Type II supernovae as probes of environment metallicity: observations of host HII regions

0.67 MB·

by  J. P. Anderson

Astronomy&Astrophysicsmanuscriptno.SNII_Z_EWs_refv_proof (cid:13)cESO2016
March7,2016
Type II supernovae as probes of environment metallicity:
ii
observations of host H 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.deJaeger2,3,H.Kuncarayakti2,3,andJ.L.Prieto9,2
1EuropeanSouthernObservatory,AlonsodeCórdova3107,Casilla19,Santiago,Chile.e-mail:[email protected]
2MillenniumInstituteofAstrophysics,Casilla36-D,Santiago,Chile
6 3DepartamentodeAstronomía,UniversidaddeChile,CaminoElObservatorio1515,LasCondes,Santiago,Chile
1 4Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Boulevard de l’Observatoire, CS 34229,
0 06304Nicecedex4,France.
2 5CarnegieObservatories,LasCampanasObservatory,Casilla601,LaSerena,Chile
6DepartmentofPhysicsandAstronomy,AarhusUniversity,NyMunkegade120,DK-8000AarhusC,Denmark
ar 7FacultaddeCienciasAstronómicasyGeofísicas,UniversidadNacionaldeLaPlata(UNLP),PaseodelBosqueS/N,B1900FWA,
M LaPlata,Argentina;InstitutodeAstrofísicadeLaPlata,IALP,CCT-CONICET-UNLP,Argentina
8KavliInstituteforthePhysicsandMathematicsoftheUniverse(WPI),TheUniversityofTokyo,Kashiwa,Chiba277-8583,Japan
9NúcleodeAstronomíadelaFacultaddeIngeniería,UniversidadDiegoPortales,Av.Ejército441Santiago,Chile
4

]
A
G ABSTRACT
h. Context.Spectral modelling of type II supernova atmospheres indicates a clear dependence of metal line strengths on progenitor
p metallicity.Thisdependencemotivatesfurtherworktoevaluatetheaccuracywithwhichthesesupernovaecanbeusedasenvironment
- metallicityindicators.
o Aims.ToassessthisaccuracywepresentasampleoftypeIIsupernovahostHii-regionspectroscopy,fromwhichenvironmentoxygen
r abundances have been derived. Theseenvironment abundances arecompared totheobserved strength of metal linesinsupernova
t
s spectra.
a Methods.Combiningoursamplewithmeasurementsfromtheliterature,wepresentoxygenabundancesof119hostHiiregionsby
[ extractingemissionlinefluxesandusingabundancediagnostics.TheseabundancesarethencomparedtoequivalentwidthsofFeii

5018Åatvarioustimeandcolourepochs.
3
Results.OurdistributionofinferredtypeIIsupernovahostHii-regionabundanceshasarangeof∼0.6dex.Weconfirmthedearth
v
oftypeIIsupernovaeexplodingatmetallicitieslowerthanthosefound(onaverage)intheLargeMagellanicCloud.Theequivalent
1
1 widthofFeii5018Åat50dayspost-explosionshowsastatisticallysignificantcorrelationwithhostHii-regionoxygenabundance.
0 Thestrengthofthiscorrelationincreasesifoneexcludesabundancemeasurementsderivedfarfromsupernovaexplosionsites.The
0 correlationsignificancealsoincreasesifweonlyanalysea‘gold’IIPsample,andifacolourepochisusedinplaceoftime.Inaddition,
0 noevidenceisfoundofacorrelationbetweenprogenitormetallicityandsupernovalight-curveorspectralproperties–exceptforthat
. statedabovewithrespecttoFeii5018Åequivalentwidths–suggestingprogenitormetallicityisnotadrivingfactorinproducingthe
2
diversitythatisobservedinoursample.
0
Conclusions.ThisstudyprovidesobservationalevidenceoftheusefulnessoftypeIIsupernovaeasmetallicityindicators.Wefinish
6
withadiscussionofthemethodologyneededtousesupernovaspectraasindependentmetallicitydiagnosticsthroughouttheUniverse.
