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NASA NTRS1981

X-ray heating and ionization of broad-line emission regions in QSOs and active galaxies

The absorption of X-rays deep within the broad-line emitting clouds in quasars and the nuclei of active galaxies produces extensive zones of warm (about 10,000 K), partially ionized gas. Because Lyman-alpha photons are trapped in these regions, the X-ray energy is efficiently channeled into Balmer lines collisionally excited from the n = 2 level. These H I regions and the H II regions created by UV photons illuminating the surfaces of the clouds gives rise to integrated Lyman-alpha/H-alpha lines emission ratios between 1 and 2. Enhanced Mg II line emission from the H I regions gives rise to integrated Mg II/H-alpha ratios near 0.5. The O I line at 8446 A is efficiently pumped by trapped H-alpha photons; and an intensity ratio of I(8446 A)/I(H-alpha) of approximately equal to and not greater than 0.1 is calculated for the X-ray heated zone.

Weisheit, J. C.↗
NASA NTRS1988

Radio and optical studies of supernova 1961V in NGC 1058

Radio observations of the Sc galaxy NGC 1058 at 20 cm and 6 cm reveal two radio sources. One of the sources is coincident with supernova 1961V and has a spectral index of -0.4 + or - 0.3. This is the second (and probably the third) detection of radio emission from a supernova decades after optical maximum. The absolute luminosity of SN 1961V at 20 cm is comparable to Cas A. A second source, about 2.3 arcsec to the west, has a spectral index of -0.3 + or - 0.3 and is likely to be the remnant of a supernova that was not optically detected. Optical images of NGC 1058 show two faint H II regions associated with the radio sources. Filter photometry in several bands indicates that these two H II regions are normal with respect to the many optically brighter, albeit radio-quiet, H II regions in NGC 1058.

Cowan, John J.↗
NASA NTRS1986

Molecular clouds and star formation in the inner galaxy - A comparison of CO, H II, and far-infrared surveys

Surveys of the galactic plane over galactic latitudes from -1 degree to +1 degree and galactic longitudes from 12 degrees to 60 degrees are compared in the CO line at 2.6 mm, in the far-infrared (FIR) continuum at 150 micrometers and 250 micrometers, and in the radio continuum and H 110-alpha recombination line at 6 cm. The main purposes are to determine the degree of association between FIR sources, H II regions, and molecular clouds in the first quadrant and to describe and analyze the stellar content of these molecular clouds. Among the conclusions it is noted that most FIR sources coincide with HII regions, and nearly all H II regions coincide with molecular clouds, and that clouds in the inner galaxy are probably several tens of millions of years old and may have been producing O stars for only about the most recent 20 percent of their lives.

Myers, P. C.↗
NASA NTRS1991

Far-infrared properties of NGC 6822

IRAS FIR maps of NGC 6822, one of the few noninteracting Magellanic irregular galaxies resolved in the FIR by IRAS, are examined and compared to optical and H I data. The diffuse FIR emission at large radii from the center of the galaxy is found to be due to Galactic cirrus. There is a general correspondence between H-alpha and FIR emission in that FIR peaks are associated with major H II complexes, but the ratio of the FIR to the H-alpha emission varies between these complexes by about a factor of five. H I mass-to-FIR luminosity ratios cover a similar range for the three major H I cloud complexes in NGC 6822. More than about half of the FIR flux is produced in regions near H II region complexes. The remaining flux comes from diffuse FIR emission that extends over the entire optically visible galaxy. A model for the galaxy is discussed in which the galaxy is fairly transparent and the FIR is strongest where there is a strong UV source and higher optical depth due to dust in gas clouds.

Gallagher, John S., III↗
NASA NTRS1977

Narrow radio recombination lines from ionization fronts

Model weak D-type ionization fronts are used to show that narrow H I radio recombination lines, such as those observed in the directions of several galactic H II regions, may be formed in the outermost layers of an ionization front bounding an H II region. Four model ionization fronts are computed with different values of magnetic-field strength, ionizing photon energy, and initial conditions. The recombination line strengths given by the models are compared with observed line strengths for the H II region NGC 2024, and the observed line profiles are found to be about twice as broad as the model profiles. Some likely explanations for the discrepancy are considered, particularly a variation in front velocity over the observing beam width (Stark broadening is ruled out).

