Wednesday, March 20, 2013

1303.4623 (Ana Monreal-Ibero et al.)

HeI in the central Giant HII Region of NGC 5253. A 2D observational approach to collisional and radiative transfer effects    [PDF]

Ana Monreal-Ibero, Jeremy R. Walsh, Mark S. Westmoquette, Jose M. Vilchez
ABRIDGED: NGC5253 is an ideal laboratory for detailed studies of starburst galaxies. We present for the first in a starburst galaxy a 2D study of the spatial behavior of collisional and radiative transfer effects in He^+. The HeI lines are analysed based on data obtained with FLAMES and GMOS. Collisional effects are negligible (i.e. 0.1-0.6%) for transitions in the singlet cascade while relatively important for those in the triplet cascade. In particular, they can contribute up to 20% of the flux in the HeIl7065 line. Radiative transfer effects are important over an extended and circular area of 30pc in diameter centered at the Super Star Clusters. HeI abundance, y^+, has been mapped using extinction corrected fluxes of six HeI lines, realistic assumptions for T_e, n_e, and the stellar absorption equivalent width as well as the most recent emissivities. We found a mean of 10^3 y^+ ~81.6 over the mapped area. The relation between the excitation and the total helium abundance, y_tot, is consistent with no abundance gradient. Uncertainties in the derivation of He abundances are dominated by the adopted assumptions. We illustrated the difficulty of detecting a putative He enrichment due to the presence of Wolf-Rayet stars in the main GHIIR. Data are marginally consistent with an excess in the N/He ratio in the N enriched area of the order of both, the atmospheric N/He ratios in WR stars and the uncertainties estimated for the N/He ratios.
View original: http://arxiv.org/abs/1303.4623

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