Spectroscopic mapping of the planetary nebula NGC 6302^⋆

¹ Laboratório de Análise Numérica e Astrofísica, Departamento de MatemáticaUniversidade Federal de Santa Maria, Av. Roraima 1000, CEP 97119-900, Santa Maria RS, Brazil
e-mail: alinerauber@gmail.com
² Universidade do Vale do Paraíba, Av. Shishima Hifumi 2911, CEP 12244-000 São José dos Campos, SP, Brazil

Received: 31 December 2013
Accepted: 17 January 2014

Abstract

Context. Most spectroscopic studies of planetary nebulae (PNe) are based on data from specific parts of the objects. Considering the high complexity of their morphological structures, integrated or average values across an observed region can only provide limited information of their ionic, density, and temperature structures.

Aims. We investigate the spatial variation of the physical and chemical properties of NGC 6302, a high-ionization PNe, with a complex bipolar structure.

Methods. Spatially resolved long-slit spectroscopic data were obtained from 11 parallel and equally spaced directions over the nebula. Maps of emission line fluxes, line ratios, electron temperature T_e, electron density N_e, and ionic abundances (He⁺, He²⁺, N⁰, N⁺, O⁰, O⁺, O²⁺, S⁺, S²⁺, Ne²⁺, Ar²⁺, Ar³⁺, and Ar⁴⁺ relative to H⁺) were constructed with a spatial resolution of 1.45″ × 1″ for a large portion of the nebula over an area of 11″ × 250″. Electron densities were estimated from the [S ii] λ 6716/λ 6731 and [Ar iv] λ 4711/λ 4740 line ratios, and electron temperatures were estimated from the [O iii] (λ 4959 + λ 5007)/λ 4363 and [N ii] (λ 6548 + λ 6584)/λ 5755 ratios. Integrated spectra were also obtained and 142 lines were identified.

Results. The electron density maps show a peaked distribution, with the densest area at the circumstellar region, reaching N_e ≈ 40 000 cm^-3, and decreasing to N_e ≤ 1000 cm^-3 at the lobes. Knots were observed in the N_e(S ii) map, with 2000 cm^-3 < N_e < 3000 cm^-3. The N_e(Ar iv) map suggests the presence of an inner region with N_e ≈ 20 000 cm^-3 that extends from the center through the east lobe. A homogeneous temperature distribution was found for both T_e(O iii) and T_e(N ii). Small temperature fluctuations on the plane of the sky of ${\mathit{t}}_{\mathrm{s}}^{\mathrm{2}}\mathrm{\left(}\mathrm{N}\hspace{0.17em}{ii}\mathrm{\right)}\mathrm{=}\mathrm{0.0071}$ and ${\mathit{t}}_{\mathrm{s}}^{\mathrm{2}}\mathrm{\left(}\mathrm{O}\hspace{0.17em}{iii}\mathrm{\right)}\mathrm{=}\mathrm{0.0043}$ were estimated.

Conclusions. In general, the total abundances obtained are similar to the maximum found in the ionic abundance maps of the dominant ionic species. This demonstrates the potential of spatially resolved abundance studies for establishing lower limits of the total abundances based on ionic abundances, for checking the ionization correlation factors, and in some cases, for determining the total abundances directly from maps of ionic abundances.