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Piersanti 2017 Apjl 836 L9.Pdf Publication Year 2017 Acceptance in OA@INAF 2020-09-02T14:46:33Z Title Type Ia Supernovae Keep Memory of their Progenitor Metallicity Authors PIERSANTI, Luciano; Bravo, Eduardo; CRISTALLO, Sergio; Domínguez, Inmaculada; STRANIERO, Oscar; et al. DOI 10.3847/2041-8213/aa5c7e Handle http://hdl.handle.net/20.500.12386/27071 Journal THE ASTROPHYSICAL JOURNAL LETTERS Number 836 The Astrophysical Journal Letters, 836:L9 (6pp), 2017 February 10 https://doi.org/10.3847/2041-8213/aa5c7e © 2017. The American Astronomical Society. All rights reserved. SNe Ia Keep Memory of Their Progenitor Metallicity Luciano Piersanti1,2, Eduardo Bravo3, Sergio Cristallo1,2, Inmaculada Domínguez4, Oscar Straniero1,5, Amedeo Tornambé6, and Gabriel Martínez-Pinedo7,8 1 INAF-Osservatorio Astronomico di Teramo, via Mentore Maggini, snc, I-64100, Teramo, Italy; [email protected] 2 INFN-Sezione di Perugia, via Pascoli, Perugia, Italy; [email protected] 3 E.T.S. Arquitectura del Vallés, Universitat Politècnica de Catalunya, Carrer Pere Serra 1-15, E-08173 Sant Cugat del Vallès, Spain; [email protected] 4 Universidad de Granada, E-18071 Granada, Spain; [email protected] 5 INFN, Laboratori Nazionali del Gran Sasso (LNGS), I-67100 Assergi, Italy; [email protected] 6 INAF-Osservatorio Astronomico di Roma, via Frascati, 33, I-00040, Monte Porzio Catone, Italy; [email protected] 7 GSI Helmholtzzentrum für Schwerioneneforschung, Planckstraße 1, D-64291 Darmstadt, Germany; [email protected] 8 Institut für Kernphysik (Theoriezentrum), Technische Universität Darmstadt, Schlossgartenstraße 2, D-64289 Darmstadt, Germany Received 2016 December 16; accepted 2017 January 26; published 2017 February 9 Abstract The ultimate understanding of SNe Ia diversity is one of the most urgent issues to exploit thermonuclear explosions of accreted White Dwarfs (WDs) as cosmological yardsticks. In particular, we investigate the impact of the progenitor system metallicity on the physical and chemical properties of the WD at the explosion epoch. We analyze the evolution of CO WDs through the accretion and simmering phases by using evolutionary models based on time-dependent convective mixing and an extended nuclear network including the most important electron captures, beta decays, and URCA processes. We find that, due to URCA processes and electron-captures, the neutron excess and density at which the thermal runaway occurs are substantially larger than previously claimed. Moreover, we find that the higher the progenitor metallicity, the larger the neutron excess variation during the accretion and simmering phases and the higher the central density and the convective velocity at the explosion. Hence, the simmering phase acts as an amplifier of the differences existing in SNe Ia progenitors. When applying our results to the neutron excess estimated for the Tycho and Kepler young supernova remnants, we derive that the metallicity of the progenitors should be in the range Z = 0.030– 0.032, close to the average metallicity value of the thin disk of the Milky Way. As the amount of 56Ni produced in the explosion depends on the neutron excess and central density at the thermal runaway, our results suggest that the light curve properties depend on the progenitor metallicity. Key words: accretion, accretion disks – nuclear reactions, nucleosynthesis, abundances – supernovae: general – supernovae: individual (Tycho, Kepler) 1. Introduction distribution of ignition points at the explosion depend on the temperature fluctuations already present in the convective core Nowadays it is largely accepted that supernovae Ia (SNe Ia) of the exploding WD (García-Senz & Woosley 1995; Wunsch are produced by the thermonuclear disruption of CO white & Woosley 2004). Finally, the pre-explosive evolution fixes dwarfs (WDs) accreting matter from their companions in binary the WD neutron excess η, thus determining the amount of 56Ni systems (Hoyle & Fowler 1960). In fact, the growth in mass of produced during the explosion and, hence, the luminosity at the degenerate object determines its compressional heating. ( ) ( -8 ˙ maximum of SNe Ia Timmes et al. 2003; Bravo et al. 2010 . For intermediate values of the accretion rate 10 M All these effects are thought to contribute to the SNe Ia --71) 10M yr when the accreting WD approaches the homogeneity, by erasing the identity hallmarks of the Chandrasekhar mass limit, C-ignition occurs at the center exploding WD progenitor (the so-called “stellar amnesia”— ( where the effects of compressional heating are larger e.g., see Höflich et al. 2003). However, such an amnesia cannot be ) Piersanti et al. 2003 . The energy delivered by C-burning global, as suggested by the correlation observed between SNe increases the local temperature and, hence, the burning rate Ia luminosity and metallicity of their environment (e.g., speeds up. The large energy excess can not be transferred Moreno-Raya et al. 2016). radiatively, so the WD inner zones become unstable for As mass is transferred, the WD contracts and density convection. As C-burning