Hill, J. K.↗
NASA NTRS1985

Optical emission-line properties of evolved galactic supernova remnants

New optical spectrophotometric data are presented for the supernova remnants CTB 1, OA 184, VRO 42.05.01, S147, the Monoceros Loop, G206.9 + 2.3, and G65.3 + 5.7. These data are combined with published spectral data to study some of the general properties of evolved galactic supernova remnants. It is found that (1) O I and O II forbidden line strengths, when used in conjunction with the usual H-alpha S II forbidden line ratio test, provide an excellent additional diagnostic for discriminating remnants from H II regions; (2) the line ratios H-alpha forbidden line N II, H-alpha forbidden line S II, and forbidden line S II 6717/6731 A generally do not vary substantially among the filaments of an individual remnant; and (3) the observed correlation of forbidden line N II/H-alpha with S II forbidden line 6717/6731 A in remnants is the result of observational selection rather than of evolutionary effects. A galactic nitrogen abundance gradient of d log (N/H)/dR = -0.088 dex/kpc, which is in agreement with that derived from H II regions. However, no abundance gradients for oxygen or sulfur are indicated from the remnant data.

Fesen, R. A.↗
NASA NTRS2000

NGC 1614: A Laboratory for Starburst Evolution

The modest extinction and reasonably face-on viewing geometry make the luminous infrared galaxy NGC 1614 an ideal laboratory for study of a powerful starburst. HST/NICMOS observations show: (1) deep CO stellar absorption, tracing a starburst nucleus about 45 pc in diameter; (2) surrounded by an approx. 600 pc diameter ring of supergiant H II regions revealed in Pa-alpha line emission; (3) lying within a molecular ring indicated by its extinction shadow in H - K; and (4) all at the center of a disturbed spiral galaxy. The luminosities of the giant H II regions in the ring axe extremely high, an order of magnitude brighter than 30 Doradus; very luminous H II regions, comparable with 30 Dor, are also found in the spiral arms of the galaxy. Luminous stellar clusters surround the nucleus and lie in the spiral arms, similar to clusters observed in other infrared luminous and ultraluminous galaxies. The star forming activity may have been initiated by a merger between a disk galaxy and a companion satellite, whose nucleus appears in projection about 300 pc to the NE of the nucleus of the primary galaxy. The relation of deep stellar CO bands to surrounding ionized gas ring to molecular gas indicates that the luminous starburst started in the nucleus and is propagating outward into the surrounding molecular ring. This hypothesis is supported by evolutionary starburst modeling that shows that the properties of NGC 1614 can be fitted with two short-lived bursts of star formation separated by 5 Myr (and by inference by a variety of models with a similar duration of star formation). The total dynamical mass of the starburst region of 1.3 x 10(exp 9) solar masses is mostly accounted for by the old pre-starburst stellar population. Although our starburst models use a modified Salpeter initial mass function (turning over near one solar mass), the tight mass budget suggests that the IMF may contain relatively more 10 - 30 solar masses stars and fewer low mass stars than the Salpeter function. The dynamical mass is nearly four times smaller than the mass of molecular gas estimated from the standard ratio of (C-12)O (1 - 0) to H2. A number of arguments place the mass of gas in the starburst region at approx. 25% of the dynamical mass, nominally about 1/15 and with an upper limit of 1/10 of the amount estimated from (C-12)O and the standard ratio.

Alonso-Herrero, A.↗
NASA NTRS1996

Hydrogenated Polycyclic Aromatic Hydrocarbons and the 2940 and 2850 Wavenumber (3.40 and 3.51 micron) Infrared Emission Features

The 3150-2700/cm (3.17-3.70 micron) range of the spectra of a number of Ar-matrix-isolated PAHs containing excess H atoms (H(sub n)-PAHS) are presented. This region covers features produced by aromatic and aliphatic C-H stretching vibrations as well as overtone and combination bands involving lower lying fundamentals. The aliphatic C-H stretches in molecules of this type having low to modest excess H coverage provide excellent fits to a number of the weak emission features superposed on the plateau between 3080 and 2700/cm (3.25 and 3.7 micron) in the spectra of many planetary nebulae, reflection nebulae, and H II regions. Higher H coverage is implied for a few objects. We compare these results in context with the other suggested identifications of the emission features in the 2950-2700/cm (3.39-3.70 micron) region and briefly discuss their astrophysical implications.

