Environs of HII Regions

1983 ◽  
Vol 5 (2) ◽  
pp. 158-162 ◽  
Author(s):  
R. S. Roger
Keyword(s):  
Molecular Clouds ◽  
Hii Regions ◽  
Low Density ◽  
Dust Grains ◽  
Early Type ◽  
Hydrogen Atoms ◽  
Interstellar Gas ◽  
Early Type Stars ◽  
Stable Surfaces ◽  
Dust Absorption

It is generally agreed that early-type stars, and hence their HII regions, appear to develop preferentially in dense concentrations within molecular clouds. These parent molecular clouds owe their existence in two ways to the dust grains in the interstellar gas, a component which comprises about 1% by mass. First, dust grains provide stable surfaces upon which hydrogen atoms can come together to form molecules. Then, once formed, the molecules are protected in cloud interiors by dust absorption of the dissociating ultraviolet radiation which pervades the low-density interstellar medium.

Preliminary Observations of Interstellar Sodium D-Line Absorption towards the Carina Nebula

1980 ◽  
Vol 4 (1) ◽  
pp. 95-97 ◽  
Author(s):  
J. B. Whiteoak ◽  
F. F. Gardner
Keyword(s):  
High Resolution ◽  
Hii Regions ◽  
Early Type ◽  
Interstellar Clouds ◽  
Carina Nebula ◽  
Line Absorption ◽  
General Investigation ◽  
Early Type Stars ◽  
Resolution Study

As part of a general investigation of interstellar clouds associated with southern HII regions we have begun a high-resolution study of the sodium D-line absorption in the directions of early-type stars that are likely to be associated with or located behind the clouds.

scholarly journals Observational Effects of Interaction in the Seyfert galaxy NGC 7469

1990 ◽  
Vol 124 ◽  
pp. 331-342
Author(s):  
I. Pronik ◽  
L. Metik
Keyword(s):  
Color Index ◽  
Seyfert Galaxy ◽  
Hii Regions ◽  
Early Type ◽  
The West ◽  
Radio Structure ◽  
Dust Clouds ◽  
Early Type Stars ◽  
Ngc 7469

AbstractSome pecularities of the circummucleus of the Seyfert galaxy NGC 7469 were revealed, plausibly caused by interaction with the satellite IC 5283 and a starlike detail, situated on the edge of the west spiral branch 14” from the nucleus. Shock excited HII regions were noted in the part of NGC 7469 turned toward the satellite IC 5283. The galaxy’s central radio structure (λ ~ 6 cm) stretches in the direction toward the satellite IC 5283 and the starlike detail. The spectum and color index of the starlike detail suggest that it is a cluster of early type stars (Mv = -19m) and dust clouds (Av = 3m), in NGC 7469.

Molecular Clouds

1977 ◽  
Vol 75 ◽  
pp. 37-54 ◽  
Author(s):  
P. Thaddeus
Keyword(s):  
Star Formation ◽  
Interstellar Medium ◽  
Molecular Clouds ◽  
Physical State ◽  
Molecular Gas ◽  
Spectroscopic Techniques ◽  
Low Density ◽  
Recombination Line ◽  
Interstellar Gas ◽  
Impossible Task

To attempt to understand star formation without knowing the physical state of the dense interstellar molecular gas from which stars are made is an almost impossible task. Star formation has developed late as a branch of astrophysics largely for lack of observational data, and in particular, has lagged badly behind the study of the atomic and ionized components of the interstellar gas because spectroscopic techniques which work well at low density have an unfortunate tendency to fail when the density is high. Optical spectroscopy, which has been applied to the interstellar medium for over 70 years, has made little progress in regions of high density because of obscuration, and the same is true a fortiori of spacecraft spectroscopy in the UV; radio 21-cm and recombination line observations, although unhampered by obscuration, are unsatisfactory because the dense condensations are almost entirely molecular in composition.