1
: Keywords. (Stars:)supernovae:general–ISM:abundances–(ISM:)HIIregions–Galaxies:abundances
v
i
X
r1. Introduction To determinethe rate of chemicalenrichmentas a function
a ofbothtimeandenvironment,metallicityindicatorsthroughout
Afundamentalparameterinourunderstandingoftheevolution theUniverseareneeded.Innearbygalaxiesonecanusespectra
ofgalaxiesisthechemicalenrichmentoftheUniverseasafunc- of individual stars to measure stellar metallicity (see, e.g.
tionoftimeandenvironment.Stellarevolutionanditsexplosive Kudritzkietal. 2012). However, further afield this becomes
enddrivetheprocesseswhichenrichtheinterstellar(andindeed impossibleandothermethodsarerequired.Inrelativelynearby
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. Gazaketal.
Universe are controlled by the speed and temporal location of 2014), or gas-phase abundances can be obtained through
chemicalenrichment.Thisisobservedinthestrongcorrelation observationsof emission lines within Hii regions produced by
between a galaxy’s mass and its gas-phase oxygen abundance the ionisation (and subsequent recombination) of the interstel-
(see Tremontietal. 2004). One also observessignificant radial lar medium (ISM) (see e.g. review of various techniques in
metallicitygradientswithingalaxies(seee.g.Henry&Worthey Kewley&Ellison2008).Athigherredshifts,thelatteremission
1999; Sánchezetal. 2014) which provides clues to their past linediagnosticsbecomethedominantsourceofmeasurements.
formationhistoryandfutureevolution. Emission line diagnostics can be broadly separated into
Articlenumber,page1of21
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two groups. The first group, so called empirical methods, are InD14,aconceptualstudyofSNeIIasenvironmentmetal-
thosewheretheratioofstrongemissionlineswithinHii-region licityindicatorswaspublished(followingDessartetal.2013in
spectra are calibrated against abundance estimations from which the impact of various stellar and explosion parameters
measurements of the electron temperature (T , referred to as a on the resulting SN radiation was examined). Model progeni-
e
direct method and derived from the ratio of faint aurorallines, tors of increasing metallicity produced spectra with metal-line
e.g.[Oiii]4363Åand5007Å,seeOsterbrock&Ferland2006). equivalentwidths(EWs)ofincreasingstrengthatagivenpost-
Someofthemostpopularempiricalrelationsarethosepresented 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-richenvelope–whichistheregionprobedduringthe
[Nii]6583Å(theN2diagnostic),oracombinationofthiswith photosphericphaseofSNIIevolution–retainsitsoriginalcom-
theratioofHβ4861Åto[Oiii] 5007Å(theO3N2diagnostic). position (given that nuclear burning during the stars life or the
explosion has negligible/weak influence on the hydrogen-rich
ThesediagnosticswereupdatedinMarinoetal.(2013)(hence-
envelopemetalcontent).Hence,thestrengthofmetallineEWs
forth M13), and we use the latter for the main analysis in this
measuredduringthe‘plateau’phaseofSNeIIisessentiallyde-
paper.Thesecondgroupofdiagnosticsarethosewhichusethe
pendent on the abundance of heavy elements contained within
comparison of observed emission line ratios with those pre-
dicted by photoionisation/stellar population-synthesis models that partof the SN ejecta, together with the temperatureof the
lineformingregion.TheresultsofD14thereforemakeapredic-
(see e.g. McGaugh 1991; Kewley&Dopita 2002). A major is-
tionthatSNeIIwithlowermetallineEWswillbefoundwithin
suecurrentlyplaguingabsolutemetallicitydeterminationsisthe
varyingresultsthatareobtainedwithdifferentline diagnostics. environmentsoflowermetallicitywithintheirhostgalaxies.The
goalofthecurrentpaperistotestsuchpredictionsbyobserving
Forexample,thephotoionisationmodelmethodsgenerallygive
SN II host Hii regions, and compare SN pseudo–EWs(pEWs)
abundances that are systematically higher than those derived
tohostHii-regionmetalabundances.
