Shocks In Molecular Clouds Research Articles

Unusually High HCO+/CO Ratios in and outside Supernova Remnant W49B

Galactic supernova remnants (SNRs) and their environments provide the nearest laboratories to study SN feedback. We performed molecular observations toward SNR W49B, the most luminous Galactic SNR in the X-ray band, aiming to explore signs of multiple feedback channels of SNRs on nearby molecular clouds (MCs). We found very broad HCO+ lines with widths of dv ∼ 48–75 km s−1 in the SNR southwest, providing strong evidence that W49B is perturbing MCs at a systemic velocity of V LSR = 61–65 km s−1, and placing the W49B at a distance of 7.9 ± 0.6 kpc. We observed unusually high-intensity ratios of HCO+ J=1–0/CO J=1–0 not only at shocked regions (1.1 ± 0.4 and 0.70 ± 0.16) but also in quiescent clouds over 1 pc away from the SNR’s eastern boundary (≥0.2). By comparing with the magnetohydrodynamics shock models, we interpret that the high ratio in the broad-line regions can result from a cosmic-ray (CR) induced chemistry in shocked MCs, where the CR ionization rate is enhanced to around 10–102 times of the Galactic level. The high HCO+/CO ratio outside the SNR is probably caused by the radiation precursor, while the luminous X-ray emission of W49B can explain a few properties in this region. The above results provide observational evidence that SNRs can strongly influence the molecular chemistry in and outside the shock boundary via their shocks, CRs, and radiation. We propose that the HCO+/CO ratio is a potentially useful tool to probe an SNR’s multichannel influence on MCs.

Discovery of Shocked Molecular Clouds Associated with the Shell-type Supernova Remnant RX J0046.5−7308 in the Small Magellanic Cloud

RX J0046.5$-$7308 is a shell-type supernova remnant (SNR) in the Small Magellanic Cloud (SMC). We carried out new $^{12}$CO($J$ = 1-0, 3-2) observations toward the SNR using Mopra and the Atacama Submillimeter Telescope Experiment. We found eight molecular clouds (A-H) along the X-ray shell of the SNR. The typical cloud size and mass are $\sim$10-15 pc and $\sim$1000-3000 $M_{\odot}$, respectively. The X-ray shell is slightly deformed and has the brightest peak in the southwestern shell where two molecular clouds A and B are located. The four molecular clouds A, B, F, and G have high intensity ratios of $^{12}$CO($J$ = 3-2) / $^{12}$CO($J$ = 1-0) $> 1.2$, which are not attributable to any identified internal infrared sources or high-mass stars. The HI cavity and its expanding motion are found toward the SNR, which are likely created by strong stellar winds from a massive progenitor. We suggest that the molecular clouds A-D, F, and G and HI clouds within the wind-blown cavity at $V_\mathrm{LSR} = 117.1$-122.5 km s$^{-1}$ are to be associated with the SNR. The X-ray spectroscopy reveals the dynamical age of $26000^{+1000}_{-2000}$ yr and the progenitor mass of $\gtrsim 30$ $M_{\odot}$, which is also consistent with the proposed scenario. We determine physical conditions of the giant molecular cloud LIRS 36A using the large velocity gradient analysis with archival datasets of the Atacama Large Millimeter/submillimeter Array; the kinematic temperature is $72^{+50}_{-37}$ K and the number density of molecular hydrogen is $1500^{+600}_{-300}$ cm$^{-3}$. The next generation of $\gamma$-ray observations will allow us to study the pion-decay $\gamma$-rays from the molecular clouds in the SMC SNR.

Spectral and morphological study of the gamma radiation of the middle-aged supernova remnant HB 21

