scholarly journals Water ice deposition and growth in molecular clouds

2020 ◽  
Vol 500 (4) ◽  
pp. 5117-5128
Author(s):  
Jonathan M C Rawlings ◽  
D A Williams
Keyword(s):  
Molecular Clouds ◽  
Column Density ◽  
Local Density ◽  
Dynamical Evolution ◽  
Threshold Value ◽  
Volume Density ◽  
Careful Consideration ◽  
Interstellar Clouds ◽  
Water Ice ◽  
Local Mean

ABSTRACT In interstellar clouds, the deposition of water ice on to grains only occurs at visual extinctions above some threshold value (Ath). At extinctions greater than Ath, there is a (near-linear) correlation between the inferred column density of the water ice and AV. For individual cloud complexes such as Taurus, Serpens, and ρ-Ophiuchi, Ath and the gradients of the correlation are very similar along all lines of sight. We have investigated the origin of this phenomenon, with careful consideration of the various possible mechanisms that may be involved and have applied a full chemical model to analyse the behaviours and sensitivities in quiescent molecular clouds. Our key results are as follows: (i) the ubiquity of the phenomenon points to a common cause, so that the lines-of-sight probe regions with similar, advanced, chemical, and dynamical evolution; (ii) for Taurus and Serpens Ath and the slope of the correlation can be explained as resulting from the balance of freeze-out of oxygen atoms and photodesorption of H2O molecules. No other mechanism can satisfactorily explain the phenomenon; (iii) Ath depends on the local density, suggesting that there is a correlation between local volume density and column density; (iv) the different values of Ath for Taurus and Serpens are probably due to variations in the local mean radiation field strength; (v) most ice is accreted on to grains that are initially very small (<0.01$\,\mu$m); and (vi) the very high value of Ath observed in ρ-Ophiuchi cannot be explained in the same way, unless there is complex microstructure and/or a modification to the extinction characteristics.

scholarly journals The Mass Spectrum of Interstellar Clouds

1989 ◽  
Vol 120 ◽  
pp. 511-517
Author(s):  
John M. Dickey ◽  
R. W. Garwood
Keyword(s):  
Cross Sections ◽  
Molecular Clouds ◽  
Column Density ◽  
Unit Volume ◽  
Empirical Relationship ◽  
Line Of Sight ◽  
Interstellar Clouds ◽  
Absorption Studies ◽  
Low Latitudes ◽  
Diffuse Clouds

AbstractThe abundance of 21-cm absorption lines seen in surveys at high latitudes can be translated into a line of sight abundance of clouds vs. column density using an empirical relationship between temperature and optical depth. As VLA surveys of 21-cm absorption at low latitudes are now becoming available, it is possible to study the variation of this function with galactic radius. It is interesting to compare the abundance of these diffuse atomic clouds (with temperatures of 50 to 100 K and masses of 1 to 10 M⊙) to the abundance of molecular clouds. To do the latter we must make assumptions about cloud cross-sections in order to convert the line of sight abundance of diffuse clouds into a number per unit volume, and to convert from cloud column density to mass. The spectrum of diffuse clouds matches fairly well the spectrum of molecular clouds, although observationally there is a gap of several orders of magnitude in cloud mass. Optical absorption studies also agree well with the 21-cm results for clouds of column density a few times 1020 M⊙.

scholarly journals A common column density threshold for scattering at 3.6μm and water-ice in molecular clouds

2014 ◽  
Vol 568 ◽  
pp. L3 ◽  
Author(s):  
M. Andersen ◽  
W.-F. Thi ◽  
J. Steinacker ◽  
N. Tothill
Keyword(s):  
Molecular Clouds ◽  
Column Density ◽  
Water Ice ◽  
Density Threshold

scholarly journals Galactic cold cores

2018 ◽  
Vol 614 ◽  
pp. A83 ◽  
Author(s):  
M. Juvela ◽  
J. Malinen ◽  
J. Montillaud ◽  
V.-M. Pelkonen ◽  
I. Ristorcelli ◽  
...  
Keyword(s):  
Radiation Field ◽  
Molecular Clouds ◽  
Column Density ◽  
Structural Parameters ◽  
Dust Emission ◽  
Three Dimensional ◽  
Interstellar Clouds ◽  
Photometric Observations ◽  
Radial Profiles ◽  
Typical Aspect

