Interstellar Gas Clouds Research Articles

The re-entrant transition from the molecular to atomic phases of dense fluids: The case of hydrogen.

A simple phenomenological thermodynamic model is developed to describe the chemical bonding and unbonding in homonuclear diatomic systems. This model describes the entire phase diagram of dimer-forming systems and shows a transition from monomers to dimers, with monomers favored at both very low and very high pressures, as well as at high temperatures. In the context of hydrogen, the former region corresponds to hydrogen present in most interstellar gas clouds, while the latter is associated with the long sought-after fluid metallic phase. The model predicts a molecular to atomic fluid transition in dense deuterium, which is in agreement with recently reported experimental measurements.

On Estimating the Masses of Star Clusters Embedded in Clouds of Interstellar Gas and Dust

On Estimating the Masses of Star Clusters Embedded in Clouds of Interstellar Gas and Dust

Descriptors for binding energies at clusters: The case of nanosilicates as models of interstellar dust grains.

Binding energies of radicals and molecules at dust grain surfaces are important parameters for understanding and modeling the chemical inventory of interstellar gas clouds. While first-principles methods can reliably be used to compute such binding energies, the complex structure and varying sizes and stoichiometries of realistic dust grains make a complete characterization of all adsorption sites exposed by their surfaces challenging. Here, we focus on nanoclusters composed of Mg-rich silicates as models of interstellar dust grains and two adsorbates of particular astrochemical relevance; H and CO. We employ a compressed sensing method to identify descriptors for the binding energies, which are expressed as analytical functions of intrinsic properties of the clusters, obtainable through a single first-principles calculation of the cluster. The descriptors are identified based on a diverse training dataset of binding energies at low-energy structures of nanosilicate clusters, where the latter structures were obtained using a first-principles-based global optimization method. The composition of the descriptors reveals how electronic, electrostatic, and geometric properties of the nanosilicates control the binding energies and demonstrates distinct physical origins of the bond formation for H and CO. The predictive performance of the descriptors is found to be limited by cluster reconstruction, e.g., breaking of internal metal-oxygen bonds, upon the adsorption event, and strategies to account for this phenomenon are discussed. The identified descriptors and the computed datasets of stable nanosilicate clusters along with their binding energies are expected to find use in astrochemical models of reaction networks occurring at silicate grain surfaces.

Traveling waves in an evolving interstellar gas cloud

This paper considers the possible emergence of traveling waves within an evolving interstellar gas cloud. To model this evolution, we use Euler–Poisson equations with the additional assumptions that the gas is incompressible, stratified, and self-gravitating. Within this framework, we establish that when the cloud has low density, the speed of these traveling waves is low. We suggest that the self-gravitational coalescence of embedded solid matter in the gas to form larger aggregates, such as cometary nuclei, may occur in the vicinity of wave crests where the mass density is highest. This idea is consistent with the widely agreed mechanism for planetary formation in proto-planetary disks, namely, that the accumulation of solids to form larger planetoids is initiated at the location of pressure maxima in the gas disk.

Tracing of magnetic fields with gradients: subsonic turbulence

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

Escaping the maze: a statistical subgrid model for cloud-scale density structures in the interstellar medium

ABSTRACT The interstellar medium (ISM) is a turbulent, highly structured multiphase medium. State-of-the-art cosmological simulations of the formation of galactic discs usually lack the resolution to accurately resolve those multiphase structures. However, small-scale density structures play an important role in the life cycle of the ISM, and determine the fraction of cold, dense gas, the amount of star formation, and the amount of radiation and momentum leakage from cloud-embedded sources. Here, we derive a statistical model to calculate the unresolved small-scale ISM density structure from coarse-grained, volume-averaged quantities such as the gas clumping factor, $\mathcal {C}$, and mean density 〈ρ〉V. Assuming that the large-scale ISM density is statistically isotropic, we derive a relation between the three-dimensional clumping factor, $\mathcal {C}_\rho$, and the clumping factor of the 4$\pi$ column density distribution on the cloud surface, $\mathcal {C}_\Sigma$, and find $\mathcal {C}_\Sigma =\mathcal {C}_\rho ^{2/3}$. Applying our model to calculate the covering fraction, i.e. the 4$\pi$ sky distribution of optically thick sightlines around sources inside interstellar gas clouds, we demonstrate that small-scale density structures lead to significant differences at fixed physical ISM density. Our model predicts that gas clumping increases the covering fraction by up to 30 per cent at low ISM densities compared to a uniform medium. On the other hand, at larger ISM densities, gas clumping suppresses the covering fraction and leads to increased scatter such that covering fractions can span a range from 20 per cent to 100 per cent at fixed ISM density. All data and example code are publicly available at GitHub.