Fossil magnetic fields and rotation of early-type stars

1994 ◽  
Vol 162 ◽  
pp. 184-185
Author(s):  
A.E. Dudorov
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Molecular Clouds ◽  
Main Sequence ◽  
Early Type ◽  
Main Sequence Stars ◽  
Early Type Stars ◽  
Quadrupole Magnetic Field ◽  
Interstellar Molecular Clouds

Observational data of the last 10 years allow two main conclusions:a) Main sequence stars can be separated in two classes: - magnetic (Bp) stars with surface strengths of a dipole or quadrupole magnetic field of Bs ≈ n · (102 − 103) G, n = 2,3,4…7, and - normal main sequence stars (F-O) with magnetic fields Bs ≈ 1 − 100 G (< 300 G);b) Typical star formation takes place in interstellar molecular clouds with magnetic field strengths B ≈ 10-5 G (See Dudorov 1990).

scholarly journals Molecules and Dust in the Magellanic Clouds

1984 ◽  
Vol 108 ◽  
pp. 319-332
Author(s):  
F. P. Israel
Keyword(s):  
Interstellar Medium ◽  
Molecular Hydrogen ◽  
Molecular Clouds ◽  
Total Mass ◽  
Spiral Galaxies ◽  
Magellanic Clouds ◽  
Major Constituent ◽  
Early Type ◽  
Disk Mass ◽  
Early Type Stars

A variety of studies over the last decade has shown molecular hydrogen to be a major constituent of the interstellar medium both in our Galaxy and in other spiral galaxies (Morris and Rickard, 1982). Our Galaxy contains roughly M(H2) = 4 × 109 M⊙; between R = 2 kpc and R = 10 kpc the H2 mass is one to three times that of HI; at the solar circle about 12 per cent of the total disk mass is in the form of H2; most of this mass is in the form of several thousand giant molecular cloud complexes (GMCs) with sizes d > 20 pc and masses M(H2) > 105 MO (Cohen et al, 1980; Sanders, 1981; Dame, 1983). These GMCs mainly consist of clumps with much smaller scales of order a few pc or less (e.g. Bally and Israel, 1983). Apart from their contribution to the total mass of the galactic interstellar medium, molecular clouds are also important as they are the major birthsite of massive early-type stars (see the review by Habing and Israel, 1979).

scholarly journals Polarized Continuum Radiation from Stellar Atmospheres

2014 ◽  
Vol 10 (S305) ◽  
pp. 395-400 ◽  
Author(s):  
J. Patrick Harrington
Keyword(s):  
Rayleigh Scattering ◽  
Accurate Method ◽  
Stellar Atmospheres ◽  
Early Type ◽  
Late Type ◽  
Hydrogen Atoms ◽  
Free Electrons ◽  
Continuum Radiation ◽  
Early Type Stars ◽  
Limb Darkening

AbstractContinuum scattering by free electrons can be significant in early type stars, while in late type stars Rayleigh scattering by hydrogen atoms or molecules may be important. Computer programs used to construct models of stellar atmospheres generally treat the scattering of the continuum radiation as isotropic and unpolarized, but this scattering has a dipole angular dependence and will produce polarization. We review an accurate method for evaluating the polarization and limb darkening of the radiation from model stellar atmospheres. We use this method to obtain results for: (i) Late type stars, based on the MARCS code models (Gustafsson et al. 2008), and (ii) Early type stars, based on the NLTE code TLUSTY (Lanz and Hubeny 2003). These results are tabulated at http://www.astro.umd.edu/~jph/Stellar_Polarization.html While the net polarization vanishes for an unresolved spherical star, this symmetry is broken by rapid rotation or by the masking of part of the star by a binary companion or during the transit of an exoplanet. We give some numerical results for these last cases.

scholarly journals On the Theory of Rotating Magnetic Stars

1970 ◽  
Vol 4 ◽  
pp. 264-268
Author(s):  
G. A. E. Wright
Keyword(s):  
Magnetic Field ◽  
Low Energy ◽  
Early Type ◽  
Interstellar Gas ◽  
Yield Information ◽  
Magnetic Stars ◽  
Strong Surface ◽  
Early Type Stars ◽  
Surface Convection ◽  
Gas Cloud

AbstractAll observations of magnetic stars necessarily yield information only about their surface features. We are ignorant of the nature of the fields in the interiors of such stars, and equally we cannot be sure of the non-existence of interior fields in stars which are superficially non-magnetic. In fact, if we assume the truth of the ‘fossil’ theory – that the magnetic flux of an Ap star is a relic of the flux initially present in the gas cloud from which the star condensed – then it is surprising that magnetic stars are not observed to be much more common, since magnetic fields appear to be ubiquitous in interstellar gas clouds. For those stars with strong surface convection zones, we might expect that a fossil field of low energy would be expelled by the turbulence and would possibly be trapped in the interior. However, the majority of early-type stars with radiative envelopes also do not exhibit any observable magnetic field.