Aims. We investigate the nature of the accelerated particles responsible for the production of the gamma-ray emission observed from the middle-aged supernova remnant (SNR) HB 21. Methods. We present the analysis of more than nine years of Fermi LAT data from the SNR HB 21. We performed morphological and spectral analysis of the SNR by means of a three-dimensional binned likelihood analysis. To assess the intrinsic properties of the parent particle models, we fit the obtained gamma-ray spectral energy distribution of the SNR by both hadronic- and leptonic-induced gamma-ray spectrum. Results. We observe an extended emission positionally in agreement with the SNR HB 21. The bulk of this gamma-ray emission is detected from the remnant; photons up to ~10 GeV show clear evidence of curvature at the lower energies. The remnant is characterized by an extension of 0°.83, that is, 30% smaller than claimed in previous studies. The increased statistics allowed us also to resolve a point-like source at the edge of the remnant, in proximity to a molecular cloud of the Cyg OB7 complex. In the southern part of the remnant, a hint of an additional gamma-ray excess in correspondence to shocked molecular clouds is observed. Conclusions. The spectral energy distribution of the SNR shows evidence of a break around 400 MeV, which can be properly fitted within both the hadronic and leptonic scenario. The pion-decay mechanism reproduces well the gamma rays, postulating a proton spectrum with a slope ~2.5 and with a steepening around tens of GeV, which could be explained by the energy-dependent escape of particles from the remnant. In the leptonic scenario the electron spectrum within the SNR matches closely the locally measured spectrum. This remarkable and novel result shows that SNR HB 21 could be a direct contributor to the population of Galactic electrons. In the leptonic scenario, we find that the local electron spectrum with a break around 2 GeV, closely evokes the best-fitting parental spectrum within this SNR. If such a scenario is confirmed, this would indicate that the SNR might be a source of Galactic background electrons.

Is HESS J1912+101 Associated with an Old Supernova Remnant?

Abstract HESS J1912+101 is a shell-like TeV source that has no clear counterpart in multiwavelength. Using CO and H i data, we reveal that V LSR ∼ +60 km s−1 molecular clouds (MCs), together with shocked molecular gas and high-velocity neutral atomic shells, are concentrated toward HESS J1912+101. The prominent wing profiles up to V LSR ∼ +80 km s−1 seen in 12CO (J = 1–0 and J = 3–2) data, as well as the high-velocity expanding H i shells up to V LSR ∼ +100 km s−1, exhibit striking redshifted-broadening relative to the quiescent gas. These features provide compelling evidences for large-scale perturbation in the region. We argue that the shocked MCs and the high-velocity H i shells may originate from an old supernova remnant (SNR). The distance to the SNR is estimated to be ∼4.1 kpc based on the H i self-absorption method, which leads to a physical radius of 29.0 pc for the ∼(0.7–2.0) × 105 years old remnant with an expansion velocity of ≳40 km s−1. The +60 km s−1 MCs and the disturbed gas are indeed found to coincide with the bright TeV emission, supporting the physical association between them. Naturally, the shell-like TeV emission comes from the decay of neutral pions produced by interactions between the accelerated hadrons from the SNR and the surrounding high-density molecular gas.

EVIDENCE FOR DECAY OF TURBULENCE BY MHD SHOCKS IN THE ISM VIA CO EMISSION

We utilize observations of sub-millimeter rotational transitions of CO from a Herschel Cycle 2 open time program ("COPS", PI: J. Green) to identify previously predicted turbulent dissipation by magnetohydrodynamic (MHD) shocks in molecular clouds. We find evidence of the shocks expected for dissipation of MHD turbulence in material not associated with any protostar. Two models fit about equally well: model 1 has a density of 10$^{3}$ cm$^{-3}$, a shock velocity of 3 km s$^{-1}$, and a magnetic field strength of 4 ${\mu}$G; model 2 has a density of 10$^{3.5}$ cm$^{-3}$, a shock velocity of $2$ km s$^{-1}$, and a magnetic field strength of 8 $\mu$G. Timescales for decay of turbulence in this region are comparable to crossing times. Transitions of CO up to $J$ of 8, observed close to active sites of star formation, but not within outflows, can trace turbulent dissipation of shocks stirred by formation processes. Although the transitions are difficult to detect at individual positions, our Herschel-SPIRE survey of protostars provides a grid of spatially-distributed spectra within molecular clouds. We averaged all spatial positions away from known outflows near seven protostars. We find significant agreement with predictions of models of turbulent dissipation in slightly denser (10$^{3.5}$ cm$^{-3}$) material with a stronger magnetic field (24 $\mu$G) than in the general molecular cloud.