Bernstein, Max P.↗
NASA NTRS1984

The stellar content of 30 Doradus

The components of the supergiant H II region Tarantula are surveyed, noting that 30 Doradus is really only the most active section of the Large Magellanic Cloud. The region contains at least 40 WR stars and numerous non-H II region late spectral type supergiants. Most of the stars are centrally located and presumably feed on the nebulosity. The closeness of the population will require fine spectroscopic scans of all the members to achieve accurate typing. Although the population is mixed, the ionizing radiation emitted by the region is consistent with its classification as part of the H II region. Finally, the brightest objects within Tarantula are suspected of being multiple systems.

Walborn, N. R.↗
NASA NTRS1981

The carbon abundance in two h 2 regions of the small Megallanic Cloud

Observations of the ultraviolet spectra of two locations in the H II region NGC 346 and of the entire H II region IC 1644 in the Small Magellanic Cloud (SMC) were made using the International Ultraviolet Explorer (IUE) satellite. The abundance of carbon in the nebulae was derived using theoretical model analysis combined with ground-based spectrophotometry of other emission lines. The abundance of C relative to H in the SMC was found to be lower by -0.9 dex compared with the Sun and lower by -0.8 dex compared with the Orion Nebula. This C deficiency is similar to that of O, Ne, S, and Ar in the SMC, but not as great as found for N. The sites and history of C nucleosynthesis in galaxies is similar to that of O, Ne, S, and Ar, in contrast to that of N, which appears to be more complex, perhaps because of a mixture of secondary primary sources or a significant contribution from intermediate-mass long-lived stars.

Dufour, R. J.↗
NASA NTRS1987

Fine-scale motion in NGC 6514 and NGC 6523

Velocity maps of the inner regions of the bright H II regions NGC 6514 and NGC 6523 were made with unprecedented spatial and spectral resolution in the 5007 A line of forbidden O III. In addition to the advantages of an instrumental full width at half-maximum intensity of only 5.4 km/s, the small thermal width of the heavy oxygen ion also allows determination of accurate line widths and velocities. The CCD spectra were numerically fitted to Gaussian line profiles and revealed two separate velocity systems in NGC 6523. The data sets of radial velocities were used to derive the dependence of the most probable turbulent velocities upon the sample sizes, and the spatial dependence of the structure function. These relationships are the basic functions for comparison with the predictions of the models for turbulence in H II regions.

O'Dell, C. R.↗
NASA NTRS1994

Nuclear Star Formation in the Hot-Spot Galaxy NGC 2903

We present high-resolution near-infrared imaging obtained using adaptive optics and HST/NICMOS and ground-based spectroscopy of the hot-spot galaxy NGC 2903. Our near-infrared resolution imaging enables us to resolve the infrared hot spots into individual young stellar clusters or groups of these. The spatial distribution of the stellar clusters is not coincident with that of the bright H II regions, as revealed by the HST/NICMOS Pace image. Overall, the circumnuclear star formation in NGC 2903 shows a ring-like morphology with an approximate diameter of 625 pc. The SF properties of the stellar clusters and H II regions have been studied using the photometric and spectroscopic information in conjunction with evolutionary synthesis models. The population of bright stellar clusters shows a very narrow range of ages, 4 to 7 x 10(exp 6) yr after the peak of star formation, or absolute ages 6.5 to 9.5 x 10(exp 6) yr (for the assumed short-duration Gaussian bursts), and luminosities similar to the clusters found in the Antennae interacting galaxy. This population of young stellar clusters accounts for some 7 - 12% of the total stellar mass in the central 625 pc of NGC 2903. The H II regions in the ring of star formation have luminosities close to that of the super-giant H II region 30 Doradus, they are younger than the stellar clusters, and will probably evolve into bright infrared stellar clusters similar to those observed today. We find that the star formation efficiency in the central regions of NGC 2903 is higher than in normal galaxies, approaching the lower end of infrared luminous galaxies.

