Reginald J. Dufour

SOFTWARE FOR THE ANALYSIS OF EMISSION LINE NEBULAE

A set of software tools has been developed for the IRAF/STSDAS environment to derive the physical conditions in a low-density (nebular) gas given appropriate diagnostic emission line ratios; and lien emissivities given appropriate emission line fluxes, the electron temperature (Te) and density (Ne). The package is based on the five-level program developed by De Robertis, Dufour and Hunt (1987), but it includes diagnostics from a greater set of ions and emission lines, most particularly those in the satellite ultraviolet that are now observable. Two fo the applications make use of a 3-zone nebular model to derive Te and N3 simultaneously in separate zones of low-, and intermediate-, and high-ionization. These applications are useful for calculating nebular densities and temperatures directly from the traditional diagnostic line ratios, either to provide some reasonable input parameters for a more complicated physical model, or to calculate ionic abundances (or other quantities) within some simplifying assumptions. Examples of the utility of these diagnostics for real nebulae are presented.

Dust in I Zw 18 from Hubble Space Telescope* Narrowband Imaging

We present new WFPC2 narrowband imaging of the blue compact dwarf galaxy I Zw 18, which is host to the lowest metallicity H II regions known. Images at Ha and Hb are combined with archival broadband images to allow the study of the ionized gas distribution and morphology. Analysis of the Ha/Hb —ux ratio reveals signi—cant enhancements in some areas of both the northwest and southeast regions of the galaxy, with ratios elevated to levels as high as 3.4. The Ha/Hb ratio varies considerably with position throughout the galaxy. Comparing this distribution with the stellar distribution indicates that the regions of enhanced Ha/Hb ratio are not due to the eUects of either collisional excitation or underlying stellar absorption and therefore are most likely interpreted as the presence of dust. This dust has an estimated mass of (2¨5) ] 103 which is consistent with the IRAS far-IR nondetection. Under M _ , the assumption that dust traces the presence of molecular gas, these results suggest that the molecular component of the interstellar medium of I Zw 18, which is needed to fuel its active star formation, is also very clumpy. Such a distribution would be consistent with the recent Far-Ultraviolet Spectroscopic Explorer nondetections of diUuse H 2 . &

The origin of broad emission lines in the extragalactic giant H II region NGC 2363

High signal-to-noise long-slit spectra have been obtained of the giant H II region NGC 2363 located in the dwarf SBm galaxy NGC 2366. A discovery of low-intensity broad spectral components (FWHM is approximately equal to 40 A or 2400 km/s) in the bright nebular lines H-alpha, H-beta, and forbidden O III is reported. The broad spectral components are detected over a large spatial extent (not less than 500 pc) centered on the nebula. Several mechanisms for broadening nebular lines are explored: stellar winds, Thomson scattering by hot gas, supernova remnants, and superbubble blowout. All mechanisms have problems. Superbubble blowout, which is the only known mechanism capable of accelerating interstellar gas over such a volume of space, does not appear consistent with the physical properties of the H II region NGC 2363 or with the nature of the host galaxy. It is concluded that the broad nebular lines are probably due to very high velocity gas whose origin is, at present, unknown.

Si/O Abundance Ratios in Extragalactic H II Regions from Hubble Space Telescope UV Spectroscopy

We have measured the Si III] intercombination doublet at 1883 ? and 1892 ? in seven extragalactic H II regions using the Faint Object Spectrograph on the Hubble Space Telescope. These measurements were used to derive the Si/C and Si/O gas phase abundance ratios in the ionized gas. We find that Si/O shows no systematic variation with O/H over the range -4.8 < log O/H < -3.4. The weighted mean value for Si/O over this range is log Si/O = -1.59 ? 0.07. For comparison, the solar value is log Si/O = -1.37, while for Galactic B stars and supergiants -1.6 < log Si/O < -1.2. Uncertainty in the stellar reference value for Si/O prevents a straightforward interpretation of our Si/O ratios. However, if the solar Si/O ratio represents the intrinsic cosmic ratio, our results imply that, on average, only about 50% of the total abundance of silicon in these H II regions is incorporated into dust grains, a value significantly smaller than is typically measured in dense interstellar clouds. Our results suggest that some grain modification is occurring within the H II region environment.

Physical Conditions in Low-Ionization Regions of the Orion Nebula

ABSTRACTWe reexamine the spectroscopic underpinnings of recent suggestions that [O I ] and[Fe II ] lines from the Orion H region are produced in gas where the iron-carryinggrains have been destroyed and the electron density is surprisingly high. Our newobservations show that previous detections of [O I ] 5577 were dominated by telluricemission. Our limits are consistent with a moderate density (≈ 10 4 cm −3 ) photoionizedgas. We show that a previously proposed model of the Orion H II region reproducesthe observed [O I ] and [Fe II ] spectrum. These lines are fully consistent with formationin a dusty region of moderate density.Subject headings: ISM: H II regions — ISM: abundances — ISM: atoms — ISM:individual (Orion Nebula)1. IntroductionThe Orion Nebula is the defining blister H II region (Zuckerman 1973; Balick, Gammon, &Hjellming 1974). A star cluster ionizes the skin of the molecular cloud, causing an expansion away 1 Based in part on observations made with the NASA/ESAHubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555

Extended emission and star formation in I Zw 18

The results of sky limited emission-line (H-alpha and forbidden O III 5007A) and line-free (6450 A and 5100 A) continuum imagery of the super-metal-poor blue compact galaxy I Zw 18 are reported. The main body of star formation in the galaxy is surrounded by a 3 kpc diameter extended envelope in H-alpha. In addition, there exist arcs of emission emanating from both central H II regions and winding around to the west, as well as a broader ridge of emission extending northward from the main body. The total mass of ionized material is estimated to be 3 x 10 to the 6th solar masses, or about 5 percent of the mass of the observed H I clouds. Several extended stellar continuum regions exist to the northwest of the main body, the brightest three forming a line along PA = 310 deg out to a 3 kpc distance from the central region of the galaxy. The correlation of the morphology and location of stellar continuum and emission regions with the distribution of H I is discussed. It is calculated that I Zw 18 has had previous, but disconnected, episodes of star formation, possibly from chance collisions of widely separated H I clouds.