proceeds, the convective zone increases. When the electron Fermi energy becomes large extends outwards in mass, while temperature in the inner enough, electron-capture processes occur. The capture thresh- zones continues to increase, until it attains ~´810K8 and the old is typically lower for odd-mass nuclei due to odd–even explosion occurs (Wunsch & Woosley 2004). The time from effects in the nuclear masses. Electron-capture is followed by the onset of convection to explosion lasts several 104 year and the beta-decay of the daughter nucleus producing an URCA it is usually called simmering phase. process, in which electron-capture on the isobar with charge Z The accretion and simmering phases determine the WD alternates with the beta-decay of the (Z - 1) isobar. During this properties at the explosion. In particular, the energy and process, the abundances are not changed, but due to the ne and chemical composition of the ejecta are determined by the n¯e emission substantial cooling occurs(Tsuruta & thermonuclear flame propagation whose properties depend on Cameron 1970). The cooling efficiency is larger at the so- ( the thermal and turbulent state of the WD Nomoto et al. 1984; called URCA density, rURCA for which electron-capture and Woosley & Weaver 1986). Moreover, the dimension and beta-decay processes operate with the same rate. Increasing 1 The Astrophysical Journal Letters, 836:L9 (6pp), 2017 February 10 Piersanti et al. density, electron-capture on even-mass nuclei becomes obtained a small increase of the central neutronization during possible. Due to pairing effects, the capture threshold for a the simmering phase (0.3´- 10--33 10 ),definitively too low second electron-capture on the produced odd–odd nucleus is to explain the neutron excess in the exploding WD as estimated much lower than for the first capture in the even–even nucleus. for the Supernova remnants in the thin disk of the Galaxy Hence, there is no URCA process but a double electron-capture (Badenes et al. 2008; Park et al. 2013). that heats as the second capture populates final states at high In this work we investigate the dependence of the physical excitation energy decaying by gamma emission. We denote properties and neutronization at the explosion on the accretion r2EC the density at which the electron Fermi energy becomes and simmering phases. To this aim we consider WD equal to the threshold for the first capture. In stellar progenitors with different initial metallicities and we adopt an environment URCA processes in odd-mass nuclei and double extended nuclear network. In Section 2 we present the initial electron-capture in even-mass nuclei operate simultaneously. CO WD models and illustrate the computational method; in Typically, URCA processes dominate the temperature evol- Section 3 we present our results; in Section 4 we summarize ution, as they operate for several cycles, and double electron- our conclusions. captures the neutronization, as even-mass nuclei are more abundant. 2. Initial Models and Assumptions Paczyński (1972) recognized early that URCA processes could play an important role during the simmering phase. In the We assume the Double Degenerate scenario for SNe Ia progenitors (Iben & Tutukov 1984) and we consider three WD regions where the density is close to rURCA, the so-called different initial binary systems, namely (MM12,)(= 3.2, 1.5 ) , URCA shell, both nuclides participating in URCA processes ( ) have similar abundances. The continuous density increase in (4.5, 1.8) and 5.3, 2.0 M having initial chemical composition -4 ( ) the accreting WD causes the URCA shell to move outward in (YZ,)(=´ 0.245, 2.45 10 ) model ZLOW , (0.269, 1.38´ 10-2) (model ZSUN),and(0.305, 4´ 10-2) (model mass, and the region interior to the URCA shell becomes ) enriched in the proton-poor isobar, A(Z - 1), i.e., it becomes ZHIG , respectively. Heavy elements in ZSUN and ZHIG models have a scaled-solar abundances, while for the ZLOW model we neutronized. Similarly, for even A nuclei the transition from 9 A assume [Fe H] =-2 and [a H0.5] = . Computations have regions enriched in the (Z - 2) isobar to those enriched in the ( AZ isobar occurs at densities close to r . During the accretion been performed with the FUNS code Straniero et al. 2006; 2EC Cristallo et al. 2009), from the pre-main sequence phase up to the phase URCA processes and double electron-captures operate tip of the AGB. The primary component of each binary system locally. After C-ignition, when convection sets in, convective evolves into a CO WD with total mass M =0.817 M ,while URCA and double electron-captures followed by double-beta WD the secondary evolves into a ~0.6 M CO WD, the exact mass decays occur. If the proton-poor isobar of an URCA pair or value depending on the considered case. The more massive WDs double electron-capture triplet is transported by convective are evolved along the cooling sequence for 1 Gyr, until the eddies across the URCA shell to lower densities, it beta-decays models attain the following physical conditions: L = to the proton-rich isobar.
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