C-type Shocks Research Articles

Shocks in the warm neutral medium

Context. Atomic and molecular line emissions from shocks may provide valuable information on the injection of mechanical energy into the interstellar medium (ISM), the generation of turbulence, and the processes of phase transition between the warm neutral medium (WNM) and the cold neutral medium (CNM). Aims. In this series of papers, we investigate the properties of shocks propagating in the WNM. Our objective is to identify the tracers of these shocks, use them to interpret ancillary observations of the local diffuse matter, and provide predictions for future observations. Methods. Shocks propagating in the WNM are studied using the Paris-Durham shock code, a multi-fluid model built to follow the thermodynamical and chemical structures of shock waves at steady-state in a plane-parallel geometry. The code, designed to take into account the impact of an external radiation field, is updated to treat self-irradiated shocks at intermediate (30 < VS < 100 km s−1) and high velocity (VS ⩾ 100 km s−1), which emit ultraviolet (UV), extreme-ultraviolet (EUV), and X-ray photons. The couplings between the photons generated by the shock, the radiative precursor, and the shock structure are computed self-consistently using an exact radiative-transfer algorithm for line emission. The resulting code is explored over a wide range of parameters (0.1 ⩽ nH ⩽ 2 cm−3, 10 ⩽ VS ⩽ 500 km s−1, and 0.1 ⩽ B ⩽ 10 μG), which covers the typical conditions of the WNM in the solar neighborhood. Results. The explored physical conditions lead to the existence of a diversity of stationary magnetohydrodynamic solutions, including J-type, CJ-type, and C-type shocks. These shocks are found to naturally induce phase transition between the WNM and the CNM, provided that the postshock thermal pressure is higher than the maximum pressure of the WNM and that the maximum density allowed by magnetic compression is greater than the minimum density of the CNM. The input flux of mechanical energy is primarily reprocessed into line emissions from the X-ray to the submillimeter domain. Intermediate- and high-velocity shocks are found to generate a UV radiation field that scales as VS3 for VS < 100 km s−1 and as VS2 at higher velocities, and an X-ray radiation field that scales as VS3 for VS ⩾ 100 km s−1. Both radiation fields may extend over large distances in the preshock depending on the density of the surrounding medium and the hardness of the X-ray field, which is solely driven by the shock velocity. Conclusions. This first paper presents the thermochemical trajectories of shocks in the WNM and their associated spectra. It corresponds to a new milestone in the development of the Paris-Durham shock code and a stepping stone for the analysis of observations that will be carried out in forthcoming works.

Maser flares driven by isothermal shock waves

ABSTRACT We use 3D computer modelling to investigate the time-scales and radiative output from maser flares generated by the impact of shock waves on astronomical unit-scale clouds in interstellar and star-forming regions, and in circumstellar regions in some circumstances. Physical conditions are derived from simple models of isothermal hydrodynamic (single-fluid) and C-type (ionic and neutral fluid) shock waves, and based on the ortho-H2O 22-GHz transition. Maser saturation is comprehensively included, and we find that the most saturated maser inversions are found predominantly in the shocked material. We study the effect on the intensity, flux density, and duration of flares of the following parameters: the pre-shock level of saturation, the observer’s viewpoint, and the shock speed. Our models are able to reproduce observed flare rise times of a few times 10 d, specific intensities of up to 105 times the saturation intensity and flux densities of order 100(R/d)2 Jy from a source of radius R astronomical units at a distance of d kiloparsec. We found that flares from C-type shocks are approximately five times more likely to be seen by a randomly placed observer than flares from hydrodynamically shocked clouds of similar dimensions. We computed intrinsic beaming patterns of the maser emission, finding substantial extension of the pattern parallel to the shock front in the hydrodynamic models. Beaming solid angles for hydrodynamic models can be as small as 1.3 × 10−5 sr, but are an order of magnitude larger for C-type models.

