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Detection of Elements at All Three R-Process Peaks in the Metal-Poor Star HD 160617

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Detection of Elements at All Three R-Process Peaks in the Metal-Poor Star HD 160617 Published in the Astrophysical Journal A Preprint typeset using LTEX style emulateapj v. 5/2/11 DETECTION OF ELEMENTS AT ALL THREE R-PROCESS PEAKS IN THE METAL-POOR STAR HD 1606171 ,2 ,3 Ian U. Roederer4 and James E. Lawler5 Published in the Astrophysical Journal ABSTRACT We report the first detection of elements at all three r-process peaks in the metal-poor halo star HD 160617. These elements include arsenic and selenium, which have not been detected previously in halo stars, and the elements tellurium, osmium, iridium, and platinum, which have been detected previously. Absorption lines of these elements are found in archive observations made with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. We present up-to-date absolute atomic transition probabilities and complete line component patterns for these elements. Additional archival spectra of this star from several ground-based instruments allow us to derive abundances or upper limits of 45 elements in HD 160617, including 27 elements produced by neutron-capture reactions. The average abundances of the elements at the three r-process peaks are similar to the predicted solar system r-process residuals when scaled to the abundances in the rare earth element domain. This result for arsenic and selenium may be surprising in light of predictions that the production of the lightest r-process elements generally should be decoupled from the heavier r-process elements. Subject headings: atomic data — nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: individual (HD 160617) — stars: Population II 1. INTRODUCTION in greater abundance during n-capture reactions. The Understanding the origin of the elements is one of the s-process path closely follows the valley of β-stability, major challenges of modern astrophysics. Heavy ele- whereas the r-process blazes a path through neutron- ments, here considered to be those more massive than the rich nuclei. The r-process encounters the closed shells iron-group, are primarily produced by neutron-capture in nuclei of lower proton number, Z, than the s-process (n-capture) reactions. The heavy elements in the so- does, so the abundance peaks of the r-process are shifted lar system (S.S.) were produced mainly by two kinds of to lower-mass atoms relative to the s-process peaks. n-capture reactions, those slow (the s-process) or rapid Most elements at the three s-process peaks are read- (the r-process) relative to the average β-decay timescales ily identified in optical spectra obtained from ground- of nuclei along the reaction chain. The nuclear structure based instruments (e.g., Merrill 1926; St. John & Moore of atoms and the conditions at the time of nucleosyn- 1928; Aller & Greenstein 1960; Wallerstein et al. 1963; thesis determine the relative abundance patterns. The Van Eck et al. 2001). These elements include rubidium, abundance patterns can be interpreted through the lens strontium, yttrium, and zirconium (37 ≤ Z ≤ 40) at of experimental or theoretical properties of atomic nuclei the first s-process peak; barium, lanthanum, cerium, to infer the conditions present in the supernovae, merg- praseodymium, and neodymium (56 ≤ Z ≤ 60) at the ing neutron stars, or red giant stars where these elements second s-process peak; and lead and bismuth (82 ≤ were forged. Z ≤ 83) at the third s-process peak. In contrast, el- Large energy gaps to the next highest nuclear energy ements at the third r-process peak—osmium, iridium, level occur in nuclei with 50, 82, and 126 neutrons. Nu- and platinum (76 ≤ Z ≤ 78)—remained undetected in halo stars until the Goddard High Resolution Spec- arXiv:1204.3901v1 [astro-ph.SR] 17 Apr 2012 clei with these “closed” nuclear shells are especially sta- ble and resist further n-capture, so they are produced trograph (GHRS) onboard the Hubble Space Telescope (HST ) revealed the ultraviolet (UV) spectra of late- type stars in high spectral resolution (Cowan et al. 1996; 1 Some of the data presented in this paper were obtained Sneden et al. 1998).6 Subsequent investigations using from the Multimission Archive at the Space Telescope Science Institute (MAST). STScI is operated by the Association of Uni- the successor to the GHRS, the Space Telescope Imag- versities for Research in Astronomy, Inc., under NASA contract ing Spectrograph (STIS), continue to offer the only re- NAS5-26555. These data are associated with Program GO-8197. liable window to detect these third r-process peak el- 2 Based on data obtained from the European Southern Obser- ements (Cowan et al. 2002, 2005; Sneden et al. 2003; vatory (ESO) Science Archive Facility. These data are associated with Programs 65.L-0507 and 82.B-0610. Roederer et al. 2009, 2010a, 2012b; Barbuy et al. 2011). 