through empirical techniques. López-Sánchezetal. (2012)
Themanuscriptisorganisedinthefollowingway.Inthenext
published a review of the systematics involved between the
section the data sample is introduced, both of the SNe II, and
variousabundancediagnostics. It should also be noted that the
ofhostHii-regionspectroscopy.Thisisfollowedbyabriefde-
majority of these techniquesuse oxygenabundance as a proxy
scriptionofspectralmodels.Insection3wesummarisetheanal-
formetallicity,neglectingelementalvariationsamongmetals.
ysismethods,andinsection4theresultsfromthatanalysisare
Given the number of issues with current metallicity diag-
presented.Insection5theimplicationsofthese resultsaredis-
nostics, any new independenttechnique is of significant value.
cussed,togetherwith futuredirectionsofthisresearch.Finally,
Dessartetal.(2013)presentedtypeIIsupernova(SNII)model
insection6wedrawourconclusions.
spectra produced from progenitors with distinct metallicity.
Dessartetal. (2014; hereafter D14) then showed how the
strength of metal lines observed within photospheric phase
2. Datasampleandcomparisonspectralmodels
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
abundancesinferredfromhostHii-regionemissionlinespectra. typesofobservations.ThefirstisofSN IIopticalspectroscopy
This comparison presents observational evidence that these obtained during their photospheric phases, i.e. from discovery
explosive events may indeed be used as metallicity indicators to at most ∼100 days post explosion. These data are used to
throughouttheUniverse. extract absorption line pEW measurements. The second data
setisemissionlinespectralobservationsofhostHiiregionsof
SNe II. These are used to estimate SN II environmentoxygen
SNe II are the most frequent stellar explosion in the Uni- abundances, which can be used as metallicity proxies. In the
verse (Lietal. 2011). Theyare the resultof massive stars (>8- course of this work we compare our observationalresults with
10M⊙)thatundergocorecollapseattheendoftheirlives.The thepredictionsfromthe spectralmodelsofD14.Thedetailsof
typeIIdesignationindicatestheseeventshavestronghydrogen thesemodelsarebrieflysummarisedbelow.
features in their spectra (see Minkowski 1941, and Filippenko
1997forareviewofSNspectralclassifications),implyingtheir
progenitorshaveretainedasignificantfractionofhydrogenprior
to exploding. Historically SNe II have been separated into II- 2.1.Supernovaobservations
Plateau (IIP), showing an almost constant luminosity for 2–
Our SN sample comprises >100 SNe II observed by the
3 months in their light-curves post maximum, and II-Linear
CarnegieSupernovaProject(CSP,Hamuyetal.2006)pluspre-
(IIL) which decline faster in a ‘linear’ manner post maximum
vious SN II follow-up surveys (‘CATS et al.’ Galbanyetal.
(Barbonetal. 1979). However, recent large samples have been
2016, sources listed in A14). A list of SNe II included in this
published which question this distinction and argue for a con-
analysisis givenin theAppendixTable A.1,togetherwith var-
tinuuminSNdeclineratesandotherproperties(Andersonetal.
iousparametersfromA14 and Gutiérrezetal. (2014). In Table
2014b; A14, and Sandersetal. 2015, although see Faranetal.
