s-process enrichment in the planetary nebula NGC 3918. Results from deep echelle spectrophotometry

Abstract

The chemical content of the planetary nebula NGC 3918 is investigated through deep, highresolution (R∼40000) UVES at VLT spectrophotometric data. We identify and measure more than 750 emission lines, making ours one of the deepest spectra ever taken for a planetary nebula. Among these lines we detect very faint lines of several neutron-capture elements (Se, Kr, Rb, and Xe), which enable us to compute their chemical abundances with unprecedented accuracy, thus constraining the efficiency of the s-process and convective dredge-up in NGC 3918s progenitor star. We find that Kr is strongly enriched in NGC 3918 and that Se is less enriched than Kr, in agreement with the results of previous papers and with predicted s-process nucleosynthesis. We also find that Xe is not as enriched by the s-process in NGC 3918 as is Kr and, therefore, that neutron exposure is typical of modestly subsolar metallicity AGB stars. A clear correlation is found when representing [Kr/O] vs. log(C/O) for NGC 3918 and other objects with detection of multiple ions of Kr in optical data, confirming that carbon is brought to the surface of AGB stars along with s-processed material during third dredge-up episodes, as predicted by nucleosynthesis models. We also detect numerous refractory element lines (Ca, K, Cr, Mn, Fe, Co, Ni, and Cu) and a large number of metal recombination lines of C, N, O, and Ne. We compute physical conditions from a large number of diagnostics, which are highly consistent among themselves assuming a three-zone ionization scheme. Thanks to the high ionization of NGC 3918 we detect a large number of recombination lines of multiple ionization stages of C, N, O and Ne. The abundances obtained for these elements by using recently-determined state-of-the-art ICF schemes or simply adding ionic abundances are in very good agreement, demonstrating the quality of the recent ICF scheme for high ionization planetary nebulae.

C/O ratios in planetary nebulae with dual-dust chemistry from faint optical recombination lines

“…This routine makes an initial guess of the T e and then, it calculates iteratively the values of n e and T e until convergence. We have used the same state-of-the-art atomic data set as in García-Rojas et al (2015) which are summarized in Table 2. Errors in the diagnostics were computed via Monte Carlo simulations.…”

“…We only compute abundances from O ii and C ii recombination lines as they are crucial for our analysis. Ionic abundances using N ii, N iii, and Ne ii ORLs can be computed following the prescriptions given in García-Rojas et al (2015). Ne ii ORLs are only detected in H 1-50, M 1-33, and M 1-60 but they are extremely faint (uncertainties are always higher than 40%) and, therefore, abundances obtained from these lines should be treated with caution.…”

Discovery of Rubidium, Cadmium, and Germanium Emission Lines in the Near-Infrared Spectra of Planetary Nebulae*

“…The situation is less clear for Rb, which has a condensation temperature of 800 K. Other alkali elements such as Na and K are depleted by factors of 2-4 in PNe (Pottasch et al 2009;García-Rojas et al 2015), including NGC7027 (Bernard Salas et al 2001). But Na and K have higher condensation temperatures (950-1000 K) than Rb.…”

“…However, the deep, high-resolution spectra needed to unambiguously identify optical n-capture element lines (Sharpee et al 2007;Otsuka et al 2011;García-Rojas et al 2015) restrict such studies to relatively bright PNe.…”

NEUTRON-CAPTURE ELEMENT ABUNDANCES IN MAGELLANIC CLOUD PLANETARY NEBULAE*

“…Emission lines of neutron(n)-capture elements (atomic number Z > 30) were first identified in a planetary nebula (PN) in 1994 (Péquignot & Baluteau 1994), and since have been detected in more than 100 Galactic PNe (e.g., Sharpee et al 2007;Sterling & Dinerstein 2008;García-Rojas et al 2015). Trans-iron elements can be produced by slow n-capture nucleosynthesis (the "s-process") in asymptotic giant branch (AGB) stars, and transported to the stellar envelope by third dredge-up (TDU) before being expelled via stellar winds and PN ejection (Karakas & Lattanzio 2014).…”