We use high signal-to-noise ratio spectra of 20 H II regions in the giant spiral galaxy M101 to derive electron temperatures for the H II regions and robust metal abundances over radii R = 019-125R0 (6-41 kpc) We compare the consistency of electron temperatures measured from the [O III] ?4363, [N II] ?5755, [S III] ?6312, and [O II] ?7325 auroral lines Temperatures from [O III], [S III], and [N II] are correlated with relative offsets that are consistent with expectations from nebular photoionization models However, the temperatures derived from the [O II] ?7325 line show a large scatter and are nearly uncorrelated with temperatures derived from other ions We tentatively attribute this result to observational and physical effects, which may introduce large random and systematic errors into abundances derived solely from [O II] temperatures Our derived oxygen abundances are well fitted by an exponential distribution over six disk scale lengths, from approximately 13 (O/H)? in the center to 1/15 (O/H)? in the outermost region studied [for solar 12 + log(O/H) = 87] We measure significant radial gradients in N/O and He/H abundance ratios, but relatively constant S/O and Ar/O Our results are in approximate agreement with previously published abundances studies of M101 based on temperature measurements of a few H II regions However, our abundances are systematically lower by 02-05 dex than those derived from the most widely used strong-line empirical abundance indicators, again consistent with previous studies based on smaller H II region samples Independent measurements of the Galactic interstellar oxygen abundance from ultraviolet absorption lines are in good agreement with the Te-based nebular abundances We suspect that most of the disagreement with the strong-line abundances arises from uncertainties in the nebular models that are used to calibrate the empirical scale, and that strong-line abundances derived for H II regions and emission-line galaxies are as much as a factor of 2 higher than the actual oxygen abundances However, other explanations, such as the effects of temperature fluctuations on the auroral line based abundances, cannot be completely ruled out These results point to the need for direct abundance determinations of a larger sample of extragalactic H II regions, especially for objects more metal-rich than solar

https://iopscience.iop.org/article/10.1086/375398/pdf

The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale

We have obtained deep optical spectra of the planetary nebula NGC 6153, both along its minor axis and by uniformly scanning a long slit across the whole nebula. The scanned spectra, when combined with the nebular total H beta flux, yield integrated fluxes for all the lines (similar to 400) in our spectra, which are rich in strong recombination lines from C, N, O and Ne ions. A weak O vi lambda 3811 emission line from the central star has been detected, suggesting that the nucleus of NGC 6153 has a hydrogen-deficient surface. The optical data, together with the ISO LWS 43-197 mu m spectrum and the archival IUE and IRAS LRS spectra, are used to study the thermal and density structure and to derive the heavy-element abundances from lines produced by different excitation mechanisms. In all cases, the C2+/H+, N2+/H+, O2+/H+ and Ne2+/H+ abundances derived from multiple optical recombination lines (ORLs) are consistently higher, by about a factor of 10, than the corresponding values deduced from optical, UV or infrared (IR) collisionally excited lines (CELs), regardless of the excitation energies or critical densities of the latter. The agreement between the temperature-sensitive optical forbidden lines and the temperature-insensitive IR fine-structure lines rules out temperature fluctuations as the cause of the large difference between the ORL and CEL abundances.We present the results of a new calculation of recombination coefficients for [O II] which lead to good agreement between the observed and predicted [O II] lambda lambda 7320, 7330 forbidden line intensities if these lines are solely excited by recombination at the Balmer jump temperature. Recombination excitation is also found to be important in exciting the [N ii] lambda 5754 line, which, if unaccounted for, would lead to an overestimated [N ii] temperature from the observed (lambda 6548+lambda 6584)/lambda 5754 ratio. Analysis of a number of C ii lines arising from levels as high as 7g in the recombination ladder reveals excellent agreement between their reddening-corrected relative intensities and those predicted by recombination theory. Spatial analysis of the long-slit spectra taken along the nebular minor axis yields a varying [O iii] temperature, whereas the hydrogen Balmer jump temperature of 6000 K is approximately constant across the nebula, and is 2000-3000 K lower than the [O iii] temperature. The observed high-n Balmer line decrement indicates that the hydrogen lines arise from material having an electron density of 2000(-1000)(+2000) cm(-3), consistent with the optical and IR forbidden-line density diagnostics, which yield average line-of-sight electron densities along the minor axis varying between 2000 and 4000 cm(-3).While the He/H ratio mapped by He I and He ii recombination lines is constant within 5 per cent across the nebula, the C2+/H+ and O2+/H+ recombination-line abundances decrease by a factor of 2-3 over a radius of 15 arcsec from the centre, pointing to the presence of abundance gradients. We consider a variety of hypotheses to account for the observed behaviour of the various thermal, density and abundance diagnostics. Empirical nebular models containing two components with differing densities and temperatures are able to account for many of the observed patterns, but only if one of the components is significantly hydrogen-deficient. One such model, which gives a good fit to the observed line intensities and patterns, has 500-K H-depleted material, presumed to be evaporating from dense neutral inclusions, embedded in 9500-K material with 'normal' abundances. An alternative model, which appears more physically plausible on a number of grounds, has high-density (2x10(6) cm(-3)), fully ionized, H-deficient knots embedded in the 'normal' component, although this model fails to account adequately for the observed low (6000 K) hydrogen Balmer jump temperature. However, the observed fact that the ORLs and CELs yield heavy-element abundance ratios that are identical within the uncertainties finds no obvious explanation in the context of H-deficient knot models.

