Astronomy & Astrophysics manuscript no. SNII_Z_EWs_refv_proof c ESO 2018 June 25, 2018 Type II supernovae as probes of environment metallicity: observations of host H ii regions J. P. Anderson1, C. P. Gutiérrez1,2,3, L. Dessart4, M. Hamuy3,2, L. Galbany2,3, N. I. Morrell5, M. D. Stritzinger6, M. M. Phillips5, G. Folatelli7,8,H.M.J.Boffin1, T. de Jaeger2,3, H. Kuncarayakti2,3, and J. L. Prieto9,2 1European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile. e-mail: [email protected] 2Millennium Institute of Astrophysics, Casilla 36-D, Santiago, Chile 3Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile 4Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Boulevard de l’Observatoire, CS 34229, 06304 Nice cedex 4, France. 5Carnegie Observatories, Las Campanas Observatory, Casilla 601, La Serena, Chile 6Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark 7Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata (UNLP), Paseo del Bosque S/N, B1900FWA, La Plata, Argentina; Instituto de Astrofísica de La Plata, IALP, CCT-CONICET-UNLP, Argentina 8Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, Kashiwa, Chiba 277-8583, Japan 9Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército 441 Santiago, Chile ABSTRACT Context. Spectral modelling of type II supernova atmospheres indicates a clear dependence of metal line strengths on progenitor metallicity. This dependence motivates further work to evaluate the accuracy with which these supernovae can be used as environment metallicity indicators. Aims. To assess this accuracy we present a sample of type II supernova host H ii-region spectroscopy, from which environment oxygen abundances have been derived. These environment abundances are compared to the observed strength of metal lines in supernova spectra. Methods. Combining our sample with measurements from the literature, we present oxygen abundances of 119 host H ii regions by extracting emission line fluxes and using abundance diagnostics. These abundances are then compared to equivalent widths of Fe ii 5018 Å at various time and colour epochs. Results. Our distribution of inferred type II supernova host H ii-region abundances has a range of ∼0.6 dex. We confirm the dearth of type II supernovae exploding at metallicities lower than those found (on average) in the Large Magellanic Cloud. The equivalent width of Fe ii 5018 Å at 50 days post-explosion shows a statistically significant correlation with host H ii-region oxygen abundance. The strength of this correlation increases if one excludes abundance measurements derived far from supernova explosion sites. The correlation significance also increases if we only analyse a ‘gold’ IIP sample, and if a colour epoch is used in place of time. In addition, no evidence is found of a correlation between progenitor metallicity and supernova light-curve or spectral properties – except for that stated above with respect to Fe ii 5018 Å equivalent widths – suggesting progenitor metallicity is not a driving factor in producing the diversity that is observed in our sample. Conclusions. This study provides observational evidence of the usefulness of type II supernovae as metallicity indicators. We finish with a discussion of the methodology needed to use supernova spectra as independent metallicity diagnostics throughout the Universe. Key words. (Stars:) supernovae: general – ISM: abundances – (ISM:) HII regions – Galaxies: abundances 1. Introduction To determine the rate of chemical enrichment as a function arXiv:1602.00011v3 [astro-ph.GA] 4 Mar 2016 of both time and environment, metallicity indicators throughout A fundamental parameter in our understanding of the evolution the Universe are needed. In nearby galaxies one can use spectra of galaxies is the chemical enrichment of the Universe as a func- of individual stars to measure stellar metallicity (see, e.g. tion of time and environment. Stellar evolution and its explosive Kudritzki et al. 2012). However, further afield this becomes end drive the processes which enrich the interstellar (and indeed impossible and other methods are required. In relatively nearby intergalactic) medium with heavy elements. Galaxy formation galaxies (<70 Mpc) one can observe the stellar light from and evolution, together with the evolution of the complete clusters to constrain stellar metallicities (see e.g. Gazak et al. Universe are controlled by the speed and temporal location of 2014), or gas-phase abundances can be obtained through chemical enrichment. This is observed in the strong correlation observations of emission lines within H ii regions produced by between a galaxy’s mass and its gas-phase oxygen abundance the