scholarly journals The Fractional Ionization in Molecular Cloud Cores

2000 ◽  
Vol 197 ◽  
pp. 41-50 ◽  
Author(s):  
Paola Caselli
Keyword(s):  
Dynamical Evolution ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Young Stellar Objects ◽  
Ionization Degree ◽  
Degree Of Ionization ◽  
Stellar Objects ◽  
Interstellar Clouds ◽  
Ion Molecule Reactions ◽  
Cloud Cores

Ions and electrons play a key role in the chemical and dynamical evolution of interstellar clouds. Gas phase ion–molecule reactions are major chemical routes to the formation of interstellar molecules. The ionization degree determines the coupling between the magnetic field and the molecular gas through ion–neutral collisions, and thus regulates the rate of star formation. In the theoretical determination of the degree of ionization we run into several sources of uncertainty, including the poorly known cosmic ray flux and metal depletion within the cores, the penetration of UV radiation deep into regions of high visual extinction due to cloud inhomogeneities, and the ionization rate increase in the proximity of young stellar objects which may be strong X–ray emitters. Observational estimates of electron (or ion) fractions x(e) (≡ n(e)/n(H2), where n(e) and n(H2) are the electron and molecular hydrogen number densities, respectively) in dense cloud cores are thus of considerable interest. In this paper, I will review recent improvements in the estimates of the ion fraction in dense cores and point out the difficulties in determining x(e).

scholarly journals The Determination of Electron Abundances in Interstellar Clouds

1980 ◽  
Vol 87 ◽  
pp. 339-340
Author(s):  
Alwyn Wootten ◽  
Ronald Snell ◽  
A. E. Glassgold
Keyword(s):  
Gas Phase ◽  
Molecular Clouds ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Interstellar Clouds ◽  
Ion Molecule Reactions ◽  
Upper Limits ◽  
Gas Phase Ion ◽  
Existing Data

A new method for estimating electron fractions in shielded molecular clouds is proposed on the basis of gas phase ion-molecule reactions which involves measuring the quantity . Applied to existing data, it yields upper limits to Xe in the range from 10−8 to 10−7 for a variety of clouds, warm as well as cool. An upper bound to the cosmic ray ionization rate is also obtained.

Laboratory investigations aimed at building a database for the interpretation of JWST spectra

2019 ◽  
Vol 15 (S350) ◽  
pp. 77-80
Author(s):  
Maria Elisabetta Palumbo ◽  
Giuseppe A. Baratta ◽  
Gleb Fedoseev ◽  
Daniele Fulvio ◽  
Carlotta Scirè ◽  
...  
Keyword(s):  
Thermal Processing ◽  
Laboratory Data ◽  
Cosmic Ray ◽  
Peak Position ◽  
Young Stellar Objects ◽  
Line Of Sight ◽  
High Quality ◽  
Stellar Objects ◽  
James Webb Space Telescope ◽  
Laboratory Investigations

AbstractThe James Webb Space Telescope (JWST) is expected to be launched in 2021. The JWST’s science instruments will provide high quality spectra acquired in the line of sight to young stellar objects whose interpretation will require a robust database of laboratory data. With this in mind, an experimental work is in progress in the Laboratory for Experimental Astrophysics in Catania to study the profile (shape, width, and peak position) of the main infrared bands of molecular species expected to be present in icy grain mantles. Our study also takes into account the modifications induced on icy samples by low-energy cosmic ray bombardment and by thermal processing. Here we present some recent results on deuterium hydrogen monoxide (HDO), N-bearing species, and carbon dioxide (CO2).

scholarly journals Dust Evolution in Protoplanetary Disks

1996 ◽  
Vol 150 ◽  
pp. 513-516
Author(s):  
Thomas Henning ◽  
Wolfgang Schmitt ◽  
Hubert Klahr ◽  
Rastislav Mucha
Keyword(s):  
Numerical Study ◽  
Dynamical Evolution ◽  
Dynamical Behaviour ◽  
Young Stellar Objects ◽  
Dust Particles ◽  
Circumstellar Disk ◽  
Stellar Objects ◽  
One Dimensional ◽  
First Results ◽  
Dust Evolution

AbstractThe evolution of dust particles in circumstellar disk-like structures around protostars and young stellar objects is discussed. We especially consider the coagulation of grains due to collisional aggregation and the influence of this process on the optical properties of the particles. These dust opacities are important for both the derivation of the circumstellar dust mass from submillimetre continuum observations and the dynamical behaviour of the disks.We present first results of a numerical study of the coagulation of dust grains in a turbulent protoplanetary accretion disk described by a time-dependent one-dimensional (radial) “alpha” model. The influence of grain opacity changes due to dust coagulation on the dynamical evolution of a protostellar disk is investigated. In addition, we consider the grain motion in two-dimensional disks.

scholarly journals 1D, 2D and 3D Collapse of Interstellar Clouds

1980 ◽  
Vol 58 ◽  
pp. 235-246
Author(s):  
W.M. Tscharnuter
Keyword(s):  
Star Formation ◽  
Gravitational Instability ◽  
Numerical Models ◽  
Young Stellar Objects ◽  
Stellar Objects ◽  
Interstellar Clouds ◽  
B Stars ◽  
Theoretical Investigations ◽  
2D And 3D

This review is concerned with recent theoretical investigations and numerical models of star formation with varions symmetries. Observations strongly support the fact that stars condense out of cool (≈10 K) and dense (103-104 atoms/cm3) interstellar clouds due to gravitational instability and collapse. Bright, young stellar objects (0- and B-stars are always found in the vicinity of coloud complexes.

