A search for intergalactic molecular hydrogen clouds

We report the identification of molecular hydrogen lines in the spectra of 4 high-redshift quasars, OQ172, 4CO5.34, 0528-250 and 1448-232. One absorption system at z = 2.27292 is found in OQ172 and two absorption systems at z = 2.37030 and 2.44478 are found in 4CO5.34.     

A search for absorption of Mg and Ca compounds in molecular clouds towards Galactic continuum sources

Absorption lines of MgH and CaH N  = 1 − 0 transitions were searched for in foreground molecular clouds towards the continuum sources associated with Sgr B2 (M) and W49A (N). None of these lines was detected with our sensitivity level of ∼20 mK. Millimetric absorption lines of MgO, MgOH, CaO and CaOH were also searched for towards Sgr B2 (M) without success. The fractional abundances relative to molecular hydrogen are ≲ 1.0 × 10⁻¹¹ for MgH, ≲ 7.9 × 10⁻¹³ for MgO, ≲ 1.6 × 10⁻¹⁰ for MgOH, ≲ 1.6 × 10⁻⁹ for CaH, ≲ 2.0 × 10⁻¹² for CaO, and ≲ 2.5 × 10⁻¹⁰ for CaOH, respectively. The low abundances measured in absorption indicate that a significant fraction of interstellar magnesium and calcium cannot be tied up in their monohydrides, monoxides and monohydroxides. The low abundance of MgH also implies that grain-surface chemistry involving magnesium is not efficient and that magnesium is depleted on to grains to a factor of ≳ 10^2.5 in well-shielded molecular clouds.     

A search for forward scattering of sunlight from lunar libration clouds

An attempt to determine the radiance of forward scattered sunlight from particles in lunar libration regions was made with the white light coronagraph on Skylab. The libration regions could not be distinguished against the solar K + F coronal background; an upper limit to the libration cloud radiance is determined to be $\text{2·5 × 10}{}^{\mathrm{?11}}\text{B}{}_{\mathrm{?}}$, where $\text{B}{}_{\mathrm{?}}$ is the mean radiance of the solar disk. Employing a model of the particle composition and size distribution which has been proposed for the interplanetary medium, we determine upper limits for the density enhancements in the libration region from the upper limit of the forward scattered radiance presented herein. Similarly, the actual spatial density enhancement is calculated using the earlier observations of the libration region backscattered radiance (Roach, 1975). Enhancements of a factor of 10²–10³ are thus determined, depending upon material composition and size distribution used. By combining the forward and backscatter observations, it is possible to eliminate from consideration clouds whose power law particle size distribution exponent k is 2·5 and complex index of refraction m is 1·33?0.05i and 1·50?0.05i (i.e. absorbing ice and quartz particles, respectively). Finally, the radiance contrast of a possible model libration cloud is calculated with respect to the K- and F-corona/zodiaal light background and is shown to be a maximum in the vicinity of solar elongation angle ～30 deg.     

An assessment of the search for dusty clouds

Some results concerning the intergalactic dust matter are presented. The results of Tarraro, considered as the first undoubted revelation of intergalactic extinction; and of Zabierowski, treated by him as a trial, seem not to be equivalent. Not experimental data, but their theoretical interpretation is responsible for this non-equivalence.Plates have to be prepared by technically uniform procedures to infer the intergalactic obscuration from pseudo-indices. Significant extragalactic obscuration participates in galaxy correlation function, Rubin-Ford effect, underpopulation of quasars, and Sierpiski's sponge structure of galaxy distribution. Contribution from supermassive pre-stars or Population III to the abundance of elements must be considered as highly plausible.     

Turbulent molecular clouds

Stars form within molecular clouds but our understanding of this fundamental process remains hampered by the complexity of the physics that drives their evolution. We review our observational and theoretical knowledge of molecular clouds trying to confront the two approaches wherever possible. After a broad presentation of the cold interstellar medium and molecular clouds, we emphasize the dynamical processes with special focus to turbulence and its impact on cloud evolution. We then review our knowledge of the velocity, density and magnetic fields. We end by openings towards new chemistry models and the links between molecular cloud structure and star-formation rates.     

