The expansion of H II regions.
H II regions expansion around O, B and A stars due to interstellar hydrogen ionization by stellar radiation absorption
Axford, W. I.↗
SEARCH · Search NASA
Results for “H II regions”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Thermal emission from grains in H-II regions
H II region dust grains thermal emission, examining effect on diffuse nebulae
Krishna Swamy, K. S.↗
New infrared sources associated with H II regions
The H II regions studied have generally been those which contain OH or H2O maser sources and which have been shown to contain compact condensations of ionized gas which may be connected with star formation processes. A brief account of two such sources, W3 and NGC 7538, is given. On the basis of the reported observations it is concluded that in general H II condensations are infrared sources in the range from 3 to 20 microns. The condensations are hidden optically by large quantities of obscuring matter.
Wynn-Williams, C. G.↗
Optical, infrared and radio studies of compact H II regions. II - The H alpha knot S88B
Explore the source record for details and available documents.
Pipher, J. L.↗
The infrared emission of G333.6-0.2 - An extremely nonspherical H II region
The southern H II region G333.6-0.2, which has a total luminosity of 3.3 million solar luminosities (for an assumed distance of 4 kpc) was mapped at 2.2, 10, 30, 50, and 100 microns. At all wavelengths, the surface brightness of the infrared radiation is unusually high and the structure of the source is compact and symmetrical. The present observations, along with previous data, suggest that G333.6-0.2 is excited by a single luminous object or a very compact cluster, which has formed on the front surface of a dense molecular cloud as seen from the earth. It is shown that the spectral and spatial characteristics of the infrared radiation can be understood in terms of this blister model.
Hyland, A. R.↗
On the formation and expansion of H II regions
The evolution of H II regions in spherical clouds with small, constant-density cores and power-law density distributions r exp -w outside the core is described analytically. It is found that there is a critical exponent above which the cloud becomes completely ionized. Its value in the formation phase depends on the initial conditions, but it has a well-defined value w(crit) = 3/2 during the expansion phase. For w less than w(crit), the radius of the H II region grows at a given rate, while neutral mass accumulates in the interphase between the ionization and shock fronts. For w = w(crit), the fronts move together without mass accumulation. Cases with w greater than w(crit) lead to the champagne phase: once the cloud is fully ionized, the expansion becomes supersonic. For self-gravitating disks without magnetic fields, the main features include a new 'variable-size' stage. The initial shape of the H II region has a critical point beyond which the disk becomes completely ionized.
Franco, Jose↗
On the determination of the number of O stars in H II regions and starburst galaxies
The hot star population in H II regions, H II galaxies, and starburst galaxies is often described in terms of the number of 'equivalent' O stars of a single representative subtype and luminosity class needed to produce the ionizing luminosity deduced from the nebular recombination lines in the optical spectra. In this paper we define conversion factors eta(sub 0), eta(sub 1), and zeta(sub 5000) with which the total number of O V stars and their flux contribution at 5000 A can be derived from the number of these 'equivalent' stars. These quantities depend primarily on three parameters: the slope and upper mass limit of the stellar mass function and the metallicity of the region. Using the latest stellar atmosphere and evolution models, we calculate eta(sub 0), eta(sub 1), and zeta(sub 5000) for a large number of values of these parameters. The results are presented in tabular as well as graphical form. We apply our results to two H II regions for which the hot star population are known and find that the predicted numbers of O stars agree well the observed counts. In addition, we describe a method by which the values of eta(sub 0) and eta(sub 1) and the observed emission-line fluxes can be used to place constraints on the allowed values of the slope and upper mass limit of the stellar mass function in a region.
Vacca, William D.↗
Far-infrared observations of H II regions in M33
Three H II regions in M33, namely, IC 133, NGC 595, and NGC 604, have been observed at far-infrared wavelengths. Only NGC 604, the strongest radio source of the three, was detected. The far-infrared radiation appears to be thermal emission from dust. The amount of dust known to be present in NGC 604 from comparison of optical and radio data is sufficient to account for the observed far-infrared emission. The total power radiated in the far-infrared is comparable to the total luminosity of the exciting stars of the H II region.
