Systematic Errors in Elemental Abundances Derived from Nebular Spectra

Moore, B. D.; Hester, J. J.; Dufour, R. J.

doi:10.1086/420801

Cited by 7 publications

(9 citation statements)

References 16 publications

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“…Our mean value of log(Ar/O) = −2.10 ± 0.06 is in agreement with the average for metal-poor blue compact dwarf galaxies (Izotov & Thuan 1999). Moore et al (2004) has recently suggested that direct modeling of photoionized nebulae should be used to infer elemental abundances with accuracies similar to observations. Abundances derived from model-based ionization correction factors were shown to exceed the range of expected errors from the original data.…”

Section: Element Ratiossupporting

confidence: 88%

The Spatial Homogeneity of Nebular and Stellar Oxygen Abundances in the Local Group Dwarf Irregular Galaxy NGC 6822

Lee

Skillman

Venn

2006

ApJ

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To test the existence of a possible radial gradient in oxygen abundances within the Local Group dwarf irregular galaxy NGC 6822, we have obtained optical spectra of 19 nebulae with the EFOSC2 spectrograph on the 3.6-m telescope at ESO La Silla. The extent of the measured nebulae spans galactocentric radii in the range between 0.05 kpc and 2 kpc (over four exponential scale lengths). In five H II regions (Hubble I, Hubble V, Kα, Kβ, KD 28e), the temperature-sensitive [O III] λ4363 emission line was detected, and direct oxygen abundances were derived. Oxygen abundances for the remaining H II regions were derived using bright-line methods. The oxygen abundances for three A-type supergiant stars are slightly higher than nebular values at comparable radii. Linear least-square fits to various subsets of abundance data were obtained. When all of the measured nebulae are included, no clear signature is found for an abundance gradient. A fit to only newly observed H II regions with [O III] λ4363 detections yields an oxygen abundance gradient of −0.14 ± 0.07 dex kpc −1 . The gradient becomes slightly more significant (−0.16 ± 0.05 dex kpc −1 ) when three additional H II regions with [O III] λ4363 measurements from the literature are added. Assuming no abundance gradient, we derive a mean nebular oxygen abundance 12+log(O/H) = 8.11 ± 0.10 from [O III] λ4363 detections in the five H II regions from our present data; this mean value corresponds to [O/H] = −0.55. 3.1.1. Oxygen Abundances: [O III] λ4363 Temperatures The "direct" conversion of emission-line intensities into ionic abundances requires a reliable estimate of the electron temperature of the ionized gas. We adopt a two-zone model for H II regions, with a low-and a high-ionization zone characterized by temperatures T e (O + ) and T e (O +2 ), respectively. The temperature in the O +2 zone is measured with the emission-line ratio I([O III] λ5007)/I([O III] λ4363) (Os terbrock 1989). The temperature in the O + zone is given bywhere t e = T e /10 4 K (Campbell et al. 1986;Garnett 1992). The total oxygen abundance by number is given by O/H = O + /H + + O +2 /H + . For conditions found in typical H II regions and those presented here, very little oxygen in neutral form is expected, and in the absence of He II emission, the O +3 contribution is considered negligible. For subsequent calculations of ionic abundances, we assume the following electron temperatures for specific ions (Garnett 1992;Thuan et al. 1995):, t e (Ar +2 ) = 0.83 t e (O +2 ) + 0.17, and t e (Ar +3 ) = t e (O +2 ).Derived ionic and total abundances are listed in Tables 6a and 6b, which include derived O + and O +2 electron temperatures, O + and O +2 ionic abundances, and the total oxygen abundances. Direct H II region oxygen abundances were derived for Hubble I, Hubble V, Kα, Kβ, and KD 28e, and were in excellent agreement with those derived from the method described by Skillman et al. (2003).Where [S II] λλ4068,4076 and [O II] λλ7320,7330 were detected in the spectra for Hubble V, Kα, and KD 28e, we used ...