A multi-transition study of molecules toward NGC 1068 based on high-resolution imaging observations with ALMA

Abstract We present 0.8-mm band molecular images and spectra obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) toward one of the nearest galaxies with an active galactic nucleus (AGN), NGC 1068. Distributions of CO isotopic species (13CO and C18O) J = 3–2, CN N = 3–2, and CS J = 7–6 are observed toward the circumnuclear disk (CND) and a part of the starburst ring with an angular resolution of ∼ 1${^{\prime\prime}_{.}}$3 × 1${^{\prime\prime}_{.}}$2. The physical properties of these molecules and shock-related molecules, such as HNCO, CH3CN, SO, and CH3OH, detected in the 3-mm band were estimated using rotation diagrams under the assumption of local thermodynamic equilibrium. The rotational temperatures of the CO isotopic species and the shock-related molecules in the CND are, respectively, 14–22 K and upper limits of 20–40 K. Although the column densities of the CO isotopic species in the CND are only from one-fifth to one-third of that in the starburst ring, those of the shock-related molecules are enhanced by a factor of 3–10 in the CND. We also discuss the chemistry of each species, and compare the fractional abundances in the CND and starburst ring with those of Galactic sources such as cold cores, hot cores, and shocked molecular clouds in order to study the overall characteristics. We find that the abundances of shock-related molecules are more similar to abundances in hot cores and/or shocked clouds than to cold cores. The CND hosts relatively complex molecules, which are often associated with shocked molecular clouds or hot cores. Because a high X-ray flux can dissociate these molecules, they must also reside in regions shielded from X-rays.

An extended source of GeV gamma rays coincident with the supernova remnant HB 21

We analyze 3.5 years of public Fermi/LAT data around the position of the supernova remnant HB 21, where four point-like sources from the 2nd Fermi/LAT catalog are located. We determine that the gamma-ray source is produced by a single extended source. We model the observed morphology as a uniform circle. The spectral energy distribution is best described by a curved power law, with a maximum at 413+-11 MeV. We divide the circle into three regions defined by previously identified shocked molecular clouds, and find that one of these regions has a softer spectrum. The >3 GeV gamma-ray emission of the soft spectrum region is bow-shaped and coincident with the supernova remnant shell seen at radio wavelengths. We suggest that the gamma-ray emission from HB 21 can be understood as a combination of emission from shocked/illuminated molecular clouds, one of them coincident with the supernova remnant shell itself.

SPITZEROBSERVATIONS OF SHOCK-EXCITED HYDROGEN DEUTERIDE IN IC 443C, HH 7, AND HH 54: PROBING THE GAS-PHASE DEUTERIUM ABUNDANCE IN THE DENSE INTERSTELLAR MEDIUM

We present and analyze observations?carried out using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope?of the R(3) and R(4) pure rotational lines of hydrogen deuteride (HD) detected from shock-heated material associated with the supernova remnant IC?443C and with the Herbig-Haro objects HH?7 and HH?54. Assuming a continuous temperature distribution for gas observed along the sight lines, we have constrained the gas density to be in the ~103-104?cm?3 range, using both spectroscopic data for H2, HD, and CO from IRS and from the Infrared Space Observatory (ISO), as well as photometric data from Spitzer's Infrared Array Camera. The derived HD abundance relative to H2 is quite sensitive to the assumed excitation conditions in the emitting gas. Assuming that HD accounts for all gas-phase deuterium in the emitting material, and using all the available spectroscopic data to constrain the excitation conditions, we obtained gas-phase deuterium abundances [D/H]gas of 0.95+0.54 ?0.27 ? 10?5 and 0.87+0.31 ?0.27 ? 10?5 (statistical errors only) for IC?443C and HH?54, respectively. The uncertainties in the HD?abundance are dominated by systematic effects related to the poorly known excitation conditions, and more accurate estimates of the HD abundance in shocked molecular clouds will require measurements of the emission in additional HD rotational transitions.