Gatley, I.↗
Infrared emission from the southern H II region H2-3
The H II region H2-3 has been found to have a large infrared flux with a luminosity of 100,000 to 200,000 solar luminosity values between 1.65 and 25 microns. Most of this flux comes from a single component with a diameter of 110 sec (2 pc). At 2.2 microns there is an unresolved source which is identified as the exciting star of the nebula; it can provide the required ionization and the total luminosity observed at infrared wavelengths. The 3- to 25 micron radiation is shown to be consistent with dust heated by L-alpha radiation within the nebula, but much of the 40- to 350-micron radiation probably originates from outside the H II region.
Becklin, E. E.↗
Infrared emission line studies of the structure and excitation of H II regions
Maps of five H II regions in one or more of the infrared fine-structure lines of Ne II (12.8 microns), Ar III (9.0 microns), and S IV (10.5 microns) have been obtained with angular resolutions ranging from 4 to 7 arcsec. The observations are used to discuss the morphology and excitation of these nebulae. Considerable diversity is found in the structures of the nebulae, probably resulting from differences in their ages and the circumstances of their formation. In all cases, more ionizing luminosity than would be provided by a single dominant ionizing star appears to be required, although uncertainties in the model nebulae make this conclusion uncertain.
Lacy, J. H.↗
The electron temperature of H II regions.
Electron temperature of H II region related to high temperature main sequence stars determined as function of electron density and radiation dilution factor
STELLAR RADIATION↗
CO observations of several galactic H II regions
Maps of the CO emission at a resolution of 8 arcmin are presented for seven H II regions or H II region groups; the results of a search for CO emission near seven more are also given. CO was detected near all but two of the observed objects. The distribution of (C-12)O and (C-13)O near S142, S157, and S159 was mapped at a resolution of 2.3 arcmin. The S147/S153 group of H II regions is shown to consist of physically related objects; it is a region of active star formation. The results show a complicated molecular cloud structure near most observed H II regions. There is a tendency for decreasing CO cloud mass when H II regions in more evolved stages are observed. The relevance of these results for the understanding of individual H II region/molecular cloud complexes, and for general aspects of the star-formation process, is briefly discussed.
Israel, F. P.↗
Far-infrared lines from G45.13 + 0.14 A and K3-50 A - Density fluctuations in compact H II regions
Properties of two compact H II regions, K3-50 A and G45.13 + 0.14 A, were investigated by measuring FIR fluxes from forbidden O III 51.8 and 88.4 micron lines, forbidden N III 57.3 micron line, forbidden S III 33.5 micron line, and forbidden Ne III 36.0 micron line of these regions, using a cooled grating spectrometer on NASA's Kuiper Airborne Observatory. For both H II regions, the ratio of the two FIR O(2+) lines indicates an electron density of about 1000/cu cm, which for K3-50 A is a factor of 10 to 100 lower than the density determined from optical line observations of the lower excitation species S(+) and N(+) and than the peak rms density deduced from radio continuum measurements by Turner and Matthews (1984). Detailed spherically symmetric models of the two sources were constructed using all available measurements.
Colgan, Sean W. J.↗
Giant H II regions in M81
H-alpha and VLA radio continuum observations at wavelengths of 6 and 20 cm are used to study the distribution of extinction and the distribution of giant radio H II regions along the spiral arms in M81. The radial distribution of visual extinction Av in the plane of M81 shows no trend, with an upper limit 0.1 mag/kpc for the radial extinction gradient and a mean Av of 1.1 + or - 0.4 mag. Nearly all the giant radio H II regions lie along the spiral arms or the inner H I ring. The radial distribution of the set of giant radio H II regions exhibits a strong maximum at a galactocentric distance R roughly 300 arcsec. Unless molecular hydrogen in M81 is also concentrated near R roughly 300 arcsec, the data disagree with Visser's (1980) model for star formation associated with a density wave. Some suggestions are made about how to change the ballistic particle model of Leisawitz and Bash to agree with the observed radial distribution of giant radio H II regions.
