CCAT-prime telescope, our results can be used to understand better the behaviour of systems in a wide range of galactic and extragalactic environments.

Deuterium chemodynamics of massive pre-stellar cores

ABSTRACT High levels of deuterium fractionation of N2H+ (i.e. $\mathrm{D^{{\mathrm{N_2H^+}}}_{\text{frac}}}$≳ 0.1) are often observed in pre-stellar cores (PSCs) and detection of N2D+ is a promising method to identify elusive massive PSCs. However, the physical and chemical conditions required to reach such high levels of deuteration are still uncertain, as is the diagnostic utility of N2H+ and N2D+ observations of PSCs. We perform 3D magnetohydrodynamics simulations of a massive, turbulent, magnetized PSC, coupled with a sophisticated deuteration astrochemical network. Although the core has some magnetic/turbulent support, it collapses under gravity in about one free-fall time, which marks the end of the simulations. Our fiducial model achieves relatively low $\mathrm{D^{{\mathrm{N_2H^+}}}_{\text{frac}}}$∼0.002 during this time. We then investigate effects of initial ortho-para ratio of H2 ($\mathrm{OPR^{H_2}}$), temperature, cosmic ray (CR) ionization rate, CO and N-species depletion factors, and prior PSC chemical evolution. We find that high CR ionization rates and high depletion factors allow the simulated $\mathrm{D^{{\mathrm{N_2H^+}}}_{\text{frac}}}$ and absolute abundances to match observational values within one free-fall time. For $\mathrm{OPR^{H_2}}$, while a lower initial value helps the growth of $\mathrm{D^{{\mathrm{N_2H^+}}}_{\text{frac}}}$, the spatial structure of deuteration is too widespread compared to observed systems. For an example model with elevated CR ionization rates and significant heavy element depletion, we then study the kinematic and dynamic properties of the core as traced by its N2D+ emission. The core, undergoing quite rapid collapse, exhibits disturbed kinematics in its average velocity map. Still, because of magnetic support, the core often appears kinematically subvirial based on its N2D+ velocity dispersion.

Monte Carlo simulation of a satellite-based detector of cosmic ray ions with elemental separation from hydrogen to nickel

Observing cosmic rays and solar energetic particles provides important information about the solar wind and solar storms. We are working toward building a Thai detector of energetic ions from space that is inspired by the Mass Spectrometer Telescope (MAST), a satellite-based detector installed on SAMPEX of cosmic ray ions with the energy range of 0.01 to 0.5 GeV/nucleon with the objective to determine the nuclear charge and kinetic energy for nuclei with atomic number of Z = 1 (hydrogen) to 28 (nickel). The energy range of the detector covers the energy range of anomalous cosmic rays, Galactic cosmic rays and solar energetic particles. Therefore, observations by such a detector can provide insight into the origin and transport of these ions in cosmic rays and solar energetic particles. During solar events, measurements of the elemental composition of solar energetic particles can be compared with the composition of the solar corona. We have created a computational model of the MAST detector to simulate ion detection using the FLUKA program to measure the energy deposition of heavy ions in multiple detector components to determine their initial kinetic energy in the simulation. From the simulation results, we can identify the ions’ energies and species using established methods and calculate the geometry factor of the detector, comparing the results with previous reports.

Preliminary results from a feasibility test of mixed alpha source detection by prototype bare silicon devices

A project on development of a satellite-borne silicon-based cosmic ray ion detector was started. Semiconductor detectors based on P-I-N junctions are widely used in space experiments due to their light weight, compact design and high stopping power. The position-sensitive parts of the detector are designed to be silicon-based detectors. A prototype silicon detector without the outermost layer of insulators was prepared. The detector is expected to have the potential to identify the elements of high energy ions using the ΔE-E technique. Thus, for the beginning step of this project, a detector response proportional to the deposited energy has to be confirmed. We set up a detection system of a mixed standard alpha source including Am-241, Pu239 and Cm-244 with our devices and a photodiode (Hamamatsu S1223 with glass removed). The detector response is indicated by the digitized signal pulse height distribution from a 2048-channel ADC. The feasibility of our devices to distinguish the difference of energy in terms of several hundred keV was confirmed. A linear relation between the pulse height peaks and the emitted energy of each radiation source was also indicated.

