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Date and Time of the Query: 2019-06-16 T19:43:27 PDT
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For refcode 1993A&A...274..730H:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1993A&A...274..730H C and O nucleosynthesis in starbursts: the connection between distant mergers, the Galaxy, and the solar system C. Henkel and R. Mauersberger Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-53 121 Bonn, Germany Received October 7, 1992, accepted March 8, 1993 Abstract. Observations of various isotopic species of interstellar CO, CN, CS, HCN, HNC, and HCO^+^ indicate that ^12^C/^13^C~40-50, ^16^O/^18^O~150-200, and ^18^O/^17^O >= 8 in the active nuclear regions of nearby galaxies. In contrast to the strong spatial variations of most kinematical, physical, and chemical properties, the isotope ratios are more smoothly distributed, thus allowing detailed studies of global anomalies. Modelling the `chemical' evolution during a starburst, the oxygen isotope ratios can be understood in terms of an `overabundance' of ^18^O. This is explained if high mass stars (>8 M_sun_) convert large amounts (>5%) of ^14^N into ^18^O and if the burst initial mass function (IMF) is biased in favor of massive stars. Relative to ^18^O, only small amounts of ^17^O are produced in massive stars. Large ^18^O/^17^O and small ^16^O/^18^O ratios should be a common signature of the advanced stages of nuclear starbursts. The ejection of large amounts of ^18^O by high mass stars also explains the peculiar solar ^18^O/^17^O ratio, if the solar system was formed in an OB association. ^12^C/^13^C ratios are less affected by nuclear starbursts. Therefore, infall of disk gas is likely the main cause for a ^12^C/^13^C enhancement. The large I(^12^CO)/I(^13^CO) line intensity ratios (>20) observed in gas-rich mergers are explained (1) by the presence of a molecular `halo' with low ^13^CO intensities, (2) by pre-burst gas masses which are negligible relative to the infalling mass or (3) by infalling gas containing relatively unprocessed material not only from the inner but also from the outer parts of the disks. While the core-halo scenario does not require particularly large ^12^C/^13^C ratios, the other effects require ^12^C/^13^C > 40 in mergers. The Galactic center ^12^C/^13^C, ^16^O/^17^O, ^18^O/^17^O, and ^14^N/^15^N ratios all support a quiescent state in the recent past. Predictions are made on the spatial distribution of the carbon and oxygen isotope ratios within the burst phase and on the ratios to be expected in ultraluminous mergers and in `normal' spirals. For the Galactic disk, the existence of small interstellar clouds with enhanced ^18^O/^17^O ratios is postulated. In starbursts, silico and sulfur isotope ratios are expected to be approximately solar. A qualitative explanation for the Galactic disk, center, and solar system ^14^N/^15^N ratios is given, implying that the corresponding ratio in nuclear starbursts may be low (< 200). Key words: nuclear reactions, nucleosynthesis, abundances - interstellar medium: molecules - galaxies: abundances - galaxies: interstellar matter - galaxies: starburst- radio lines: galaxies - solar system: formation
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