Laboratory investigation of shock-induced dissociation of buckminsterfullerene and astrophysical insights

Fullerene C60 is one of the most iconic forms of carbon found in the interstellar medium (ISM). The interstellar chemistry of carbon-rich components, including fullerenes, is driven by a variety of energetic processes including UV and X-ray irradiation, cosmic-ray (CR) bombardment, electron impact, and shock waves. These violent events strongly alter the particle phase and lead to the release of new molecular species in the gas phase. Only a few experimental studies on the shock processing of cosmic analogs have been conducted so far. We explored in the laboratory the destruction of buckminsterfullerene C60 using a pressure-driven shock tube coupled with optical diagnostics. Our efforts were first devoted to probing in situ the shock-induced processing of C60 at high temperatures (≤ 4500 K) by optical emission spectroscopy. The analysis of the spectra points to the massive production of C2 units. A broad underlying continuum was observed as well and was attributed to the collective visible emission of carbon clusters, generated similarly in large amounts. This proposed assignment was performed with the help of calculated emission spectra of various carbon clusters. The competition between dissociation and radiative relaxation, determined by statistical analysis, alludes to a predominance of clusters with less than 40 carbon atoms. Our laboratory experiments, supported by molecular dynamics simulations performed in the canonical ensemble, suggest that C60 is very stable, and that high-energy input is required to process it under interstellar low-density conditions and to produce C2 units and an abundance of intermediate-sized carbon clusters. These results provide some insights into the life cycle of carbon in space. Our findings hint that only J-type shocks with velocities above ~100 km s−1 or C-type shocks with velocities above 9 km s−1 can lead to the destruction of fullerenes. Observational tracers of this process remain elusive, however. Our work confirms the potential of shock tubes for laboratory astrophysics.

Shock excitation of H2 in the James Webb Space Telescope era

Context. Molecular hydrogen, H2, is the most abundant molecule in the Universe. Thanks to its widely spaced energy levels, it predominantly lights up in warm gas, T ≳ 102 K, such as shocked regions externally irradiated or not by interstellar UV photons, and it is one of the prime targets of James Webb Space Telescope (JWST) observations. These may include shocks from protostellar outflows, supernova remnants impinging on molecular clouds, all the way up to starburst galaxies and active galactic nuclei. Aims. Sophisticated shock models are able to simulate H2 emission from such shocked regions. We aim to explore H2 excitation using shock models, and to test over which parameter space distinct signatures are produced in H2 emission. Methods. We here present simulated H2 emission using the Paris-Durham shock code over an extensive grid of ~14 000 plane-parallel stationary shock models, a large subset of which are exposed to a semi-isotropic external UV radiation field. The grid samples six input parameters: the preshock density, shock velocity, transverse magnetic field strength, UV radiation field strength, the cosmic-ray-ionization rate, and the abundance of polycyclic aromatic hydrocarbons, PAHs. Physical quantities resulting from our self-consistent calculations, such as temperature, density, and width, have been extracted along with H2 integrated line intensities. These simulations and results are publicly available on the Interstellar Medium Services platform. Results. The strength of the transverse magnetic field, as quantified by the magnetic scaling factor, b, plays a key role in the excitation of H2. At low values of b (≲0.3, J-type shocks), H2 excitation is dominated by vibrationally excited lines; whereas, at higher values (b ≳ 1, C-type shocks), rotational lines dominate the spectrum for shocks with an external radiation field comparable to (or lower than) the solar neighborhood. Shocks with b ≥ 1 can potentially be spatially resolved with JWST for nearby objects. H2 is typically the dominant coolant at lower densities (≲104 cm−3); at higher densities, other molecules such as CO, OH, and H2O take over at velocities ≲20 km s−1 and atoms, for example, H, O, and S, dominate at higher velocities. Together, the velocity and density set the input kinetic energy flux. When this increases, the excitation and integrated intensity of H2 increases similarly. An external UV field mainly serves to increase the excitation, particularly for shocks where the input radiation energy is comparable to the input kinetic energy flux. These results provide an overview of the energetic reprocessing of input kinetic energy flux and the resulting H2 line emission.

