Dark Molecular Clouds Research Articles

The effect of the ionization rate on the chemical composition of dense cores of dark molecular clouds

The effect of variations in the rate of ionization of neutral chemical species by cosmic rays, ζ, on the abundances of some observed molecules in the dense cores of dark molecular clouds is studied. Changes in molecular abundances accompanying an increased (decreased) ionization rate have a single origin: the acceleration (deceleration) of processes that are affected directly or indirectlybychemical reactions with charged species. In addition to affecting the gas-phase chemistry, an increased cosmic-ray flux leads to the more efficient destruction of dust-grain mantles and also accelerates the freezing of some components onto dust. In particular, in a model with an increased ζ, the destruction of the volatile N2 molecule by ionized helium leads to the rapid accumulation of nitrogen atoms in dust-phase ammonia, which has a higher desorption energythan N2. As a result, the gas-phase abundance of NH3 and N2H+ decreases significantly. This mechanism can explain the unusual chemical structures of some dense globules, such as B68, where surprisingly low abundances of nitrogen-bearing molecules are observed together with a central drop in the NH3 and N2H+ column densities. Observations of clouds in HCN and HNC lines can discriminate between the two possible origins of the reduced NH3 and N2H+ abundances: an increased cosmic-ray flux or N2 freezing due to the higher desorption energy of this molecule.

Influence of uncertainties in the rate constants of chemical reactions on astrochemical modeling results

We analyze the influence of errors in the rate constants of gas-phase chemical reactions on the model abundances of molecules in the interstellar medium using the UMIST 95 chemical database. By randomly varying the rate constants within the limits of the errors given in UMIST 95, we have estimated the scatters in theoretical abundances for dark and diffuse molecular clouds. All of the species were divided into six groups by the scatter in their model equilibrium abundances when varying the rate constants of chemical reactions. The distribution of the species in groups depends on the physical conditions. The scatters in the abundances of simple species lie within 0.5–1 order of magnitude, but increase significantly as the number of atoms in the molecule increases. We suggest a simple method for identifying the reactions whose rate constants have the strongest effect on the abundance of a selected species. This method is based on an analysis of the correlations between the abundance of species and the reaction rate constants and allows the extent to which an improvement in the rate constant of a specific reaction reduces the uncertainty in the abundance of the species concerned to be directly estimated.

Cyanopolyynes: carbon chains formation in a carbon arc mimicking the formation of carbon chains in the circumstellar medium

Monocyanopolyynes and dicyanopolyynes can be synthesized quite easily by the submerged electric arc. Monocyanopolyynes having the general formula H[bond](C[triple bond]C)n[bond]C[triple bond]N can be synthesized together with ordinary polyynes series H[bond](C[triple bond]C)n[bond]H by arcing graphite electrodes in acetonitrile. Dicyanopolyynes N[triple bond]C[bond](C[triple bond]C)n[bond]C[triple bond]N are produced almost pure by arcing graphite electrodes directly into liquid nitrogen. These molecules are present in the envelope of post-AGB (asymptotic giant branch), carbon-rich giant stars and also in dark molecular clouds. They are incorporated into comets and also into other primitive materials and may play a role in the prebiotic synthesis of more complex organic molecules having a biological significance. Furthermore, the cyanopolyynes are involved in the atmospheric chemistry of some bodies of the solar system. The discovery of the easy formation of these molecules under laboratory conditions may explain why these molecules are so ubiquitous in space and may also stimulate new ideas about the mechanism of their formation.

ISOCAM observations of the L1551 star formation region

The results of a deep mid-IR ISOCAM survey of the L1551 dark molecular cloud are presented. The aim of this survey is a search for new YSO (Young Stellar Object) candidates, using two broad-band filters centred at 6.7 and 14.3 micron. Although two regions close to the centre of L1551 had to be avoided due to saturation problems, 96 sources were detected in total (76 sources at 6.7 micron and 44 sources at 14.3 micron). Using the 24 sources detected in both filters, 14 were found to have intrinsic mid-IR excess at 14.3 micron and were therefore classified as YSO candidates. Using additional observations in B, V, I, J, H and K obtained from the ground, most candidates detected at these wavelengths were confirmed to have mid-IR excess at 6.7 micron as well, and three additional YSO candidates were found. Prior to this survey only three YSOs were known in the observed region (avoiding L1551 IRS5/NE and HL/XZ Tau). This survey reveals 15 new YSO candidates, although several of these are uncertain due to their extended nature either in the mid-IR or in the optical/near-IR observations. Two of the sources with mid-IR excess are previously known YSOs, one is a brown dwarf MHO 5 and the other is the well known T Tauri star HH30, consisting of an outflow and an optically thick disk seen edge on.

