NGC 6153: Reality is complicated

Abstract

We study the kinematics of emission lines that arise from many physical processes in NGC 6153 based upon deep, spatially-resolved, high resolution spectra acquired with the UVES spectrograph at the ESO VLT. Our most basic finding is that the plasma in NGC 6153 is complex, especially its temperature structure. The kinematics of most emission lines defines a classic expansion law, with the outer part expanding fastest (normal nebular plasma). However, the permitted lines of \ion{O}{1}, \ion{C}{2}, \ion{N}{2}, \ion{O}{2}, and \ion{Ne}{2} present a constant expansion velocity that defines a second kinematic component (additional plasma component). The physical conditions imply two plasma components, with the additional plasma component having lower temperature and higher density. The [\ion{O}{2}] density and the [\ion{N}{2}] temperature are anomalous, but may be understood considering the contribution of recombination to these forbidden lines. The two plasma components have very different temperatures. The normal nebular plasma appears to be have temperature fluctuations in part of its volume (main shell), but only small fluctuations elsewhere. The additional plasma component contains about half of the mass of the N${}^{2+}$ and O${}^{2+}$ ions, but only $3-5$\% of the mass of H${}^{+}$ ions, so the two plasma components have very different chemical abundances. We estimate abundances of $12+\log ({\mathrm{O}}^{2+}/{\mathrm{H}}^{+})\sim 9.2$\,dex and $\mathrm{He}/\mathrm{H}\sim 0.13$. Although they are all complications, multiple plasma components, temperature fluctuations, and the contributions of multiple physical processes to a given emission line are all part of the reality in NGC 6153, and should generally be taken into account.

arXiv comment55 pages, 51 figures, 9 tables, accepted for publication in The Astronomical Journal

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