NGC 6153: a super-metal-rich planetary nebula?

We have combined the detailed He I recombination model of Smits with the collisional transitions of Sawey & Berrington in order to produce new accurate helium emissivities that include the effects of collisional excitation from both the 2 3S and 2 1S levels. We present a grid of emissivities for a range of temperature and densities along with analytical fits and error estimates. These grids eliminate the necessity of making corrections for collisional enhancements as in the work of Clegg or Kingdon & Ferland for lines with upper levels below n=5. For densities greater than ne≈106 cm-3, inclusion of collisional excitation from the 2 1S level is also necessary if accuracies of greater than a few percent are required. Atomic data for a model atom with 29 levels (nmax=5) are presented that match the recombination model of Smits to within 5% over the temperature range T=5000-20,000 K. Collisional effects are calculated self-consistently using the algorithm of Almog & Netzer. This model atom will be useful in models of radiative transfer. A notable feature of this technique is an algorithm that calculates the indirect recombination rates, the recombination to individual levels that go through n>nmax first. Fits accurate to within 1% are given for the emissivities of the brightest lines over a restricted range for estimates of primordial helium abundance. We characterize the analysis uncertainties associated with uncertainties in temperature, density, fitting functions, and input atomic data. We estimate that atomic data uncertainties alone may limit abundance estimates to an accuracy of ~1.5%; systematic errors may be greater than this. This analysis uncertainty must be incorporated when attempting to make high-accuracy estimates of the helium abundance. For example, in recent determinations of the primordial helium abundance, uncertainties in the input atomic data have been neglected. Finally, we compare our theoretical calculations to the measured strengths of a few dozen helium emission lines in three nebulae, Orion (NGC 1976) and the planetary nebulae NGC 6572 and IC 4997. Incorporation of collisional effects yields noticeable improvements for some lines, but some notable discrepancies remain.

http://iopscience.iop.org/article/10.1086/306923/pdf

Improving Predictions for Helium Emission Lines

We present Very Large Telescope (VLT) UVES echelle spectrophotometry of the Orion nebula in the 3100-10 400 A range. We have measured the intensity of 555 emission lines, many of them corresponding to permitted lines of different heavy-element ions. This is the largest set of spectral emission lines ever obtained for a Galactic or extragalactic H II region. We have derived He + , C 2+ , O + , O 2+ and Ne 2+ abundances from pure recombination lines. This is the first time that O + and Ne 2+ abundances have been obtained from these kinds of lines in the nebula. We have also derived abundances from collisionally excited lines for a large number of ions of different elements. In all cases, ionic abundances obtained from recombination lines are larger than those derived from collisionally excited lines. We have obtained remarkably consistent independent estimations of the temperature fluctuation parameter, t 2 , from different methods, which are also similar to other estimates from the literature. This result strongly suggests that moderate temperature fluctuations (t 2 between 0.02 and 0.03) are present in the Orion nebula. We have compared the chemical composition of the nebula with those of the Sun and other representative objects. The heavy-element abundances in the Orion nebula are only slightly higher than the solar ones, a difference that can be explained by the chemical evolution of the solar neighbourhood.

