Ionization fronts in interstellar gas and the expansion of HII regions

1961 ◽  
Vol 253 (1028) ◽  
pp. 277-300 ◽  
Keyword(s):  
Equations Of Motion ◽  
Initial Density ◽  
Neutral Hydrogen ◽  
Similarity Solutions ◽  
Ionization Front ◽  
H Ii Regions ◽  
Interstellar Gas ◽  
Ionized Gas ◽  
Ionization Fronts ◽  
H Ii Region

The gas dynamical effects of an expanding nearly fully ionized hydrogen region (H II region), which is associated with the formation of O and B stars, are investigated. The radiation from the hot star is absorbed by the surrounding interstellar gas (mainly neutral hydrogen) and leads to its ionization. Previous analyses have disregarded the internal motions set up in expanding H II regions. Similarity solutions of the equations of motion are presented for spherical and cylindrical problems, thus enabling the effects of groups of stars as well as individual stars to be discussed. For similarity to be applicable the initial density variations of the undisturbed neutral gas have to be like 1/ r ® in the spherical case and like 1/ r in the cylindrical case. This does not, however, limit their use in describing the general picture of events for any other given density distribution. Recombination of the ions and electrons and subsequent re-ionization by radiation within the H II region is allowed for; cooling processes such as that due to the excitation of O<super>+</super> ions are also taken into account. It is shown that the temperature of the ionized gas in the H II region is approximately uniform even though the region as a whole is expanding. Rates of expansion are calculated and it is also determined whether a shock propagates ahead of the ionized gas. In particular for rates of expansion less than about 20 km/s a shock wave occurs ahead, but for speeds greater than about 20 km/s, which would occur in the initial motion, the rate of expansion of the ionized gas is too great and an ‘isothermal’ shock occurs within the H II region. The boundary between the ionized and neutral gases can be regarded as a discontinuity and is termed an ionization front. The present paper is concerned with the propagation of such fronts and accompanying shocks; a companion paper by W. I. Axford investigates the structures of ‘isothermal shock’ and ionization fronts. The lack of uniqueness, which occurs in the present paper, is removed when the results are combined with Axford’s work.

scholarly journals The Ionization and Expansion of a Globule of Interstellar Gas overun by a Weak R Type Ionization Front

1977 ◽  
Vol 32 (7) ◽  
pp. 692-696
Author(s):  
J. S. Berry
Keyword(s):  
Density Distribution ◽  
Similarity Solution ◽  
Spherical Symmetry ◽  
Initial Density ◽  
Neutral Hydrogen ◽  
Ionization Front ◽  
Interstellar Gas ◽  
Ionized Gas ◽  
Initial Density Distribution

Abstract The flow of the ionized gas behind a contracting ionization front is investigated for spherical symmetry. A similarity solution is given when the initial density distribution in the neutral hydrogen is ω0/rα- where r is the distance from the centre of contraction.

Ionization front in interstellar gas: The structure of ionization fronts

1961 ◽  
Vol 253 (1029) ◽  
pp. 301-333 ◽  
Keyword(s):  
Cylindrical Symmetry ◽  
Ionization Front ◽  
Complete Analysis ◽  
Interstellar Gas ◽  
Ionized Gas ◽  
Important Condition ◽  
Ionization Fronts ◽  
Gas Dynamic ◽  
Cooling Effects ◽  
Stagnation Enthalpy

A complete analysis of the structures of all types of ionization front is given, together with computed examples. It is shown that the solutions given by Goldsworthy for the propagation of ionization fronts, considered as discontinuities, can be made unique, and the unique solutions are given for the case of cylindrical symmetry. Ionization fronts of all types are shown to be possible, depending on the density of the ionized gas and the spectral type of the radiation. In particular strong D -type and weak R -type ionization fronts (corresponding to strong deflagrations and weak detonations, respectively, in combustion theory) prove to be of importance. The existence of these discontinuities conflicts with the Chapman-Jouguet hypothesis and the reasons for this behaviour are examined in detail. It is concluded that the most important condition for waves of this type to occur is that strong cooling effects should be present, which allow the stagnation enthalpy of the flow to have a maximum before decreasing to an equilibrium value at the rear of the wave. It is suggested that this may be of significance in the theory of other gas-dynamic discontinuities, and in determining the validity of the Chapman—Jouguet hypothesis in general.