Robert H. Rubin (1941–2013)

Rubin developed widely used physical models of interstellar clouds of gas ionized by nearby hot stars. His work provided evidence for the previously unsuspected presence of hot stars in the vicinity of the Galactic Center.

Interaction of a singular surface with a strong shock in the interstellar gas clouds

We investigate the interaction between a singular surface and a strong shock in the self-gravitating interstellar gas clouds with the assumption of spherical symmetry. Using the method of the Lie group of transformations, a particular solution of the flow variables and the cooling–heating function for an infinitely strong shock is obtained. This paper explores an application of the singular surface theory in the evolution of an acceleration wave front propagating through an unperturbed medium. We discuss the formation of an acceleration, considering the cases of compression and expansion waves. The influence of the cooling–heating function on a shock formation is explained. The results of a collision between a strong shock and an acceleration wave are discussed using the Lax evolutionary conditions. doi:10.1017/S1446181121000328

Multiple photodetachment of silicon anions via K -shell excitation and ionization

Experimental cross sections for $m$-fold photodetachment ($m=3-6$) of silicon anions via $K$-shell excitation and ionization were measured in the photon-energy range of 1830-1900 eV using the photon-ion merged-beams technique at a synchrotron light source. All cross sections exhibit a threshold behavior that is masked by pre-threshold resonances associated with the excitation of a $1s$ electron to higher, either partly occupied or unoccupied atomic subshells. Results from multi-configuration Dirac-Fock (MCDF) calculations agree with the experimentally derived cross sections for photo-absorption if small energy shifts are applied to the calculated resonance positions and detachment thresholds. Moreover, a systematic approach is applied for modeling the deexcitation cascades that set in after the initial creation of a $K$-shell hole. The resulting product charge-state distributions compare well with the measured ones for direct $K$-shell detachment but less well for resonant $K$-shell excitation. The present results are potentially useful for identifying silicon anions in cold plasmas such as interstellar gas clouds.

INTERACTION OF A SINGULAR SURFACE WITH A STRONG SHOCK IN THE INTERSTELLAR GAS CLOUDS

AbstractWe investigate the interaction between a singular surface and a strong shock in the self-gravitating interstellar gas clouds with the assumption of spherical symmetry. Using the method of the Lie group of transformations, a particular solution of the flow variables and the cooling–heating function for an infinitely strong shock is obtained. This paper explores an application of the singular surface theory in the evolution of an acceleration wave front propagating through an unperturbed medium. We discuss the formation of an acceleration, considering the cases of compression and expansion waves. The influence of the cooling–heating function on a shock formation is explained. The results of a collision between a strong shock and an acceleration wave are discussed using the Lax evolutionary conditions.

Detecting composite dark matter with long-range and contact interactions in gas clouds

Cold interstellar gas clouds provide an exciting new method to discover dark matter. Their immense size makes them uniquely sensitive to interactions from the heaviest, most rarefied dark matter models. Using gas cloud observations, we derive constraints on heavy composite dark matter coupled to the Standard Model through a light dark photon for dark matter up to a thousand solar masses. We find gas clouds are also sensitive to very large composite dark matter that interacts with nuclei through a fixed contact interaction cross section. We also study the contact interaction model and implement multiscatter and overburden analyses to obtain bounds from experiments like CDMS, CRESST, DAMA, XQC, and XENON1T.