The Unusual AGN Host NGC 1266: Evidence for Shocks in a Molecular Gas Rich S0 Galaxy with a Low Luminosity Nucleus

NGC 1266 is a lenticular galaxy (S0) hosting an active galactic nucleus (AGN), and known to contain a large amount of shocked gas. We compare the luminosity ratio of mid-J CO lines to IR continuum with star-forming galaxies (SFGs), and then model the CO spectral line energy distribution (SLED). We confirm that in the mid- and high-J regions (J up = 4–13), the C-type shock (v s = 25 km s−1, n H = 5 × 104 cm−3) can reproduce the CO observations well. The galaxy spectral energy distribution (SED) is constructed and modeled by the code X-CIGALE and obtains a set of physical parameters including the star formation rate (SFR, 1.17 ± 0.47 M ⊙ yr−1). Also, our work provides SFR derivation of [C ii] from the neutral hydrogen regions only (1.38 ± 0.14 M ⊙yr−1). Previous studies have illusive conclusions on the AGN or starburst nature of the NGC 1266 nucleus. Our SED model shows that the hidden AGN in the system is intrinsically low-luminosity, consequently the infrared luminosity of the AGN does not reach the expected level. Archival data from NuSTAR hard X-ray observations in the 3–79 keV band shows a marginal detection, disfavoring presence of an obscured luminous AGN and implying that a compact starburst is more likely dominant for the NGC 1266 nucleus.

Modelling grain-size distributions in C-type shocks using a discrete power-law model

In this paper we discuss the implementation of a discrete, piecewise power-law grain-size distribution method into a numerical multifluid MHD code as described in Sumpter and Van Loo (2020). Such a description allows to capture the full size range of dust grains and their dynamical effects. The only assumptions are that grains within a single discrete bin have the same velocity and charge. We test the implementation by modelling plane-parallel C-type shocks and compare the results with shock models of multispecies grain models. We find that both the discrete and multispecies grain models converge to the same shock profile. However, the convergence for the discrete models is faster than for the multispecies grain models. For the pure advection models a single discrete bin is sufficient, while the multispecies grain models need a minimum of 8 grain species. When including grain sputtering the necessary number of discrete bins increases to 4, as the grain distribution cannot be described by a single power-law as in the advection models. The multispecies grain models still need more grain species to model the distribution, but the number does not increase compared to the pure advection models. Our results show that modelling the grain distribution function using a discrete distribution reduces the computational cost needed to capture the grain physics significantly.

Studying a precessing jet of a massive young stellar object within a chemically rich region

Aims. In addition to the large surveys and catalogs of massive young stellar objects (MYSOs) and outflows, dedicated studies are needed of particular sources in which high angular observations, mainly at near-IR and (sub)millimeter wavelengths, are analyzed in depth, to shed light on the processes involved in the formation of massive stars. The galactic source G079.1272+02.2782 is a MYSO at a distance of about 1.4 kpc that appears in several catalogs, and is hereafter referred to as MYSO G79. It is an ideal source to carry out this kind of study because of its relatively close distance and the intriguing structures that the source shows at near-IR wavelengths. Methods. Near-IR integral field spectroscopic observations were carried out using NIFS at Gemini North. The spectral and angular resolutions, about 2.4–4.0 Å, and 0.″15–0.″22, allow us to perform a detailed study of the source and its southern jet, resolving structures with sizes between 200 and 300 au. As a complement, millimeter data retrieved from the James Clerk Maxwell Telescope and the IRAM 30 m telescope databases were analyzed to study the molecular gas around the MYSO on a larger spatial scale. Results. The detailed analysis of a jet extending southward from MYSO G79 shows corkscrew-like structures at 2.2 μm continuum, strongly suggesting that the jet is precessing. The jet velocity is estimated at between 30 and 43 km s−1 and its kinematics indicates that it is blueshifted, that the jet is coming to us along the line of sight. We suggest that the precession may be produced by the gravitational tidal effects generated in a probable binary system, and we estimate a jet precession period of about 103 yr, indicating a slow-precessing jet, which is in agreement with the observed helical features. An exhaustive analysis of H2 lines at the near-IR band along the jet allows us to investigate in detail a bow shock produced by this jet. We find that this bow shock is indeed generated by a C-type shock and it is observed coming to us, at an inclination angle, along the line of sight. This is confirmed by the analysis of molecular outflows on a larger spatial scale. A brief analysis of several molecular species at millimeter wavelengths indicates a complex chemistry developing at the external layers of the molecular clump in which MYSO G79 is embedded. We note that we are presenting interesting observational evidence that can give support to theoretical models of bow shocks and precessing jets.