Generation of zonal magnetic fields by low-frequency dispersive electromagnetic waves in a nonuniform dusty magnetoplasma.

It is shown that zonal magnetic fields can be parametrically excited by low-frequency dispersive driftlike compressional electromagnetic (DDCEM) modes in a nonuniform dusty magnetoplasma. For this purpose, we derive a pair of coupled equations which exhibits the nonlinear coupling between DDCEM modes and zonal magnetic fields. The coupled mode equations are Fourier analyzed to derive a nonlinear dispersion relation. The latter depicts that zonal magnetic fields are nonlinearly generated at the expense of the low-frequency DDCEM wave energy. The relevance of our investigation to the transfer of energy from short scale DDCEM waves to long scale zonal magnetic field structures in dark molecular clouds is discussed.

Submillimeter wave astronomy satellite science highlights

Since its launch on 5 December 1998, the Submillimeter Wave Astronomy Satellite ( SWAS) has surveyed more than 5000 lines-of-sight toward more than 200 Galactic objects in transitions of five astrophysically important atomic and molecular species: H 2 16O, H 2 18O, O 2, CI, and 13CO. SWAS sources include giant molecular clouds, dark molecular clouds and starless cores, diffuse and translucent interstellar clouds, planetary nebulae, supernova remnants, oxygen-rich evolved stars, extreme carbon stars, and solar system objects such as Jupiter, Saturn, Mars and comets. A review of SWAS results relating to each of these sources is beyond the scope of this paper. However, a sampling of discoveries is offered through a summary of three diverse areas of SWAS studies – new insights into interstellar chemistry, cometary studies, and extra-solar comets.

Physical Processes in Star–Gas Systems

AbstractWe first present a recently developed three-dimensional chemodynamic code for galaxy evolution from the Kiev–Kiel collaboration. It follows the evolution of all components of a galaxy, such as dark matter, stars, molecular clouds and diffuse interstellar matter. Dark matter and stars are treated as collisionless N-body systems. The interstellar matter is numerically described by a smoothed particle hydrodynamics approach for the diffuse (hot) gas and a sticky particle scheme for the (cool) molecular clouds. Physical processes, such as star formation, stellar death, or condensation and evaporation processes of clouds interacting with the ISM are described locally. An example application of the model to a star forming dwarf galaxy will be shown for comparison with other codes. Secondly, we will discuss new kinds of exotic chemodynamic processes, as they occur in dense gas–star systems in galactic nuclei, such as non-standard ‘drag’-force interactions, destructive and gas-producing stellar collisions. Their implementation in one-dimensional dynamic models of galactic nuclei is presented. Future prospects to generalise these to three dimensions are in progress and will be discussed.

Chemistry ahead of Herbig-Haro objects

There is now compelling evidence that dark molecular clouds are clumpy. Much of the clumpiness is unresolved by single-dish telescopes but is apparent in the data from array telescopes. Molecular clumps may also be observed close to Herbig-Haro (HH) objects. These clumps are easily observable because they are ‘illuminated’ due to the UV radiation from the shock front of the HH jet. A detailed observational and theoretical study of one HH clump has been performed and it indicates that this clump must be transient and has a similar density and temperature to those clumps detected in the cloud interior. Thus, HH clumps may be used as an independent method of determining physical parameters of the clumpiness of molecular clouds.

Is RX J1856.5−3754 a Quark Star?

Deep Chandra Low Energy Transmission Grating and High Resolution Camera spectroscopic observations of the isolated neutron star candidate RX J1856.5-3754 have been analyzed to search for metallic and resonance cyclotron spectral features and for pulsation behavior. As found from earlier observations, the X-ray spectrum is well represented by an ~60 eV (7 × 105 K) blackbody. No unequivocal evidence of spectral line or edge features has been found, arguing against metal-dominated models. The data contain no evidence for pulsation, and we place a 99% confidence upper limit of 2.7% on the unaccelerated pulse fraction over a wide frequency range from 10-4 to 100 Hz. We argue that the derived interstellar medium neutral hydrogen column density of 8 × 1019 cm-2 ≤ NH ≤ 1.1 × 1020 cm-2 favors the larger distance from two recent Hubble Space Telescope parallax analyses, placing RX J1856.5-3754 at ~140 pc instead of ~60 pc and in the outskirts of the R CrA dark molecular cloud. That such a comparatively rare region of high interstellar matter (ISM) density is precisely where an isolated neutron star reheated by accretion of ISM would be expected is either entirely coincidental or current theoretical arguments excluding this scenario for RX J1856.5-3754 are premature. Taken at face value, the combined observational evidence—a lack of spectral and temporal features and an implied radius of R∞ = 3.8-8.2 km that is too small for current neutron star models—points to a more compact object, such as allowed for quark matter equations of state.

