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

By Xiaowei Liu, P. Storey, M. Barlow, I. J. Danziger, Marvin L. Cohen, M. Bryce. 2000

287 citations recorded by the source corpus.

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.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics

View Paper: https://www.semanticscholar.org/paper/03fb341a0d6e401ac260cdc44556868963d03f68

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Citations in the corpus

PyNeb: a new tool for analyzing emission lines - I. Code description and validation of results (2014, methodology): “3 of Liu et al. (2000)) by the same factor and, consequently, an artificial decrease in the diagnostic line ratio at a given electron temperature.”
Chemical abundances of planetary nebulae from optical recombination lines – II. The neon abundance of NGC 7009 (2001, methodology): “The ratios of heavy-element abundances derived from ORLs on the one hand to those from CELs on the other hand, cover a wide range, from unity (i.e. near agreement) to more than a factor of 20 (Liu et al. 1995, 1999, 2000, 2001b).”
The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale (2003, background): “…of infrared forbidden lines (which have weak temperature dependences) in the spectra of planetary nebulae yield abundances that are in good agreement with those from optical forbidden lines, even where there is a large discrepancy with recombination line abundances (Liu et al. 2000, 2001).”
A VLT study of metal-rich extragalactic H II regions - I. Observations and empirical abundances (2005, background): “…[O ] λ7325 line, while it is true that accounting for a recombination component goes in the right direction to alleviate the discrepancy (for example using the empirical formula given by Liu et al. 2000), the effect, when corrected as in Kennicutt et al. (2003), would be negligible in our sample.”
A reappraisal of the chemical composition of the Orion nebula based on Very Large Telescope echelle spectrophotometry (2004, background): “In the case of the [N ii] λ 5755 Å line, we have corrected its intensity for the contribution of recombination following Liu et al. (2000).”
Biases in abundance derivations for metal-rich nebulae (2005, methodology): “This has been shown to be important in planetary nebula with cool, hydrogen-poor inclusions (Liu et al. 2000, Tsamis et al. 2003a).”
KECK HIRES SPECTROSCOPY OF EXTRAGALACTIC H ii REGIONS: C AND O ABUNDANCES FROM RECOMBINATION LINES (2009, methodology): “Te([O ii]) and Te([N ii]) have been corrected from the contribution to [O ii] λλ7319, 7330 and [N ii] λ5755, respectively, due to recombination following the formulae derived by Liu et al. (2000).”
The Chemical Composition of the Small Magellanic Cloud H II Region NGC 346 and the Primordial Helium Abundance (2000, methodology): “To obtain the total Ar abundance we adopted the following equation N(Ar) N(H) = ICF (Ar) N(Ar++) +N(Ar3+) N(H+) ; (18) where ICF (Ar) includes the Ar+/H+ contribution and according to Liu et al. (2000) can be approximated by ICF (Ar) = ( 1− N(O+) N(O) ) −1 .”
EXTRAGALACTIC CHEMICAL ABUNDANCES: DO H ii REGIONS AND YOUNG STARS TELL THE SAME STORY? THE CASE OF THE SPIRAL GALAXY NGC 300 (2009, background): “We note that we have corrected the auroral line intensity [O II]λ7325/Hβ for recombination into the upper level (following Liu et al. 2000), although the effect is found to be at most 6%.”
Abundances of Metal-rich H II Regions in M51 (2004, background): “In addition, the temperature in the [NII ] zone could be overestimated from neglecting the recombination component of theλ5755 line (see Liu et al. 2000).”
THE MASS–METALLICITY RELATION WITH THE DIRECT METHOD ON STACKED SPECTRA OF SDSS GALAXIES (2012, background): “…may be affected by recombination of O++, although they find that the typical contribution to the [O ii] λλ7320, 7330 line fluxes is <5% (based on the correction formulae from Liu et al. 2000) and that Te[O ii] is affected by ∼2%–3%, which corresponds to <400 K for the H ii regions in their study.”