Revealing the Drag Instability in One-fluid Nonideal Magnetohydrodynamic Simulations of a 1D Isothermal C-shock

C-type shocks are believed to be ubiquitous in turbulent molecular clouds thanks to ambipolar diffusion. We investigate whether the drag instability in 1D isothermal C-shocks, inferred from the local linear theory of Gu & Chen, can appear in nonideal magnetohydrodynamic simulations. Two C-shock models (with narrow and broad steady-state shock widths) are considered to represent the typical environment of star-forming clouds. The ionization-recombination equilibrium is adopted for the one-fluid approach. In the 1D simulation, the inflow gas is continuously perturbed by a sinusoidal density fluctuation with a constant frequency. The perturbations clearly grow after entering the C-shock region until they start being damped at the transition to the post-shock region. We show that the profiles of a predominant Fourier mode extracted locally from the simulated growing perturbation match those of the growing mode derived from the linear analysis. Moreover, the local growth rate and wave frequency derived from the predominant mode generally agree with those from the linear theory. Therefore, we confirm the presence of the drag instability in simulated 1D isothermal C-shocks. We also explore the nonlinear behavior of the instability by imposing larger-amplitude perturbations to the simulation. We find that the drag instability is subject to wave steepening, leading to saturated perturbation growth. Issues concerning local analysis, nonlinear effects, one-fluid approach, and astrophysical applications are discussed.

Modelling cosmic masers in C-type shock waves – the coexistence of Class I CH3OH and 1720 MHz OH masers

ABSTRACT The collisional pumping of CH3OH and OH masers in non-dissociative C-type shock waves is studied. The chemical processes responsible for the evolution of molecule abundances in the shock wave are considered in detail. The large velocity gradient approximation is used to model radiative transfer in molecular lines. We present calculations of the optical depth in maser transitions of CH3OH and OH for a grid of C-type shock models that vary in cosmic ray ionization rate, gas density, and shock speed. We show that pre-shock gas densities nH, tot = 2 × 104–2 × 105 cm−3 are optimal for the pumping of methanol maser transitions. A complete collisional dissociation of methanol at the shock front takes place for shock speeds us ≳ 25 km s−1. At high pre-shock gas density nH, tot = 2 × 106 cm−3, the collisional dissociation of methanol takes place at shock speeds just above the threshold speed us ≈ 15–17.5 km s−1, corresponding to sputtering of icy mantles of dust grains. We show that the methanol maser transition E 4−1 → 30 at 36.2 GHz has an optical depth |τ| higher than that of the transition A+ 70 → 61 at 44.1 GHz at high cosmic ray ionization rate $\zeta _\mathrm{H_2} \gtrsim 10^{-15}$ s−1 and pre-shock gas density nH, tot = 2 × 104 cm−3. These results can be applied to the interpretation of observational data on methanol masers near supernova remnants and in molecular clouds of the Central Molecular Zone. At the same time, a necessary condition for the operation of 1720 MHz OH masers is a high ionization rate of molecular gas, $\zeta _\mathrm{H_2} \gtrsim 10^{-15}$ s−1. We find that physical conditions conducive to the operation of both hydroxyl and methanol masers are cosmic ray ionization rate $\zeta _\mathrm{H_2} \approx 10^{-15}$–3 × 10−15 s−1 and a narrow range of shock speeds 15 ≲ us ≲ 20 km s−1. The simultaneous observations of OH and CH3OH masers may provide restrictions on the physical parameters of the interstellar medium in the vicinity of supernova remnants.

