Hydrogen In Interstellar Clouds Research Articles

Low-energy cosmic rays: regulators of the dense interstellar medium

Low energy cosmic rays (up to the GeV energy domain) play a crucial role in the physics and chemistry of the densest phase of the interstellar medium. Unlike interstellar ionising radiation, they can penetrate large column densities of gas, and reach molecular cloud cores. By maintaining there a small but not negligible gas ionisation fraction, they dictate the coupling between the plasma and the magnetic field, which in turn affects the dynamical evolution of clouds and impacts on the process of star and planet formation. The cosmic-ray ionisation of molecular hydrogen in interstellar clouds also drives the rich interstellar chemistry revealed by observations of spectral lines in a broad region of the electromagnetic spectrum, spanning from the submillimetre to the visual band. Some recent developments in various branches of astrophysics provide us with an unprecedented view on low energy cosmic rays. Accurate measurements and constraints on the intensity of such particles are now available both for the very local interstellar medium and for distant interstellar clouds. The interpretation of these recent data is currently debated, and the emerging picture calls for a reassessment of the scenario invoked to describe the origin and/or the transport of low energy cosmic rays in the Galaxy.

Jet propagation through inhomogeneous media and shock ionization

AbstractIn this contribution, we present the first numerical simulations of a relativistic outflow propagating through the inner hundreds of parsecs of its host galaxy, including atomic and ionized hydrogen, and the cooling effects of ionization. Our results are preliminary, but we observe efficient shock ionization of atomic hydrogen in interstellar clouds. The mean density of the interstellar medium in these initial simulations is lower than that expected in typical galaxies, which makes cooling times longer and thus no recombination is observed inside the shocked region. The velocities achieved by the shocked gas in the simulations are in agreement with observational results, although with a wide spectrum of values.

The Transition from Atomic to Molecular Hydrogen in Interstellar Clouds: 21 cm Signature of the Evolution of Cold Atomic Hydrogen in Dense Clouds

We have investigated the timescale for formation of molecular clouds by examining the conversion of H I to H2 using a time-dependent model with H2 photodissociation including self-shielding. H2 formation on dust grains and cosmic-ray destruction are also included in one-dimensional model slab clouds that incorporate time-independent density and temperature distributions. We calculate 21 cm spectral line profiles seen in absorption against a background provided by general Galactic H I emission and compare the model spectra with H I narrow self-absorption (HINSA) profiles absorbed in a number of nearby molecular clouds. The time evolution of the H I and H2 densities is dramatic, with the atomic hydrogen disappearing in a wave propagating from the central, denser regions, which have a shorter H2 formation timescales, to the edges, where the density is lower and the timescales for H2 formation longer. The model 21 cm spectra are characterized by very strong absorption at early times. Emission at early times produced by the warm edges of the cloud is difficult to separate from variations in the background spectrum, when the background temperature is low. The minimum time for cloud evolution based on the model spectra is set by the requirement that most of the H I in the outer portions of the cloud be removed. The characteristic time that has elapsed since cloud compression and initiation of the H I → H2 conversion is a few ×1014 s, or 107 yr. This sets a minimum time for the age of these molecular clouds and thus for star formation that may take place within them.

Effective grain surface area in the formation of molecular hydrogen in interstellar clouds

Aims. In the interstellar clouds, molecular hydrogens are formed from atomic hydrogen on grain surfaces. An atomic hydrogen hops around till it finds another one with which it combines. This necessa rily implies that the average recombination time, or equivalently, the effective grain surface area depends on the relative numbers of atomic hydrogen influx rate and the number of sites on the grain. Our a im is to discover this dependency. Methods. We perform a numerical simulation to study the recombination of hydrogen on grain surfaces in a variety of cloud conditions. We use a square lattice (with a periodic boundary condition) of various sizes on two types of grains, namely, amorphous carbon and olivine. Results. We find that the steady state results of our simulation match v ery well with those obtained from a simpler analytical consideration ,

Efficiency of Molecular Hydrogen Formation on Silicates

We report on laboratory measurements of molecular hydrogen formation and recombination on an olivine slab as a function of surface temperature under conditions relevant to those encountered in the interstellar medium. On the basis of our experimental evidence, we recognize that there are two main regimes of H coverage that are of astrophysical importance; for each of them we provide an expression giving the production rate of molecular hydrogen in interstellar clouds.