FAR-INFRARED LUMINOUS SUPERNOVA REMNANT Kes 17

We present the results of infrared (IR; 2.5–160 μm) observations of the supernova remnant (SNR) Kes 17 based on the data obtained with the AKARI and Spitzer satellites. We first detect bright continuum emission of its western shell in the mid- and far-IR wavebands together with its near-IR molecular line emission. We also detect hidden mid-IR emission of its southern shell after subtraction of the background emission in this region. The far-IR luminosity of the western shell is ∼8100 L☉, which makes Kes 17 one of the few SNRs of significant far-IR emission. The fittings of the spectral energy distribution indicate the existence of two dust components: ∼79 K (hot) and ∼27 K (cold) corresponding to the dust masses of ∼6.2 × 10−4 M☉ and ∼6.7 M☉, respectively. We suggest that the hot component represents the dust emission of the material swept up by the SNR to its western and southern boundaries, compatible with the distribution of radio continuum emission overlapping the mid-IR emission in the western and southern shells. The existence of hot (∼2000 K), shocked dense molecular gas revealed by the near-IR molecular line emission in the western shell, on the other hand, suggests that the cold dust component represents the dust emission related to the interaction between the SNR and nearby molecular gas. The excitation conditions of the molecular gas appear to be consistent with those from shocked, clumpy admixture gas of different temperatures. We discuss three possibilities for the origin of the bright far-IR emission of the cold dust in the western shell: the emission of dust in the inter-clump medium of shocked molecular clouds, the emission of dust in evaporating flows of molecular clouds engulfed by hot gas, and the emission of dust of nearby molecular clouds illuminated by radiative shocks.

OBSERVATION OF SUPERNOVA REMNANT IC 443 WITH THE FERMI LARGE AREA TELESCOPE

ABSTRACT We report observation of the supernova remnant (SNR) IC 443 (G189.1+3.0) with the Fermi Gamma-ray Space Telescope Large Area Telescope (LAT) in the energy band between 200 MeV and 50 GeV. IC 443 is a shell-type SNR with mixed morphology located off the outer Galactic plane where high-energy emission has been detected in the X-ray, GeV and TeV gamma-ray bands. Past observations suggest IC 443 has been interacting with surrounding interstellar matter. Proximity between dense shocked molecular clouds and GeV–TeV gamma-ray emission regions detected by EGRET, MAGIC, and VERITAS suggests an interpretation that cosmic-ray (CR) particles are accelerated by the SNR. With the high gamma-ray statistics and broad energy coverage provided by the LAT, we accurately characterize the gamma-ray emission produced by the CRs accelerated at IC 443. The emission region is extended in the energy band with θ68 = 0.°27 ± 0.°01(stat) ± 0.°03(sys) for an assumed two-dimensional Gaussian profile and overlaps almost completely with the extended source region of VERITAS. Its centroid is displaced significantly from the known pulsar wind nebula (PWN) which suggests the PWN is not the major contributor in the present energy band. The observed spectrum changes its power-law slope continuously and continues smoothly to the MAGIC and VERITAS data points. The combined gamma-ray spectrum (200 MeV <E< 2 TeV) is reproduced well by decays of neutral pions produced by a broken power-law proton spectrum with a break around 70 GeV.

PROTOPLANETARY DISK EVOLUTION AROUND THE TRIGGERED STAR-FORMING REGION CEPHEUS B

The Cepheus B (Cep B) molecular cloud and a portion of the nearby Cep OB3b OB association, one of the most active regions of star formation within 1 kpc, have been observed with the Infrared Array Camera detector on board the Spitzer Space Telescope. The goals are to study protoplanetary disk evolution and processes of sequential triggered star formation in the region. Out of ~400 pre-main-sequence (PMS) stars selected with an earlier Chandra X-ray Observatory observation, ~95% are identified with mid-infrared sources and most of these are classified as diskless or disk-bearing stars. The discovery of the additional >200 IR-excess low-mass members gives a combined Chandra+Spitzer PMS sample that is almost complete down to 0.5 M ? outside of the cloud, and somewhat above 1 M ? in the cloud. Analyses of the nearly disk-unbiased combined Chandra and Spitzer-selected stellar sample give several results. Our major finding is a spatio-temporal gradient of young stars from the hot molecular core toward the primary ionizing O star HD 217086. This strongly supports the radiation-driven implosion (RDI) model of triggered star formation in the region. The empirical estimate for the shock velocity of ~1 km s?1 is very similar to theoretical models of RDI in shocked molecular clouds. The initial mass function (IMF) of the lightly obscured triggered population exhibits a standard Galactic field IMF shape. The unusually high apparent value of 70% star formation efficiency inferred from the ratio of star mass to current molecular gas mass indicates that most of the Cep B molecular cloud has been already ablated or transformed to stars. Contrary to the current RDI simulations, our findings indicate that star formation triggering by H II region shocks is not restricted to a single episode but can continue for millions of years. Other results include: (1) agreement of the disk fractions, their mass dependency, and fractions of transition disks with other clusters; (2) confirmation of the youthfulness of the embedded Cep B cluster; (3) confirmation of the effect of suppression of time-integrated X-ray emission in disk-bearing versus diskless systems.