/pdf/a-reappraisal-of-the-chemical-composition-of-the-orion-4h8nt29ba0.pdf

A reappraisal of the chemical composition of the Orion nebula based on Very Large Telescope echelle spectrophotometry

The evolution of the content of heavy elements in galaxies, the relative chemical abundances, their spatial distribution, and how these scale with various galactic properties, provide unique information on the galactic evolutionary processes across the cosmic epochs. In recent years major progress has been made in constraining the chemical evolution of galaxies and inferring key information relevant to our understanding of the main mechanisms involved in galaxy evolution. In this review we provide an overview of these various areas. After an overview of the methods used to constrain the chemical enrichment in galaxies and their environment, we discuss the observed scaling relations between metallicity and galaxy properties, the observed relative chemical abundances, how the chemical elements are distributed within galaxies, and how these properties evolve across the cosmic epochs. We discuss how the various observational findings compare with the predictions from theoretical models and numerical cosmological simulations. Finally, we briefly discuss the open problems and the prospects for major progress in this field in the nearby future.

/pdf/de-re-metallica-the-cosmic-chemical-evolution-of-galaxies-1mjmpxmn5s.pdf

De re metallica: the cosmic chemical evolution of galaxies

We present new spectrophotometric observations of the rich O II optical recombination line spectrum of the planetary nebula NGC 7009, obtained at a spectral resolution of about 1 Angstrom (FWHM). New intermediate coupling quantal calculations of O II radiative recombination coefficients for the 3d-3p and 4f-3d transitions are presented. The effect of departure from pure LS-coupling is shown to be important. Excellent agreement is found between the observed relative intensities of the O II lines and those calculated from recombination theory allowing for intermediate coupling effects. C, N and O abundances based on our recombination line measurements are derived. In all cases, they are about a factor of 5 higher than the corresponding values deduced from collisionally excited lines, indicating that the discrepancy between the abundances derived from these two different types of emission Lines, previously known to exist for C2+, is a common phenomenon, and is probably caused by the same physical process. The nature of this process is still not known. If the discrepancy is due to temperature fluctuations, the implied rms temperature fluctuation parameter t(2) is about a factor of 2 larger than that derived by comparing the temperatures deduced from the [O III] forbidden line ratio and from the ratio of the nebular continuum Balmer discontinuity to H beta. However, if we adopted the electron temperature derived from nebular continuum Balmer discontinuity instead of that from the [O III] forbidden line ratio, the C and N abundances deduced from ultraviolet collisionally excited lines would come into agreement with those deduced from the optical recombination lines, although the abundance of oxygen deduced from the optical forbidden lines would still be a factor of 2 lower than the corresponding value obtained from the optical recombination lines. The O/H abundance ratio derived from our recombination line analysis of NGC 7009 is more than a factor of 2 higher than the solar oxygen abundance.

The rich O II recombination spectrum of the planetary nebula NGC 7009: new observations and atomic data

Les nebuleuses planetaires NGC 6543 et NGC 6826 sont membres du groupe des nebuleuses planetaires a enveloppes multiples et classees comme nebuleuses planetaires a halos faibles. Des spectres a fentes longues pour plusieurs positions dans ces halos sont analyses afin d'etablir les proprietes moyennes de ces 2 regions