Alonso-Herrero, A.↗
NASA NTRS1992

Ultraviolet observations of clusters of Wolf-Rayet stars in the SBm3 galaxy NGC 4214 and Ultraviolet and optical observations of LINER's

The purpose of the grant was to obtain and analyze IUE (UV) and ground-based (optical) spectra of the central bar of NGC 4214, which contains several bright H II regions, in order to further explore the properties of the Wolf-Rayet stars in this galaxy. Several spatially distinct regions, with widely different equivalent widths of optical Wolf-Rayet lines, could be sampled by the large IUE entrance aperture. By using newly developed extraction techniques, the spectra of these H II regions could be isolated, and differences in their stellar populations would be systematically studied. Data were obtained with IUE in late February and early March, 1992. Some of the shifts were successful, but a few were not -- apparently the blind offset from the nearby star did not work equally well in all cases. Thus, the signal-to-noise ratio is somewhat lower than we had hoped. This necessitated a more careful extraction of the spectra of individual H II regions from the two-dimensional spectra. (A program that models the point spread function in the spatial direction was used to deblend the distinct H II regions.) The IUE data are currently being analyzed in conjunction with ground-based optical spectra. There appear to be obvious variations in the stellar population over angular scales of only a few arc seconds. The second part of the research performed under this grant was a continuation of a project that uses IUE (UV) and ground-based (optical) spectra to infer the physical conditions in Low-Ionization Nuclear Emission-Line Regions (LINER's). We have obtained spectra of a few key objects that cover a representative range in LINER continuum and emission-line properties. The overall goals are to (1) separate the emission into spatially distinct components, (2) establish whether the observed nuclear ultraviolet continua indicate sufficient photoionizing fluxes to account for the emission lines, (3) determine whether the nuclear emission can be explained by hot stars alone, (4) detect and measure the strengths of UV emission lines, and (5) search for systematic differences in the UV spectra of LINER's whose other properties differ in some respects.

Filippenko, Alexei V.↗
NASA NTRS2000

Populations of High-Luminosity Density-Bounded HII Regions in Spiral Galaxies? Evidence and Implications

In this paper we present evidence that the H II regions of high luminosity in disk galaxies may be density bounded, so that a significant fraction of the ionizing photons emitted by their exciting OB stars escape from the regions. The key piece of evidence is the presence, in the Ha luminosity functions (LFs) of the populations of H iI regions, of glitches, local sharp peaks at an apparently invariant luminosity, defined as the Stromgren luminosity Lstr), LH(sub alpha) = Lstr = 10(sup 38.6) (+/- 10(sup 0.1)) erg/ s (no other peaks are found in any of the LFs) accompanying a steepening of slope for LH(sub alpha) greater than Lstr This behavior is readily explicable via a physical model whose basic premises are: (a) the transition at LH(sub alpha) = Lstr marks a change from essentially ionization bounding at low luminosities to density bounding at higher values, (b) for this to occur the law relating stellar mass in massive star-forming clouds to the mass of the placental cloud must be such that the ionizing photon flux produced within the cloud is a function which rises more steeply than the mass of the cloud. Supporting evidence for the hypothesis of this transition is also presented: measurements of the central surface brightnesses of H II regions for LH(sub alpha) less than Lstr are proportional to L(sup 1/3, sub H(sub alpha)), expected for ionization bounding, but show a sharp trend to a steeper dependence for LH(sub alpha) greater than Lstr, and the observed relation between the internal turbulence velocity parameter, sigma, and the luminosity, L, at high luminosities, can be well explained if these regions are density bounded. If confirmed, the density-bounding hypothesis would have a number of interesting implications. It would imply that the density-bounded regions were the main sources of the photons which ionize the diffuse gas in disk galaxies. Our estimates, based on the hypothesis, indicate that these regions emit sufficient Lyman continuum not only to ionize the diffuse medium, but to cause a typical spiral to emit significant ionizing flux into the intergalactic medium. The low scatter observed in Lstr, less than 0.1 mag rms in the still quite small sample measured to date, is an invitation to widen the data base, and to calibrate against primary standards, with the aim of obtaining a precise, approx. 10(exp 5) solar luminosity widely distributed standard candle.

Beckman, J. E.↗
NASA NTRS1996

Model Stellar Atmospheres and Real Stellar Atmospheres and Status of the ATLAS12 Opacity Sampling Program and of New Programs for Rosseland and for Distribution Function Opacity

I discuss errors in theory and in interpreting observations that are produced by the failure to consider resolution in space, time, and energy. I discuss convection in stellar model atmospheres and in stars. Large errors in abundances are possible such as the factor of ten error in the Li abundance for extreme Population II stars. Finally I discuss the variation of microturbulent velocity with depth, effective temperature, gravity, and abundance. These variations must be dealt with in computing models and grids and in any type of photometric calibration. I have also developed a new opacity-sampling version of my model atmosphere program called ATLAS12. It recognizes more than 1000 atomic and molecular species, each in up to 10 isotopic forms. It can treat all ions of the elements up through Zn and the first 5 ions of heavier elements up through Es. The elemental and isotopic abundances are treated as variables with depth. The fluxes predicted by ATLAS12 are not accurate in intermediate or narrow bandpass intervals because the sample size is too small. A special stripped version of the spectrum synthesis program SYNTHE is used to generate the surface flux for the converged model using the line data on CD-ROMs 1 and 15. ATLAS12 can be used to produce improved models for Am and Ap stars. It should be very useful for investigating diffusion effects in atmospheres. It can be used to model exciting stars for H II regions with abundances consistent with those of the H II region. These programs and line files will be distributed on CD-ROMs.