3 This research has made use of the Keck Observatory Archive Only one element at the second r-process peak, tellurium (KOA), which is operated by the W.M. Keck Observatory and (Z = 52), was recently detected in three halo stars by the NASA Exoplanet Science Institute (NExScI), under contract with the National Aeronautics and Space Administration. These 6 Numerous heavy elements, including those at the third data are associated with Program H6aH (P.I. Boesgaard). r-process peak, have long been recognized in the stratified at- 4 Carnegie Observatories, Pasadena, CA 91101, USA; mospheres of chemically peculiar stars; e.g., Morgan (1933) or [email protected] Dworetsky (1969). We do not regard these stars as useful for inter- 5 Department of Physics, University of Wisconsin, Madison, preting the nucleosynthetic record, however, so we shall not discuss WI 53706, USA; [email protected] them further. 2 Roederer & Lawler Roederer et al. (2012a). provides some motivation for extensive lab work on these The need for UV access can be explained using a com- elements (e.g., Lawler et al. 2009). Alkaline earth ele- bination of principles from astrophysics, nuclear physics, ments, such as strontium (Z = 38) and barium (Z = 56), statistical mechanics, and atomic physics. Here we focus with two valence s-shell electrons are also accessible to on photospheric temperatures typical for F, G, and K ground-based observations, even though they exist pri- stars. Many of the useful visible and UV lines of iron- marily as ionized species in typical photospheres. group atoms and ions have excitation potentials (E.P.) Elements at the r-process peaks are critical to un- of ∼ 1 eV or more. Even the most abundant lighter derstanding r-process nucleosynthesis. These serve as n-capture elements tend to have abundances several or- key normalization points to constrain the conditions ders of magnitude below those of iron-group elements. that produce a successful r-process (Kratz et al. 1993). This simple fact eliminates the utility of atomic lines Experimental data are available for only a limited connecting highly excited levels of n-capture elements. number of short-lived nuclei along the r-process path It necessitates the use of resonance lines (E.P. = 0) or (Dillmann et al. 2003; Baruah et al. 2008). The Facility lines connected to low metastable levels if such levels for Rare Isotope Beams, under construction at Michigan exist in the atom or ion of interest. A simple consider- State University, and similar facilities in other countries ation of Boltzmann factors reveals the ground and low will alleviate this shortage of r-process nuclear data in metastable levels tend to serve as the major population the future. Nuclei at the so-called “waiting-points” are reservoirs for both neutral and singly ionized atoms in the parent isobars of elements at the r-process peaks, a typical photosphere. Depending on the ionization po- including selenium and tellurium. Yet, until now, the tential (I.P.) of the neutral atom, either the neutral or only secure measurements of their abundances in en- singly ionized specie is dominant in F, G, and K stars. vironments suitable for inferring the r-process abun- Consider n-capture elements near and at the right edge dance distribution have come from the S.S. r-process of the periodic table. Rare gases at the right edge (kryp- residuals. This method of computing these residu- ton, Z = 36, and xenon, Z = 54) have deeply bound als assumes that the r-process produced some frac- valence electrons due to their closed p-shells and a huge tion of all heavy isotopes in the S.S. except those at- gap between the ground and lowest excited levels. The tributed to s-process nucleosynthesis or inaccessible to atomic structure of rare gases prohibits any photospheric the r-process (e.g., Seeger et al. 1965; Cameron 1982; abundance observations even on the strongly dominant K¨appeler et al. 1989). Light n-capture nuclei may have neutral ionization stage, whose resonance lines lie be- a variety of origins besides the s- and r-processes (e.g., low 1500 A˚ (e.g., Morton 2000). The elements selenium α-rich freeze-out from nuclear statistical equilibrium; (Z = 34) and bromine (Z = 35), adjacent to the rare gas Woosley & Hoffman 1992). Since the assumptions un- krypton, as well as tellurium and iodine (Z = 53), adja- derlying the S.S. r-process residual distribution for the cent to the rare gas xenon, have partially filled p-shells. light nuclei are tenuous, detection of these elements in These have some similarities to the rare gases. There are r-process material beyond the S.S. is greatly desired. gaps of ∼ 48,000 cm−1 and 67,000 cm−1, respectively, Here, we report the detection of two light n-capture between levels in the ground 4pn configurations and the elements, arsenic (Z = 33) and selenium in an archive lowest opposite parity 4pn−1 5s configurations of atomic STIS UV spectrum of the bright, metal-poor subgiant selenium and bromine.
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