HCN/HNC chemistry in shocks: a study of L1157-B1 with ASAI

ABSTRACT Hydrogen cyanide (HCN) and its isomer hydrogen isocyanide (HNC) play an important role in molecular cloud chemistry and the formation of more complex molecules. We investigate here the impact of protostellar shocks on the HCN and HNC abundances from high-sensitivity IRAM 30 m observations of the prototypical shock region L1157-B1 and the envelope of the associated Class 0 protostar, as a proxy for the pre-shock gas. The isotopologues H12CN, HN12C, H13CN, HN13C, HC15N, H15NC, DCN, and DNC were all detected towards both regions. Abundances and excitation conditions were obtained from radiative transfer analysis of molecular line emission under the assumption of local thermodynamical equilibrium. In the pre-shock gas, the abundances of the HCN and HNC isotopologues are similar to those encountered in dark clouds, with an HCN/HNC abundance ratio ≈1 for all isotopologues. A strong D-enrichment (D/H ≈ 0.06) is measured in the pre-shock gas. There is no evidence of 15N fractionation neither in the quiescent nor in the shocked gas. At the passage of the shock, the HCN and HNC abundances increase in the gas phase in different manners so that the HCN/HNC relative abundance ratio increases by a factor 20. The gas-grain chemical and shock model uclchem allows us to reproduce the observed trends for a C-type shock with pre-shock density n(H) = $10^5\hbox{cm$^{-3}$}$ and shock velocity $V_\mathrm{ s}= 40\hbox{kms$^{-1}$}$. We conclude that the HCN/HNC variations across the shock are mainly caused by the sputtering of the grain mantle material in relation with the history of the grain ices.

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.

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.

Structure and kinematics of shocked gas in Sgr B2: further evidence of a cloud–cloud collision from SiO emission maps

ABSTRACTWe present SiO J = 2–1 maps of the Sgr B2 molecular cloud, which show shocked gas with a turbulent substructure comprising at least three cavities at velocities of $[10,40]\, \rm km\, s^{-1}$ and an arc at velocities of $[-20,10]\, \rm km\, s^{-1}$. The spatial anticorrelation of shocked gas at low and high velocities, and the presence of bridging features in position-velocity diagrams suggest that these structures formed in a cloud–cloud collision. Some of the known compact H ii regions spatially overlap with sites of strong SiO emission at velocities of $[40,85]\, \rm km\, s^{-1}$, and are between or along the edges of SiO gas features at $[100,120]\, \rm km\, s^{-1}$, suggesting that the stars responsible for ionizing the compact H ii regions formed in compressed gas due to this collision. We find gas densities and kinetic temperatures of the order of $n_{\rm H_2}\sim 10^5\, \rm cm^{-3}$ and $\sim 30\, \rm K$, respectively, towards three positions of Sgr B2. The average values of the SiO relative abundances, integrated line intensities, and line widths are ∼10−9, $\sim 11\, \rm K\, km\, s^{-1}$, and $\sim 31\, \rm km\, s^{-1}$, respectively. These values agree with those obtained with chemical models that mimic grain sputtering by C-type shocks. A comparison of our observations with hydrodynamical simulations shows that a cloud–cloud collision that took place $\lesssim 0.5\, \rm Myr$ ago can explain the density distribution with a mean column density of $\bar{N}_{\rm H_2}\gtrsim 5\times 10^{22}\, \rm cm^{-2}$, and the morphology and kinematics of shocked gas in different velocity channels. Colliding clouds are efficient at producing internal shocks with velocities $\sim 5\!-\!50\, \rm km\, s^{-1}$. High-velocity shocks are produced during the early stages of the collision and can readily ignite star formation, while moderate- and low-velocity shocks are important over longer time-scales and can explain the widespread SiO emission in Sgr B2.

