EDP Sciences
Free access
Volume 471, Number 1, August III 2007
Page(s) 193 - 204
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20065675

A&A 471, 193-204 (2007)
DOI: 10.1051/0004-6361:20065675

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

G. Stasinska1, G. Tenorio-Tagle2, M. Rodríguez2, and W. J. Henney3

1  LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
    e-mail: grazyna.stasinska@obspm.fr
2  Instituto Nacional de Astrofísica Óptica y Electrónica, AP 51, 72000 Puebla, Mexico
3  Centro de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Campus Morelia, Apartado Postal 3-72, 58090 Morelia, Mexico

(Received 23 May 2006 / Accepted 25 May 2007)

We critically examine a scenario for the enrichment of the interstellar medium (ISM) in which supernova ejecta follow a long (108 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 H II 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 H II regions. We derive bounds of 1013-1015 cm on the droplet sizes inside H II 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 H II 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.

Key words: galaxies: abundances -- galaxies: ISM -- ISM: abundances -- ISM: H II regions

© ESO 2007

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access.

An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account.
    In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.