CNO abundances and temperature fluctuations in the Orion Nebula

The largest UV-optical data base to date of the Orion Nebula has been assembled, and the CNO abundances their distribution at a statistically significant number of positions to the north and west of the Trapezium and out to a distance of 5 arcmin has been examined. The forbidden O III temperature increases with distance from Theta1 Ori C while the forbidden N II temperature decreases with increasing distance. The density decreases as a function of increasing distance from Theta1 Ori C. The C/H, N/H, O/H, and C/O distributions are constant across the inner 5 arcmin of the northwestern quadrant of the nebula. The empirically determined mean C/H abundance calculated from UV lines is almost a factor of two lower in the number density than earlier estimates. Photoionization model calculations are consistent with a solar abundance for N/H and O/H.

Spitzer observations of M83 and the hot star, H ii region connection

We have undertaken a programme to observe emission lines of [Siv] 10.51, [NeII] 12.81, [Ne III] 15.56, and [S III] 18.71 μm in a number of extragalactic HII regions with the Spitzer Space Telescope. Here we report our results for the nearly face-on spiral galaxy M83. A subsequent paper will present our data and analysis for another substantially face-on spiral galaxy M33. The nebulae selected cover a wide range of galactocentric radii (R G ). The observations were made with the infrared spectrograph in the short wavelength, high dispersion configuration. The above set of four lines is observed cospatially, thus permitting a reliable comparison of the fluxes. From the measured fluxes, we determine the ionic abundance ratios including Ne ++ /Ne + , S 3+ /S ++ and S ++ /Ne + and find that there is a correlation of increasingly higher ionization with larger R G . By sampling the dominant ionization states of Ne and S for Hit II regions, we can approximate the Ne/S ratio by (Ne + + Ne ++ )/(S ++ + S 3+ ). Our findings of ratios that significantly exceed the benchmark Orion Nebula value, as well as a decrease in this ratio with increasing R G , are more likely due to other effects than a true gradient in Ne/S. Two effects that will tend to lower these high estimates and to flatten the gradient are first, the method does not account for the presence of S + and second, S but not Ne is incorporated into grains. Both Ne and S are primary elements produced in α-chain reactions, following C and O burning in stars, making their yields depend very little on the stellar metallicity. Thus, it is expected that Ne/S remains relatively constant throughout a galaxy. We stress that this type of observation and method of analysis does have the potential for accurate measurements of Ne/S, particularly for H II regions that have lower metallicity and higher ionization than those here, such as those in M33. Our observations may also be used to test the predicted ionizing spectral energy distribution (SED) of various stellar atmosphere models. We compare the ratio of fractional ionizations / and / versus / with predictions made from our photoionization models using several of the state-of-the-art stellar atmosphere model grids. The overall best fit appears to be the nebular models using the supergiant stellar atmosphere models of Pauldrach, Hoffmann & Lennon and Sternberg, Hoffmann & Pauldrach. This result is not sensitive to the electron density and temperature range expected for these M83 nebulae. Considerable computational effort has gone into the comparison between data and models, although not all parameter studies have yet been performed on an ultimate level (e.g. in the present paper the stellar atmosphere model abundances have been fixed to solar values). A future paper, with the benefit of more observational data, will continue these studies to further discriminate how the ionic ratios depend on the SED and the other nebular parameters.

The abundances of carbon and nitrogen in I Zw 18

The first measurements of the semiforbidden C III 1909 A and forbidden N II 6583 A emission lines in I Zw 18 are reported. These data are combined with previously published spectrophotometry in order to derive the carbon and nitrogen abundances in I Zw 18 with the aid of a nebular model. The CNO abundances in I Zw 18 are found to exceed those found in the Magellanic Clouds and other well-observed oxygen-poor irregular galaxies and are more similar to those in the sun and nearby Galactic H II regions. It is argued that I Zw 18 may be older and more chemically evolved than previously believed, and that its extremely low O/H results from escape of O-rich supernova ejecta from the Galaxy or from dilution by accretion of primordial gas. Alternatively, I Zw 18 may have formed from a preenriched intergalactic medium. Spectroscopic diagnostics for composite photoionization models to fit the spectrum of I Zw 18 are discussed. 35 references.

Extinction and scattering in the Orion Nebula : comparison of optical and VLA 21 centimeter studies

We have compared VLA 21 cm continuum maps of M42 with extinction-corrected H-beta images and found that much of M42 has an excess of H-beta surface brightness. This is interpreted as reflection by particles lying close to the principal ionization front. Comparison of column densities of H I near M42 and extinction (C(H-beta)) indicates an excellent area-by-area correlation, establishing that most of the extinction in M42 arises in the neutral lid that lies on the near side of the nebula. A more quantitative comparison of (C(H-beta)) and N(H I), the column density, is consistent with this conclusion and indicates a dust scattering cross section per hydrogen atom that is similar to the value for the general interstellar medium. The usual model for extinction in the Trapezium region is that it occurs very near the stars and produces an anomalous reddening curve through modification of the nature or size distribution of the particles. These new results question the basic assumptions and conclusions of that model.