Chemical models of interstellar cyanomethanimine isomers

ABSTRACT The E-isomer of cyanomethanimine (HNCHCN) was first identified in Sagittarius B2(N) (Sgr B2(N)) by a comparison of the publicly available Green Bank Telescope (GBT) PRIMOS survey with laboratory rotational spectra. Recently, Z-cyanomethanimine was detected in the quiescent molecular cloud G+0.693−0.027 with the IRAM 30-m telescope. Cyanomethanimine is a chemical intermediate in the proposed synthetic routes of adenine, and may play an important role in forming biological molecules in the interstellar medium. Here we present a new modelling study of cyanomethanimine, using the nautilus gas–grain reaction network and code with the addition of over 400 chemical reactions of the three cyanomethanimine isomers and related species. We apply cold isothermal core, hot core, and C-type shock models to simulate the complicated and heterogeneous physical environment in and in front of Sgr B2(N), and in G+0.693−0.027. We identify the major formation and destruction routes of cyanomethanimine, and find that the calculated abundances of the cyanomethanimine isomers and the ratio of Z-isomer to E-isomer are both in reasonable agreement with observations for selected environments. In particular, we conclude that these isomers are most likely formed within or near the hot core without the impact of shocks, or in the cold regions with shocks.

Tracing shock type with chemical diagnostics

Aims.The physical structure of a shock wave may take a form unique to its shock type, implying that the chemistry of each shock type is unique as well. We aim to investigate the different chemistries of J-type and C-type shocks in order to identify unique molecular tracers of both shock types. We apply these diagnostics to the protostellar outflow L1157 to establish whether the B2 clump could host shocks exhibiting type-specific behaviour. Of particular interest is the L1157-B2 clump, which has been shown to exhibit bright emission in S-bearing species and HNCO.Methods.We simulate, using a parameterised approach, a planar, steady-state J-type shock wave using UCLCHEM. We compute a grid of models using both C-type and J-type shock models to determine the chemical abundance of shock-tracing species as a function of distance through the shock and apply it to the L1157 outflow. We focus on known shock-tracing molecules such as H2O, HCN, and CH3OH.Results.We find that a range of molecules including H2O and HCN have unique behaviour specific to a J-type shock, but that such differences in behaviour are only evident at lowvsand lownH. We find that CH3OH is enhanced by shocks and is a reliable probe of the pre-shock gas density. However, we find no difference between its gas-phase abundance in C-type and J-type shocks. Finally, from our application to L1157, we find that the fractional abundances within the B2 region are consistent with both C-type and J-type shock emission.

G−0.02−0.07, the compact H ii region complex nearest to the galactic center with ALMA

Abstract We have observed the compact H ii region complex nearest to the dynamical center of the Galaxy, G−0.02−0.07, using ALMA in the H42α recombination line, CS J = 2–1, H13CO+J = 1–0, and SiO v = 0, J = 2–1 emission lines, and the 86 GHz continuum emission. The H ii regions HII-A to HII-C in the cluster are clearly resolved into a shell-like feature with a bright half and a dark half in the recombination line and continuum emission. The analysis of the absorption features in the molecular emission lines show that H ii-A, B, and C are located on the near side of the “Galactic center 50 km s−1 molecular cloud” (50MC), but HII-D is located on the far side of it. The electron temperatures and densities ranges are Te = 5150–5920 K and ne = 950–2340 cm−3, respectively. The electron temperatures in the bright half are slightly lower than those in the dark half, while the electron densities in the bright half are slightly higher than those in the dark half. The H ii regions are embedded in the ambient molecular gas. There are some molecular gas components compressed by a C-type shock wave around the H ii regions. From the line width of the H42α recombination line, the expansion velocities of HII-A, HII-B, HII-C, and HII-D are estimated to be Vexp = 16.7, 11.6, 11.1, and 12.1 km s−1, respectively. The expansion timescales of HII-A, HII-B, HII-C, and HII-D are estimated to be tage ≃ 1.4 × 104, 1.7 × 104, 2.0 × 104, and 0.7 × 104 yr, respectively. The spectral types of the central stars from HII-A to HII-D are estimated to be O8V, O9.5V, O9V, and B0V, respectively. These derived spectral types are roughly consistent with the previous radio estimation. The positional relation among the H ii regions, the SiO molecule enhancement area, and Class-I maser spots suggest that a shock wave caused by a cloud–cloud collision propagated along the line from HII-C to HII-A in the 50MC. The shock wave would have triggered the massive star formation.

