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NASA NTRS1978

Far-infrared observations of H II regions near the galactic center

This paper presents and discusses far-infrared observations of nine H II regions within 1 deg of the galactic center, including Sgr A, Sgr B2, Sgr C, and G0.5-0.0. The far-infrared luminosity, color temperature, and optical depth of these regions and the ratio of infrared flux to radio-continuum flux lie in the range characteristic of spiral-arm H II regions. The far-infrared results are therefore consistent with the idea that the galactic-center H II regions are ionized by luminous early-type stars. Steep systematic gradients in far-infrared color temperature and optical depth are seen along the galactic plane between Sgr B2 and G0.5-0.0; the appearance of this area is similar to that of regions of star formation in the spiral arms.

Gatley, I.↗
NASA NTRS1990

Cometary compact H II regions are stellar-wind bow shocks

Comet-shaped H II regions, like G34.3 + 0.2, are easily explained as bow shocks created by wind-blowing massive stars moving supersonically through molecular clouds. The required velocities of the stars through dense clumps are less than about 10 km/s, comparable to the velocity dispersion of stars in OB associations. An analytic model of bow shocks matches the gross characteristics seen in the radio continuum and the velocity structure inferred from hydrogen recombination and molecular line observations. The champagne flow model cannot account for these structures. VLBI observations of masers associated with the shells of cometary compact H II regions should reveal tailward proper motions predominantly parallel to the shell, rather than perpendicular. It is predicted that over a decade baseline, high signal-to-noise VLA observations of this class of objects will show headward pattern motion in the direction of the symmetry axis, but not expansion. Finally, shock-generated and coronal infrared lines are also predicted.

Van Buren, Dave↗
NASA NTRS1994

H II regions and the abundance properties of spiral galaxies

We investigate the relationships between the characteristic oxygen abundance, the radial abundance gradient, and the macroscopic properties of spiral galaxies by examining the properties of individual H II regions within those galaxies. Our observations of the line flux ratio (O II) lambda lambda 3726, 3729 + (O III) lambda lambda 4959, 5007)/H beta for 159 H II regions in 14 spiral galaxies are combined with published data to provide a sample of 39 disk galaxies for which (O II) + (O III)/H beta has been measured for at least five H II regions. We find that the characteristic gas-phase abundances and luminosities of spiral galaxies are strongly correlated. This relationship maps almost directly onto the luminosity-metallicity relationship of irregular galaxies and is also quite similar to that found for elliptical and dwarf spheroidal galaxies. Within our sample of spirals, a strong correlation between characteristic abundance and Hubble type also exists. The correlation between luminosity and Hubble type complicates the issue, but we discuss several interpretations of the correlations. The relationship between circular velocity and characteristic abundance is also discussed. We find that the slopes of the radial abundance gradients, when expressed in units of dex/isophotal radius, do not significantly correlate with either luminosity or Hubble type. However, the hypothesis that both early and very late type spirals have shallower gradients than intermediate spirals is consistent with the data. We find suggestive evidence that the presence of a bar induces a flatter gradient and also briefly discuss whether abundance gradients are exponential, as is usually assumed. We investigate the properties of individual H II regions in a subset of 42 regions for which we have spectra that cover almost the entire spectral range from 3500 to 9800 A. We use those data to estimate the densitites and ionizing spectra within the H II regions. We confirm that the ionizing spectrum hardens with increasing radius and decreasing abundance. We find no correlation between the ionization parameter and either radius or abundance, but this may be due to significant scatter introduced by the simple conversion of line ratios to ionization parameter.

Zaritsky, Dennis↗
NASA NTRS1989

High resolution observations of compact H II regions at 230 GHz

Based on the idea that star formation goes on progressively in molecular clouds, a search was conducted for protostars by mapping compact H II regions at a frequency of 250 GHz. The IRAM 30 m radio telescope was used with a (3)He cooled bolometer. Twenty compact H II regions usually obtaining twice the expected free-free flux density, positionally coincident with the H II region, were observed. Even fine structure within the H II regions can be traced in the maps as in the case of G75.84+0.40 near ON-2. The high degree of coincidence between the 250 and 5 GHz map of Harris shows that the excess flux density observed must come from dust mixed with the ionized gas. Part of the dust must however be accumulated in the outer part of the H II region, since in some cases the contours are shifted outwards relative to the radio maps. This is consistent with the fact that in those cases where enough information is available to make a model fit, temperatures were derived of 80 + or - 30 K.

Wink, JOERN-E.↗
NASA NTRS1985

VLA continuum observations of compact H II regions

The VLA interferometer has been used to obtain 5 GHz continuum observations of the galactic H II regions W48, G25.4 - 0.2S, G8.1 + 0.2, and G89.1 + 1.3, at less than 6-arcsec spatial resolution. From the source maps presented, H II region physical parameters are derived and tentative models for the G25.4-0.2S H II region, employing the VLA radio interferometric map and available IR data, are discussed.

