Electron temperature fluctuations in NGC 346

¹ Laboratório de Análise Numérica e Astrofísica, Departamento de Matemática, Universidade Federal de Santa Maria, 97119-900 Santa Maria, RS, Brazil e-mail: vinic99@mail.ufsm.br
² Southern Astrophysical Research Telescope, c/o AURA Inc., Casilla 603, La Serena, Chile

Received: 8 July 2008
Accepted: 28 September 2008

Abstract

Context. The existence and origin of large spatial temperature fluctuations in H ii regions and planetary nebulae are assumed to explain the differences between the heavy element abundances inferred from collisionally excited and recombination lines, although this interpretation remains significantly controversial.

Aims. We investigate the spatial variation in electron temperature inside NGC 346, the brightest H ii region in the Small Magellanic Cloud.

Methods. Long slit spectrophotometric data of high signal-to-noise were employed to derive the electron temperature from measurements derived from localized observations of the [O iii](λ4959 + λ5007)/λ4363 ratio in three directions across the nebula.

Results. The electron temperature was estimated in 179 areas of 5´´ 15 of size distributed along three different declinations. A largely homogeneous temperature distribution was found with a mean temperature of 12 269 K and a dispersion of 6.1%. After correcting for pure measurements errors, a temperature fluctuation on the plane of the sky of = 0.0021 (corresponding to a dispersion of 4.5%) was obtained, which indicates a 3D temperature fluctuation parameter of t² ≈ 0.008. A large scale gradient in temperature of the order of -5.7 ± 1.3 K arcsec^-1 was found.

Conclusions. The magnitude of the temperature fluctuations observed agrees with the large scale variations in temperature predicted by standard photoionization models, but is too small to explain the abundance discrepancy problem. However, the possible existence of small spatial scale temperature variations is not excluded.