Electron temperature fluctuations in H II regions

The feasibility of t² estimates from point-to-point observations

Laboratório de Análise Numérica e Astrofísica, Departamento de Matemática, Universidade Federal de Santa Maria, 97119-900 Santa Maria, RS, Brazil e-mail: mvfc@lana.ccne.ufsm.br

Received: 20 December 2005
Accepted: 24 February 2006

Abstract

Context. The spatial electron temperature fluctuations in ionized nebulae that were initially proposed to explain the discrepancies among temperatures obtained from different sensors have been pointed to as the cause of huge inconsistencies among abundances of heavy elements calculated from recombination and forbidden emission lines. Recently, there have been some attempts of direct detection and quantification of spatial temperature fluctuations with point-to-point temperature measurements across the nebula.

Aims. In this paper, we assess the feasibility of estimating the temperature fluctuation parameter t², the total variance of the spatial distribution of temperature relative to the mean, from the distribution of temperatures measured on the plane of the sky with different sensors.

Methods. Point-to-point measurements of the electron temperature at a series of contiguous and equally spaced rectangular apertures along a radius of homogeneous and spherically symmetric nebulae were numerically simulated for six different temperature sensors.

Results. Variances of projected temperature distributions were obtained and compared with total t² for different values for the density, ionization parameter and temperature of the ionizing star. The projected profiles of electron temperature obtained from indicators associated with ions occurring at large fractions of the nebula, such as [] (, [] , and the Balmer jump /Hβ, are good tracers of the internal gradient of temperature. The variances of the projected temperature distributions measured from these sensors correspond to significant fractions of the total temperature variance, for typical nebulae of the order of 30%, 25%, and 15% for the Balmer jump and the [] and [] ratios, in that order. On the other hand, the temperature profiles obtained from sensors corresponding to low ionization ions, such as [] (, [] , and [] , are almost constant at the values of temperature of the outer parts of the nebula, fail to reproduce the true temperature gradient, and have a that is always less than 10% and is usually around 1–3% of t².