Shock Models Revisited

We review progress in the area of the modelling of shocks in molecular clouds. In particular, we consider what has been learnt about shock structure evolution in situations where the steady-state assumption is no longer valid. We discuss the interpretation of the observed water abundance from SWAS, Odin, and ISO. We also consider the erosion of grains in shocks as well as the effect of the presence of grains on shock structure.

The shocking properties of supersonic flows: Dependence of the thermal overstability on M, α, and T$\mathsf{_{c}}\,$/ T$\mathsf{_{0}}$

We present hydrodynamical calculations of radiative shocks with low Mach numbers and find that the well-known global overstability can occur if the temperature exponent (α) of the cooling is sufficiently negative. We find that the stability of radiative shocks increases with decreasing Mach number, with the result that shocks require in order to be overstable. Such values occur within a limited temperature range of many cooling curves. We observe that Mach numbers of order 100 are needed before the strong shock limit of is reached, and we discover that the frequency of oscillation of the fundamental mode also has a strong Mach number dependence. We find that feedback between the cooling region and the cold dense layer (CDL) further downstream is a function of Mach number, with stronger feedback and oscillation of the boundary between the CDL and the cooling region occuring at lower Mach numbers. This feedback can be quantified in terms of the reflection coefficient of sound waves, and in those cases where the cooling layer completely disappears at the end of each oscillation cycle, the initial velocity of the waves driven into the upstream pre-shock flow and into the downstream CDL, and the velocity of the the boundary between the CDL and the cooling layer, can be understood in terms of the solution to the Riemann problem. An interesting finding is that the stability properties of low Mach number shocks can be dramatically altered if the shocked gas is able to cool to temperatures less than the pre-shock value (i.e. when , where χ is the ratio of the temperature of the cold dense layer to the pre-shock temperature). In such circumstances, low Mach number shocks have values of which are comparable to values obtained for higher Mach number shocks when . For instance, when and , comparable to that when and . Thus, it is probable that low Mach number astrophysical shocks will be overstable in a variety of situations. We also explore the effect of different assumptions for the initial hydrodynamic set up and the type of boundary condition imposed downstream, and find that the properties of low Mach number shocks are relatively insensitive to these issues. The results of this work are relevant to astrophysical shocks with low Mach numbers, such as supernova remnants (SNRs) immersed in a hot interstellar medium (e.g., within a starburst region), and shocks in molecular clouds, where time-dependent chemistry can lead to overstability.

A numerical scheme for multifluid magnetohydrodynamics

This paper describes a numerical scheme for multi-fluid hydrodynamics in the limit of small mass densities of the charged particles. The inertia of the charged particles can then be neglected, which makes it possible to write an evolution equation for the magnetic field that can be solved using an implicit scheme. This avoids the severe restriction on the stable timestep that would otherwise arise at high resolution, or when the Hall effect is large. Numerical tests show that the scheme can accurately model steady multi-fluid shock structures both with and without sub-shocks. Although the emphasis is on shocks in molecular clouds, a multi-dimensional version of this code could be applied to any Astrophysical flow in which ambi-polar diffusion or the Hall effect, or both play a significant role.

The instability of fast shocks in molecular clouds

We report on the discovery that moderately fast shocks in dense molecular clouds with low transverse magnetic fields are likely to be unstable. The instability is triggered by the promoted cooling that results from the formation of carbon monoxide and water molecules in an extended warm shock section. Numerical methods are employed to demonstrate that, in the absence of magnetic fields, the instability regime is restricted to densities above n0= 104 cm−3, velocities between 30–70 km s−1, and O or C abundances above ∼10−4, so that cooling from reforming molecules dominates in the warm gas without being suppressed by ultraviolet dissociation. The result is either a quasi-periodic or chaotic collapse and re-establishment of the warm shock layer on a typical time-scale of 106 cm−3/n0 yr with variations on shorter time-scales and changes in period being possible. Infrared emission lines from the unstable region, including the H2 lines, exhibit orders of magnitude variability. Atomic lines such as Hα display constant fluxes but undergo rapid radial velocity variations.