The rotationalexcitation by helium of methanol in its ground and first excitedtorsional states

We have improved and extended our previous calculations of cross sections for the rotational excitation of methanol by helium. Inthe case of methanol in its torsional ground state, we extendedour coupled states computations of the cross sections to highercollision energies and larger rotational basis sets. Theaccuracy of the rotational energy levels and eigenfunctions hasbeen reviewed and improved for A-type methanol. The calculationshave been extended to A- and E-type methanol in their firstexcited torsional states by averaging the CH3OH-Heinteraction potential over the excited state torsionaleigenfunctions. Thermal rate coefficients have been calculatedat low temperatures. Predictions are made of line intensityratios which are sensitive to the density of the He perturberand which lend themselves to the determination of the perturberdensities in the dark molecular clouds of the interstellarmedium.

Waves and instabilities in dark interstellar molecular clouds containing ferromagnetic dust grains

The propagation characteristics of magnetization waves, as well as the instabilities of sound waves in a self-gravitating dark interstellar molecular cloud containing ferromagnetic dust grains and baryonic gas clouds, have been theoretically investigated by including the dynamics of both ferromagnetic dust grains and baryonic gases. It has been shown that there exist two types of subsonic or supersonic (depending on the field strength of the magnetization) transverse magnetization waves, which can be regarded as counterparts of Alfven waves (for the parallel propagation) and magnetosonic waves (for the perpendicular propagation) in a magnetoactive plasma. It has also been found that, in addition to the usual Jeans instability, the sound waves suffer a new type of instability, which is due to the combined effects of the baryonic gas dynamics and self-gravitational field in both weakly and highly collisional regimes.

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.

Velocities of Magneto-Elastic and Magneto-Electron Waves in Dark Molecular Clouds

We present analytic and numerical estimates for group velocity of wave motions in two models of cold interstellar medium presumably constituting the interior of cores of magnetically supported dark molecular clouds. Namely, in the model of gas-based ferrocolloidal soft matter and in the model of noncompensated electron magnetoplasma. The predictions of these models are given in juxtaposition with data on recent Zeeman measurements of the molecular linewidths detected from dark central regions of star-forming interstellar clouds.

Gas-grain chemical models of star-forming molecular clouds as constrained by ISO and SWAS observations

We have investigated the gaseous and solid state molecular composition of dense interstellar material that periodically experiences processing in the shock waves associated with ongoing star formation. Our motivation is to confront these models with the stringent abundance constraints on CO2, H2O and O2, in both gas and solid phases, that have been set by ISO and SWAS. We also compare our results with the chemical composition of dark molecular clouds as determined by ground-based telescopes. Beginning with the simplest possible model needed to study molecular cloud gas-grain chemistry, we only include additional processes where they are clearly required to satisfy one or more of the ISO-SWAS constraints. When CO, N2 and atoms of N, C and S are efficiently desorbed from grains, a chemical quasi-steady-state develops after about one million years. We find that accretion of CO2 and H2O cannot explain the ISO observations; as with previous models, accretion and reaction of oxygen atoms are necessary although a high O atom abundance can still be derived from the CO that remains in the gas. The observational constraints on solid and gaseous molecular oxygen are both met in this model. However, we find that we cannot explain the lowest abundances seen by SWAS or the highest atomic carbon abundances found in molecular clouds; additional chemical processes are required and possible candidates are given. One prediction of models of this type is that there should be some regions of molecular clouds which contain high gas phase abundances of H2O, O2 and NO. A further consequence, we find, is that interstellar grain mantles could be rich in NH2OH and NO2. The search for these regions, as well as NH2OH and NO2 in ices and in hot cores, is an important further test of this scenario. The model can give good agreement with observations of simple molecules in dark molecular clouds such as TMC-1 and L134N. Despite the fact that S atoms are assumed to be continously desorbed from grain surfaces, we find that the sulphur chemistry independently experiences an "accretion catastrophe" . The S-bearing molecular abundances cease to lie within the observed range after about years and this indicates that there may be at least two efficient surface desorption mechanisms operating in dark clouds -one quasi-continous and the other operating more sporadically on this time-scale. We suggest that mantle removal on short time-scales is mediated by clump dynamics, and by the effects of star formation on longer time-scales. The applicability of this type of dynamical-chemical model for molecular cloud evolution is discussed and comparison is made with other models of dark cloud chemistry.