Context. The Galactic Cold Cores (GCC) project has made Herschel photometric observations of interstellar clouds where Planck detected compact sources of cold dust emission. The fields are in different environments and stages of star formation. Aims. Our aim is to characterise the structure of the clumps and their parent clouds, and to study the connections between the environment and the formation of gravitationally bound objects. We also examine the accuracy to which the structure of dense clumps can be determined from sub-millimetre data. Methods. We use standard statistical methods to characterise the GCC fields. Individual clumps are extracted using column density thresholding. Based on sub-millimetre measurements, we construct a three-dimensional radiative transfer (RT) model for each field. These are used to estimate the relative radiation field intensities, to probe the clump stability, and to examine the uncertainty of column density estimates. We examine the structural parameters of the clumps, including their radial column density profiles. Results. In the GCC fields, the structure noise follows the relations previously established at larger scales and in lower-density clouds. The fractal dimension has no significant dependence on column density and the values DP = 1.25 ± 0.07 are only slightly lower than in typical molecular clouds. The column density probability density functions (PDFs) exhibit large variations, for example, in the case of externally compressed clouds. At scales r > 0.1 pc, the radial column density distributions of the clouds follow an average relation of N ~ r−1. In spite of a great variety of clump morphologies (and a typical aspect ratio of 1.5), clumps tend to follow a similar N ~ r−1 relation below r ~ 0.1 pc. RT calculations indicate only factor 2.5 variation in the local radiation field intensity. The fraction of gravitationally bound clumps increases significantly in regions with AV > 5 mag but most bound objects appear to be pressure-confined. Conclusions. The host clouds of the cold clumps in the GCC sample have statistical properties similar to general molecular clouds. The gravitational stability, peak column density, and clump orientation are connected to the cloud background while most other statistical clump properties (e.g. DP and radial profiles) are insensitive to the environment. The study of clump morphology should be continued with a comparison with numerical simulations.

scholarly journals Insights into the physics when modelling cold gas clouds in a hot plasma

2019 ◽  
Vol 490 (1) ◽  
pp. L52-L56
Author(s):  
Bastian Sander ◽  
Gerhard Hensler
Keyword(s):  
Adaptive Mesh Refinement ◽  
Adaptive Mesh ◽  
Careful Consideration ◽  
Necessary Condition ◽  
Interstellar Clouds ◽  
Hot Plasma ◽  
Magnetic Dipole Field ◽  
Set Up ◽  
Self Gravity ◽  
Cold Gas

ABSTRACT This paper aims at studying the reliability of a few frequently raised, but not proven, arguments for the modelling of cold gas clouds embedded in or moving through a hot plasma and at sensitizing modellers to a more careful consideration of unavoidable acting physical processes and their relevance. At first, by numerical simulations we demonstrate the growing effect of self-gravity on interstellar clouds and, by this, moreover argue against their initial set-up as homogeneous. We apply the adaptive-mesh refinement code flash with extensions to metal-dependent radiative cooling and external heating of the gas, self-gravity, mass diffusion, and semi-analytic dissociation of molecules, and ionization of atoms. We show that the criterion of Jeans mass or Bonnor–Ebert mass, respectively, provides only a sufficient but not a necessary condition for self-gravity to be effective, because even low-mass clouds are affected on reasonable dynamical time-scales. The second part of this paper is dedicated to analytically study the reduction of heat conduction by a magnetic dipole field. We demonstrate that in this configuration, the effective heat flow, i.e. integrated over the cloud surface, is suppressed by only 32 per cent by magnetic fields in energy equipartition and still insignificantly for even higher field strengths.

scholarly journals Photoproduction of H3+ from gaseous methanol inside dense molecular clouds

2008 ◽  
Vol 4 (S251) ◽  
pp. 369-370
Author(s):  
S. Pilling ◽  
D. P. P. Andrade ◽  
A. C. F. Santos ◽  
H. M. Boechat-Roberty
Keyword(s):  
Cross Sections ◽  
Molecular Clouds ◽  
Mass Spectra ◽  
Column Density ◽  
X Rays ◽  
Alternative Source ◽  
X Ray ◽  
Flight Mass Spectrometry ◽  
Dense Molecular Cloud ◽  
Synchrotron Light