scholarly journals Rapid elimination of small dust grains in molecular clouds

2020 ◽  
Vol 641 ◽  
pp. A39 ◽  
Author(s):  
Kedron Silsbee ◽  
Alexei V. Ivlev ◽  
Olli Sipilä ◽  
Paola Caselli ◽  
Bo Zhao
Keyword(s):  
Size Distribution ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Microwave Emission ◽  
Dust Grains ◽  
Gas Temperature ◽  
Neutral Gas ◽  
Cloud Cores ◽  
The Impact ◽  
Small Dust

We argue that impact velocities between dust grains with sizes of less than ∼0.1 μm in molecular cloud cores are dominated by drift arising from ambipolar diffusion. This effect is due to the size dependence of the dust coupling to the magnetic field and the neutral gas. Assuming perfect sticking in collisions up to ≈50 m s−1, we show that this effect causes rapid depletion of small grains, consistent with starlight extinction and IR and microwave emission measurements, both in the core center (n ∼ 106 cm−3) and envelope (n ∼ 104 cm−3). The upper end of the size distribution does not change significantly if only velocities arising from this effect are considered. We consider the impact of an evolved grain-size distribution on the gas temperature, and argue that if the depletion of small dust grains occurs as expected from our model, then the cosmic ray ionization rate must be well below 10−16 s−1 at a number density of 105 cm−3.

scholarly journals On the line profiles of shell-shaped bipolar outflows

1987 ◽  
Vol 122 ◽  
pp. 87-88
Author(s):  
G. Silvestro ◽  
M. Robberto
Keyword(s):  
Shell Structure ◽  
High Velocity ◽  
Young Stellar Objects ◽  
Structure Characteristic ◽  
Line Profiles ◽  
Bipolar Outflows ◽  
Stellar Objects ◽  
Interstellar Clouds ◽  
Molecular Outflows

High velocity molecular outflows with bipolar morphology are detected in association with young stellar objects within dense interstellar clouds. Recent observations suggest that the flow could be “confined to a relatively thin, swept-up shell surrounding an evacuated wind cavity” (1). A shell structure characteristic of the wind-cloud interaction had been predicted in earlier theoretical works (see for instance (2)). More recently, models with different (not shell-shaped) geometries were presented, e.g. (3).

scholarly journals The distribution of cosmic-ray ionization rates in diffuse molecular clouds as probed by H 3 +

2012 ◽  
Vol 370 (1978) ◽  
pp. 5142-5150 ◽  
Author(s):  
Nick Indriolo
Keyword(s):  
Molecular Clouds ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Line Of Sight ◽  
Interstellar Clouds ◽  
Upper Limits ◽  
The Galaxy ◽  
Cosmic Ray Flux ◽  
Made In ◽  
Ionization Rates

Owing to its simple chemistry, H is widely regarded as the most reliable tracer of the cosmic-ray ionization rate in diffuse interstellar clouds. At present, H observations have been made in over 50 sight lines that probe the diffuse interstellar medium (ISM) throughout the Galaxy. This small survey presents the opportunity to investigate the distribution of cosmic-ray ionization rates in the ISM, as well as any correlations between the ionization rate and line-of-sight properties. Some of the highest inferred ionization rates are about 25 times larger than the lowest upper limits, suggesting variations in the underlying low-energy cosmic-ray flux across the Galaxy. Most likely, such variations are caused predominantly by the distance between an observed cloud and the nearest site of particle acceleration.

scholarly journals The Cosmic-Ray Ionization Rate

1992 ◽  
Vol 150 ◽  
pp. 471-475 ◽  
Author(s):  
Stephen Lepp
Keyword(s):  
Cosmic Rays ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Reaction Networks ◽  
Interstellar Clouds ◽  
Heating Mechanism

A wide variety of molecules have been observed in the interstellar clouds. They are believed to be formed by reaction networks which begin with ionization by cosmic-rays. Cosmic-rays are also an important heating mechanism for many astrophysical regions. In this paper I shall review the methods used to infer the cosmic-ray ionization rate and the values which have been measured.

in the diffuse interstellar medium

2006 ◽  
Vol 364 (1848) ◽  
pp. 3049-3062 ◽  
Author(s):  
Harvey S Liszt
Keyword(s):  
Interstellar Medium ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Time Dependent ◽  
Static Equilibrium ◽  
Equilibrium Conditions ◽  
Interstellar Clouds ◽  
Comprehensive Theory ◽  
Trace Species ◽  
Optically Transparent

Three forms of solely hydrogen-bearing molecules—H 2 , HD and —are observed in diffuse or optically transparent interstellar clouds. Although no comprehensive theory exists for the diffuse interstellar medium or its chemistry, the abundances of these species can generally be accommodated locally within the existing static equilibrium frameworks for heating/cooling, H 2 -formation on large grains, etc. with one modification demanded equally by observations of HD and , i.e. a pervasive low-level source of H and H 2 ionization ca 10 times faster than the usual cosmic ray ionization rate ζ H ≈10 −17  s −1 per free H-atom. We discuss this situation with reference to observation and time-dependent modelling of H 2 and formation. While not wishing to appear ungrateful for the success of what are very simplistic notions of the interstellar medium, we point out several reasons not to feel smug. The equilibrium conditions which foster high H 2 and abundances are very slow to appear and these same simple ideas of static equilibrium cannot explain any, but a few, of the simplest of the trace species, which are ubiquitously embedded in H 2 -bearing diffuse gases.

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