Kaufman, Michele↗
Reddening and extinction towards H II regions
The light emitted by the gas in H II regions is attenuated by dust. This extinction can be measured by comparing H alpha, H beta, and radio continuum fluxes, since the intrinsic ratios of the Balmer line and thermal radio continuum emissivities are nearly constant for reasonable conditions in H II regions. In the case of giant extragalactic H II regions, the extinction was found to be considerably greater than expected. The dust between the Earth and the emitting gas may have an optical thickness which varies. The dust may be close enough to the source that scattered light contributes to the flux, or the dust may be actually mixed with the emitting gas. It is difficult to decide which configuration is correct. A rediscussion of this question in light of recent observations, with the Fabry-Perot spectrophotometers, of the large Galactic H II region is presented. The color excesses are compared for stars embedded in these H II regions with those derived (assuming the standard law) from the nebular extinction and reddening.
Caplan, James↗
A kinematic search for supernova remnants in giant extragalactic H II regions
We have obtained velocity fields of the Giant H II complexes NGC 5471 in M101, NGC 2363 in NGC 2366, and the largest H II region in NGC 2403 from H-alpha observations using the TAURUS imaging Fabry-Perot interferometer. We have detected five H-alpha sources with velocity profiles which are broad when compared with the surrounding H II region. Region B in NGC 5471 has been previously determined to contain a supernova remnant by the presence of nonthermal radio continuum radiation and enhanced (O I) and (S II) emission (Skillman 1985) and broad H-alpha emission (Chu & Kennicutt 1986). Two broad H-alpha sources in NGC 2363 coincide with regions where strong splitting has been found in the (O III) line (Roy et al. 1991). Two more broad H-alpha sources have been identified in the largest H II region in NGC 2403. Very Large Array (VLA) radio continuum observations with a resolution of 2 sec at lambda(6) and lambda(20) cm of all 3 H II complexes are presented. In addition, high resolution (subarcsecond) VLA images of NGC 5471 were made at lambda(2) and lambda(6) cm. The presence of a nonthermal source in region NGC 5471 B was confirmed while region NGC 5471 A appears to be dominated by thermal emission. The nonthermal spectral index in NGC 2363 A indicates the existence of none or more supernova remnants at the position of a large velocity width source detected in H-alpha emission. No similar nonthermal sources were detected in NGC 2403 #1. Supernovae explosions and stellar winds are considered as causes for these large velocity width sources (LVWS). If the emission from the LVWSs is attributed to single supernova remnants, they are unusually luminous in both nonthermal radio continuum and H-alpha emision. The very large H-alpha luminosities could be a result of high velocity gas being ionized by the neighboring stellar cluster.
Yang, Hui↗
The morphologies and physical properties of ultracompact H II regions
Seventy-five ultracompact (UC) H II regions are observed 0.4 arcsec resolution at 2 and 6 cm using the Very Large Array, and their radio continuum brightness distributions are presented in the form of contour plots. H-76 alpha recombination line profiles are obtained for six sources using a 4.4 arcsec beam. It is shown that these UC H II regions have electron densities of at least approximately 10,000 per cubic cm, emission measures of at least approximately 10 to the 7th pc cm to the -6th, and diameters no more than approximately 0.1 pc, consistent with their being small photoionized nebulae produced by O and B stars embedded in clouds of molecular gas and dust. At high angular resolution five different morphologies of the ionized gas are seen: spherical or unresolved (43 percent), cometary (20 percent), core-halo (16 percent), shell (4 percent), and irregular or multiply peaked (17 percent). Evidence is presented that the UC H II region phase of a massive star must last for a significant fraction of its main-sequence lifetime.
Wood, Douglas O. S.↗
Bow shock models of ultracompact H II regions
This paper presents models of ultracompact H II regions as the bow shocks formed by massive stars, with strong stellar winds, moving supersonically through molecular clouds. The morphologies, sizes and brightnesses of observed objects match the models well. Plausible models are provided for the ultracompact H II regions G12.21 - 0.1, G29.96 - 0.02, G34.26 + 0.15, and G43.89 - 0.78. To do this, the equilibrium shape of the wind-blown shell is calculated, assuming momentum conservation. Then the shell is illuminated with ionizing radiation from the central star, radiative transfer for free-free emission through the shell is performed, and the resulting object is visualized at various angles for comparison with radio continuum maps. The model unifies most of the observed morphologies of ultracompact H II regions, excluding only those objects with spherical shells. Ram pressure confinement greatly lengthens the life of ultracompact H II regions, explaining the large number that exist in the Galaxy despite their low apparent kinematic ages.
Mac Low, Mordecai-Mark↗