# Post-doctorate fellowship in Astrophysics:

## Investigation of the intermediate neutron capture process nucleosynthesis

Rationale
One of the major issues in modern astrophysics concerns the analysis and understanding of the present composition of the Universe and its various constituting objects. Nucleosynthesis models at explaining the origin of the nuclei observed and to identify the processes able to forge them. Though the origin of most nuclides lighter than iron is now quite well understood (through the various hydrostatic and explosive burning stages in stars), the synthesis of the heavy elements (i.e. heavier than iron) remains unsatisfactory, or even unexplained, in many respects.

Elements heavier than iron are mainly produced by neutron capture reactions. Two main processes have been identified depending on the neutron flux: the slow neutron-capture process, or s-process, associated with low neutron densities between 106 and 1010 cm-3 and the rapid neutron-capture process, or r-process, with neutron densities exceeding 1020 cm-3. The intermediate neutron-capture process, or i-process, operates at neutron densities between those of s- and r-processes. Neutrons from the i-process are released when protons are injected in a convective region powered by helium burning. In this event, hydrogen is entrained by the convective flow to regions of higher temperature where it is depleted via the reaction 12C(p,$\gamma$)13N($\beta$+)13C, followed by the reaction 13C($\alpha$,n)16O which produces neutron densities up to ~1015 cm-3.

Several sites have been identified to host such a unique nucleosynthesis. In particular, rapidly accreting white dwarfs in close binary systems as well as low-mass low-metallicity AGB stars have recently been proposed as contributors to the chemical evolution of trans-iron elements in the Galaxy.

The aim of this project is to study this nuclear process not only in the accretion of matter onto compact stars but also in other promising astrophysical sites including proton ingestions in very metal-poor low-mass stars during the core He flash or at the beginning of the AGB phase and in super-AGB stars. For nucleosynthesis calculations, we will use up-to-date nuclear experimental data supplemented with consistent inputs from detailed nuclear models as microscopically grounded as possible. The role of nuclear and astrophysics uncertainties will be assessed and predictions will be compared with astronomical observations.

The Post-doctorate position is available at the Institut d’Astronomie et d’Astrophysique (IAA) of the Université Libre de Bruxelles (ULB) in Belgium to work with L. Siess and S. Goriely. The position is funded for 3 years. Screening of applications begins immediately and continues until an outstanding candidate is selected. The position is to start on March 1, 2020.

Applicant's Profile:

• The candidate must have a PhD in physics, preferentially astrophysics or nuclear physics awarded no later than 5 years before the start of the contract
• The candidate must have a good programming knowledge in Fortran and a strong interest in numerical calculations;
• The applicant should have good a taste for interdisciplinary research, excellent scientific writing and presenting skills and be able to work independently.
• Working in our international team requires capacity of team work and a good level of English language

Interested candidates should send their CV and request two referees to send their recommendation letter directly to L. Siess at Lionel.Siess@ulb.ac.be

Contact:
L. Siess & S. Goriely
Institut d’Astronomie et d’Astrophysique , Université Libre de Bruxelles
Campus de la Plaine CP 226
B-1050 Brussels, Belgium
Tel: +32 2 6505516 - Fax: +32 2 6504226
Email: Lionel.Siess@ulb.ac.be