scholarly journals The hydroxyl satellite-line ‘flip’ as a tracer of expanding H ii regions

2020 ◽  
Vol 497 (4) ◽  
pp. 4066-4076
Author(s):  
Anita Petzler ◽  
J R Dawson ◽  
Mark Wardle
Keyword(s):  
Spectral Feature ◽  
Stimulated Emission ◽  
State Transitions ◽  
H Ii Regions ◽  
Satellite Line ◽  
Post Shock ◽  
H Ii Region ◽  
Non Local ◽  
Background Continuum ◽  
Cool Gas

ABSTRACT Observations of the four 2Π3/2,  J = 3/2 ground state transitions of the hydroxyl radical (OH) have emerged as an informative tracer of molecular gas in the Galactic interstellar medium (ISM). We discuss an OH spectral feature known as the ‘flip’, in which the satellite lines at 1612 and 1720 MHz flip – one from emission to absorption and the other the reverse – across a closely blended double feature. We highlight 30 examples of the flip from the literature, 27 of which exhibit the same orientation with respect to velocity: the 1720-MHz line is seen in emission at more negative velocities. These same examples are also observed towards bright background continuum, many (perhaps all) show stimulated emission, and 23 of these are coincident in on-sky position and velocity with H ii radio recombination lines. To explain these remarkable correlations, we propose that the 1720-MHz stimulated emission originates in heated and compressed post-shock gas expanding away from a central H ii region, which collides with cooler and more diffuse gas hosting the 1612-MHz stimulated emission. The foreground gas dominates the spectrum due to the bright central continuum; hence, the expanding post-shock gas is blue-shifted relative to the stationary pre-shock gas. We employ non-local thermodynamic equilibrium (LTE) excitation modelling to examine this scenario and find that indeed FIR emission from warm dust adjacent to the H ii region radiatively pumps the 1612-MHz line in the diffuse, cool gas ahead of the expanding shock front, while collisional pumping in the warm, dense shocked gas inverts the 1720-MHz line.

scholarly journals On the Nature of R 136, the Central Object of 30 Dor. A Comparison by the Galactic Cluster NGC 3603

1984 ◽  
Vol 108 ◽  
pp. 257-258
Author(s):  
Michael Rosa ◽  
Jorge Melnick ◽  
Preben Grosbol
Keyword(s):  
Core Region ◽  
H Ii Regions ◽  
Central Object ◽  
The Core ◽  
Dominant Component ◽  
Galactic Cluster ◽  
H Ii Region

The massive H II region NGC 3603 is the closest galactic counterpart to the giant LMC nebula 30 Dor. Walborn (1973) first compared the ionizing OB/WR clusters of the two H II regions and suggested that R 136, the unresolved luminous WR + 0 type central object of 30 Dor, might be a multiple system like the core region of NGC 3603. Suggestions that the dominant component of R 136, i.e. R 136A, might be either a single or a very few supermassive and superluminous stars (Schmidt-Kaler and Feitzinger 1982, Savage et al. 1983) have recently been disputed by Moffat and Seggewiss (1983) and Melnick (1983), who have presented spectroscopic and photometric evidence to support the hypothesis of an unresolved cluster of stars. We have extended Walborn's original comparison of the apparent morphology of the two clusters by digital treatment of the images to simulate how the galactic cluster would look like if it were located in the LMC

scholarly journals Three generations of stars: a possible case of triggered star formation

2020 ◽  
Vol 496 (1) ◽  
pp. 870-874
Author(s):  
M B Areal ◽  
A Buccino ◽  
S Paron ◽  
C Fariña ◽  
M E Ortega
Keyword(s):  
Star Formation ◽  
Young Stellar Objects ◽  
H Ii Regions ◽  
Stellar Objects ◽  
Three Generations ◽  
The North ◽  
Western Border ◽  
H Ii Region ◽  
New Generation ◽  
North Western

ABSTRACT Evidence for triggered star formation linking three generations of stars is difficult to assemble, as it requires convincingly associating evolved massive stars with H ii regions that, in turn, would need to present signs of active star formation. We present observational evidence for triggered star formation relating three generations of stars in the neighbourhood of the star LS II +26 8. We carried out new spectroscopic observations of LS II +26 8, revealing that it is a B0 III-type star. We note that LS II +26 8 is located exactly at the geometric centre of a semi-shell-like H ii region complex. The most conspicuous component of this complex is the H ii region Sh2-90, which is probably triggering a new generation of stars. The distances to LS II +26 8 and to Sh2-90 are in agreement (between 2.6 and 3 kpc). Analysis of the interstellar medium on a larger spatial scale shows that the H ii region complex lies on the north-western border of an extended H2 shell. The radius of this molecular shell is about 13 pc, which is in agreement with what an O9 V star (the probable initial spectral type of LS II +26 8 as inferred from evolutive tracks) can generate through its winds in the molecular environment. In conclusion, the spatial and temporal correspondences derived in our analysis enable us to propose a probable triggered star formation scenario initiated by the evolved massive star LS II +26 8 during its main-sequence stage, followed by stars exciting the H ii region complex formed in the molecular shell, and culminating in the birth of young stellar objects around Sh2-90.

scholarly journals What do planetary nebulae and H II regions reveal about the chemical evolution of nearby dwarf galaxies?

2018 ◽  
Vol 14 (S344) ◽  
pp. 161-177 ◽  
Author(s):  
Denise R. Gonçalves
Keyword(s):  
Chemical Evolution ◽  
Local Group ◽  
Planetary Nebulae ◽  
H Ii Regions ◽  
Chemical Abundances ◽  
Local Universe ◽  
Time Line ◽  
Nearby Galaxies ◽  
H Ii Region ◽  
Age Range

AbstractThe Local Group contains a great number of dwarf irregulars and spheroidals, for which the spectroscopy of individual stars can be obtained. Thus, the chemical evolution of these galaxies can be traced, with the only need of finding populations spanning a large age range and such that we can accurately derive the composition. Planetary nebulae (PNe) are old- and intermediate-age star remnants and their chemical abundances can be obtained up to 3-4 Mpc. H ii regions, which are brighter and much easily detected, represent galaxies young content. PNe and H ii regions share similar spectroscopic features and are analysed in the same way. Both are among the best tracers of the chemical evolution allowing to draw the chemical time line of nearby galaxies. The focus in this review are the PN and H ii region populations as constraints to the chemical evolution models and the mass-metallicity relation of the local universe.

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 Interferometric Studies of OH Sources Associated with H II Regions and IR Objects

1973 ◽  
Vol 52 ◽  
pp. 423-432
Author(s):  
R. D. Davies
Keyword(s):  
Zeeman Splitting ◽  
H Ii Regions ◽  
Mass Outflow ◽  
H Ii Region

An interferometer with a 24 km baseline has been used to map components of OH sources to a precision of a few hundredths of a second of arc. The W3 OH source which is associated with an H II region appears to be rotating slowly. There is evidence for Zeeman splitting in the components of this source. The IR/OH sources NML Cygnus and VY CMa are found to be expanding objects with a large rate of mass outflow. Rotation is also evident in both these objects.

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