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Section: Element Ratiossupporting

confidence: 88%

The Spatial Homogeneity of Nebular and Stellar Oxygen Abundances in the Local Group Dwarf Irregular Galaxy NGC 6822

Lee

Skillman

Venn

2006

ApJ

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“…It is important to note that the complexity and variability of the stratified ionization structure of filaments in the Crab complicates interpretation of filament spectra, including measurement of nebular abundances. Similar effects have been seen in modeling of other photoionized environments such as H II regions, where changing conditions across sharply stratified ionization structure can lead to significant errors in abundances inferred using traditional methods (e.g., Sankrit & Hester 2000;Moore, Hester, & Dufour 2004). As noted above, the appearance of the Crab in the light of low-and high-ionization lines is radically different, down to subarcsecond scales (Sankrit et al 1998, Blair et al 1997, Loll et al 2007.…”

Section: Spectroscopic Studies Of the Crab Show Abundance Variationsmentioning

confidence: 72%

The Crab Nebula: An Astrophysical Chimera

Hester

2008

Annu. Rev. Astron. Astrophys.

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380

401

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The Crab Nebula, henceforth the Crab, the remnant of the historical supernova of 1054 AD, has long been of intense interest. The pulsar at the center of the Crab has a spin-down luminosity ∼10 5 times that of the Sun. The outer nebula holds several solar masses of material ejected by the explosion. Between the two lies the trapped pulsar wind, visible as synchrotron radiation at radio wavelengths through X-ray wavelengths. Recent observations with the Hubble Space Telescope, the Chandra X-ray Observatory, and a host of other instruments have provided a wealth of information about the extraordinary structure and dynamics of the Crab. Understanding those data requires thinking of the Crab not in terms of its individual components, but instead as a single interconnected physical system formed as the axisymmetrical wind from the pulsar pushes its way outward through a larger freely expanding supernova remnant.

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“…We note here the recent work by Moore et al (2004), who 10 Note that part of the difference is due to a 14% difference in the ratio of the [O III]λ5007 and [Ne III]λ3869 emissivities used by Izotov & Thuan (1999) and that computed by the IONIC program in the NEBULAR code of Shaw & Dufour (1995). This 14% difference, which translates into a 0.06 dex difference (in the sense observed), is probably an indication of the minimum systematic uncertainty in the atomic data which are used for calculating nebular abundances (see Garnett 2004).…”

Section: Element Ratiosmentioning

confidence: 83%

Investigating the Possible Anomaly between Nebular and Stellar Oxygen Abundances in the Dwarf Irregular Galaxy WLM

Lee

Skillman

Venn

2005

ApJ

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We obtained new optical spectra of 13 H II regions in WLM with EFOSC2; oxygen abundances are derived for nine H II regions. The temperature-sensitive [O III]λ4363 emission line was measured in two bright H II regions HM 7 and HM 9. The direct oxygen abundances for HM 7 and HM 9 are 12+log(O/H) = 7.72±0.04 and 7.91 ± 0.04, respectively. We adopt a mean oxygen abundance of 12+log(O/H) = 7.83 ± 0.06. This corresponds to [O/H] = −0.83 dex, or 15% of the solar value. In H II regions where [O III]λ4363 was not measured, oxygen abundances derived with bright-line methods are in general agreement with direct values of the oxygen abundance to an accuracy of about 0.2 dex. In general, the present measurements show that the H II region oxygen abundances agree with previous values in the literature. The nebular oxygen abundances are marginally consistent with the mean stellar magnesium abundance ([Mg/H] = −0.62). However, there is still a 0.62 dex discrepancy in oxygen abundance between the nebular result and the A-type supergiant star WLM 15 ([O/H] = −0.21). Non-zero reddening values derived from Balmer line ratios were found in H II regions near a second H I peak. There may be a connection between the location of the second H I peak, regions of higher extinction, and the position of WLM 15 on the eastern side of the galaxy.

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Systematic Errors in Elemental Abundances Derived from Nebular Spectra

Cited by 7 publications

References 16 publications

The Spatial Homogeneity of Nebular and Stellar Oxygen Abundances in the Local Group Dwarf Irregular Galaxy NGC 6822

The Spatial Homogeneity of Nebular and Stellar Oxygen Abundances in the Local Group Dwarf Irregular Galaxy NGC 6822

The Crab Nebula: An Astrophysical Chimera

Investigating the Possible Anomaly between Nebular and Stellar Oxygen Abundances in the Dwarf Irregular Galaxy WLM

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