A.1 we also list the host galaxy properties: recession velocity
2014b,a;Arcavietal.2012fordistinctconclusions).(Intherest
andabsoluteB-bandmagnitude.Themeanhostgalaxyabsolute
of the manuscriptwe simply refer to all types as ‘SNe II’, and
magnitudeis–20.5,andthelowesthostmagnitudeis–17.7.The
differentiate events by specific photometric/spectroscopic pa-
vast majority of the SN sample have host galaxies intrinsically
rameters where needed.) It is clear that SNe II show signifi- brighter than the Large Magellanic Cloud (LMC),1 suggesting
cantdispersionintheirlight-curvesandspectralproperties(see
the vast majority of the sample have environmentmetallicities
e.g. A14, Andersonetal. 2014aand Gutiérrezetal. 2014), and
higherthanthosegenerallyfoundintheLMC(assumingtheac-
A14andGutiérrezetal.(2014)havespeculated(followingear-
ceptedluminosity–metallicityrelation).Thisisimportantforthe
lier predictions; see Blinnikov&Bartunov 1993) that this ob-
discussionpresentedlaterwithrespecttoalackofSNeIIinlow
served dispersion could be the result of explosionsof progeni-
torswithdistincthydrogenenvelopemassesatdeath. 1 HyperLeda:http://leda.univ-lyon1.fr/
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Andersonetal.:SNeIIasmetallicitytracers
wavelength spectroscopy. Measurements from these emission
linespectraarealsocombinedwiththoseofotherSNeIIwhich
were previously presented in Andersonetal. (2010) (where
many values were taken from Covarrubias2007). In Appendix
Table B.2 the source of the abundance measurements(here, or
fromAndersonetal.2010)isindicated.
SN II host Hii regions were observed using VLT–FORS2
(Appenzelleretal. 1998) in long-slitmode (LSS). We used the
300Vgratingtogetherwith the GG435blockingfilter anda 1′′
slit. This set-up provideda wavelength range of 4450–8650Å,
witharesolutionof1.68Åpixel−1.AsourtargetSNeIIareno
longervisible (a requirementfor our observationsand analysis
methods),tocentretheslitonSNIIexplosionsitesthetelescope
was first aligned to a nearby bright star. Blind offsets to the
SN location were then applied and the slit position angle was
chosentointersecttheSNhost-galaxynucleus.
Datareductionwasperformedinthestandardmannerusing
iraf, intheformof:bias-subtractions;flat-field normalisations;
1dspectralextractionandsky-subtractionofemissionlinespec-
tra;andfinallywavelengthandfluxcalibration.Onedimensional
spectralextractionwasfirstachievedontheexactregionwhere
each SN exploded. However, in many cases no emission lines
were detected in that region (consistent with the non-detection
ofHαwithinSNIIenvironmentsasreportedinAndersonetal.
Fig.1.Modelspectra(D14)fromfourdistinctprogenitormodelsat50
2012), and extractions were attempted further along the slit in
days post explosion: 0.1, 0.4, 1 and 2 × solar metallicity. The dotted
either direction until sufficient lines (at a minimum Hα and
black lines bracket the Feii 5018Å absorption feature we use in this
[Nii]) could be detected. The distances of these extraction
analysis.
regions from those of SN explosion coordinates are listed in
the AppendixTable B.2, and the effectof includingHii-region
metallicityenvironments. measurementsoffsetfromexplosionsitesisdiscussedbelow.
Opticallow-resolution(typicalspectralresolutionsbetween
5and8Å,FWHM)spectroscopictimeserieswereobtainedfor
SNe II from epochs close to explosion out to nebular phases
2.3.SyntheticspectraofSNIIPatdifferentmetallicities
throughanumberofSNfollow-upcampaigns.Wedonotgointo
thedetailsofthefollow-upsurveyshere,howevermoreinforma-
Theobservationalresearchpresentedinthismanuscriptwasmo-
tioncanbefoundinanumberofpreviouspublications(seee.g.
tivated by the study of D14. That study used four models with
Hamuy 2003; Hamuyetal. 2006, 2009; Contrerasetal. 2010;
distinct progenitor metallicities, producing synthetic spectral
Folatellietal.2010).Initialanalysesofthesespectroscopicdata
time series. Progenitors of 15M initial mass, and progenitor
focussing on the nature of the dominant Hα line can be found ⊙
metallicitiesof0.1,0.4,1,and2timessolar(Z )wereevolved
inGutiérrezetal.(2014)andAndersonetal.(2014a),whilethe ⊙
from the main sequence until death with mesa (Paxtonetal.