Cooling behind mildly supersonic shocks in molecular clouds

Collisions between stable, thermally supported gas clumps produce shock-compressed layers. However, these layers then undergo gravitational fragmentation only if the gas cools — on a dynamical time-scale or faster — to below its pre-shock temperature. Here we present an approximate analytic treatment of post-shock cooling which demonstrates that, under the conditions prevailing in molecular clouds, where the typical collision speeds are ~ 1 km s−1, cooling by dust is likely to be the dominant cooling mechanism, and is sufficiently fast to satisfy this requirement. Cooling by CO appears to be of secondary importance.

Grains in shocks in weakly ionized clouds - II. Magnetic field rotation in oblique shocks

A four-fluid treatment of oblique C-type shocks in magnetized dark molecular clouds is presented. The four fluids are those consisting of neutrals, ions, electrons and negatively charged spherical grains of uniform size and composition. The charged particles couple to the neutrals by frictional interaction and their motion is described by the drift approximation. In an oblique shock, a current is set up which has a component parallel to the magnetic field component transverse to the shock velocity; this component of the current induces rotation of the magnetic field around the shock propagation direction. We give steady solutions for oblique C-type shocks for which the propagation speed is only slightly above the coupled Alfvén speed of about 2.2 km |$\text s^{-1}$|⁠. in these shocks, the magnetic field component transverse to the shock velocity rotates by |$180^\circ$| through the shock. The rotation occurs for the upstream perturbations chosen in previous studies of steady oblique shocks in molecular clouds. it is possible that the set of upstream perturbations that do not lead to large rotation in these shocks is nonempty, but we find that the set is limited. We also find that, for reasonable dark cloud parameters, the rotation ill a shock with a speed of a few km |$\text s^{-1}$| or more is sufficient to cause the velocity component parallel to the shock velocity of at least one of the charged fluids to drop sharply to zero in the shock frame.

Magnetic Fields and Shocks in Molecular Clouds

AbstractWe discuss the expected properties of shocks in molecular clouds when the effect of a magnetic field is included. The results suggest that shocks should have magnetic precursors, and may be C-shocks rather than J-shocks, but the observations are not obviously consistent with this prediction. The effect of these shock properties on cloud collisions triggering star formation is discussed.

Jump shocks in molecular clouds: speed limits and excitation levels

Below a speed limit, hydromagnetic jump shocks propagate through the interstellar medium without dissociating the molecules. The resulting excitation of the molecules is modelled here, where several factors, including the shock shape and magnetic field strength (i.e. the Alfven speed), are explored. The speed limit for molecular hydrogen is in the range ∼ 24-39 km s −1 for low Alfven speeds but reaches 50 km s −1 for an Alfven speed of 10 km s −1 . High Alfven speeds may well occur within active regions, such as already found for HH 34, where the results of multiple outflow events are being observed. Origins for the low but significant level of pre-shock ionization, a prerequisite for a jump shock, are considered.

Switch shocks in molecular clouds. II - Implications

view Abstract Citations (23) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Switch Shocks in Molecular Clouds. II. Implications Smith, Michael D. Abstract Some of the observable properties of switch shocks in molecular clouds with oblique magnetic fields are derived. These shocks, in which the initial motion is parallel to the field lines, are explored and compared to the transverse case. The process of switching on the tangential component, to enable field-cushioning, leads to higher temperatures over a narrower shocked layer. Nevertheless, ambipolar diffusion and acceleration of molecular material via shocks remains effective even along field lines. Implications for molecular emission lines are explored. Ramifications for star formation and circumstellar disks are discussed. Torsional switches and poloidal switches extract angular momentum and magnetic flux from molecular cloud cores at super-Alfvenic speeds. Publication: The Astrophysical Journal Pub Date: April 1993 DOI: 10.1086/172464 Bibcode: 1993ApJ...406..520S Keywords: Interstellar Magnetic Fields; Magnetohydrodynamics; Molecular Clouds; Shock Waves; Ambipolar Diffusion; Angular Momentum; Magnetic Flux; Molecular Excitation; Propagation Velocity; Star Formation; Transverse Waves; Astrophysics; ISM: CLOUDS; ISM: MOLECULES; MAGNETOHYDRODYNAMICS: MHD; SHOCK WAVES full text sources ADS | Related Materials (1) Part 1: 1992ApJ...390..447S