Enrichment of the interstellar medium by metal-rich droplets and the abundance bias in HII regions

被引:45
作者
Stasinska, G.
Tenorio-Tagle, G.
Rodriguez, M.
Henney, W. J.
机构
[1] Univ Paris Diderot, CNRS, Observ Paris, LUTH, F-92190 Meudon, France
[2] Inst Nacl Astrofis Opt & Electr, Puebla 72000, Mexico
[3] Univ Nacl Autonoma Mexico, Ctr Radioastron & Astrofis, Morelia 58090, Michoacan, Mexico
关键词
galaxies : abundances; galaxies : ISM; ISM : abundances; ISM : HII regions;
D O I
10.1051/0004-6361:20065675
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
We critically examine a scenario for the enrichment of the interstellar medium ( ISM) in which supernova ejecta follow a long (10(8) yr) journey before falling back onto the galactic disk in the form of metal-rich "droplets", These droplets do not become fully mixed with the interstellar medium until they become photoionized in HII regions. We investigate the hypothesis that the photoionization of these highly metallic droplets can explain the observed " abundance discrepancy factors" (ADFs), which are found when comparing abundances derived from recombination lines and from collisionally excited lines, both in Galactic and extragalactic HII regions. We derive bounds of 10(13)-10(15) cm on the droplet sizes inside HII regions in order that ( 1) they should not have already been detected by direct imaging of nearby nebulae, and ( 2) they should not be too swiftly destroyed by diffusion in the ionized gas. From photoionization modelling we find that, if this inhomogeneous enrichment scenario holds, then the recombination lines strongly overestimate the metallicities of the fully mixed HII regions. The abundances derived from collisionally excited lines also suffer some bias, although to a much lesser extent. In the absence of any recipe for correcting these biases, we recommend the discarding of all objects showing large ADFs from studies of galactic chemical evolution. These biases must also be kept in mind when comparing the galactic abundance gradients for elements derived from recombination lines with those derived from collisionally excited lines. Finally, we propose a set of observations that could be undertaken to test our scenario and improve our understanding of element mixing in the ISM.
引用
收藏
页码:193 / 204
页数:12
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