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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.

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At least 127 records · Page 7
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NASA NTRS1987

Far-infrared measurements of N/O in H II regions - Evidence for enhanced CN process nucleosynthesis in the inner Galaxy

Measurements of the far-infrared lines of forbidden O III 51.8 microns, 88.4 microns, and forbidden N III 57.3 microns are presented for 13 H II regions covering a wide range in Galactocentric distance. These lines are used to measure the variation of N(2+)/O(2+) with Galactic radius from the center out to 12 kpc. It is argued that this ratio is indicative of the elemental ratio N/O. It is shown that the line flux ratio 57.3 microns forbidden N III/51.7 microns forbidden O III is nearly equal to the N(2+)/O(2+) abundance ratio and is only weakly dependent on nebular conditions. The derived N(2+)/O(2+) values show an inverse correlation with distance from the Galactic center. N(2+)/O(2+) tends to increase as the H II region electron temperature decreases, a behavior consistent with a model of Galactic enrichment in which N-14 is secondary to primary O-16, a scenario that is the natural result of CN processing.

Lester, D. F.↗
NASA NTRS1993

H II regions as probes of galaxy evolution and the properties of massive stars

The use of H II regions as probes to study the chemical evolution of galaxies and the spectral properties of hot, massive stars is reviewed. The observable parameters for this task are the physical conditions, elemental abundances, and ionization balance in the ionized gas. Some outstanding uncertainties in the determination of these parameters and some approaches to remedy or circumvent the problems are discussed.

Garnett, Donald R.↗
NASA NTRS1974

Far-infrared emission from H II regions. II - Multicolor photometry of selected sources and 2.2 min resolution maps of M42 and NGC 2024

Medium- and broad-bandwidth photometric measurements have been made of seven compact H II regions at wavelengths between 30 and 650 microns. The results are consistent with the hypothesis that the far-infrared flux is continuum radiation from dust grains. Color temperatures range from 33 to 85 K. For most of the sources, the infrared flux is low enough so that the dust could conceivably be mixed with the ionized gas. In at least one case, however, the far-infrared spectrum and luminosity of the source cannot be reconciled with reasonable grain models unless the far-infrared flux comes from a shell surrounding the H II region. M42 (Orion) and NGC 2024 (Orion B) have been mapped at 90 microns with a resolution (half-power beam diameter) of 2.2 min. Although the peak far-infrared brightness in M42 occurs at the position of the Kleinmann-Low infrared nebula, most of the flux originates in a more extended (about 6 x 8 min) source. The energy for the diffuse component could be supplied by the stars in the Trapezium cluster. However, the single early-type star visible in NGC 2024 is not luminous enough to account for either the infrared flux or the observed free-free flux.

Harper, D. A.↗
NASA NTRS1985

Star formation in the inner Galaxy - A far-infrared and radio study of two H II regions

Far-infrared and radio continuum maps have been made of two inner-Galaxy H II region complexes, G30.8-0.0 and G25.4-0.2, along with radio and molecular line measurements at selected positions. The far-IR emission from each region is dominated by two sources. For both G25.4 and G30.8, the distribution of the emission is similar to that of the radio emission, indicating that OB stars provide most of the heating. There is evidence that extinction plays an important role in G30.8, even in the far-IR. A near-IR point source has been detected in G30.8 at the position of peak far-IR color temperature. This source may be the ionizing star for the core of G30.8. Measurement of forbidden S III 9532 A from G25.4SE indicates that the extinction toward this source is very low, which is difficult to reconcile with previously determined distance measurements to this source.

Lester, D. F.↗
NASA NTRS1995

Accreting, Isolated Neutron Stars: Preheated of Infalling Gas and Cometary H II Regions - 3

Nearby, isolated, old neutron stars (IONSs) accreting interstellar material might be detectable as sources of UV and soft X-ray radiation. We investigate the interaction between this radiation field and the surrounding medium, assumed to consist of purely hydrogen and helium. This interaction results in a cometary H II region around the star which we model in detail by including all the relevant time-dependent ionization, recombination, heating, and cooling processes. We find that preheating of the ambient gas to temperatures higher than 10(exp 4) K may significantly quench the accretion rate onto these neutron stars, thereby reducing the IONS number counts in the EUV and soft X-ray bands predicted by previous investigations. However, the reprocessing of hard radiation by the surrounding H II regions may enhance the detectability of IONSs at optical wavelengths. Searches for nearby accreting IONSs are currently being undertaken.

