Ionization Correction Factors Research Articles

Inhomogeneous H II Regions and the Helium Abundance

When calculating the helium (4He) abundance in low metallicity H ii regions, the ionization correction factor (icf) for unseen neutral helium (and hydrogen) is usually assumed to be unity. In this paper, we explore this factor for H ii regions ionized by young stellar clusters. Our main result is that icf < 1 for homogeneous H ii regions and, that the effect of density condensations in the H ii regions is to further reduce the icf. For icf < 1, the primordial helium abundance inferred from observations of low-metallicity, extragalactic H ii regions is decreased.

Emission‐Line Helium Abundances in Highly Obscured Nebulae

ABSTRACTThis paper outlines a way to determine the ionization correction factor (ICF) using only infrared data. We identify four line pairs, [Ne iii] 36 μm/[Ne ii] 12.8 μm, [Ne iii] 15.6 μm /[Ne ii] 12.8 μm, [Ar iii] 9 μm/[Ar ii] 6.9 μm, and [Ar iii] 21 μm/[Ar ii] 6.9 μm, that are sensitive to the He ICF. This happens because the ions cover a wide range of ionization, the line pairs are not sensitive to electron temperature, they have similar critical densities, and they are formed within the He+/H+ region of the nebula. We compute a very wide range of photoionization models appropriate for galactic H ii regions. The models cover a wide range of densities, ionization parameters, and stellar temperatures and use continua from four very different stellar atmospheres.The results show that each line pair has a critical intensity ratio above which the He ICF is always small. Below these values the ICF depends very strongly on details of the models for three of the ratios, and so other information would be needed to determine the helium abundance. The [Ar iii] 9 μm/[Ar ii] 6.9 μm ratio can indicate the ICF directly as a result of the near exact match in the critical densities of the two lines. Finally, continua predicted by the latest generation of stellar atmospheres are sufficiently hard that they routinely produce significantly negative ICFs.

The remarkably high excitation planetary nebula GC 6537.

NGC 6537 is an unusually high excitation point symmetric planetary nebula with a rich spectrum. Its kinematical structures are of special interest. We are here primarily concerned with the high resolution spectrum as revealed by the Hamilton echelle Spectrograph at Lick Observatory (resolution approximately 0.2 A) and supplemented by UV and near-UV data. These extensive data permit a determination of interstellar extinction, plasma diagnostics, and ionic concentrations. The photoionization models that have been used successfully for many planetary nebulae are not entirely satisfactory here. The plasma electron temperature of a photoionization model cannot much exceed 20,000 K, but plasma diagnostics show that regions emitting radiation of highly ionized atoms such as [NeIV] and [NeV] are much hotter, showing that shock excitation must be important, as suggested by the remarkable kinematics of this object. Hence, instead of employing a strict photoionization model, we are guided by the nebular diagnostics, which reveal how electron temperature varies with ionization potential and accommodates density effects. The predictions of the photoionization model may be useful in estimating ionization correction factor. In effect, we have estimated the chemical composition by using both photoionization and shock considerations.

Spectroscopic Observations of the Planetary Nebula NGC 6818

NGC 6818 is a bright, high-excitation, fairly regular, planetary nebula with a rather rich spectrum. Its rather unusual kinematical structure, studied by Wilson, Weedman, and particularly by Sabbadin, is of special interest. We are here primarily concerned with its high-resolution spectrum as revealed by the Hamilton Echelle Spectrograph at Lick Observatory (resolution ~0.2 A) and supplemented by UV and IR data. These extensive data permit a determination of interstellar extinction, plasma diagnostics, and ionic concentrations. Establishing the chemical composition requires finding the ionization correction factors that may be estimated empirically for only a few elements. In general we rely on a composite theoretical model. The chemical composition appears to be unremarkable. C and N appear to be more abundant than in the Sun. Most other elements seem to have roughly a solar abundance, except for those depleted by grain formation. NGC 6818 may have originated from a star resembling the Sun at least in chemical composition. The central star is rather faint and possibly of the Wolf-Rayet type. Comparisons with model predictions suggest a L of about 3,500 L and T of at least 140,000 K. Comparison with theoretical tracks suggests an age of about 9000 yr. Spectroscopic studies show that the bright shell appears to consist of tangentially moving filaments of differing excitation, a complication that may explain why theoretical nebular models have difficulties.

