A New Class of Compact H II Regions Associated with OH Emission Sources

Although fundamental for astrophysics, the processes that produce massive stars are not well understood. Large distances, high extinction, and short timescales of critical evolutionary phases make observations of these processes challenging. Lacking good observational guidance, theoretical models have remained controversial. This review offers a basic description of the collapse of a massive molecular core and a critical discussion of the three competing concepts of massive star formation:• monolithic collapse in isolated cores• competitive accretion in a protocluster environment• stellar collisions and mergers in very dense systems We also review the observed outflows, multiplicity, and clustering properties of massive stars, the upper initial mass function and the upper mass limit. We conclude that high-mass star formation is not merely a scaledup version of low-mass star formation with higher accretion rates, but partly a mechanism of its own, primarily owing to the role of stellar mass and radiation pressure in controlling the dynamics.

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“…• Compact and classical HII regions (Mezger et al 1967, Yorke 1986). Their gas is ionized globally, often by several ionizing sources.…”

Section: Observable Stagesmentioning

confidence: 99%

Toward Understanding Massive Star Formation

Zinnecker

Yorke

2007

Annu. Rev. Astron. Astrophys.

1,184

1,141

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“…Points in Fig. 5 represent observed data with flux densities taken from the literature (Menon & Terzian 1965;Slee & Orchiston 1965;Terzian 1966;Hughes 1967;Mezger et al 1967;Thompson et al 1967;Terzian 1968;Berge & Seielstad 1969;Kaftan-Kassim 1969;Le Marne 1969;Ribes 1969;Terzian 1969;Thomasson & Davies 1970; Table 1. Adopted input parameters for the fitting of the radio spectra.…”

Section: Comparing Calculated and Observed Radio Spectramentioning

confidence: 99%

Radio-continuum spectrum, brightness temperature, and planetary nebulae properties

Gruenwald¹,

Aleman²

2006

A&A

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Context. Radio-continuum spectra are frequently used to infer the density distribution of ionized gaseous regions, while observed correlations between the brightness temperature and other distance-independent parameters are used to test evolutionary models of planetary nebulae. Aims. We check empirical correlations among features derived from the observed radio data and the inferred conclusions available in the literature, using self-consistent photoionization models. Methods. Photoionization models are computed for the physical conditions of planetary nebulae in order to derive self-consistent radio-continuum spectra, as well as the brightness temperature. Results. The temperature and ionization distributions throughout the nebulae explain the observed range of spectral indexes in the thick region of the spectrum, even for a uniform density distribution, usually challenged in the literature. The obtained models fit the observed radio spectra for planetary nebulae in a large range of spectral indexes. Our calculations show a correlation between the spectral index obtained in a given frequency range and the nebula size, as well as reproduce the observed relations between the brightness temperature and other distance-independent parameters. Such diagrams are frequently used to check evolutionary models of the central star and/or of the nebula. Conclusions. Since PNe images clearly show that the density is not constant inside the nebulae, and models with uniform and nonuniform density distributions can both reproduce the observed radio spectra, we conclude that it is not possible to favour one of them from the radio data, or to infer a particular density distribution for planetary nebulae.

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“…The appearance of massive stars in molecular clouds is heralded by compact H n regions or IR sources, temperature peaks in the molecular gas, the presence of water or OH masers, and CO selfabsorption. Mezger et al (1967) were the first to suggest that compact H n regions are the signatures of recently formed O stars. Compact H n regions are some of the most luminous objects in the Galaxy, and some of the strongest IRAS point sources; linking these objects to the subsequent appearance of massive luminous stars is extremely interesting.…”

Section: Protostar Formation and Pre-main-sequence Starsmentioning

confidence: 99%

OB associations: Massive stars in context

Garmany¹

1994

PASP

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ABSTRACT. Massive stars are important in a variety of contexts: individually they are examples of stars that undergo significant mass loss which is driven by line radiation pressure; as denizens of the Galaxy they play a significant role in the galactic recycling effort; as ensembles of stars they contain clues to massive star evolution in general and the question of supernova progenitors in particular. Finally, under extreme conditions of pathological star formation massive stars participate in the star burst phenomena seen in some galaxies. If we wish to understand star bursts, we must first understand locally observed variations in the mass spectrum. But all of these diverse areas of study rely on certain fundamental parameters of the stars themselves-here we include temperature, luminosity, mass, and chemical composition. And central to accurate determinations of these parameters are the distances to the stars. This review will focus on what is known about the loose clustering of massive stars termed OB associations. These associations contain many, but by no means all, of the stars with masses greater than 15Mq. Most of our knowledge of the stellar content of the OB associations in our own Galaxy is surprisingly sketchy due to their large angular size, but large-field CCD detectors are changing this. Magellanic Cloud associations can now be compared with OB associations in our own Galaxy, and the initial mass function shows surprising variations. The physical cause of these variations is still unknown. Another development is the study of embedded star clusters: infrared array detectors have been probing the buried stars in molecular clouds and finding an unexpectedly large population of these objects. Both these young IR clusters, and a sprinkling of evolved red supergiants, suggest that the star-formation history of OB associations may be more complicated than we have supposed.

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A New Class of Compact H II Regions Associated with OH Emission Sources

Cited by 50 publications

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Toward Understanding Massive Star Formation

Toward Understanding Massive Star Formation

Radio-continuum spectrum, brightness temperature, and planetary nebulae properties

OB associations: Massive stars in context

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