Madau, Piero↗
NASA NTRS1984

The high-ionization and excited-state interstellar lines in the Carina Nebula - A giant H II region in absorption

The dominant component of the highly ionized and excited-state interstellar species seen in the ultraviolet toward stars within the Carina Nebula has been found to have a heliocentric radial velocity of about -30 km/s. This velocity is known to correspond to the near edge of the expanding H II region, as shown by the double optical nebular emission lines and the single He I lambda 3889 nebular absorption line. It is also seen as a (nondominant) component in both the optical and ultraviolet low-ionization interstellar lines, discussed previously. Hence, the Carina Nebula provides a clear case in which the principal high-ionization interstellar features are associated with the hot stars against which they are observed; and a considerable range of absorption lines can provide physical information about the H II region. Also, the weaker zero-velocity, line-of-sight component in the high-ionization species can be measured in this direction relatively free from confusion by the material associated with the stars. There is, in addition, a pervasive high-ionization feature with a velocity of -90 km/s in this region, which has the relatively large value of N(C IV)/N(Si IV) = 4.5.

Walborn, N. R.↗
NASA NTRS1984

Models of four highly obscured compact H II regions

Models are constructed for three stars in order to determine their properties and elemental abundances through comparisons of model predictions and observations. By comparing predicted line fluxes with those observed, it is noted that oxygen is apparently enhanced by a factor of 2 in W3A, and of 4 in G75.84 + 0.40A and G29.9 - 0.0. The first two cases are based on the premise that the O(++)-emitting regions are as clumped as those of S(++). The H II regions all need more than one main sequence star or one unusually large star to provide the observed radio luminosity.

Simpson, J. P.↗
NASA NTRS1990

The Wolf-Rayet star population in the most massive giant H II regions of M33

Narrow-band images of NGC 604, NGC 595, and NGC 592, the most massive giant H II regions (GHRs) in M33 have been obtained, in order to study their Wolf-Rayet content. These images reveal the presence of nine candidates in NGC 604 (seven WN, two WC), 10 in NGC 595 (nine WN, one WC), and two in NGC 592 (two WN). Precise positions and estimated magnitudes are given for the candidates, half of which have so far been confirmed spectroscopically as genuine W-R stars. The flux in the emission lines of all candidates is comparable to that of normal Galactic W-R stars of similar subtype. A few of the putative superluminous W-R stars are shown to be close visual double or multiple stars; their newly estimated luminosities are now more compatible with those of normal W-R stars. NGC 595 seems to be overabundant in W-R stars for its mass compared to other GHRs, while NGC 604 is normal. Factors influencing the W-R/O number ratio in GHRs are discussed: metallicity and age appear to be the most important.

Drissen, Laurent↗
NASA NTRS1989

A survey for PAH emission in H II regions, planetary and proto-planetary nebulae

The results of a systematic investigation of polycyclic aromatic hydrocarbon (PAH) emission in H II regions, planetary nebulae (PN), and proto-planetary nebulae (PNN), are reported. Data is obtained from the low resolution spectra (LRS) of IRAS. The results show that: PAHs are formed in carbon rich objects; and PAH emission is ubiquitous in general interstellar medium and requires the presence of ultraviolet photons, in planetary and proto-planetary nebulae, PAH emission is seen only where an ionizing flux is present and in carbon rich objects.

Demuizon, M.↗
NASA NTRS1988

Determination of N/O from far-infrared line observations of Galactic H II regions

Measurements of the 52- and 88-micron forbidden O III line ratios and the 57-micron forbidden N III line ratio for six H II regions have been obtained with the NASA Kuiper Airborne Observatory. The derived N/O ratio for most Galactic regions (of about 0.2-0.3) is greater than that obtained by optical studies, and it exceeds the solar value. Although enhancement of N/O at a galactocentric distance of about 6 kpc may be possible, no evidence is found for an overall linear gradient in N/O with galactocentric distance, suggesting that N-14 is predominantly produced by primary rather than secondary nucleosynthesis.