Molecular line signatures of cloud–cloud collisions

ABSTRACT Collisions between interstellar gas clouds are potentially an important mechanism for triggering star formation. This is because they are able to rapidly generate large masses of dense gas. Observationally, cloud collisions are often identified in position–velocity (PV) space through bridging features between intensity peaks, usually of CO emission. Using a combination of hydrodynamical simulations, time-dependent chemistry, and radiative transfer, we produce synthetic molecular line observations of overlapping clouds that are genuinely colliding, and overlapping clouds that are just chance superpositions. Molecules tracing denser material than CO, such as NH3 and HCN, reach peak intensity ratios of 0.5 and 0.2 with respect to CO in the ‘bridging feature’ region of PV space for genuinely colliding clouds. For overlapping clouds that are just chance superpositions, the peak NH3 and HCN intensities are co-located with the CO intensity peaks. This represents a way of confirming cloud collisions observationally and distinguishing them from chance alignments of unrelated material.

Interaction of an acceleration wave with a characteristic shock in Interstellar gas clouds

Interaction of an acceleration wave with a characteristic shock in Interstellar gas clouds

One-dimensional spherical shock waves in an interstellar dusty gas clouds

Abstract A system of partial differential equations describing the one-dimensional motion of an inviscid self-gravitating and spherical symmetric dusty gas cloud, is considered. Using the method of the kinematics of one-dimensional motion of shock waves, the evolution equation for the spherical shock wave of arbitrary strength in interstellar dusty gas clouds is derived. By applying first order truncation approximation procedure, an efficient system of ordinary differential equations describing shock propagation, which can be regarded as a good approximation of infinite hierarchy of the system. The truncated equations, which describe the shock strength and the induced discontinuity, are used to analyze the behavior of the shock wave of arbitrary strength in a medium of dusty gas. The results are obtained for the exponents from the successive approximation and compared with the results obtained by Guderley’s exact similarity solution and characteristic rule (CCW approximation). The effects of the parameters of the dusty gas and cooling-heating function on the shock strength are depicted graphically.

Kinematics of spherical shock waves in an interstellar ideal gas clouds with dust particles

In this article, a system of partial differential equations governing the one‐dimensional motion of inviscid, self‐gravitating, spherical dusty gas cloud is considered. The evolutionary behavior of spherical shock waves of arbitrary strength in an interstellar dusty gas cloud is examined. By utilizing the method based on the kinematics of the one‐dimensional motion of shock waves, we derived an infinite set of transport equations governing the strength of shock waves and induced discontinuity behind it. By applying the truncation procedure to the infinite set of transport equations, we get an efficient system of finite number ordinary differential equations describing shock propagation, which can be regarded as a good approximation of the infinite hierarchy of the system. The truncated equations describing the shock strength and the induced discontinuity are used to analyze the growth and decay behavior of shock waves of arbitrary strength in the dusty gas medium. We considered the first two truncation approximations and the obtained results for the exponent from the successive approximation and compared our results with Guderley's exact similarity solution and the characteristic rule.

ALMA CO Observations of the Gamma-Ray Supernova Remnant RX J1713.7–3946: Discovery of Shocked Molecular Cloudlets and Filaments at 0.01 pc Scales

Abstract RX J1713.7−3946 is a unique core-collapse supernova remnant (SNR) that emits bright TeV gamma-rays and synchrotron X-rays caused by cosmic rays, in addition to interactions with interstellar gas clouds. We report here on results of Atacama Large Millimeter/submillimeter Array 12CO(J = 1–0) observations toward the northwestern shell of the SNR. We newly found three molecular complexes consisting of dozens of shocked molecular cloudlets and filaments with typical radii of ∼0.03–0.05 pc and densities of ∼104 cm−3. These cloudlets and filaments are located not only along synchrotron X-ray filaments, but also in the vicinity of X-ray hotspots with month- or year-scale time variations. We argue that X-ray hotspots and filaments were generated by shock–cloudlet interactions through magnetic-field amplification up to mG. The interstellar medium density contrast of ∼105, coexistence of molecular cloudlets and low-density diffuse medium of ∼0.1 cm−3, is consistent with such a magnetic field amplification as well as a wind-bubble scenario. The small-scale cloud structures also affect hadronic gamma-ray spectra considering the magnetic field amplification on surface and inside clouds.