General characteristics of interstellar clouds and their interpretation

1967 ◽  
Vol 31 ◽  
pp. 95-115
Author(s):  
F. D. Kahn
Keyword(s):  
Equations Of Motion ◽  
Orion Nebula ◽  
Interstellar Gas ◽  
Ionized Gas ◽  
Interstellar Clouds ◽  
Ionization Fronts ◽  
Heating And Cooling ◽  
Exciting Star ◽  
Early Type Stars ◽  
Time Required

This report is concerned with recent progress on various aspects of interstellar gas dynamics, including an investigation of the Orion Nebula, studies of H 11 regions and of ionization fronts, a statistical model of H 1 clouds and a re-appraisal of the heating and cooling processes in H 1 regions.Dyson has worked out a new model for the Orion Nebula, in which it is supposed that dense pockets of non-ionized gas exist within the H 11 region. He makes predictions concerning the flow of ionized gas around these pockets, and about the possible presence of shock-waves nearby.Mendis has extended Axford's classical description of ionization fronts to include the case in which a front advances into a region where the hydrogen is molecular, and finds that there are a number of consequent changes in the nature of the front. Hjellming has given a comprehensive account of the processes by which a thermal balance is achieved in an H 11 region, and has shown how the temperature in its various parts depends on the distance from the exciting star and on the stellar surface temperature. Mathews has integrated the equations of motion for a growing H 11 region, and has described how ionization fronts develop and when shocks will occur. Lasker has followed the motion further, and has given examples to show how older H 11 regions evolve.By means of a simple model of H 1 clouds, Field and Saslaw have tried to estimate how long it would take to form a massive interstellar complex which would become gravitationally unstable. They find that the time required is relatively short. One must therefore assume that only a small fraction of an unstable cloud can condense into stars, or else the rate of star formation would be too rapid.Finally an estimate is made of the rate at which early-type stars supply kinetic energy to the H 1 clouds, via the expansion of H 11 regions. One cannot, with the present data, decide definitely whether this process is important. A possible alternative would be the acceleration of gas clouds by supernova shells.

scholarly journals Imaging and spectroscopy of the LMC He II nebula N 44C and its ionizing star

1999 ◽  
Vol 193 ◽  
pp. 480-481
Author(s):  
Vanessa C. Galarza ◽  
Donald R. Garnett ◽  
You-Hua Chu
Keyword(s):  
Large Magellanic Cloud ◽  
Convincing Evidence ◽  
H Ii Regions ◽  
Uv Spectrum ◽  
Ionized Gas ◽  
X Ray ◽  
H Ii Region ◽  
Recombination Emission ◽  
Imaging And Spectroscopy ◽  
Echelle Spectroscopy

We present results from new HST imaging and spectroscopy of the peculiar Large Magellanic Cloud H II region N 44C and its ionizing star. While this nebula exhibits strong He II recombination emission, the source of the He+ ionizing photons has not been found. The UV spectrum of the ionizing star suggests an approximate spectral class of 07–08; the UV Si IV, He II, and N IV features do not show P-Cygni profiles, indicating that the ionizing star is not a supergiant. No companion star has yet been detected. Ground-based and HST optical spectroscopy of the ionized gas shows that the nebular abundances of C, N, O and He are not anomalous relative to other LMC H II regions, suggesting that no previous WR/SN companion has disappeared. Echelle spectroscopy has also ruled out the presence of high velocity shocked gas. Deep ROSAT imaging shows no X-ray point source in this location. The “fossil X-ray binary” hypothesis of Pakull & Motch (1989) remains the best explanation for the ionization of this nebula; however, convincing evidence for this hypothesis remains elusive.

scholarly journals Magnetic fields, stellar feedback, and the geometry of H II regions