Woodward, C. E.↗
NASA NTRS1979

Far-infrared observations of the Carina I and II H II regions

Far-infrared (35-175 microns) mapping and photometry are presented of the Carina I and II H II regions in the Carina Nebula (NGC 3372). The far-infrared source associated with Carina I has a half-power width of 2.7 arcmin and has no compact high-surface-brightness condensations, unlike many northern H II regions such as W51 or W49. The peak emission region is offset from the radio continuum peak in the direction of a dust lane to the west. It is proposed that the H II regions and far-IR source are both powered by the O-star cluster Tr 14 about 4 arcmin to the east. The H II region then represents an ionization front at the boundary of the dense cloud west of Tr 14, and the far-infrared emission is due to thermal radiation from the heated dust in the dark cloud. A similar model may also be applicable to the Carina II region.

Harvey, P. M.↗
NASA NTRS1995

Far-infrared lines from H II regions: Abundance variations in the galaxy

Far-infrared lines of (N III) (57 microns), (O III) (52, 88 microns), (Ne III) (36 microns), and (S III) (19, 33 microns) have been measured in the H II regions G1.13 - 0.11, W31B, G23.95 + 0.15, G25.38 - 0.18, G29.96 - 0.02, W43, W51e, S156, S158, NGC 3576, NGC 3603, and G298.22-0.34. These observations were made with the facility Cryogenic Grating Spectrometer on the Kuiper Airborne Observatory to examine variations in abundances throughout the Galaxy. Previously published observations of G0.095 + 0.012, G333.60 - 0.21, G45.13 + 0.14A, K3-50, and M17 are also discussed. The giant H II region 30 Doradus in the Large Magellanic Cloud (LMC) was observed for comparison. Fluxes for (Ne II) (12.8 microns), (S IV) (10.5 microns), and the radio free-free continuum were collected from the literature for those sources. Electron densities were estimated from FIR line-pair ratios, and ionic abundances were estimated from the FIR line and radio fluxes. The excitation was estimated from the O(2+)/S(2+) ratio. Corrections for unseen ionization stages were calculated with the use of constnat-density H II region models. The validity and range of applicability of such semiempirical ionization correction schemes are discussed. The abundances with respect to hydrogen exhibit gradients with R(sub G) comparable to those previously measured for our Galaxy and for other galaxies. The overall gradients are d (log N/H)/dR = -0.10 +/- 0.02 dex/kpc, d(log Ne/H)/dR = -0.08 +/- 0.02 dex/kpc and d(log S/H)/dR = 0.07 +/- 0.02 dex/kpc. Compared to the Orion Nebula, the intermediate R(sub G) H II regions with 6 is less than R(sub G) is less than 11 kpc have similar or lower S/H and N/O ratios. The N/O ratios in the inner Galaxy are more than twice those observed in the Orion Nebula and intermediate R(sub G) H II regions. In fact, all the abundance ratios are as well or better fitted by a step fit with two levels than by a linear gradient. As has been noted in previous studies, the N/O ratio estimated from infrared observations of the doubly ionized N and O lines in H II regions is larger than the ratio estimated from optical observations of the singly ionized N and O lines. The Ne(2+)/O(2+) ratio is observed to be essentially constant over a wide range of excitation. This contradicts predictions of model H II regions calculated with the use of Local Thermodynamic Equilibrium (LTE) model stellar atmospheres. We conclude that these stellar atmospheres significantly underestimate the actual emergent fluxes for energies greater than 41 eV.

Simpson, Janet P.↗
NASA NTRS1980

Spectrophotometry of H II regions and the nucleus of NGC 1566

The paper presents spectrophotometric observations of the nucleus and nine H II regions in the spiral arms of the Sbc galaxy NGC 1566. The emission line spectra of the H II regions are characterized by a low grade of ionization, and most of the line intensities do not show radial gradients; the N II/O II forbidden line ratio appears to increase toward the nucleus, and the forbidden O III/H-beta ratio may decrease in the same direction. This is explained by the increase of the oxygen abundance and decrease of the electron temperature toward the nucleus; the spectra confirm the presence of the weak, broad emission wings at H-alpha, and the nuclear spectrum of NGC 1566 resembles that of a giant H II region rather than that of a Seyfert galaxy.

Hawley, S. A.↗
NASA NTRS1987

Evidence for turbulence in H II regions

The results of extensive velocity mapping across the face of Galactic H II regions are used to make an accurate determination of velocity dispersions and structure functions for a number of nebulae. A dependence of random velocities upon scale, indicating the presence of turbulence, is found. These relationships are compared with the predictions of models for Kolmogorov turbulence in H II regions, and poor agreement is found. A more general formulation shows that there is no single power law for the velocity-distance relation that fits the observations. From the variations in the shape of the observed structure function, it is concluded that the turbulent energy enters the nebulae at several different scales. It is also found that velocity broadening of the observed lines is significantly greater than would be inferred from the velocity scatter across the face of the nebulae.

