NGC 6153: Reality is complicated

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.Comment: 55 pages, 51 figures, 9 tables, accepted for publication in The Astronomical Journa

Optical Recombination Lines of Heavy-elements in Giant Extragalactic HII Regions

We present high resolution observations of the giant extragalactic H II regions NGC 604, NGC 2363, NGC 5461 and NGC 5471, based on observations taken with the ISIS spectrograph on the William Herschel Telescope. We have detected -by the first time- C II and O II recombination lines in these objects. We find that recombination lines give larger C^{++} and O^{++} abundances than collisionallly excited lines, suggesting that temperature variations can be present in the objects. We detect [Fe IV] lines in NGC 2363 and NGC 5471, the most confident detection of optical lines of this kind in H II regions. Considering the temperature structure we derive their H, He, C, N, O, Ne, S, Ar, and Fe abundances. From the recombination lines of NGC 5461 and NGC 5471 we determine the presence of C/H and O/H gradients in M101. We calculate the Delta Y/Delta O and Delta Y/Delta Z values considering the presence of temperature variations and under the assumption of constant temperature. We obtain a better agreement with models of galactic chemical evolution by considering the presence of temperature variations than by assuming that the temperature is constant in these nebulae.Comment: 42 pages, 5 figures. To be published in Ap

Microsoft Word - N.37-V.6-DIC-71.doc

SUMARIO Se estudia la variación de la composición química de las capas exteriores de las estrellas a partir de modelos evolutivos. Las secuencias analizadas varían de 1 a 1.45 M y de Z = 0.023 a 0.10. Los modelos en la rama de las gigantes rojas alcanzan un punto en que la envolvente convectiva llega a su máxima extensión; esto último acontece un poco antes de la conversión de helio en carbono en el núcleo de estas estrellas. Se discute como la composición química que adquiere la envolvente cuando alcanza su máxima extensión corresponde a la de las nebulosas planetarias. Se sugiere que las nebulosas planetarias producen el gradiente de N/O observado a lo largo de los discos de galaxias espirales. Basándose en los resultados de la evolución estelar y en las líneas de emisión observadas en los núcleos de M51 y M81 se encuentra que las abundancias relativas de N/O y O/H son mayores que las de la vecindad solar. ABSTRACT Based on stellar evolution models and on observations of planetary nebulae we have studied the change of chemical abundances in the interstellar medium. It is found that the contribution to the N/O abundance gradient present across disks of spiral galaxies by planetary nebulae is considerably more important than that produced by supernovae. We analyze the chemical abundances of the nuclei of M51 and M81 from stellar evolution results and the observed emission lines