Seeds of Life in Space (SOLIS)

Context. The isotopic ratio of nitrogen presents a wide range of values in the Solar System: from ~140 in meteorites and comets to 441 in the solar wind. In star-forming systems, we observe even a higher spread of ~150–1000. The origin of these differences is still unclear. Aims. Chemical reactions in the gas phase are one of the possible processes that could modify the 14N/15N ratio. We aim to investigate if and how the passage of a shock wave in the interstellar medium, which activates a rich chemistry, can affect the relative fraction of nitrogen isotopes. The ideal place for such a study is the chemically rich outflow powered by the L1157-mm protostar, where several shocked clumps are present. Methods. We present the first measurement of the 14N/15N ratio in the two shocked clumps, B1 and B0, of the protostellar outflow L1157. The measurement is derived from the interferometeric maps of the H13CN (1–0) and the HC15N (1–0) lines obtained with the NOrthern Extended Millimeter Array (NOEMA) interferometer as part of the Seeds of Life in Space (SOLIS) programme. Results. In B1, we find that the H13CN (1–0) and HC15N (1–0) emission traces the front of the clump, that is the apex of the shocked region, where the fast jet impacts the lower velocity medium with an averaged column density of N(H13CN) ~ 7 × 1012 cm−2 and N(HC15N) ~ 2 × 1012 cm−2. In this region, the ratio H13CN (1–0)/HC15N (1–0) is almost uniform with an average value of ~5 ± 1. The same average value is also measured in the smaller clump B0e. Assuming the standard 12C/13C = 68, we obtain 14N/15N = 340 ± 70. This ratio is similar to those usually found with the same species in prestellar cores and protostars. We analysed the prediction of a chemical shock model for several shock conditions and we found that the nitrogen and carbon fractionations do not vary much for the first period after the shock. The observed H13CN/HC15N can be reproduced by a non-dissociative, C-type shock with pre-shock density n(H) = 105 cm−3, shock velocity Vs between 20 and 40 km s−1, and cosmic-ray ionization rate of 3 × 10−16 s−1; this agrees with previous modelling of other chemical species in L1157-B1. Conclusions. Both observations and chemical models indicate that the rich chemistry activated by the shock propagation does not affect the nitrogen isotopic ratio, which remains similar to that measured in lower temperature gas in prestellar cores and protostellar envelopes.

Non-linear diffusive shock acceleration: A recipe for injection of electrons

Prescriptions for electron injection into the diffusive shock acceleration process are required in many practical considerations of cosmic-ray astrophysics, particularly in modeling of the synchrotron emission of astrophysical sources. In particle-in-cell simulations of quasi-parallel magnetized collisionless shocks, we analyse the evolution of particle spectra. We find that in the later stages of shock evolution, the initially strong suprathermal part in the ion spectra fades, thus leaving the spectra composed of a Maxwellian and a power law. Once the electron and ion spectra flatten and become parallel, we find that the amounts of cosmic ray ions and electrons become similar. We make the step towards relating the micro and macro-scale physics by applying this injection rule to Blasi’s semi-analytical model of non-linear diffusive shock acceleration, in order to obtain the particle spectra and electron-to-proton ratio Kep at high energies. By using shock jump conditions that include the electron heating, we find Kep as a function of Mach number. For Mach number ∼ 100, our model finely reproduce the typically observed ratio for Galactic cosmic-rays Kep∼ 1:100 in the test particle regime.

Constraining of physical conditions in the cold neutral medium using HD/H2 relative abundance

We present constraints on physical conditions in diffuse HD/H2-bearing molecular clouds using measured HD/H2 column density ratio. We used recently published formalism that describes how this ratio changes with ultraviolet field intensity, number density and cosmic ray ionization rate. Out of them the most important parameter is the cosmic ray ionization rate, as it determines the chemistry in neutral medium and is still poorly constrained in ISM. Using the result of absorption line analysis in our Galaxy and in several high-redshift systems we found that value of cosmic ray ionization rate is in our Galaxy and is few × 10−18 s−1 in DLAs at high redshifts.

Survey of ortho-H2D+ in high-mass star-forming regions

Context. Deuteration has been suggested to be a reliable chemical clock of star-forming regions due to its strong dependence on density and temperature changes during cloud contraction. In particular, the H3+ isotopologues (e.g. ortho-H2D+) seem to act as good proxies of the evolutionary stages of the star formation process. While this has been widely explored in low-mass star-forming regions, in the high-mass counterparts only a few studies have been pursued, and the reliability of deuteration as a chemical clock remains inconclusive. Aims. We present a large sample of o-H2D+ observations in high-mass star-forming regions and discuss possible empirical correlations with relevant physical quantities to assess its role as a chronometer of star-forming regions through different evolutionary stages. Methods. APEX observations of the ground-state transition of o-H2D+ were analysed in a large sample of high-mass clumps selected from the ATLASGAL survey at different evolutionary stages. Column densities and beam-averaged abundances of o-H2D+ with respect to H2, X(o-H2D+), were obtained by modelling the spectra under the assumption of local thermodynamic equilibrium. Results. We detect 16 sources in o-H2D+ and find clear correlations between X(o-H2D+) and the clump bolometric luminosity and the dust temperature, while only a mild correlation is found with the CO-depletion factor. In addition, we see a clear correlation with the luminosity-to-mass ratio, which is known to trace the evolution of the star formation process. This would indicate that the deuterated forms of H3+ are more abundant in the very early stages of the star formation process and that deuteration is influenced by the time evolution of the clumps. In this respect, our findings would suggest that the X(o-H2D+) abundance is mainly affected by the thermal changes rather than density changes in the gas. We have employed these findings together with observations of H13CO+, DCO+, and C17O to provide an estimate of the cosmic-ray ionisation rate in a sub-sample of eight clumps based on recent analytical work. Conclusions. Our study presents the largest sample of o-H2D+ in star-forming regions to date. The results confirm that the deuteration process is strongly affected by temperature and suggests that o-H2D+ can be considered a reliable chemical clock during the star formation processes, as proved by its strong temporal dependence.