2008 ◽  
Vol 4 (S259) ◽  
pp. 25-34
Author(s):  
Gary J. Ferland
Keyword(s):  
Magnetic Field ◽  
Magnetic Pressure ◽  
H Ii Regions ◽  
Line Profiles ◽  
Ionized Gas ◽  
Stellar Feedback ◽  
H Ii Region ◽  
Field Lines ◽  
The Magnetic Field ◽  
Simple Picture

AbstractMagnetic pressure has long been known to dominate over gas pressure in atomic and molecular regions of the interstellar medium. Here I review several recent observational studies of the relationships between the H+, H0 and H2 regions in M42 (the Orion complex) and M17. A simple picture results. When stars form they push back surrounding material, mainly through the outward momentum of starlight acting on grains, and field lines are dragged with the gas due to flux freezing. The magnetic field is compressed and the magnetic pressure increases until it is able to resist further expansion and the system comes into approximate magnetostatic equilibrium. Magnetic field lines can be preferentially aligned perpendicular to the long axis of quiescent cloud before stars form. After star formation and pushback occurs ionized gas will be constrained to flow along field lines and escape from the system along directions perpendicular to the long axis. The magnetic field may play other roles in the physics of the H II region and associated PDR. Cosmic rays may be enhanced along with the field and provide additional heating of atomic and molecular material. Wave motions may be associated with the field and contribute a component of turbulence to observed line profiles.

scholarly journals When H ii regions are complicated: considering perturbations from winds, radiation pressure, and other effects

2019 ◽  
Vol 492 (1) ◽  
pp. 915-933 ◽  
Author(s):  
Sam Geen ◽  
Eric Pellegrini ◽  
Rebekka Bieri ◽  
Ralf Klessen
Keyword(s):  
Radiation Pressure ◽  
Large Range ◽  
Massive Stars ◽  
Relative Effect ◽  
H Ii Regions ◽  
Gas Temperature ◽  
Algebraic Models ◽  
Ionized Gas ◽  
Principal Mode ◽  
H Ii Region

ABSTRACT We explore to what extent simple algebraic models can be used to describe H ii regions when winds, radiation pressure, gravity, and photon breakout are included. We (a) develop algebraic models to describe the expansion of photoionized H ii regions under the influence of gravity and accretion in power-law density fields with ρ ∝ r−w, (b) determine when terms describing winds, radiation pressure, gravity, and photon breakout become significant enough to affect the dynamics of the H ii region where w = 2, and (c) solve these expressions for a set of physically motivated conditions. We find that photoionization feedback from massive stars is the principal mode of feedback on molecular cloud scales, driving accelerating outflows from molecular clouds in cases where the peaked density structure around young massive stars is considered at radii between ∼0.1 and 10–100 pc. Under a large range of conditions the effect of winds and radiation on the dynamics of H ii regions is around 10 per cent of the contribution from photoionization. The effect of winds and radiation pressure is most important at high densities, either close to the star or in very dense clouds such as those in the Central Molecular Zone of the Milky Way. Out to ∼0.1 pc they are the principal drivers of the H ii region. Lower metallicities make the relative effect of photoionization even stronger as the ionized gas temperature is higher.

scholarly journals On the Propagation and Structure of Ionization Fronts

1958 ◽  
Vol 8 ◽  
pp. 1062-1068
Author(s):  
F. A. Goldsworthy
Keyword(s):  
Shock Wave ◽  
Ionizing Radiation ◽  
Atomic Hydrogen ◽  
Hydrogen Gas ◽  
Ionization Front ◽  
Ionized Gas ◽  
Hii Region ◽  
Ionization Fronts ◽  
Neutral Gas ◽  
Gravitational Equilibrium

The problem discussed here is that of determining the motion of a cloud of neutral atomic hydrogen gas, when it is subjected to ionizing radiation from a star embedded in it. Initially the gas is in gravitational equilibrium at a constant temperature of about 100°K. It is supposed that at time t=0 the star suddenly begins to radiate with a certain intensity, which remains constant thereafter. Part of the surrounding gas will be ionized and an ionization front (separating the ionized gas from the neutral gas) will move outwards into the neutral gas. A shock wave may also propagate ahead of the ionization front into the neutral gas. There will therefore be two regions to consider—a region of ionized gas (HII region) and a region of neutral gas (HI region) in which there may be a shock.