fulldatareleaseandanalysiswillbepublishedinupcomingpa-
2011)–adopting Z =0.02.Uponreachingcore-collapse,pro-
pers(Gutiérrezetal.inpreparation). ⊙
genitors were exploded and synthetic spectral sequences com-
Thedatawereobtainedwitharangeofinstrumentsinvari-
puted using cmfgen (Dessart&Hillier 2010; Hillier&Dessart
ousformsoflongslitspectroscopy.Datareductionwasachieved
in the standard manner using routines within iraf2, including 2012).ThereaderisreferredtoDessartetal.(2013)andD14for
adetailedexplanationofthemodellingprocedure(Dessartetal.
bias-subtraction;flat-field normalisation;1d spectral extraction
2013 explore a large range of progenitor parameters and their
and sky-subtraction; and finally, wavelength and flux calibra-
subsequent effects on the model SNe II produced, while D14
tion.MoredetailsofthisprocessasappliedtoCSPSNIaspec-
concentratesontheeffectofprogenitormetallicity).
troscopycanbefoundinFolatellietal.(2013).
InFigure1 modelspectraare plotted,oneforeachprogen-
itormetallicity,takenat50dayspostexplosion(50d).Onecan
2.2.Hii-regionspectroscopy clearly see the effects of increasing metallicity on the model
spectra,inparticularatbluerwavelengths(i.e.bluewardsofHα,
InESOperiodP94(October2014–March2015)50hrsofVLT
<6000Å). The higher metallicity models exhibit many more
(+FORS2)timeatCerroParanalwereallocatedtothisproject. ∼
lines which are also significantly stronger in EW. As one goes
Thiswastoobserve∼100Hiiregionscoincidentornearthesite
to the lower metallicity models spectra appear much ‘cleaner’
ofSNeII.SNeIIweretakenfromthepublicationsofA14,plus
beingdominatedby Balmer linesandshowingweakersignsof
other SNe II from the CSP (a small number of ‘normal’SN II
metallineblanketing.
whichwerenotpresentedinA14,plusafewIInandIIb),i.e.the
D14 explored the effects of changing progenitor metallic-
samesampleasdiscussedabovewithrespecttotransientoptical
ity with all other parameters constant (initial mass, mass-loss
2 iraf isdistributedbytheNationalOpticalAstronomy Observatory, andmixinglengthprescriptions).However,thereare otherpre-
whichisoperatedbytheAssociationofUniversitiesforResearchinAs- SNparameterswhichmaysignificantlyaffecttheevolutionand
tronomy(AURA)under cooperativeagreementwiththeNationalSci- strengthof spectralline EWs (theimportantfeatureswe use in
enceFoundation. the current work), and produce degeneracies in SN measure-
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Fig.2.TwoexamplesofbothSNIIspectra(upperpanels,atepochscloseto50dayspostexplosion)andhostHii-regionspectra(lowerpanels).
Left:SN2003bl,aSNwithrelativelyhighFeii5018ÅpEWandO3N2abundance,andRight:SN2007ocwithrelativelylowFeii5018ÅpEW
and O3N2 abundance. The position of the Feii 5018Å absorption feature in the SN spectra isindicated in blue. In the Hii-region spectra we
indicatethepositionoftheemissionlinesusedforabundanceestimations:Hβingreen,[Oiii]incyan,Hαinblue,and[Nii]inmagenta.