/pdf/the-giant-halos-of-ngc-6543-and-6826-45496im56e.pdf

The giant halos of NGC 6543 and 6826

Optical spectroscopic data for 71 Planetary Nebulae (PN) in the Large and Small Magellanic Clouds have been analysed. The line fluxes have been used to determine nebular temperatures, densities, and the abundances of He, N, O, Ne, and Ar, relative to H. In our sample there are 12 nebulae with N/O≽0.5, resembling Peimbert's Type I PN; six low-excitation (LE) objects [1≼I(5007)/I(Hβ)≼4]; and four very-low-excitation (VLE) nebulae [I(Hβ)>I(5007)], similar to the Galactic VLE class. Mean abundances have been calculated for the nebulae not in these special groups. After correction for collisional excitation contributions to the nebular He I lines, PN in the SMC and LMC yield mass fractions of Y=0.249±0.025 and Y=0.258±0.015, respectively. Compared with PN in our own Galaxy, the abundances of Ne and Ar, which are the elements in our sample least affected by nucleosynthesis, are lower by 0.6 and 0.35 dex for the SMC and LMC respectively. The oxygen and neon abundances in the Magellanic Cloud PN are the same as those previously found for H II regions in the LMC and SMC, but the nitrogen in PN is enhanced by 0.9 and 1.0 dex in each galaxy, respectively. This is found to be consistent with the processing of all of the original carbon to nitrogen by the CN cycle, operating in the progenitor stars at the time of the first dredge-up. This process seems to have operated much more efficiently in the metal-poor Magellanic Clouds than in the Milky Way, in agreement with theoretical predictions. Five Wolf–Rayet central stars are detected in the sample (two SMC, three LMC). The frequency of occurrence of these helium-rich central stars in low- and medium-excitation PN (15 per cent) is very similar to that of helium-rich white dwarfs in the solar neighbourhood, suggesting that events at the end of the AGB phase may be reponsible for the observed fractions of helium- and hydrogen-rich white dwarf stars.

An optical spectrophotometric survey of abundances in Magellanic Cloud Planetary Nebulae

A detailed study of the planetary nebula NGC 3918, based on UV, optical, and radio observations, is presented. The central star has a predicted V magnitude of 14.6 and a luminosity of 6000 solar luminosities for a distance of 1.5 kpc. A contour map of the nebula in H-beta light is given. Velocity profiles of forbidden O II optical lines show that O(+) ions are mostly located at the front and rear of the nebula. A composite, biconical model is constructed which consists of optically thick cones at the front and rear with a low-density 'equatorial' region. The nebular expansion age of about 3000 yr is in reasonable agreement with the age of the nucleus found from evolutionary tracks. Silicon, magnesium, and iron are depleted by factors of 4, 3, and 100 respectively. It is shown that the Mg II 2800 A lines are affected by interstellar absorption in this and many other planetaries and thus often cannot be used for abundance determinations.

The planetary-nebula ngc 3918

HST Faint Object Camera images of hydrogen-poor gas in the planetary nebulae A30 and A78 have revealed remarkable 'cometary' structures in the O III 5007-A forbidden line. Most of these cometary structures, consisting of compact (0.15-0.5 arcsec) knots with radial tails several arcsec in length, are located in an equatorial plane in both nebulae. In addition, two bright, compact (0.3 arcsec) polar knots are present in A30, one of them forming a bow shock. Corresponding polar features in A78 are more diffuse. The central stars of both nebulae have energetic winds which are most likely responsible for the 'cometary' knot morphology. We interpret this morphology in terms of dense (several thousand electrons per cu cm) H-poor condensations whose outer expanding layers are swept outward by stellar winds. Photoionization modeling indicates that while dense knot cores are mostly heated by atomic photoionization, expanding tenuous gas is heated by photoelectrons ejected from abundant dust grains. Our models predict steep temperature gradients for which there is observational evidence and possible abrupt phase transitions in the expanding gas.

R. E. S. Clegg

Papers

The rich O II recombination spectrum of the planetary nebula NGC 7009: new observations and atomic data

The giant halos of NGC 6543 and 6826

An optical spectrophotometric survey of abundances in Magellanic Cloud Planetary Nebulae

The planetary-nebula ngc 3918

HST imaging of hydrogen-poor ejecta in Abell 30 and Abell 78 - Wind-blown cometary structures