Heavy‐Element Abundances in Blue Compact Galaxies

We present high-quality ground-based spectroscopic observations of 54 supergiant H II regions in 50 low-metallicity blue compact galaxies with oxygen abundances 12+log O/H between 7.1 and 8.3. We use the data to determine abundances for the elements N, O, Ne, S, Ar, and Fe. We also analyze Hubble Space Telescope (HST) Faint Object Spectrograph archival spectra of 10 supergiant H II regions to derive C and Si abundances in a subsample of seven BCGs. The main result of the present study is that none of the heavy element-to-oxygen abundance ratios studied here (C/O, N/O, Ne/O, Si/O, S/O, Ar/O, Fe/O) depend on oxygen abundance for BCGs with 12+log O/H≤7.6 (Z≤Z☉/20). This constancy implies that all of these heavy elements have a primary origin and are produced by the same massive (M≥10 M☉) stars responsible for O production. The dispersion of the ratios C/O and N/O in these galaxies is found to be remarkably small, being only ±0.03 and ±0.02 dex, respectively. This very small dispersion is strong evidence against any time-delayed production of C and primary N in the lowest metallicity BCGs (secondary N production is negligible at these low metallicities). The absence of a time-delayed production of C and N is consistent with the scenario that galaxies with 12+logO/H≤7.6 are now undergoing their first burst of star formation, and that they are therefore young, with ages not exceeding 40 Myr. If very low metallicity BCGs are indeed young, this would argue against the commonly held belief that C and N are produced by intermediate-mass (3 M☉≤M≤9 M☉) stars at very low metallicities, as these stars would not have yet completed their evolution in these lowest metallicity galaxies. In higher metallicity BCGs (7.6<12+log O/H<8.2), the abundance ratios Ne/O, Si/O, S/O, Ar/O, and Fe/O retain the same constant value they had at lower metallicities. By contrast, there is an increase of C/O and N/O along with their dispersions at a given O. We interpret this increase as due to the additional contribution of C and primary N production in intermediate-mass stars, on top of that by high-mass stars. The above results lead to the following timeline for galaxy evolution: (1) all objects with 12+logO/H≤7.6 began to form stars less than 40 Myr ago; (2) after 40 Myr, all galaxies have evolved so that 12+logO/H>7.6; (3) by the time intermediate-mass stars have evolved and released their nucleosynthetic products (100-500 Myr), all galaxies have become enriched to 7.6<12+logO/H<8.2. The delayed release of primary N at these metallicities greatly increases the scatter in N/O; (4) later, when galaxies get enriched to 12+logO/H>8.2, secondary N production becomes important. BCGs show the same O/Fe overabundance with respect to the Sun (~0.4 dex) as Galactic halo stars, suggesting the same chemical enrichment history. We compare heavy elements yields derived from the observed abundance ratios with theoretical yields for massive stars and find general good agreement. However, the theoretical models are unable to reproduce the observed N/O and Fe/O. Further theoretical developments are necessary, in particular to solve the problem of primary nitrogen production in low-metallicity massive stars. We discuss the apparent discrepancy between abundance ratios N/O measured in BCGs and those in high-redshift damped Lyα galaxies, which are up to 1 order of magnitude smaller. We argue that this large discrepancy may arise from the unknown physical conditions of the gas responsible for the metallic absorption lines in high-redshift damped Lyα systems. While it is widely assumed that the absorbing gas is neutral, we propose that it could be ionized. In this case, ionization correction factors can boost N/O in damped Lyα galaxies into the range of those measured in BCGs.

