i-process Contribution of Rapidly Accreting White Dwarfs to the Solar Composition of First-peak Neutron-capture Elements

Benoit Côté1,2,3,10, Pavel Denissenkov1,3,10, Falk Herwig1,3,10, Ashley J. Ruiter4,5,6, Christian Ritter1,3,7,10, Marco Pignatari3,8,10, and Krzysztof Belczynski9

Astrophysical Journal 854, 131 Link to Article [DOI: 10.3847/1538-4357/aaaae8]
1Department of Physics and Astronomy, University of Victoria, Victoria, BC, V8W 2Y2, Canada
2Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly Thege Miklos ut 15-17, H-1121 Budapest, Hungary
3Joint Institute for Nuclear Astrophysics—Center for the Evolution of the Elements, USA
4Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 0200, Australia
5ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), Australia
6School of Physical, Environmental and Mathematical Sciences, University of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600, Australia
7Keele University, Keele, Staffordshire ST5 5BG, UK
8E.A. Milne Centre for Astrophysics, Department of Physics & Mathematics, University of Hull, HU6 7RX, UK
9Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
10NuGrid Collaboration, http://nugridstars.org.

Rapidly accreting white dwarfs (RAWDs) have been proposed as contributors to the chemical evolution of heavy elements in the Galaxy. Here, we test this scenario for the first time and determine the contribution of RAWDs to the solar composition of first-peak neutron-capture elements. We add the metallicity-dependent contribution of RAWDs to the one-zone galactic chemical evolution code OMEGA according to RAWD rates from binary stellar population models combined with metallicity-dependent i-process stellar yields calculated following the models of Denissenkov et al. With this approach, we find that the contribution of RAWDs to the evolution of heavy elements in the Galaxy could be responsible for a significant fraction of the solar composition of Kr, Rb, Sr, Y, Zr, Nb, and Mo ranging from 2% to 45% depending on the element, the enrichment history of the Galactic gas, and the total mass ejected per RAWD. This contribution could explain the missing solar Lighter Element Primary Process for some elements (e.g., Sr, Y, and Zr). We do not overproduce any isotope relative to the solar composition, but 96Zr is produced in a similar amount. The i process produces efficiently the Mo stable isotopes 95Mo and 97Mo. When nuclear reaction rate uncertainties are combined with our GCE uncertainties, the upper limits for the predicted RAWD contribution increase by a factor of 1.5–2 for Rb, Sr, Y, and Zr, and by 3.8 and 2.4 for Nb and Mo, respectively. We discuss the implication of the RAWD stellar evolution properties on the single-degenerate SN Ia scenario.

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