The Environmental Dependence of the XCO Conversion Factor

Abstract is the most widely used observational tracer of molecular gas. The observable luminosity is translated to mass via a conversion factor, , which is a source of uncertainty and bias. Despite variations in , the empirically determined solar neighborhood value is often applied across different galactic environments. To improve understanding of , we employ 3D magnetohydrodynamics simulations of the interstellar medium (ISM) in galactic disks with a large range of gas surface densities, allowing for varying metallicity, far-ultraviolet (FUV) radiation, and cosmic-ray ionization rate (CRIR). With the TIGRESS simulation framework we model the three-phase ISM with self-consistent star formation and feedback, and post-process outputs with chemistry and radiation transfer to generate synthetic CO (1–0) and (2–1) maps. Our models reproduce the observed CO excitation temperatures, line widths, and line ratios in nearby disk galaxies. decreases with increasing metallicity, with a power-law slope of −0.8 for the (1–0) line and −0.5 for the (2–1) line. also decreases at higher CRIR and is insensitive to the FUV radiation. As density increases, first decreases owing to increasing excitation temperature and then increases when the emission is fully saturated. We provide fits between and observable quantities such as the line ratio, peak antenna temperature, and line brightness, which probe local gas conditions. These fits, which allow for varying beam size, may be used in observations to calibrate out systematic biases. We also provide estimates of the CO-dark fraction at different gas surface densities, observational sensitivities, and beam sizes.

Extending the view of ArH+chemistry in diffuse clouds

Context.One of the surprises of theHerschelmission was the detection of ArH+towards the Crab Nebula in emission and in absorption towards strong Galactic background sources. Although these detections were limited to the first quadrant of the Galaxy, the existing data suggest that ArH+ubiquitously and exclusively probes the diffuse atomic regions of the interstellar medium.Aims.In this study, we extend the coverage of ArH+to other parts of the Galaxy with new observations of itsJ= 1−0 transition along seven Galactic sight lines towards bright sub-millimetre continuum sources. We aim to benchmark its efficiency as a tracer of purely atomic gas by evaluating its correlation (or lack of correlation as suggested by chemical models) with other well-known atomic gas tracers such as OH+and H2O+and the molecular gas tracer CH.Methods.The observations of theJ= 1−0 line of ArH+near 617.5 GHz were made feasible with the new, sensitive SEPIA660 receiver on the APEX 12 m telescope. Furthermore, the two sidebands of this receiver allowed us to observe theNKaKc= 11,0−10,1transitions of para-H2O+at 607.227 GHz simultaneously with the ArH+line.Results.We modelled the optically thin absorption spectra of the different species and subsequently derived their column densities. By analysing the steady state chemistry of OH+and o-H2O+, we derive on average a cosmic-ray ionisation rate,ζp(H), of (2.3 ± 0.3) × 10−16s−1towards the sight lines studied in this work. Using the derived values ofζp(H) and the observed ArH+abundances we constrain the molecular fraction of the gas traced by ArH+to lie below 2 × 10−2with a median value of 8.8 × 10−4. Combined, our observations of ArH+, OH+, H2O+, and CH probe different regimes of the interstellar medium, from diffuse atomic to diffuse and translucent molecular clouds. Over Galactic scales, we see that the distribution ofN(ArH+) is associated with that ofN(H), particularly in the inner Galaxy (within 7 kpc of the Galactic centre) with potentially even contributions from the warm neutral medium phase of atomic gas at larger galactocentric distances. We derive an average ortho-to-para ratio for H2O+of 2.1 ± 1.0, which corresponds to a nuclear spin temperature of 41 K, consistent with the typical gas temperatures of diffuse clouds.

Exclusion of Cosmic Rays from Molecular Clouds by Self-generated Electric Fields

Abstract It was recently discovered that in some regions of the Galaxy, the cosmic-ray (CR) abundance is several orders of magnitude higher than previously thought. Additionally, there is evidence that in molecular cloud envelopes, the CR ionization may be dominated by electrons. We show that for regions with high, electron-dominated ionization, the penetration of CR electrons into molecular clouds is modulated by the electric field that develops as a result of the charge they deposit. We evaluate the significance of this novel mechanism of self-modulation and show that the CR penetration can be reduced by a factor of a few to a few hundred in high-ionization environments, such as those found near the Galactic center.