AbstractWe present experimental results obtained from photoionization and photodissociation processes of abundant interstellar methanol (CH3OH) as an alternative route for the production of H3+ in dense clouds. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS) employing soft X-ray and time-of-flight mass spectrometry. Mass spectra were obtained using the photoelectron-photoion coincidence techniques. Absolute averaged cross sections for the production of H3+ due to molecular dissociation of methanol by soft X-rays (C1s edge) were determined. The H3+'s photoproduction rate and column density were been estimated adopting a typical soft X-ray luminosity inside dense molecular and the observed column density of methanol. Assuming a steady state scenario, the highest column density value for the photoproduced H3+ was about 1011 cm2, which gives the ratio photoproduced/observed of about 0.05%, as in the case of dense molecular cloud AFGL 2591. Despite the small value, this represent a new and alternative source of H3+ into dense molecular clouds and it is not been considered as yet in interstellar chemistry models.

scholarly journals Molecular Clouds and Star Formation at Large R

1991 ◽  
Vol 144 ◽  
pp. 121-130
Author(s):  
J. Brand ◽  
J.G.A. Wouterloot
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Density Wave ◽  
Cosmic Ray ◽  
Volume Density ◽  
Kinetic Temperature ◽  
Galactocentric Distance ◽  
The Galaxy ◽  
The Mean ◽  
Cosmic Ray Flux

In the outer Galaxy (defined here as those parts of our system with galactocentric radii R>R0) the HI gas density (Wouterloot et al., 1990), the cosmic ray flux (Bloemen et al, 1984) and the metallicity (Shaver et al., 1983) are lower than in the inner parts. Also, the effect of a spiral density wave is much reduced in the outer parts of the Galaxy due to corotation. This changing environment might be expected to have its influence on the formation of molecular clouds and on star formation within them. In fact, some differences with respect to the inner Galaxy have been found: the ratio of HI to H2 surface density is increasing from about 5 near the Sun to about 100 at R≈20kpc (Wouterloot et al., 1990). Because of the “flaring” of the gaseous disk, the scale height of both the atomic and the molecular gas increases by about a factor of 3 between R0 and 2R0 (Wouterloot et al., 1990), so the mean volume density of both constituents decreases even more rapidly than their surface densities. The size of HII regions decreases significantly with increasing galactocentric distance (Fich and Blitz, 1984), probably due to the fact that outer Galaxy clouds are less massive (see section 3.3), and therefore form fewer O-type stars than their inner Galaxy counter parts. There are indications that the cloud kinetic temperature is lower by a few degrees (Mead and Kutner, 1988), although it is not clear to what extent this is caused by beam dilution.

scholarly journals Observability of the Magnetic Field in Molecular Clouds

1990 ◽  
Vol 140 ◽  
pp. 304-304
Author(s):  
N. Bel ◽  
B. Leroy
Keyword(s):  
Magnetic Field ◽  
Molecular Clouds ◽  
Diatomic Molecules ◽  
Zeeman Effect ◽  
Interstellar Clouds ◽  
The Magnetic Field

We have done detailed calculations of the Zeeman effect in the dozen diatomic molecules identified in interstellar clouds.

scholarly journals Growth after the streaming instability

2019 ◽  
Vol 624 ◽  
pp. A114 ◽  
Author(s):  
Beibei Liu ◽  
Chris W. Ormel ◽  
Anders Johansen
Keyword(s):  
Dynamical Evolution ◽  
Bimodal Distribution ◽  
Stokes Number ◽  
Volume Density ◽  
Mass Star ◽  
Type I ◽  
Solar Mass ◽  
Streaming Instability ◽  
Two Component ◽  
Self Gravity