O'Dell, C. R.↗
NASA NTRS1969

Dynamical study of an H II region.

Dynamical effects in structure of H II equilibrium region left behind expanding ionization front, showing temperature as function of distance from star

Sofia, S.↗
NASA NTRS1992

The molecular interstellar medium in two giant H II regions in M33 - NGC 604 and NGC 595

This study reports single-dish and interferometric CO observations for the two largest H II regions in M33, NGC 604, and NGC 595. Six individual molecular clouds are identified in the H II regions. Molecular and virial masses for three resolved clouds in NGC 604 agree very well, from which it is inferred that the CO-to-H2 conversion factor in this region is the same as it is in the inner disk of M33 and in the Galaxy, despite a metallicity in NGC 604 that is three times below solar. The total gas mass of the molecular complex associated with NGC 604 estimated from single-dish data is dissimilar to those of molecular complexes in the inner disk of M33, while the molecular gas mass of NGC 505 is roughly an order or magnitude smaller. Both regions contain a large mass fraction of atomic hydrogen, which suggests that photodissociation of molecular hydrogen is an important process in these energetic H II regions.

Wilson, Christine D.↗
NASA NTRS1987

High-resolution radio observations of the supernova remnant G24.7 + 0.6 and the discovery of an ultracompact H II region

The Crab-like SNR G24.7 + 0.6 with 12 arcmin resolution at 20 cm wavelength has been observed. The image reveals a bright Crab-like core surrounded by a faint shell. An ultracompact H II region lies adjacent to the remnant core and several other compact H II regions lie just outside the remnant shell. Optical, far-infrared, and X-ray observations of the region are also presented. The specific attributes of this remnant and its place in the classification of composite versus Crab-like remnants as well as the general question of the association between Crab-like remnants and H II regions are discussed.

Becker, R. H.↗
NASA NTRS1994

A very metal poor H II region in the outer disk of M101

We present new spectroscopic observations of an H II region in the extreme outer disk of the spiral galaxy M101, more than 32 kpc from the nucleus, or 25% farther out than the well-studied giant H II region NGC 5471. From a derived (O III) electron temperature of 13,900 K, we derive log O/H = -4.1, only 10% of the solar value, and smaller than the abundance measured in NGC 5471. Log N/O = -1.49, similar to the values seen in metal-poor dwarf galaxies, while log S/O = -1.74, essentially identical to the solar value, confirming the trend of constant S/O observed in earlier studies. With the inclusion of this new object, the composition gradient in M101 from published spectroscopic observations shows no evidence for either a break in the gradient from 4 to 33 kpc, or a rise in the abundances in the outer parts of the disk.

Garnett, Donald R.↗
NASA NTRS1994

G5.89-0.39: A compact H II region with a very dense circumstellar dust torus

We present the results of high angular resolution infrared observations of the compact H II region G5.89-0.39. Subarcsecond imaging at 1.65, 2.2, 3.6, and 4.8 microns and diffraction-limited scans at 50 and 100 microns have been obtained at NASA's Infrared Telescope Facility and Kupier Airborne Observatory and the Canada-France-Hawaii Telescope. We find a bipolar source morphology at the shortest infrared wavelengths, a compact single source at 4-5 microns, similar to that seen at 10 microns, and a very high optical depth, barely resolved far-infrared source. Spherical radiative transfer models have been used to estimate dust envelope properties; in spite of the obvious nonspherical geometry at short wavelengths, these models provide strong constraints on the overall properties of the circumnebular envelope. We find that the bulk of the circumstellar dust must be relatively close to the H II region, with a relatively steep gradient. The high column densities and steep density gradient imply densities at the inner edge of the cloud of order 10(exp 7)-10(exp 8)/cc. These results suggest that the compact H II region is surrounded by a very dense dust torus.

Harvey, Paul M.↗
NASA NTRS1985

Ionization equilibrium in isolated H II regions

Theoretical ionization equilibrium models are combined with Copernicus ultraviolet observations to study a particular H II region absorption-line component in the spectra of Lambda Sco and Upsilon Sco. The Lambda Sco observations imply that the region is characterized by a hydrogen density of approximately 0.2/cu cm and an extent along the line of sight of approximately 16 pc. Three choices for the relative placement of Lambda Sco and Upsilon Sco are considered. Conclusions are that Lambda Sco is the primary source of ionization and that there is a separation of approximately 5 pc between the H II region and Lambda Sco. Observational data support an enhanced dielectronic recombination rate for S III. Comparison of the Upsilon Sco and Lambda Sco observations suggests that spatial variations of density may be present.

Eder, D. C.↗