ments.D14showedhowSNeIIwithdistinctpre-SNradii(cre- tween either side of the emission line. The uncertainty created
atedusingthesameprogenitors,butevolvedwithadistinctmix- by this process is taken into account when estimating flux er-
ing length prescription for convection) produced different EW rors. pEWs are measured in all spectra obtained within 0–100
strengthsandevolutionsforthesameprogenitormetallicity.This dayspostexplosion(seeA14fordetailsofexplosionepoches-
is seen in Figs. 4 and 10, where the models are referred to as timations). To measure pEWs we proceed to define the pseudo
m15mlt1(largerradius)andm15mlt3(smallerradius,m15mlt2 continuum(theadjacentmaximathatboundtheabsorption)ei-
isthesolarmetallicitymodelalreadydiscussedabove).We use thersideofthebroadSNabsorptionfeatureandfitaGaussian.
suchmodelstocomparethemetallineEWsresultingfrommetal Thesearedefinedas‘pseudo’EWsduetothedifficultyinknow-
abundance variations or from changes in the progenitor struc- ing/definingthetruecontinuumlevel.Thisprocedureisachieved
ture. multipletimes,eachtimeremovingnarrowemissionlineswhen
present.AmeanpEWisthencalculatedtogetherwithastandard
deviation, with the latter being taken as the pEW error. In this
3. Analysis way we obtaina pEWfor each spectralepoch.The same mea-
surementprocedureundertakenforobservationsisachievedfor
As outlined above, our data comprises two distinct sets, and
modelspectra, meaning that for models we also present pEWs
hence our analysis is split into two distinct types of measure-
tomakeconsistentcomparisonswithobservations(eventhough
ments. These are now outlined in more detail. In Fig. 2 we
in the case of models we know the true continuum and could
presentexamplesofourdata,indicatingthepositionofthespec-
measuretrueEWs).
trallinesusedinouranalysis.
3.1.SNeIIFeii5018ÅEWmeasurements 3.2.HostHii-regionabundancemeasurements
To quantifythe influenceof progenitormetallicity on observed Fluxes of all detected narrow emission lines within host Hii-
linestrengths,inthispublicationweconcentrateonthestrength regionspectraaremeasuredbydefiningthecontinuumoneither
of the Feii 5018Å line. This line is prominent in the majority sideoftheemissionandfittingaGaussiantotheline.Thelines
of SNe II from relatively early times, i.e. at the onset of hy- ofinterestforourabundanceestimationsare:Hβ,[Oiii],Hα,and
drogenrecombination,and stays presentthroughoutthe photo- [Nii].Usingthesefluxes,gas-phaseoxygenabundancesarecal-
sphericphase.Inaddition,itisnotsignificantlycontaminatedby culatedusingtheN2andO3N2diagnosticsofbothM13andthe
otherSNlines.Oneissuewiththislineisthatitisinthewave- earliercalibrationsfromPettini&Pagel(2004).Thesearelisted
lengthrangewhereoneobservesnarrowHβand[Oiii]emission in Table B.2. Abundance errors are estimated by calculating
linesfromhostHii regions.Oftenitisdifficultto fullyremove theminimumandmaximumlineratiostakingintoaccountline
thesefeaturesinspectralreductionandextraction,andtheycan fluxerrors,i.e. the ‘analytic’approachoutlinedin Biancoetal.
contaminate the broad spectral features of the SN. When nar- (2015) (and used on the previous sample of Andersonetal.
row Hii-region emission lines are present in our SN spectra, 2010). In addition,in Figs. 8 and 9 the systematic errorsfrom
they are removed by simply interpolating the SN spectra be- theM13N2andO3N2diagnosticsarealsoshown.
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Andersonetal.:SNeIIasmetallicitytracers
Fig.3.EvolutionofFeii5018ÅpEWswithtimeforallSNeIIwithin Fig.4.SameasFig.3,butnowtheobservationaldataarebinnedintime.
thesample.Individualmeasurementsareshowninblue,togetherwith The black sold line represents the mean pEW within each time bin,
theirerrors.Theseareconnectedbyblacklines.Alsopresentedarethe while the dashed blue lines indicate the standard deviation. Together
timesequenceofpEWsmeasuredfromsyntheticspectra(Dessartetal. withthefourdistinctmetallicitymodels(Dessartetal.2013shownin
2013),forfourmodelsofdistinctprogenitormetallicity. solidredlines)wealsopresentspectral seriesproduced fromtwoad-
ditional solar metallicity progenitors, but with distinct mixing length
prescriptions, leading to smaller (mlt3, shown as the dotted red line)
and larger (mlt1, shown as the dashed red line) RSG progenitor radii
We are restricted in the abundance diagnostics we can use
(seeSection 2.3formoredetails).
