The Rôle Of Astrochemistry In Low Mass Star Formation

Molecular processes play both active and passive/diagnostic rôles in the process of star formation. Various molecular behaviours can be identified in star-forming regions with the result that different molecular species can be used to constain different aspects of the infall process, such as the density structures, the kinematics and the evolutionary history of star-forming cores. The main limitations in the chemical analysis of infall sources arise from poorly constrained boundary conditions; in particular the chemical and physical initial conditions are usually very uncertain. The most promising application of astrochemical modelling is probably in the analysis of the infall dynamics through combined chemical/radiative transfer modelling of molecular emission line profiles.     

Formation of a Fractal Structure of Giant Molecular Clouds in the Galaxy

Observations show that molecular clouds in the interstellar medium are fractals with a dimensionality close to 2.35. A model for the formation of clouds from cloudlets ejected from stars is examined in this paper. It is shown that the motion of cloudlets in the interstellar medium is described by a model of generalized brownian motion, so that the resulting clouds should have a fractal structure. We examine the hypothesis that the fractal dimensionality of a cloud is completely determined by the way the mass of the cloudlets varies. The generalized brownian motion of an ensemble of particles is described as a random process with a time dependent parameter. The relationship among the growth of the cloudlet mass, the properties of the process by which the cloudlets move, and the fractal dimensionality of the structures resulting from this process is examined. It is shown that the fractal dimensionality of the formed clouds corresponds to the natural aggregation of mass assuming random cloudlet collisions.     

The 10-μm profile of molecular-cloud and diffuse ISM silicate dust

High signal-to-noise ratio spectra are presented of the 10-μm silicate absorption feature in lines of sight towards Elias 16 and 18 in the Taurus dark cloud, and towards the heavily reddened supergiant Cyg OB2 no. 12. The observations are fitted with laboratory and astronomical spectra to produce intrinsic absorption profiles. These features, which represent molecular-cloud and diffuse ISM dust respectively, are better fitted with emissivity spectra of the Trapezium and μ Cephei than they are with those of laboratory, terrestrial, or other observations of circumstellar silicates. The difference in width between the silicate band in the two environments can be almost entirely ascribed to a broad excess absorption in the long-wavelength wing of the profiles, which is much stronger in the molecular-cloud lines of sight, and possibly reflects grain growth in the denser environment. Limits are placed on the strength of fine spectral structure; if there is a crystalline silicate component in these spectra, it is most likely to be serpentine. Column-density upper limits for methanol and the photolysis product hexamethylenetetramine (HMT) are less than a few per cent of those of water ice and silicates.     

Desorption processes and the deuterium fractionation in molecular clouds

Several processes have been suggested as ways of returning accreted grain mantles to the gas, thus preventing the total removal of molecules from the gas phase in dark quiescent clouds. We attempt to distinguish between them by considering not only the calculated gas-phase abundances, but also the ratio of the abundances of deuterated species to non-deuterated species. We find that the D/H ratio in molecules is relatively model-independent, but that desorption due to the formation of H₂ on grains gives the best overall agreement with the observations.     

HE 1127–1304: the largest radio quasar

By considering the propagation of low-amplitude magnetohydrodynamic waves in partially ionized plasmas, it is shown that the ion-neutral drift (ambipolar diffusion) induced by the waves can have specific effects on the molecular chemistry of cold material. The chemistry occurring in gas swept by Alfvén waves is described and it is shown that this leads to spatial variations in the deuterium fractionation ratios of, for example, HCO⁺ and N₂H⁺, on spatial scales of a few hundredths of a parsec, depending upon the fractional ionization of the ambient medium. The possibility of detecting interstellar Alfvén waves by molecular spectroscopy and their effect of producing small-scale chemical abundance gradients in molecular clouds are briefly discussed.     

Evidence for a large population of shocked interstellar clouds

A 21 cm absorption measurement over a long path length free of the effects of differential galactic rotation indicates the existence of two distinct cloud populations in the plane. One of them consisting of cold, dense clouds has been well studied before. The newly found hot clouds appear to be at least five times more numerous. They have a spin temperature of ~ 300 K, an rms velocity of ~ 35 km s^-1, twice the total mass, and hundred times the kinetic energy of the cold clouds. Over long path lengths, the hot clouds haveN _H/kpc ~ 2 X 10²¹ cm^-2 Kpc^-1, and are estimated to have individual column densities ≤ 10²⁰ cm^-2. We propose that they are shocked clouds found only within supernova bubbles and that the cold clouds are found in the regions in-between old remnants, immersed in an intercloud medium. We conclude that the solar neighbourhood must be located between old supernova remnants rather than within one.     

Collapse and fragmentation of rotating magnetized clouds – I. Magnetic flux–spin relation

We discuss the evolution of the magnetic flux density and angular velocity in a molecular cloud core, on the basis of three-dimensional numerical simulations, in which a rotating magnetized cloud fragments and collapses to form a very dense optically thick core of  >5 × 10¹⁰ cm⁻³  . As the density increases towards the formation of the optically thick core, the magnetic flux density and angular velocity converge towards a single relationship between the two quantities. If the core is magnetically dominated its magnetic flux density approaches  1.5( n /5 × 10¹⁰ cm⁻³)^1/2 mG  , while if the core is rotationally dominated the angular velocity approaches  2.57 × 10⁻³ ( n /5 × 10¹⁰ cm⁻³)^1/2 yr⁻¹  , where n is the density of the gas. We also find that the ratio of the angular velocity to the magnetic flux density remains nearly constant until the density exceeds  5 × 10¹⁰ cm⁻³  . Fragmentation of the very dense core and emergence of outflows from fragments will be shown in the subsequent paper.     

Observational characteristics of dense cores with deeply embedded young protostars

Class 0 objects, which are thought to be the youngest protostars, are identified in terms of NIR or radio emission and/or the presence of molecular outflows. We present combined hydrodynamic and radiative transfer simulations of the collapse of a star‐forming molecular core, which suggest two criteria for identifying dense cores with deeply embedded very young protostars that may not be observable in the NIR or radio with current telescopes. We find that cores with protostars are relatively warm (T > 15 K) with their SEDs peaking at wavelengths <170 µm, and they tend to appear circular. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)