Mathematics – Probability
Scientific paper
Mar 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...21115804p&link_type=abstract
American Astronomical Society, AAS Meeting #211, #158.04
Mathematics
Probability
Scientific paper
Empirical upper limits to the dispersion in Galactic metallicity at a given age are less than 0.05 dex at present, and probably not much larger in the past. We ask how the Galaxy could remain so homogeneous in the face of continuing localized sources of new metals and infall of low-metallicity gas. We use a physically realistic mixing model that recognizes microscopic diffusion as the only physical process that can alter the abundance pdf, and an interstellar turbulent velocity field that produces gradient structures steep and small enough for diffusive smoothing. We solve a kinetic equation that evolves the probability distribution of metallicity, including nucleosynthetic sources, star formation, infall, and turbulent mixing, with no diffusive approximation made for the mixing. The finite time to mix accreted gas with disk gas produces scatter at least 5 times larger than the observed upper limits for infall models, while models without infall are consistent with observations. To reduce the mixing time, we added schematic efficient mixing within SNR and superbubble shells, considered preenriched infall and episodic infall, and pushed all parameters to their limits, but the results still require a mixing timescale smaller by a factor of 20. The discrepancy can be removed if the infall is primarily in the form of clouds of size 1-5 pc, small enough to mix with the disk in less than a million years. Similar, but larger, clouds have been predicted to form by thermal instability in halos. The small sizes are below current detection limits, consistent with the fact that HVC census' give an infall mass flux two orders of magnitude smaller than the current disk star formation rate, and with the expected erosion of larger clouds that have been proposed to explain the velocity dispersions in the outer regions of disk galaxies.
Pan Liubin
Scalo John
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