scholarly journals The Magellanic Clouds and the Primordial Helium Abundance

2000 ◽  
Vol 198 ◽  
pp. 194-203 ◽  
Author(s):  
Manuel Peimbert ◽  
Antonio Peimbert
Keyword(s):  
Neutral Hydrogen ◽  
Magellanic Clouds ◽  
Heavy Elements ◽  
H Ii Regions ◽  
Helium Abundance ◽  
Temperature Variations ◽  
Systematic Effects ◽  
H Ii Region ◽  
Primordial Helium

A new determination of the pregalactic helium abundance based on the Magellanic Clouds H II regions is discussed. This determination amounts to Yp = 0.2345 ± 0.0030 and is compared with those derived from giant extragalactic H II regions in systems with extremely low heavy elements content. It is suggested that the higher primordial value derived by other authors from giant H II region complexes could be due to two systematic effects: the presence of neutral hydrogen inside the helium Strömgren sphere and the presence of temperature variations inside the observed volume.

scholarly journals Stacking redshifted 21 cm images of H ii regions around high-redshift galaxies as a probe of early reionization

2020 ◽  
Vol 501 (1) ◽  
pp. 146-156
Author(s):  
James E Davies ◽  
Rupert A C Croft ◽  
Tiziana Di-Matteo ◽  
Bradley Greig ◽  
Yu Feng ◽  
...  
Keyword(s):  
High Redshift ◽  
Neutral Hydrogen ◽  
Neutral Fraction ◽  
H Ii Regions ◽  
Fourier Space ◽  
Signal To Noise ◽  
First Stars ◽  
High Redshift Galaxies ◽  
H Ii Region ◽  
The Universe

ABSTRACT A number of current and future experiments aim to detect the reionization of neutral hydrogen by the first stars and galaxies in the Universe via the redshifted 21 cm line. Using the bluetides simulation, we investigate the measurement of an average ionized region towards the beginning of reionization by stacking redshifted 21 cm images around optically identified bright galaxies using mock observations. We find that with an SKA 1000 h observation, assuming perfect foreground subtraction, a 5σ detection of a stacked H ii region can be made with 30 images around some of the brightest galaxies in bluetides (brighter than MUV &lt; −22.75) at z = 9 (corresponding to a neutral fraction of 90.1 per cent in our model). We present simulated relationships between the UV magnitude of galaxies, the sizes of the ionized regions they reside in, and the shape of the stacked profiles. These mock observations can also distinguish between scenarios where the intergalactic medium is in net emission or absorption of 21 cm photons. Once 21 cm foreground contamination is included, we find that even with up to 200 images around these rare, bright galaxies, only a tentative &gt;1σ detection will be possible. However, partial foreground subtraction substantially improves signal to noise. For example, we predict that reducing the area of Fourier space dominated by foregrounds by 50 (80) per cent will allow &gt;3σ (&gt;5σ) detections of ionized regions at z = 9.

On the Interaction of a Weak-R Type Ionization Front and a Contact Discontinuity

1978 ◽  
Vol 33 (4) ◽  
pp. 393-397
Author(s):  
J. S. Berry
Keyword(s):  
Shock Waves ◽  
Numerical Solution ◽  
Contact Discontinuity ◽  
Initial Density ◽  
Ionization Front ◽  
Ionization Fronts ◽  
And Shock Waves ◽  
Mesh Length

A numerical solution is given to the problem of a weak R type ionization front approaching a contact discontinuity across which the density increases. During the interaction it is found that there are three possible configurations produced; the weak-R type ionization front becomes (a) weak-R type but slower for ρ1/ρ0 < 1.15, (b) strong-D type for 1.15 < ρ1/ρ0 < 3.4 or (c) weak-D type for ρ1/ρ0 > 3.4, where the original ionization front travels through a region of initial density go and across the contact discontinuity the density becomes ρ1. A fine spacial mesh length is chosen so that the structures of the resulting ionization fronts and shock waves may be identified clearly at any time.

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