Testing CNO Enrichment Scenarios in Metal‐poor Galaxies withHubble Space TelescopeSpectroscopy

Using Hubble Space Telescope ultraviolet and ground-based optical spectroscopy, we measure the C/O and N/O ratios of three metal-poor galaxies with similar metallicity but differing N/O. These observations, in conjunction with photoionization models, indicate that the C/O ratios estimated from C III] λ1909 and [O III] λ5007 lines are consistent with those measured from C II] λ2326 and [O II] λ3727 lines. Although inherently more uncertain than C++/O++ due to poorly known ionization correction factors, we develop the use of the C+/O+ ratio as a reasonable substitute diagnostic of the carbon abundance in H II regions. The C II] λ2326 multiplet is typically quite weak, but its proximity to the [O II] λλ3726, 3729 lines makes these transitions a potential tool for measuring the carbon abundance in high-redshift emission-line objects.

The Optical Spectrum of the Planetary Nebula NGC 2440

ABSTRACTWavelengths and identifications have been provided for approximately 300 lines between 3660 and 10125 Å in the spectrum of the bipolar planetary nebula NGC 2440. These lines measured with the Hamilton echelle spectrograph at Lick Observatory and supplemented by published IUE results are used to construct diagnostic diagrams and derive ionic concentrations. The electron temperature indicated by [Oiii] is ∼14,200 K; the electron density is ∼5000 cm−1. With the aid of a standard model and an ionization correction factor method, we obtain a set of abundances that are compared with the Sun and with results of previous studies. As with other Peimbert Type I's, He and N are enriched as compared with the Sun; C is likewise overabundant. NGC 2440 evolved from a progenitor star that was more massive than the Sun. The central star temperature seems close to 180,000 K.

A New Look at Carbon Abundances in Planetary Nebulae. III. DDDM1, IC 3568, IC 4593, NGC 6210, NGC 6720, NGC 6826, and NGC 7009

This paper is the third in a series reporting on a study of carbon abundances in a carefully chosen sample of planetary nebulae representing a large range in progenitor mass and metallicity. We make use of the IUE Final Archive database containing consistently reduced spectra to measure line strengths of C III] λ1909 along with numerous other UV lines for the planetary nebulae DDDM1, IC 3568, IC 4593, NGC 6210, NGC 6720, NGC 6826, and NGC 7009. We combine the IUE data with line strengths from optical spectra obtained specifically to match the IUE slit positions as closely as possible, in order to determine values for the abundance ratios He/H, O/H, C/O, N/O, and Ne/O. The ratio of C III] λ1909/C II λ4267 is found to be effective for merging UV and optical spectra when He II λ1640/λ4686 is unavailable. Our abundance determination method includes a five-level program whose results are fine-tuned by corrections derived from detailed photoionization models constrained by the same set of emission lines. All objects appear to have subsolar levels of O/H, and all but one show N/O levels above solar. In addition, the seven planetary nebulae span a broad range in C/O values. We infer that many of our objects are matter-bounded, and thus the standard ionization correction factor for N/O may be inappropriate for these PNs. Finally, we estimate C/O using both collisionally excited and recombination lines associated with C+2 and find the well-established result that abundances from recombination lines usually exceed those from collisionally excited lines by several times.

Abundances in planetary nebulae near the galactic centre

Abundance determinations of about 110 planetary nebulae, which are likely to be in the Galactic Bulge are presented. Plasma diagnostics have been performed by making use of the available forbidden line ratios combined with radio continuum measurements. Chemical abundances of He, O, N, Ne, S, Ar, and Cl are then derived by employing theoretical nebular models as interpolation devices in establishing the ionization correction factors (ICFs) used to estimate the distribution of atoms among unobserved ionization stages. The overall agreement between the results derived by using the model-ICFs and those obtained from the theoretical models is reasonably good. The uncertainties related to the total abundances show a clear dependence on the level of excitation. In most cases, the abundances of chlorine can be derived only in objects with a relatively high Cl-abundance. Contrary to the conclusion previously drawn by Webster (1988), we found the excitation classes are not uniformly distributed. A clear peak at about classes 5 and 6 is noticed. The distribution is shifted toward a lower excitation range with respect to that of the nearby nebulae, reflecting the difference in the central star temperature distribution between the two samples.