Ionized Gas in the NGC 3077 Galaxy

The nearby dwarf galaxy NGC 3077 is known for its peculiar morphology, which includes numerous dust lanes and emission-line regions. The interstellar medium in this galaxy is subject to several perturbing factors. These are primarily the central starburst and tidal structures in the M 81 group. We present a comprehensive study of the state of ionization, kinematics, and chemical composition of ionized gas in NGC 3077, including both star-forming regions and diffuse ionized gas (DIG) at the periphery. We study gas motions in the H$\alpha$ line via high-resolution ($R\approx15\,000$) 3D spectroscopy with the scanning Fabry-Perot interferometer installed into SCORPIO-2 instrument attached to the 6-m telescope of the SAO RAS. Images in the main optical emission lines were acquired with MaNGaL photometer with a tunable filter at the 2.5-m telescope of the Caucasian Mountain Observatory of SAI MSU. We also used SCORPIO-2 to perform long-slit spectroscopy of the galaxy with a resolution of $R\approx1\,000$. Our estimate of the gas metallicity, $Z=0.6Z_\odot$, is significantly lower than the earlier determination, but agrees with the "luminosity--metallicity" dependence. Spatially resolved diagnostic diagrams of the emission-line ratios do not show correlations between the gas ionization state and its velocity dispersion, and this is most likely due to strong ionization by young stars, whereas the contribution of shocks to the excitation of emission lines is less important. We also studied the locations of multicomponent H$\alpha$ profiles and provide arguments suggesting that they are mostly associated with individual kinematic components along the line of sight and not with expanding shells as it was believed earlier. We also observe there a combination of wind outflow from star-forming regions and accretion from interstellar gas clouds in the M 81 group.

On the evolution of a primordial interstellar gas cloud

In 1796, Laplace conjectured that a primitive interstellar gas cloud may evolve under the influence of its own gravity to form a system of isolated rings, which may in turn lead to the formation of planetary systems. In fact, such a system of rings around a protostar has been observed recently in the constellation Taurus. In this paper, we use the time dependent Euler–Poisson equations to find out under what conditions such a pattern formation can take place. To this end, we consider these equations assuming that the gas is composed of incompressible stratified fluid. Under these assumptions, the original six governing equations are then reduced to three. These equations are then solved analytically (approximately), and these solutions are used to determine the evolution of the mass density distribution within the interstellar gas cloud. We show that under proper conditions, mass density waves can emerge within the cloud.

The effect of dark matter on the Jeans instability with the q-nonextensive velocity distribution

Based on the framework of nonextensive statistics and its related kinetic theory, the Jeans instability in a self-gravitational system composed of dark and bright matters is restudied. The results show that the nonextensivity, the presence of dark matter, has significant effects on the dynamics of Jeans modes. With the increase in the q parameter and the ratio of dispersion velocities, as well as the decreased ratio of densities of dark to bright matters, both critical wave-numbers and growth rates of the instabilities are increased. In addition, as expected, when q → 1, the corresponding results for Maxwellian velocity distribution are recovered. The present results may shed new light on the comprehension of the intrinsic physical mechanism responsible for the collapse of interstellar gas clouds.

Solar System Collisions with Dense Interstellar Gas Clouds and Radiocarbon Traces of Periods with “Abnormally Low” Solar Modulation of Cosmic Rays

The causes of a sharp increase in the radiocarbon content in the Earth’s atmosphere for periods with abnormally low solar modulation of galactic cosmic rays (GCRs) are considered. It is shown that such periods around 660 BC and 5480 BC can also be caused (in addition to the low activity of the Sun itself) by heliospheric compression as a result of the passage of the Solar System through a compact interstellar cloud with a density of nH ~ 100 cm–3 and characteristic dimensions of 10–100 AU. In this case, GCR modulation is suppressed, while solar cyclic processes are preserved. Wavelet analysis of radiocarbon data shows the presence of the 11-year solar-activity cycle in the period around 5480 BC. Based on the model of the interaction of cosmic rays with the Earth’s atmosphere, the 14C production rates are calculated for various GCR model spectra typical for the local interstellar medium.