Tuesday, 1/23/13

A theme for today: metallicity gradients in disks and what is the role of supernova feedback (via galactic fountains) in maintaining them. First a paper on modeling the abundance gradient in M31, including SN-driven fountains; secondly, a paper on witnessing the effects of cooling (induced by SN-driven galactic fountains) in the MW disk-corona interface and then finally a paper comparing the PNe and HII-region derived metallicity gradients in NGC300 and some conclusions on what that tells us about AGB star evolution.

Title: The Effects of radial inflow of gas and galactic fountains on the chemical evolution of M31
Authors: Spitoni, E.; Matteucci, F.; Marcon-Uchida, M. M.
Bibliographic Code: 2013arXiv1301.5153S


Galactic fountains and radial gas flows are very important ingredients in modeling the chemical evolution of galactic disks. Our aim here is to study the effects of galactic fountains and radial gas flows in the chemical evolution of the disk of M31. We adopt a ballistic method to study the effects of galactic fountains on the chemical enrichment of the M31 disk. We find that the landing coordinate for the fountains in M31 is no more than 1 kpc from the starting point, thus producing negligible effect on the chemical evolution of the disk. We find that the delay time in the enrichment process due to fountains is no longer than 100 Myr and this timescale also produces negligible effects on the results. Then, we compute the chemical evolution of the M31 disk with radial gas flows produced by the infall of extragalactic material and fountains. We find that a moderate inside-out formation of the disk coupled with radial flows of variable speed can very well reproduce the observed gradient. We discuss also the effects of other parameters such a threshold in the gas density for star formation and an efficiency of star formation varying with the galactic radius. We conclude that the most important physical processes in creating disk gradients are the inside-out formation and the radial gas flows. More data on abundance gradients both locally and at high redshift are necessary to confirm this conclusion.

Title: Ionized absorbers as evidence for supernova-driven cooling of the lower Galactic corona
Authors: Fraternali, Filippo; Marasco, Antonino; Marinacci, Federico; Binney, James
Bibliographic Code: 2013arXiv1301.4998F


We show that the ultraviolet absorption features, newly discovered in HST spectra, are consistent with being formed in a layer that extends a few kpc above the disk of the Milky Way. In this interface between the disk and the Galactic corona, high-metallicity gas ejected from the disk by supernova feedback can mix efficiently with the virial-temperature coronal material. The mixing process triggers the cooling of the lower corona down to temperatures encompassing the characteristic range of the observed absorption features, producing a net supernova-driven gas accretion onto the disk at a rate of a few Msun/yr. We speculate that this mechanism explains how the hot-mode of cosmological accretion feeds star formation in galactic disks.

Title: Clues on the evolution of abundance gradients and on AGB nucleosynthesis
Authors: Stasinska, G.; Pena, M.; Bresolin, F.; Tsamis, Yi.
Bibliographic Code: 2013arXiv1301.5280S


We have obtained deep spectra of 26 planetary nebulae (PNe) and 9 compact HII regions in the nearby spiral galaxy NGC 300, and analyzed them together with those of the giant HII regions previously observed. We have determined the physical properties of all these objects and their He, N, O, Ne, S and Ar abundances in a consistent way. We find that, globally, compact HII regions have abundance ratios similar to those of giant HII regions, while PNe have systematically larger N/O ratios and similar Ne/O and Ar/O ratios. We demonstrate that the nitrogen enhancement in PNe cannot be only due to second dredge-up in the progenitor stars, since their initial masses are around 2–2.5\,\msun. An extra mixing process is required, perhaps driven by stellar rotation. Concerning the radial abundance distribution, PNe behave differently from HII regions: in the central part of the galaxy their average O/H abundance ratio is 0.15 dex smaller. Their abundance dispersion at any galactocentric radius is significantly larger than that shown by HII regions and many of them have O/H values higher than HII regions at the same galactocentric distance. This suggests that not only nitrogen, but also oxygen is affected by nucleosynthesis in the PN progenitors, by an amount which depends at least on the stellar rotation velocity and possibly other parameters. The formal O/H, Ne/H and Ar/He abundance gradients from PNe are significantly shallower that from HII regions. We argue that this indicates a steepening of the metallicity gradient in NGC 300 during the last Gyr, rather than an effect of radial stellar motions, although the large observed dispersion makes this conclusion only tentative.

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