ionisation (and subsequent recombination) of the interstel- (see Tremonti et al. 2004). One also observes significant radial lar medium (ISM) (see e.g. review of various techniques in metallicity gradients within galaxies (see e.g. Henry & Worthey Kewley & Ellison 2008). At higher redshifts, the latter emission 1999; Sánchez et al. 2014) which provides clues to their past line diagnostics become the dominant source of measurements. formation history and future evolution. Emission line diagnostics can be broadly separated into Article number, page 1 of 21 A&A proofs: manuscript no. SNII_Z_EWs_refv_proof two groups. The first group, so called empirical methods, are In D14, a conceptual study of SNe II as environment metal- those where the ratio of strong emission lines within H ii-region licity indicators was published (following Dessart et al. 2013 in spectra are calibrated against abundance estimations from which the impact of various stellar and explosion parameters measurements of the electron temperature (Te, referred to as a on the resulting SN radiation was examined). Model progeni- direct method and derived from the ratio of faint auroral lines, tors of increasing metallicity produced spectra with metal-line e.g. [O iii] 4363Å and 5007Å, see Osterbrock & Ferland 2006). equivalent widths (EWs) of increasing strength at a given post- Some of the most popular empirical relations are those presented explosion time or colour. This is the result of the fact that the in Pettini & Pagel (2004), which use the ratio of Hα 6563Å to hydrogen-rich envelope – which is the region probed during the [N ii] 6583Å (the N2 diagnostic), or a combination of this with photospheric phase of SN II evolution – retains its original com- the ratio of Hβ 4861Å to [O iii] 5007Å (the O3N2 diagnostic). position (given that nuclear burning during the stars life or the These diagnostics were updated in Marino et al. (2013) (hence- explosion has negligible/weak influence on the hydrogen-rich forth M13), and we use the latter for the main analysis in this envelope metal content). Hence, the strength of metal line EWs paper. The second group of diagnostics are those which use the measured during the ‘plateau’ phase of SNe II is essentially de- comparison of observed emission line ratios with those pre- pendent on the abundance of heavy elements contained within dicted by photoionisation/stellar population-synthesis models that part of the SN ejecta, together with the temperature of the (see e.g. McGaugh 1991; Kewley & Dopita 2002). A major is- line forming region. The results of D14 therefore make a predic- sue currently plaguing absolute metallicity determinations is the tion that SNe II with lower metal line EWs will be found within varying results that are obtained with different line diagnostics. environments of lower metallicity within their host galaxies. The For example, the photoionisation model methods generally give goal of the current paper is to test such predictions by observing ii abundances that are systematically higher than those derived SN II host H regions, and compare SN pseudo–EWs (pEWs) ii through empirical techniques. López-Sánchez et al. (2012) to host H -region metal abundances. published a review of the systematics involved between the The manuscript is organisedin the following way. In the next various abundance diagnostics. It should also be noted that the section the data sample is introduced, both of the SNe II, and ii majority of these techniques use oxygen abundance as a proxy of host H -region spectroscopy. This is followed by a brief de- for metallicity, neglecting elemental variations among metals. scription of spectral models. In section 3 we summarise the anal- Given the number of issues with current metallicity diag- ysis methods, and in section 4 the results from that analysis are nostics, any new independent technique is of significant value. presented. In section 5 the implications of these results are dis- Dessart et al. (2013) presented type II supernova (SN II) model cussed, together with future directions of this research. Finally, spectra produced from progenitors with distinct metallicity. in section 6 we draw our conclusions. Dessartetal. (2014; hereafter D14) then showed how the strength of metal lines observed within photospheric phase 2. Data sample and comparison spectral models spectra are strongly dependent on progenitor metallicity. Here we present spectral observations of SNe II in comparison to The data analysed in this publication comprise two distinct abundances inferred from host H ii-region emission line spectra. types of observations. The first is of SN II optical spectroscopy