The high-excitation planetary nebula NGC 6741

The compact, bright Peimbert type I planetary nebula NGC 6741 shows an unusually rich spectrum which is studied here from 3700 to 10 370 Å with the Hamilton Echelle spectrograph at the Lick Observatory. By combining Hamilton Echelle observations with archive UV data secured with the International Ultraviolet Explorer (IUE), and with available IR data, we can obtain improved diagnostics and chemical composition of this high-excitation planetary nebula. The diagnostic diagram gives Tϵ = 12 500 K, Nϵ= 6300 cm−3 for most ions. An improved ‘composite’ theoretical nebular model is employed. The stellar temperature quoted in the earlier literature seems too high; the highest likely temperature appears to be 140 000 K. Elemental abundances are estimated from ionic concentrations, using an ionization correction factor in the usual way, and also from a theoretical model. These different methods show large discordances, implying that one must use great caution in choosing Tϵ for the determination of the ionic concentrations. For every 10 000 H atoms, there appear to be 8.0 atoms of C, 2.4 of N, 5.4 of 0,1.3 of Ne, 0.068 of S, 0.0023 of Cl and 0.035 of Ar. NGC 6741 is rich in He, C and N, compared with the solar abundance. O is probably depleted by ‘hot bottom’ burning. NGC 6741 may be also depleted in metals like Ca, Mg, Si and Fe, which may be tied up in grains. The progenitor star may have been an object in which C, N and Ne had essentially solar abundances, while heavier elements were less plentiful than in the Sun.

Ionization correction factors from an extensive grid of photoionization models of Planetary Nebulae

Using a new photoionization code PAN (Yu. Malkov et al, this book) we have built a grid of 250 photoionization models for planetary nebulae (PN). The stellar evolutionary tracks for PN nuclei with M∗ = 0.566, 0.598, 0.644 and 0.8M⊙ are taken from the computations of the Blöcker (A&amp;A, 299, 1995). On the L∗ – Teff diagram our grid covers whole region in which the PN nuclei are observed. The spectra of the central stars at λ ≤ 912 Å corresponded to non-LTE model atmospheres (R.E.S. Clegg and D. Middlemass, MNRAS, 228, 1987), corrected for presence of the stellar wind. The nebular shell was assumed to be spherically symmetric, and the profile of the radial gas density distribution in it was taken equal to the empirical approximate expression found from the analysis of isophotal maps of real PN (V. Golovaty and Yu. Malkov, Astron. Zh., 69, 1992). Each model was calculated for three choises of Mshell and each of them for two choises of shell expansion velosity −15 and 30 km/sec respectively. The relative elemental abundances were the same for all models: H:He:C:N:O:Ne:Mg:Si:S:Ar = 10000:1200:5.0:1.0:6.0:0.8:0.3:0.3:0.15:0.04 (by number of atoms).

SNAP: Spreadsheet nebular analysis package

A flexible and extensible software package for analyzing two-dimensional and multi-object spectra of nebulae has been implemented within Microsoft Excel (version 5). From lists of calibrated fluxes (and errors) for lines in a series of spectra, the program first identifies transitions by referring to wavelengths generated from atomic data. Colour excesses can be computed from a reddening law and ratio of total to selective extinction of the user's choice. Line fluxes can be corrected for stellar absorption and reddening and reported in a format suitable for publication. Temperatures, densities, ionic abundances, ionization correction factors, and total abundances can be computed. (The interface for the temperture/density calculation is shown below.) Emission coefficients for forbidden transitions are calculated using an adaptation of the FIVEL five-level atom routine (DeRobertis, 1987). As well, emission coefficients are supplied for an extensive set of recombination lines. The user has complete control over which spectral lines are used in an analysis – there are no pre-ordained methods. Abundance analyses can be performed using the physical conditions either measured or specified for each spectrum. Uncertainties are propagated throughout. The atomic data used in all of the calculations are easily updated or augmented by the user. Since the software is embedded in Microsoft Excel, graphical representations of the data are easily created, and extended analyses using the full functionality of the spreadsheet are easily implemented.