Context. Streaming instability is a key mechanism in planet formation, clustering pebbles into planetesimals with the help of self-gravity. It is triggered at a particular disk location where the local volume density of solids exceeds that of the gas. After their formation, planetesimals can grow into protoplanets by feeding from other planetesimals in the birth ring as well as by accreting inwardly drifting pebbles from the outer disk. Aims. We aim to investigate the growth of planetesimals into protoplanets at a single location through streaming instability. For a solar-mass star, we test the conditions under which super-Earths are able to form within the lifetime of the gaseous disk. Methods. We modified the Mercury N-body code to trace the growth and dynamical evolution of a swarm of planetesimals at a distance of 2.7 AU from the star. The code simulates gravitational interactions and collisions among planetesimals, gas drag, type I torque, and pebble accretion. Three distributions of planetesimal sizes were investigated: (i) a mono-dispersed population of 400 km radius planetesimals, (ii) a poly-dispersed population of planetesimals from 200 km up to 1000 km, (iii) a bimodal distribution with a single runaway body and a swarm of smaller, 100 km size planetesimals. Results. The mono-dispersed population of 400 km size planetesimals cannot form protoplanets of a mass greater than that of the Earth. Their eccentricities and inclinations are quickly excited, which suppresses both planetesimal accretion and pebble accretion. Planets can form from the poly-dispersed and bimodal distributions. In these circumstances, it is the two-component nature that damps the random velocity of the large embryo through the dynamical friction of small planetesimals, allowing the embryo to accrete pebbles efficiently when it approaches 10−2 M⊕. Accounting for migration, close-in super-Earth planets form. Super-Earth planets are likely to form when the pebble mass flux is higher, the disk turbulence is lower, or the Stokes number of the pebbles is higher. Conclusions. For the single site planetesimal formation scenario, a two-component mass distribution with a large embryo and small planetesimals promotes planet growth, first by planetesimal accretion and then by pebble accretion of the most massive protoplanet. Planetesimal formation at single locations such as ice lines naturally leads to super-Earth planets by the combined mechanisms of planetesimal accretion and pebble accretion.

scholarly journals Magnetized filament models for diverging plasma lenses

2020 ◽  
Vol 493 (2) ◽  
pp. 1736-1752
Author(s):  
Adam Rogers ◽  
Abdul Mohamed ◽  
Bailey Preston ◽  
Jason D Fiege ◽  
Xinzhong Er
Keyword(s):  
Magnetic Field ◽  
Column Density ◽  
Volume Density ◽  
Magnetized Plasma ◽  
Gravitational Lens ◽  
Line Of Sight ◽  
Density Distributions ◽  
Rotation Measure ◽  
Spherical Plasma ◽  
Helical Magnetic Field

ABSTRACT Spherical plasma lens models are known to suffer from a severe overpressure problem, with some observations requiring lenses with central pressures up to millions of times in excess of the ambient interstellar medium. There are two ways that lens models can solve the overpressure problem: a confinement mechanism exists to counter the internal pressure of the lens, or the lens has a unique geometry, such that the projected column-density appears large to an observer. This occurs with highly asymmetric models, such as edge-on sheets or filaments, with potentially low volume–density. In the first part of this work we investigate the ability of non-magnetized plasma filaments to mimic the magnification of sources seen behind spherical lenses and we extend a theorem from gravitational lens studies regarding this model degeneracy. We find that for plasma lenses, the theorem produces unphysical charge density distributions. In the second part of the work, we consider the plasma lens overpressure problem. Using magnetohydrodynamics, we develop a non self-gravitating model filament confined by a helical magnetic field. We use toy models in the force-free limit to illustrate novel lensing properties. Generally, magnetized filaments may act as lenses in any orientation with respect to the observer, with the most high-density events produced from filaments with axes near the line of sight. We focus on filaments that are perpendicular to the line of sight that show the toroidal magnetic field component may be observed via the lens rotation measure.

scholarly journals Measurements of Isotopic Abundances in Interstellar Clouds

1980 ◽  
Vol 87 ◽  
pp. 397-404 ◽  
Author(s):  
Arno A. Penzias
Keyword(s):  
Molecular Clouds ◽  
General Framework ◽  
Galactic Center ◽  
Galactic Plane ◽  
Giant Molecular Clouds ◽  
Interstellar Clouds ◽  
The Galaxy ◽  
Isotopic Abundances ◽  
Si Isotopes

While an examination of the available data reveals some seemingly contradictory results, a general framework having the following outlines can be put forward:1. With the exception of the two galactic center sources SgrA and SgrB, the relative isotopic abundances exhibited by the giant molecular clouds in our Galaxy exhibit few, if any, significant variations from the values obtained by averaging the data from all these sources.2. The 13C/12C and 14N/15N abundance ratios are ∼130% and ∼150%, respectively, of their terrestrial values throughout the galactic plane and somewhat higher, ∼300%, near the galactic center.3. The 16O/18O and 17O/18O abundance ratios are ∼130% and ∼160%, respectively, of their terrestrial values throughout the Galaxy, although the former may be somewhat lower near the galactic center.4. The S and Si isotopes have generally terrestrial abundances.

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