Supernova Shocks in Molecular Clouds: Velocity Distribution of Molecular Hydrogen

Abstract Supernovae from core collapse of massive stars drive shocks into the molecular clouds from which the stars formed. Such shocks affect future star formation from the molecular clouds, and the fast-moving, dense gas with compressed magnetic fields is associated with enhanced cosmic rays. This paper presents new theoretical modeling, using the Paris–Durham shock model, and new observations at high spectral resolution, using the Stratospheric Observatory for Infrared Astronomy, of the H2 S(5) pure rotational line from molecular shocks in the supernova remnant IC 443. We generate MHD models for nonsteady-state shocks driven by the pressure of the IC 443 blast wave into gas of densities 103–105 cm−3. We present the first detailed derivation of the shape of the velocity profile for emission from H2 lines behind such shocks, taking into account the shock age, preshock density, and magnetic field. For preshock densities 103–105 cm−3, the H2 emission arises from layers that extend 0.01–0.0003 pc behind the shock, respectively. The predicted shifts of line centers, and the line widths, of the H2 lines range from 20–2 and 30–4 km s−1, respectively. The a priori models are compared to the observed line profiles, showing that clumps C and G can be explained by shocks into gas with density 103 to 2 × 104 cm−3 and strong magnetic fields. Two positions in clump B were observed. For clump B2 (a fainter region near clump B), the H2 spectrum requires a J-type shock into moderate-density gas (∼102 cm−3) with the gas accelerated to 100 km s−1 from its preshock location. Clump B1 requires both a magnetic-dominated C-type shock (like for clumps C and G) and a J-type shock (like for clump B2) to explain the highest observed velocities. The J-type shocks that produce high-velocity molecules may be locations where the magnetic field is nearly parallel to the shock velocity, which makes it impossible for a C-type shock (with ions and neutrals separated) to form.

Analysis of Some Parameters and of Possible Types of Shocks in a New Sample of Spitzer/GLIMPSE/EGO Regions of Formation of Massive Stars

A comparative analysis is presented for the physical parameters of gas in various types of molecular clouds, derived from observations and theoretical calculations. Cool, dense fragments of infrared dense clouds (IRDCs)—rare star-forming regions with OH radical emission in the 1720-MHz satellite line, extended green objects (EGOs)— diffuse bipolar outflows discovered by the Spitzer mission at short IR wavelengths, and gas-dust condensations hosting class I and II methanol masers (class I MM and class II MM) are considered. The magnetic fields of these regions are calculated using the empirical criterion of Crutcher (ApJ, 520, 706, 1999) and the equation for numerical modeling of cold turbulent clouds of Ostriker et al. (ApJ, 546, 980, 2001). This modeling is done for three values of the parameter β = (cs/cA)2 relating the sound and Alfven speeds, which describes the effect of the magnetic field: strong (β = 0.01), intermediate (β = 0.1), and weak (β = 1). In 69 EGOs with densities of 104–105.6 cm−3, derived earlier from observations, and with an acceptable temperature range, from Tk ≈ 30 K to Tk ≈ 100 K, the results of the calculations are most consistent with β = 0.1 (|B|= 0.26 mG) and β = 1 (|B|= 0.15 mG) for intermediate and weak magnetic fields, respectively. For class I methanol masers, with gas densities not exceeding 106 cm−3 and a temperature Tk = 80 K, the most plausible value is β = 1 (|B < 0.4 mG), i.e., the expected effect of the magnetic field is weak. Comparison of the sound and Alfven speeds suggests that very faint, low-velocity C-type shocks may exist in these objcts, with the magnetic field exerting little control over the chaotic motions of the material. The shocks are continuous, and do not affect the parameters in the maser condensations. The magnetic intensities found for class I MMs and OH (1720 MHz) regions differ only slightly; according to this parameter, which is related to the density of the medium and the kinetic temperature of the gas, they may be observed in either different or the same gas-dust condensations.