simply because of small number of detected emission lines in
our data. Our exposure times were relatively short so in many
cases only Hα and [Nii] are detected, making the N2 diagnos- 4.1.Feii5018ÅpEWdistributionandevolution
tictheonlypossibility.AnadvantageofboththeN2andO3N2
diagnosticsisthattheyareessentiallyunaffectedbyeitherhost- ThetimeevolutionofSNpEWsisshowninFig.3,togetherwith
galaxyextinctionand/orrelativefluxcalibration,duetotheuse thosefromspectralmodels.InFig.4wepresentthesesamemea-
of ratios of emission lines close in wavelength.While both the surementsbutnowwiththeobservationsbinned(withbinsof0–
M13 and Pettini&Pagel (2004) abundances are listed in Ta- 20 days, then 20–30,30–40,40–50,50–60,60–70,70–80,and
ble B.2, we use the M13 values for our analysis given the re- 80–100 days). In both figures the evolution of model pEWs is
calibrationofthediagnosticsincludingadditionalHii-regionTe alsopresented.ThesefiguresshowtheincreaseintimeofpEWs
measurements3. (usingtheconventionthatadeeperabsorptionisdocumentedas
alargerpositivepEW),butalsothelargedispersionbetweendif-
ferentSNeII.Onecanalsoseethedistinctstrengthandevolution
ofFeii5018ÅpEWsfoundwithinmodelspectrafromthefour
4. Results distinctprogenitormetallicities. Theeffectonmodelspectraof
changingpre-SNradii(atafixed,solar,metallicity)canbeseen
Above we have presented two sets of observations: spectral inFig.4(m15mlt1largerradius,andm15mlt3smallerradius).
line Feii 5018Å pEW measurements during the photospheric In Fig. 5 the pEW– time pastexplosiontrendsfromFig. 3
phase of SNe II, and emission line spectral measurements of areagainpresentedbutnowsplitinto 4panels,separatingSNe
SN II host Hii regions, with the latter being used to obtain bytheirpEWsat 50d postexplosion(seebelow fordiscussion
environment oxygen abundances. The distributions of these ofthismeasurement).Itisnowpossibletoobservethetrendsof
are now both presented. Then we proceed to correlate both individualSNeinmoredetail.Thisconfirmsthemonotonicbe-
parameters, and confront model predictions with SN and host haviourofFeii5018ÅpEWswithtimepastexplosionthrough-
Hii-region observations. In addition, we analyse how pEWs outthe‘plateau’phaseofSNeIIevolution(<100dayspostex-
are related to other SN II light-curve and spectral parameters, plosion), in qualitative agreement with models. The plot also
and finally we search for correlations between environment shows that SNe with similar pEWs at 50d evolve in a similar
metallicityandSNIItransientproperties. mannerwithrelativelylowdispersion.
To proceed with our analysis pEWs are required at consis-
tent epochs between SNe. Our epoch of choice is 50d (more
details below). To estimate the pEW at this epoch, interpola-
3 IfweweretousethePettini&Pagel(2004)valuesinsteadthenour tion/extrapolation is needed. This is achieved for SNe II with
resultsandconclusionsremainunchanged. ≥2measurementsavailableandwherein thecase ofextrapola-
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Fig.5.EvolutionofFeii5018ÅpEWswithtime,aspresentedinFig.3,separatedinto4panelssplittingthesamplebytheirFeii5018ÅpEWs
at50d.ThedistributionsgofromlowpEWinpanela)throughtothehighestpEWsampleinpaneld).Theoverallsampleispresentedinblue,
whilethe‘Gold’samplediscussedlaterisplottedinmagenta.(Errorsarenotplottedtoenablebettervisualisationofthetrends.)