The spectrum of the planetary nebula IC 351

International Ultraviolet Explorer (WE) and optical region spectroscopic observations are presented for the moderately high-excitation planetary nebula IC 351. Most of the atoms are distributed over a narrow range of ionization. Thus [N ii] and [S ii] are weak; [Ne iv] is prominent, but [Ne v] is missing. It appears that the metal/hydrogen ratio is smaller than in the Sun. No Si lines are seen either in optical or UV regions. It is suggested that virtually all Si atoms are locked in grains. The ionization correction factor method fails utterly for N, for example. Hence IUE data are essential for chemical composition estimates. Until high spatial resolution images are available, construction of a new theoretical model does not seem warranted.

Abundances at high redshift: ionization correction factors

In the last few years, improvements in high-resolution observations of quasi-stellar objects (QSOs) are enabling the use of absorption line systems as probes of the metallicity at early epochs, in particular through the observation of damped Lyman α systems. However, the determination of chemical abundances from the ionic column densities obtained from the observed absorption lines depends on unobserved ions. In this paper, it is shown that the ionization correction factors for OI, NI, CII, MgII and SiII, relative to HI, may be significant only for systems with hydrogen column densities N(HI) < 1020 cm-2 for a soft radiation field. For a harder radiation field, the correction factors for the singly ionized ions are significant for systems with column densities up to 1021 cm-2.

The spectrum of the variable planetary nebula IC 4997

The compact, dusty, presumably young planetary nebula (PN) IC 4997 has been studied extensively since the variability of the lambda 4363/lambda 4340 ratio was established in 1956. Since 1938, other nebular lines have shown changes. IC 4997 is also unique because of the great density range revealed by its spectrum which goes in excitation from Mg I to (Ar IV). We present a detailed listing of spectral lines from 360 to 1005 nm. The diagnostic diagram shows that the spectrum can be interpreted only in terms of strata with a huge density gamut. Essential spectral features can be reproduced approximately by a model consisting of a geometrically thin shell of density around 10(exp 7) atoms cm(exp -3), surrounded by a much larger shell with a density of about 10(exp 4) atoms cm(exp -3). The actual, certainly more complex structure can be evaluated only when high resolution spatial imaging is at hand. The usual method of getting abundances from N(ion)/N(H(+)) and ionization correction factors (ICFs) cannot be applied here. It is argued that a reasonable theoretical model that represents the spectrum provides a valid initial approximation to nebular abundances. We propose that the chemical composition of IC 4997 does not differ greatly from that of the Sun. The finally adopted model suggests that the ejection of the material destined to form the inner shell occurred between 1900 and 1960, but observational evidence of such an ejection event is lacking. Perhaps the shell was accelerated. A need for further study is emphasized, especially the role of dust which appears to contribute 2% of the total mass. More attention to this object is recommended. An accurate measurement of its distance is especially desirable.

Chemical Composition of Planetary Nebulae: A New Determination

A new method of the determination of planetary nebulae abundances is proposed. Unobserved ionization stages are taken into account with aid of the correlations between relative abundances of various ions which had been obtained from the grid of the photoionization models of planetary nebulae luminescence calculated by us. Simple approximative expressions for the determination of He/H, C/H, N/H, O/H, Ne/H, Mg/H, Si/H, S/H, and Ar/H are found. The chemical composition of 130 galactic planetary nebulae is revised. The observational data were compiled from 73 papers of many authors published in 1972–1991. Our mean abundances of C, N, O, Ne, S, Ar are 0.1–0.3 dex lower than the mean abundances of these elements found previously by other authors. Such a discrepancy may be due to an overestimation of “empirical” ionization correction factors in previous works. It is shown that the abundance of oxygen in massive stars-precursors may be reduced by 0.2 dex on average due to the ON-cycle, but the abundance of neon remains practically unchanged. A comparative analysis of the abundances in the galactic disk, bulge and halo nebulae is carried out. We found that helium is enhanced in the galactic bulge nebulae relative to the disk ones, but the mean nitrogen abundance and mean ratio N/O are lower in the bulge. Our data suggest that the second dredge-up did not take place in the stars-precursors of the bulge planetaries, and the helium and nitrogen enhancement in these nebulae is due to other mixing processes. The mean abundances of O, Ne, S, Ar in the galactic disk and galactic bulge nebulae are quite similar, and we must conclude that the stars-precursors of the bulge planetaries had been formed during the same burst of star formation as the stars-precursors of the disk nebulae, while the halo nebulae correspond to the previous burst.