Kurucz, Robert L.↗
NASA NTRS2007

Variations of the Mid-IR Aromatic Features Inside and Among Galaxies

We present the results of a systematic study of mid-IR spectra of Galactic regions, Magellanic H II regions, and galaxies of various types (dwarf, spiral, starburst), observed by the satellites ISO and Spitzer. We study the relative variations of the 6.2, 7.7, 8.6 and 11.3 micron features inside spatially resolved objects (such as M 82, M 51, 30 Doradus, M 17 and the Orion bar), as well as among 90 integrated spectra of 50 objects. Our main results are that the 6.2, 7.7 and 8.6 micron bands are essentially tied together, while the ratios between these bands and the 11.3 micron band varies by one order of magnitude. This implies that the properties of the PAHs are remarkably universal throughout our sample, and that the relative variations of the band ratios are mainly controlled by the fraction of ionized PAHs. In particular, we show that we can rule out the modification of the PAH size distribution as an explanation of these variations. Using a few well-studied Galactic regions (including the spectral image of the Orion bar), we give an empirical relation between the I(6.2)/I(11.3) ratio and the ionization/recombination ratio Go/n(sub e) x square root of(T(sub gas)). Finally, we discuss the physical interpretation of the I(6.2)/I(11.3) ratio, on galactic size scales.

Galliano, F.↗
NASA NTRS1975

On the ionization of the intercloud medium by runaway O-B stars

H II regions around seven high-velocity stars of type O5 to B1 (lambda Cep, xi Per, 68 Cyg, alpha Cam, AE Aur, zeta Oph, mu Col) are investigated assuming 'on-the-spot' reabsorption of the diffuse UV and time-dependent cooling and recombination behind the stars. Ratios of column densities integrated through the H II regions are obtained for stationary and moving stars, and the ionization structure is calculated for an H II region around a runaway star moving perpendicular to the galactic plane. The total volume ionized is found to be practically independent of the star's velocity. Global properties of the intercloud medium are examined; it is found that the presence of clouds may reduce the volume ionized by as much as a factor of 7 and that as much as 30% of the total ionizing radiation from early-type stars may leak out of the galaxy. Copernicus satellite observations are shown to be consistent with lines of sight crossing only the H II regions around the observed stars and a neutral intercloud medium as well as with the existence of a 'hole' in the distribution of O-B5 stars in the vicinity of the sun.

Thuan, T. X.↗
NASA NTRS1994

Nebular properties from far-infrared spectrosopy

We describe a semiempirical methodology-based on measurements of far-infrared (FIR) lines-that yields information on electron densities in regions where various ionic species exist, effective temperatures (T(sub eff)) for stars ionizing H II regions, and gas-phase heavy element abundances. Although this capability has long been available via optical data, the special features of FIR lines-relative insensitivity to extinction and electron temperature variations-extend the analysis ability. Several line ratios serve as diagnostics of electron density, N(sub e), probing different ionization conditions and different density regimes. The more N(sub e)-diagnostic observations made, the more reliable will be the deciphering of the actual variation in density throughout a nebula. A method to estimate T(sub eff) from the FIR (N III)/(N II) line ratio requires that the nebula be ionization bounded and that substantially all of the flux from the revevant lines be observed. However, to estimate T(sub eff) by a second method that uses the ratio of FIR (S III)/(O III) lines, an ionization-bounded nebula is a sufficient, but not necessary, condition. These restrictions are unnecessary for estimating densities and heavy element abundances. We show that a fairly general determination of metallicity, via the S/H ratio, may be made for H II regions with observations of just two lines-(S III) 19 micron and a hydrogen recombination line (or appropriate substitute). These techniques are applied to recent FIR data for the G333.6-0.2 H II region, including application to the recently measured (N II) 122 and 205 micron lines.

Rubin, Robert H.↗