Comet formation in molecular clouds

The observed properties of the long-period comet system, and its periodic disturbance by galactic forces manifesting as terrestrial impact episodes, may be indicative of a comet capture/escape cycle as the Solar System orbits the Galaxy. A mean number density of comets in molecular clouds of ～10^?1±1 AU^?3 is implied. This is sufficient to deplete metals from the gaseous component of the interstellar medium, as observed, but leads to the problem of how stars are formed nevertheless with solar metal abundances. Formation of comets prior to stars in dense systems of near-zero energy may be indicated, and isotope signatures in cometary particles may be diagnostic of conditions in young spiral arm material.     

Turbulent motions in molecular clouds

We have studied the behavior of the inner motions of OH, H₂CO, and CO molecular clouds. This study shows the existence of two main components of these clouds: the narrow one, associated to dense small clouds and a wide one representing the large diffuse clouds seen in neutral hydrogen, the large clouds are the vortex and intermediate state between turbulent and hydrodynamic motions in the Galaxy.For the dense clouds with sizesd<10 pc we have found a relationship d ^0.38 consistent with the Kolmogorov law of turbulence; the densities and sizes of these clouds behave asnd ^–1. This last relation for these molecular clouds is compared with theHII one. Also, we discuss the effects of the inner magnetic field in these clouds.     

Small-scale clumping in molecular clouds

Summary This review consists of five sections. In the introduction, we briefly review the development of the study of molecular clouds. In the second section, we review the theories of molecular cloud structure and compare these predictions with the statistical properties of the clouds. In Sect. 3 we give an overview of current approaches to determinations of mass and local density in clouds. In the fourth part, we discuss the observations of a selected sample of individual sources. The emphasis here is on high resolution studies of regions of star formation. The final section contains a discussion of instrumental limitations and mentions some future developments.     

A search for molecular gas in restarting radio galaxies

Several radio galaxies are known that show radio morphological signatures that are best interpreted as restarting of nuclear activity after a period of quiescence. The conditions surrounding the phenomenon of nuclear recurrence are not understood. In this paper we have attempted to address this question by examining the nuclear fuelling characteristics in a sample of restarting radio galaxies. We have examined the detection rate for molecular gas in a representative sample of nine restarting radio galaxies, for seven of which we present new upper limits to the molecular gas mass derived from CO line observations we made with the IRAM 30-m telescope. We derive a low CO detection rate for the relatively young restarted radio galaxies suggesting that the cessation of the nuclear activity and its subsequent restarting may be a result of instabilities in the fuelling process rather than a case of depletion of fuel followed by a recent fuel acquisition. It appears that abundant molecular gas content at the level of few  10⁸–10⁹ M_⊙  does not necessarily accompany the nuclear restarting phenomenon. For comparison we also discuss the molecular gas properties of five normal giant radio galaxies, three of which we observed using Swedish-ESO Millimetre Telescope (SEST). Despite obvious signs of interactions and nuclear dust discs none of them has been found to host significant quantities of molecular gas.     

Star formation and molecular clouds

The hypothesis advanced by V. A. Ambartsumyan according to which stars are formed from prestellar superdense objects-- protostars-- was an alternative to the hypothesis of the 1950's (and even now, not much changed) according to which stars are formed by accretion with subsequent collapse (in various modifications). Ambartsumyan's basic inferences were based on an analysis of the observational data available at that time. This paper presents both Ambartsumyan's pioneering ideas and some modern hypotheses of star formation. Some results from studies of molecular clouds and star formation regions are also discussed. One of the distinctive features of young stellar objects (YSO) is the outflow of matter from these objects (molecular, in the form of jets, etc.), a phenomenon whose importance for the evolution of stars was noted by Ambartsumyan as long ago as 1937. Radial systems of dark globules are examined, as well as H-H objects associated with star formation regions, cometary nebulae, and close Trapeziumtype systems (consisting of YSO). Translated from Astrofizika, Vol. 52, No. 2, pp. 185–202 (May 2009).     

The formation of molecular clouds

In a recent paper, Elmegreen has made a cogent case, from an observational point of view, that the lifetimes of molecular clouds are comparable to their dynamical time-scales. If so, this has important implications for the mechanisms by which molecular clouds form. In particular, we consider the hypothesis that molecular clouds may form not by in situ cooling of atomic gas, but rather by the agglomeration of the dense phase of the interstellar medium, much, if not most, of which is already in molecular form.     