THE [NE III]-[O II] SPECTRUM AS AN IONIZATION INDICATOR IN NEBULAE

ABSTRACT The usefulness of the Ne III-O II forbidden-line spectrum as a helium ionization correction factor (ICF) indicator in low-density nebulae, such as H II regions, is examined, and the results of an extensive grid of photoionization simulations which have a 'blister'-like geometry are presented. Fully filled constant pressure gas, Orion dust, and H II region abundances are used, along with a very wide variety of ionizing continua. As suggested by the ionization potentials, the He ICF is small when the Ne III 3869/O II 3727 forbidden line ratio, an easily measured line pair, reaches above 0.20. The 'eta' parameter, a ratio of ratios of oxygen and sulfur lines, is probably strongly affected by low-temperature dielectronic recombination, a basic process which has no reliable rate coefficients for third-row elements.

A New Ionization Correction scheme for He+/H+ and the determination of the helium abundance inHii regions

We have developed a New Ionization Correction Factor (ICF) scheme for He+ based on the definition of the radiation softness parameter η (Vilchez and Pagel, 1988) which is very sensitive to the effective temperature of the ionizing star(s) and, therefore, a good indicator of the He+−He0 ionization structure whilst remaining insensitive to other nebular parameters. The ICF(η) relationship is obtained making use of the Stasinska (1980, 1982) photoionization models. After comparison with previous semiempirical approaches in the literature and its application to a selected sample of abundances it is found that this ICF scheme seems a powerful tool for the determination of He abundances, particularly for objects of intermediate to low excitation.

Some Statistics of Nebular Chemical Compositions

Data from a survey of 51 planetary nebulae (PN) observed with the image tube scanner on Shane Telescope at Lick Observatory are combined with those for a comparable number of objects previously reported. For nearly all of the PN included in the later program, it was possible to obtain adequately accurate plasma diagnostics and line intensities to derive ionic concentrations for He, N, O, Ne, S, Cl, and Ar. To get ionization correction factors we calculated theoretical nebular models to fit the excitation level and the intensities of individual important lines. Final model parameters include the stellar radius, emergent flux, Fv(*), from Husfeld et al. (1984, Astron. Astrophys., 134, 139), nebular size, the optical depth at the hydrogenic Lyman limit, and chemical abundances. Many PN do not appear to be optically very thick in the Lyman continuum.

TWO COMPACT PLANETARY NEBULAE OF MODERATE EXCITATION: NGC 6565 (3-4.5) AND NGC 6644 (8-7.2)

ABSTRACT Data obtained with an image-tube scanner at the 3-m Shane telescope are combined with IUE data to obtain plasma diagnostics and chemical compositions for two planetary nebulae of moderately high excitation. Theoretical nebular models were calculated using stellar fluxes given by Husfeld et al. (1984) for Tasterisk = 85,000 K, log g = 4.72, and elemental abundances were obtained by fitting theoretical to observed line intensities and also by using the model to determine ionization correction factors to be applied to observed ionic concentrations. Although C appears to be about 1.5 times as abundant in NGC 6644 as in NGC 6565, N, O, Ne, S, Cl, and Ar are depleted by factors ranging from 2 to 6 in NGC 6644 as compared to NGC 6565. The high-velocity object, NGC 6644, was evidently made from a less metal-rich mixture than the sun.