Astronomy. The complex story of H2.

Molecular hydrogen is the most abundant species in the universe. In his Perspective, [Klemperer][1] explains how the complex electronic properties of hydrogen helps explain its behavior in space, including the formation of a rich chemistry of organic molecules in dark molecular clouds. [1]: http://www.sciencemag.org/cgi/content/full/293/5531/815

L 1274: A multiwavelength study of a dark cloud in the Cep -Cas void

We present a multiwavelength study of L 1274, a dark molecular cloud in the Cep { Cas void. From objective prism spectroscopy we derive a distance of 200 30 pc to the cloud. CO maps reveal a slightly elongated molecular cloud with a diameter of about 2 pc. Maps in 12 CO (1{0), 13 CO (1{0) and 12 CO (2{1) indicate two distinct features: a \main and an adjacent \ridge. The kinetic temperature of the CO gas is 10{12 K, calculated by a non { LTE method. The total gas mass of the main body is estimated from the CO data to be 25 { 45M. We nd a large dierence between CO { inferred gas mass and virial mass, a property usually attributed to HLCs. However, the derived mean density of H2 is at least an order of magnitude larger than typical for HLCs, i.e. similar to ordinary dark cloud cores. We discuss the validity of our mass estimates and the possibility of an external disturbance, e.g. a passage of a supernova induced shock{front.

Internal structure of a cold dark molecular cloud inferred from the extinction of background starlight.

Stars and planets form within dark molecular clouds, but little is understood about the internal structure of these clouds, and consequently about the initial conditions that give rise to star and planet formation. The clouds are primarily composed of molecular hydrogen, which is virtually inaccessible to direct observation. But the clouds also contain dust, which is well mixed with the gas and which has well understood effects on the transmission of light. Here we use sensitive near-infrared measurements of the light from background stars as it is absorbed and scattered by trace amounts of dust to probe the internal structure of the dark cloud Barnard 68 with unprecedented detail. We find the cloud's density structure to be very well described by the equations for a pressure-confined, self-gravitating isothermal sphere that is critically stable according to the Bonnor-Ebert criteria. As a result we can precisely specify the physical conditions inside a dark cloud on the verge of collapse to form a star.

Supersonic waves of magnetization in dark interstellar molecular clouds

We suggest that supersonic linewidths inferred from recent measurements of magnetic fields toward the core position in dark interstellar molecular clouds may be due to transverse waves of magnetization propagating in a poorly ionized, magnetically ordered gas—dust medium composed of tiny ferromagnetic dusty grains suspended in a cold gaseous cloud of molecular hydrogen.

Far‐InfraredISOMaps of Active Galaxies

Far-IR Infrared Space Observatory (ISO) maps of four Seyfert galaxies are presented and compared with their optical counterpart images. The far-IR maps resemble the optical images of these galaxies when degraded to the ISO resolution. The spatial extension of the far-IR maps is also comparable to the optical extension of these objects, an indication of the extended nature of their far-IR emission. The 90 μm intensity profiles of these galaxies show that there is a disk component noticeable in the far IR. NGC 3079 shows a 90 μm halo extending far out of the galaxy plane. NGC 4051 shows a dark molecular cloud whose far-IR output could be as large as that of the active nucleus itself, demonstrating that very bright IR sources other than the active nucleus may be responsible for a large fraction of the far-IR emission in some Seyfert galaxies.

Temperature Rise at the Edges of Dark Molecular Clouds

Two-fluid magnetohydrodynamic equations are applied to dark molecular clouds that are composed of neutrals mixed with minor charged particles, weakly ionized gas systems. The result shows the temperatures are higher at the cloud edges than at their inner regions, the cause of which is that the cloud potential, released as clouds contract particularly at their edges, along with some dissipated rotational kinetic energy is converted into thermal. The cloud contracting is due to the loss of the magnetic field that threads it through ambipolar diffusion. Nevertheless, without the support of the magnetic and the centrifugal forces in the direction of the magnetic field assumed to be parallel to the cloud rotating axis, the cloud collapses in that direction when its mass is over the Jeans mass.