C-type shock modelling – the effect of new H2–H collisional rate coefficients

ABSTRACTWe consider collisional excitation of H2 molecules in C-type shocks propagating in dense molecular clouds. New data on collisional rate coefficients for (de-)excitation of H2 molecule in collisions with H atoms and new H2 dissociation rates are used. The new H2–H collisional data are state of the art and are based on the most accurate H3 potential energy surface. We re-examine the excitation of rotational levels of H2 molecule, the para-to-ortho-H2 conversion, and H2 dissociation by H2–H collisions. At cosmic ray ionization rates ζ ≥ 10−16 s−1 and at moderate shock speeds, the H/H2 ratio at the shock front is mainly determined by the cosmic ray ionization rate. The H2–H collisions play the main role in the para-to-ortho-H2 conversion and, at ζ ≥ 10−15 s−1, in the excitation of vibrationally excited states of H2 molecule in the shock. The H2ortho-to-para ratio of the shocked gas and column densities of rotational levels of vibrationally excited states of H2 are found to depend strongly on the cosmic ray ionization rate. We discuss the applicability of the presented results to interpretation of observations of H2 emission in supernova remnants.

Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud

With an aim of probing the physical conditions and excitation mechanisms of warm molecular gas in individual star-forming regions, we performed Herschel SPIRE Fourier Transform Spectrometer (FTS) observations of 30 Doradus in the Large Magellanic Cloud. In our FTS observations, important far-infrared (FIR) cooling lines in the interstellar medium, including CO J = 4–3 to J = 13–12, [C I] 370 μm, and [N II] 205 μm, were clearly detected. In combination with ground-based CO J = 1–0 and J = 3–2 data, we then constructed CO spectral line energy distributions (SLEDs) on ~10 pc scales over a ~60 pc × 60 pc area and found that the shape of the observed CO SLEDs considerably changes across 30 Doradus. For example, the peak transition Jp varies from J = 6–5 to J = 10–9, while the slope characterized by the high-to-intermediate J ratio α ranges from ~0.4 to ~1.8. To examine the source(s) of these variations in CO transitions, we analyzed the CO observations, along with [C II] 158 μm, [C I] 370 μm, [O I] 145 μm, H2 0–0 S(3), and FIR luminosity data, using state-of-the-art models of photodissociation regions and shocks. Our detailed modeling showed that the observed CO emission likely originates from highly compressed (thermal pressure P∕kB ~ 107–109 K cm−3) clumps on ~0.7–2 pc scales, which could be produced by either ultraviolet (UV) photons (UV radiation field GUV ~ 103–105 Mathis fields) or low-velocity C-type shocks (pre-shock medium density npre ~ 104–106 cm−3 and shock velocity vs ~ 5–10 km s−1). Considering the stellar content in 30 Doradus, however, we tentatively excluded the stellar origin of CO excitation and concluded that low-velocity shocks driven by kiloparsec-scale processes (e.g., interaction between the Milky Way and the Magellanic Clouds) are likely the dominant source of heating for CO. The shocked CO-bright medium was then found to be warm (temperature T ~ 100–500 K) and surrounded by a UV-regulated low-pressure component (P∕kB ~ a few (104 –105) K cm−3) that is bright in [C II] 158 μm, [C I] 370 μm, [O I] 145 μm, and FIR dust continuum emission.