scholarly journals Disks formed by Rotation Induced Bi-stability

1999 ◽  
Vol 169 ◽  
pp. 159-168 ◽  
Author(s):  
Henny J.G.L.M. Lamers ◽  
Jorick S. Vink ◽  
Alex de Koter ◽  
Joseph P. Cassinelli
Keyword(s):  
Loss Rate ◽  
Mass Loss Rate ◽  
Equatorial Region ◽  
Stellar Winds ◽  
Low Density ◽  
Escape Velocity ◽  
Rotating Stars ◽  
Early Type ◽  
Early Type Stars ◽  
Rapidly Rotating Stars

AbstractWe discuss the evidence for the existence of bi-stable stellar winds of early type stars, both theoretically and observationally. The ratio between the terminal wind velocity and the escape velocity drops steeply from about 2.6 for stars with Teff > 21 000 K to about 1.3 at Teff < 21 000 K. This is the bi-stability jump, which is due to a change in the ionization of the wind and in the wind driving lines. The mass loss rate increases across the jump by about a factor 2 to 5 from the hotter to the cooler stars. The mass flux from rapidly rotating stars can also show the bi-stability jump at some lattitude between the pole and the equator, with a slow high density wind in the equatorial region and a faster low density wind from the poles. This might explain the disks of rapidly rotating B[e] stars, formed by the Rotation Induced Bi-stability mechanism. We discuss the RIB mechanism and its properties. We also describe some future improvements of the model.

The formation of glycine and other complex organic molecules in exploding ice mantles

2014 ◽  
Vol 168 ◽  
pp. 369-388 ◽  
Author(s):  
J. M. C. Rawlings ◽  
D. A. Williams ◽  
S. Viti ◽  
C. Cecchi-Pestellini ◽  
W. W. Duley
Keyword(s):  
Gas Phase ◽  
Molecular Clouds ◽  
Organic Molecules ◽  
Alternative Mechanism ◽  
Hydrogen Atoms ◽  
Interstellar Gas ◽  
Dark Cloud ◽  
Short Period ◽  
Molecular Abundances ◽  
Complex Organic

Complex Organic Molecules (COMs), such as propylene (CH3CHCH2) and the isomers of C2H4O2 are detected in cold molecular clouds (such as TMC-1) with high fractional abundances (Marcelino et al., Astrophys. J., 2007, 665, L127). The formation mechanism for these species is the subject of intense speculation, as is the possibility of the formation of simple amino acids such as glycine (NH2CH2COOH). At typical dark cloud densities, normal interstellar gas-phase chemistries are inefficient, whilst surface chemistry is at best ill defined and does not easily reproduce the abundance ratios observed in the gas phase. Whatever mechanism(s) is/are operating, it/they must be both efficient at converting a significant fraction of the available carbon budget into COMs, and capable of efficiently returning the COMs to the gas phase. In our previous studies we proposed a complementary, alternative mechanism, in which medium- and large-sized molecules are formed by three-body gas kinetic reactions in the warm high density gas phase. This environment exists, for a very short period of time, after the total sublimation of grain ice mantles in transient co-desorption events. In order to drive the process, rapid and efficient mantle sublimation is required and we have proposed that ice mantle ‘explosions’ can be driven by the catastrophic recombination of trapped hydrogen atoms, and other radicals, in the ice. Repeated cycles of freeze-out and explosion can thus lead to a cumulative molecular enrichment of the interstellar medium. Using existing studies we based our chemical network on simple radical addition, subject to enthalpy and valency restrictions. In this work we have extended the chemistry to include the formation pathways of glycine and other large molecular species that are detected in molecular clouds. We find that the mechanism is capable of explaining the observed molecular abundances and complexity in these sources. We find that the proposed mechanism is easily capable of explaining the large abundances of all three isomers of C2H4O2 that are observationally inferred for star-forming regions. However, the model currently does not provide an obvious explanation for the predominance of methyl formate, suggesting that some refinement to our (very simplistic) chemistry is necessary. The model also predicts the production of glycine at a (lower) abundance level, that is consistent with its marginal detection in astrophysical sources.

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