Infrared Response of H2to X‐Rays in Dense Clouds

The excitation by X-rays and cosmic rays of molecular hydrogen in interstellar clouds is analyzed. We carried out detailed calculations of entry efficiencies in rovibrational levels of H2 following impact with fast electrons produced by X-ray ionization of the gas. The competing effect of collisional excitation, and quenching by the ambient gas is examined in detail. Up to date values for H-H2 collisional rate coefficients are adopted, and some derivations of H2-H2 rovibrational rate coefficients from existing literature data are proposed. Several models as a function of temperature, density, and ionization rate are presented. We found that H2 infrared emission in X-ray dominated regions (XDR) is potentially observable for temperatures and ionization rates lower than certain critical values (typically T < 1000 K and zeta/n(sub H) < 10(exp -15) cc/s where zeta is the ionization rate). At higher temperatures, collisional excitation by the ambient gas dominates the population of low vibrational levels, and at higher values of zeta/n(sub H) the abundance of H2 is negligible. If such conditions are satisfied, the resulting infrared emission spectrum can be used as a diagnostic of nearby X-ray sources such as in cooling flows in galaxy clusters, quasars, Seyfert galaxies and supernova remnants. The intensity ratio of the 2-1S(1) and 1-0S(1) lines measured for the Seyfert galaxy NGC 1275 is consistent with X-ray pumping.

On the Mechanism of H2 Formation in the Interstellar Medium

The problem of the formation of molecular hydrogen in interstellar clouds is revisited. the role played by cosmic ray bombardment under certain circumstances is considered mainly in the light of the low formation rate of H2 on grains due to the reduced mobility of adsorbed H atoms on their amorphous surfaces at interstellar temperatures.

Dependence of interstellar depletion on hydrogen column density - Possibilities and implications

A reexamination of the observed column densities of various elements in diffuse clouds suggests that almost all elements including oxygen, nitrogen, sulfur, and argon may be depleted with respect to hydrogen in interstellar clouds with large hydrogen column density. The amount of depletion varies from element to element and increases with increasing column density of hydrogen nuclei. This result is in qualitative agreement with the depletion of oxygen and sulfur independently inferred from the gas phase chemistry of sulfur in dense clouds. The rate of increase of depletion with hydrogen column density implied by the present study is large. It is possible that observational selection effects may have amplified the real dependence on N(H). A broad spectrum of C/O ratios ranging from values greater than unity to values less than unity appears possible for interstellar clouds, which would have the effect of a large variation in chemical composition from cloud to cloud.

Interstellar H2 - The population of excited rotational states and the infrared response to ultraviolet radiation

Molecular hydrogen in interstellar clouds absorbs ultraviolet radiation in lines of the Lyman and Werner systems. The subsequent fluorescence leads to dissociation or to the population of excited rotational-vibrational levels of the ground electronic state. The rotational-vibrational levels decay by means of quadrupole transitions which result in the emission of infrared photons and populate excited rotational levels of the lowest vibrational state. An additional input into the cascade is provided by the formation of molecules on grain surfaces. A concise tabular description of the rotation-vibration cascade is presented. Calculations of the infrared emission spectrum of H$sub 2$ are given for an illustrative cloud model which includes the processes of fluorescence and hot molecule formation. In favorable circumstances, some of the infrared lines may have detectable intensities. Due to distribution of lines, it may prove possible to detect interstellar H$sub 2$ using narrow-band filter photometry at a wavelength of 2.4 $mu$. (AIP)

A Dust-Radiation Instability in Ionized Nebulae

view Abstract Citations (6) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Dust-Radiation Instability in Ionized Nebulae Nordsieck, Kenneth H. Abstract Inhomogeneities in planetary nebulae and H ii regions may be formed by the pressure of trapped resonance4ine radiation on dust grains For a large range in physical conditions in such nebulae, the formation time of sizable regions of abnormal dust content is considerably less than the lifetime of the nebulae. The dust and gas, however, are only weakly coupled: the dust perturbation is ahvays at least 10 times the ionized-gas perturbation. The resulting grain-enriched gas may later form into a star with peculiar abundance characteristics, if the region of perturbation is large enough to include a stellar mass of gas. A conventional linear perturbation approach is used to obtain the e-folding time of a dust-density perturbation from a set of differential equations describing the motion and interaction of the dust and gas fluids, the ionization balance of hydrogen, and the diffusion of La line radiation. Also, an estimate of the La density caused by cosmic-ray ionization and excitation of neutral hydrogen in interstellar clouds shows that the instability may be marginally active there for regions less than 1 pc in size (corresponding to included gas masses / < 1). Publication: The Astrophysical Journal Pub Date: January 1971 DOI: 10.1086/150767 Bibcode: 1971ApJ...163..287N full text sources ADS |

Clustering of Cold Hydrogen Gas on Protons

KRASOVSKII1 has suggested that the presence of ion-clusters of hydrogen in gaseous nebulae could account for the absorption and polarization of stellar radiation. It has since been argued2 that interstellar gas densities are too low for such processes. Hoyle and co-workers3,4 have, however, proposed that condensation of hydrogen on graphite grains at ∼3 K might promote the formation of galaxies and stars. The densities of hydrogen in interstellar clouds may thus be great enough for ion-clusters to form by interaction with radiation.