Gamma rays from molecular clouds

It is believed that the observed diffuse gamma-ray emission from the galactic plane is the result of interactions between cosmic rays and the interstellar gas. Such emission can be amplified if cosmic rays penetrate into dense molecular clouds. The propagation of cosmic rays inside a molecular cloud has been studied assuming an arbitrary energy and space dependent diffusion coefficient. If the diffusion coefficient inside the cloud is significantly smaller compared to the average one derived for the galactic disk, the observed gamma-ray spectrum appears harder than the cosmic ray spectrum, mainly due to the slower penetration of the low energy particles towards the core of the cloud. This may produce a great variety of gamma-ray spectra.     

Helicoidal magneto-electron waves in interstellar molecular clouds

We investigate the evolution of the magnetic flux density in a magnetically supported molecular cloud driven by Hall and Ohmic components of the electric field generated by the flows of thermal electrons. Particular attention is given to the wave transport of the magnetic field in a cloud whose gas dynamics is dominated by electron flows; the mobility of neutrals and ions is regarded as heavily suppressed. It is shown that electromagnetic waves penetrating such a cloud can be converted into helicons – weakly damped, circularly polarized waves in which the densities of the magnetic flux and the electron current undergo coherent oscillations. These waves are interesting in their own right, because for electron magnetohydrodynamics the low-frequency helicoidal waves have the same physical significance as the transverse Alfvén waves do for a single-component magnetohydrodynamics. The latter, as is known, are considered to be responsible for the widths of molecular lines detected in dark, magnetically supported clouds. From our numerical estimates for the group velocity and the rate of dissipation of helicons it follows that a possible contribution of these waves to the broadening of molecular lines is consistent with the conditions typical of dark molecular clouds.     

Hydrated sulphuric acid in dense molecular clouds

We consider sulphur depletion in dense molecular clouds, and suggest hydrated sulphuric acid, H₂SO₄ ·  n H₂O, as a component of interstellar dust in icy mantles. We discuss the formation of hydrated sulphuric acid in collapsing clouds and its instability in heated regions in terms of the existing hot core models and observations. We also show that some features of the infrared spectrum of hydrated sulphuric acid have correspondence in the observed spectra of young stellar objects.     

The interpretation of flows in molecular clouds

Stellar winds interacting with gas in dense molecular clouds produce flows which may be initially energy or momentum driven. A criterion for this is derived which depends sensitively on the wind velocity. Flows may change from one regime to another depending on the gas distribution about the wind source and these changes are discussed for power law density distributions. In general, the flows observed in CO associated with infrared point sources seem to be in the energy driven regime. By combining CO observations with radio continuum flux measurements, wind parameters are derived for several of these sources. There is some evidence from the derived parameters that high (L _*2×10³ L ) luminosity sources have radiatively-driven winds. Lower luminosity source winds are driven by some agency as yet unknow. We suggest that the widths of infrared lines from wind sources seriously underestimate the wind terminal velocities.     

Nonlinear thin shell instabilities in molecular clouds

Observations of molecular clouds point to the existence of supersonic, turbulent flows. Therefore, any theory which attempts to describe molecular cloud evolution and star formation must include a consideration of the dynamics of colliding flows. Previous studies have considered the collision of supersonic streams or clouds. The resultant instabilities provide a mechanism which may give rise to observable cloud morphologies and enhance the star formation rate. One such instability is the nonlinear thin shell instability (NTSI) of a shock-bounded slab. This process is driven by ram pressure and efficient cooling. In this study, I use numerical simulations to examine the head-on collision of supersonic gas streams in a cold, molecular gas. A dense slab forms in the collision midplane and is prone to a number of instabilities, including the NTSI. The thermodynamic processes involved are found to have a controlling influence upon the instability and fragmentation of the slab. Although some minimal amount of cooling is needed to drive the instability, too rapid a cooling rate gives rise to smaller wavelength instabilities which wipe out the NTSI. The growth rate of the NTSI in a gas undergoing molecular cooling corresponds to a timescale of order 10¹² s, in general agreement with the theoretical value for an isothermal gas. The NTSI may provide a viable mechanism for the instigation of rapid star formation.