Rubin, R. H.↗
NASA NTRS1989

Dust in a few southern H II regions

The property of dust in four southern H II region/molecular cloud complexes (RCW 108, RCW 57, RCW 122, and G351.6-1.3) was discussed. These regions were observed at an effective wavelength of 150 micron using TIFR balloon borne 1 m telescope and deconvolved maps with a resolution of 1 min were obtained. The data were combined with other available data to derive the properties of the infrared emitting dust in these regions.

Ghosh, Swarna Kanti↗
NASA NTRS1994

A multiline, multiresolution VLA study of the ultracompact H II region G29.96-0.02

A non-local thermodynamic equilibrium (non-LTE) analysis is presented for high spatial resolution observations of H 110 alpha, H 66 alpha, and H 42 alpha toward the cometary ultracompact (UC) H II region G29.96-0.02. We find a mean Tau(sub e) approximately 6500 K and local n(sub e) approximately 5 x 10(exp 4)/cu cm in the leading arc and n(sub e) approximately 2 x 10(exp 4)/cu cm in the tail. Impact broadening is significant in H 110 alpha, implied by a reduction in the peak line amplitude by as much as 50% in the leading arc. The morphology of the line emission and the line profiles are consistent with a bow shock.

Afflerbach, A.↗
NASA NTRS1974

Infrared studies of H II regions and OH sources

This paper presents the observational results of a high spatial-resolution mapping and photometric study in the wavelength range from 1.65 to 20 microns of four H II regions and seven OH sources. Infrared emission indicative of the presence of heated dust was found from the compact H II condensations in W51, DR21, and NGC7538. Infrared emission was also found from the positions of maser sources in NGC 7538, W75(N) W75(S) and Sharpless 269, but not from that in W51. An extended infrared source was found coincident with the peculiar OH source OH 0739-14.

Wynn-Williams, C. G.↗
NASA NTRS1993

A Hubble Space Telescope planetary camera view of giant H II regions - The Wolf-Rayet content of NGC 595 and NGC 604 in M33

We present images of NGC 595 and NGC 604, the most massive giant H II regions in M33, obtained with the Planetary Camera aboard the HST in order to study their WR population. Fourteen WR and/or Of candidates are detected in NGC 604, and eleven in NGC 595. All previously claimed 'superluminous' WR stars are found to be tight (diameter less than 3 pc) stellar aggregates containing one (or sometimes more) normal WR star. As suspected from ground-based data, the WR/O number ratio is significantly higher in NGC 595 (about 0.3) than in NGC 604 (about 0.1). The WR stars may be major contributors to the output of mechanical power and energy into the interstellar medium in both clusters. Over the observable initial mass range, the initial mass functions (IMFs) have similar slopes. These IMFs are somewhat flatter than those generally derived for massive stars in the Galaxy or the Magellanic Clouds.

Drissen, Laurent↗
NASA NTRS1985

Far-infrared observations of two southern H II regions - RCW 122 and G 351.6-1.3

The luminosities in excess of 1,000,000 solar luminosities obtained in the present 1-arcmin resolution mapping of the two southern H II regions RCW 122 and G 351.6-1.3 requires the presence of OB star clusters. A luminosity/unit mass of the associated cloud of about 6 solar luminosities/solar mass is obtained by combining near-IR, far-IR, and CO-emission properties for RCW 122, and it is determined that the emitting dust grains are amorphous silicate-like. G 351.6-1.3 has very similar properties, although the fraction of 1-25 micron luminosity corrected for silicate absorption is about five times less than for RCW 122.

Mcbreen, B.↗
NASA NTRS1972

Infrared objects in H II regions.

The discovery of 2-micron emission sources in NGC 2264, IC 2087, and M1-82, and of prominent infrared emission in Lk H-alpha 198 and V376 Cas is reported. Of the 25 objects found by scanning around the H II regions, less than half would appear to be late-type stars.

Allen, D. A.↗