tion,aspectrumisavailableat50±10dayspostexplosion.Then diagnostics is problematic, and it is probable that the N2 and
a low order polynomialfit is made to the pEW measurements, O3N2 diagnostics give systematically lower abundances than
andthisisusedtoobtainapEWat50d.WeusetheRMSerror thetrueintrinsicvalues(seee.g.López-Sánchezetal.2012and
ofthisfitastheerrorontheinterpolatedpEW.Thisinterpolation referencestherein).
waspossiblein82cases,andahistogramofthesemeasurements
ispresentedinFig.6.
4.3.SNIIFeii5018ÅpEWsandhostHiiregionabundance
4.2.HostHii-regionabundances In Fig. 8 SN II Feii 5018Å pEWs at 50d are plotted against
the N2 diagnostic on the M13 scale. To test the significance
In Fig. 7 we present the distribution of all SN II emission line
of this trend, and all subsequent correlations, we run a Monte
abundancemeasurementsforboththeN2andO3N2M13diag-
Carlo simulation randomly selecting events from the distribu-
nostics. The N2 distribution has a mean value of 12+log[O/H]
tioninabootstrapwithreplacementsmanner10000times.The
=8.49dex,andamedianof8.52dex.Thedistributionshowsa
meanPearson’s correlationr value is determinedtogetherwith
peakatjustbelow∼8.6dex,andatailouttolowerabundances
its standard deviation using the 10000 random sets of pEW –
withthelowestvalueof8.03dex.TheO3N2distributionhasa
abundances. The lower limit of the chance probability of find-
meanof8.41dex,andamedianvalueof8.44dex.Thedistribu-
ingacorrelation, p,istheninferredusingthesevalues4.Witha
tionshowsapeakat∼8.5dex,arangeof∼0.6dex,andatailout
to lower abundances with the lowest value of 8.06 dex. Using
4 Itisgenerallyconsideredthatforrvaluesbetween0.0and0.2there
asolarvalueof12+log[O/H]=8.69dex(Asplundetal.2009),
iszeroornegligiblecorrelation;between0.2and0.3weakcorrelation;
the O3N2 distribution thus ranges between 0.23 and 0.87Z⊙, between0.3and0.5moderatecorrelation;andabove0.5signifiesstrong
withameanof0.51Z⊙.However,westressthatanydiscussion correlation.The pvaluegivestheprobabilitythatthislevelofcorrela-
of absolute metallicity scale when dealing with emission line tionisfoundbychance.
Articlenumber,page6of21
Andersonetal.:SNeIIasmetallicitytracers
Fig. 6. Histogramof Feii 5018Å pEW measurements for our sample Fig.8.pEWoftheFeii5018Åabsorptionlinemeasuredat50dplotted
ofSNIIspectra,interpolatedto50d.ThepositionofthemeanpEWis against host Hii-region oxygen abundance on theN2 M13 scale. The
indicatedbytheverticaldashedline. dashedlineindicatesthemeanbestfittothedata.Errorbarsonindivid-
ualmeasurementsarethestatisticalerrorsfromlinefluxmeasurements.
ThelargeblackerrorbargivestheN2diagnosticerrorfromM13.
Hαand[Nii])theHβand[Oiii]fluxesneededtocomputeabun-
dances on the O3N2 scale. We are able to do this in 44 cases,
andhereweobtainameanr valueof0.50±0.10,whichgivesa
chanceprobabilityof≤0.7%.Statisticsusingonlytheobserved
distributiongive:r=0.50,p=0.05%.Thecorrelationisshown
inFig.9.WhilethestatisticalsignificanceishigherfortheO3N2
diagnostic(andthereforewe use thatdiagnosticforsubsequent
sub-samples), both of these figures show there is a statistically
significant trend in the direction predicted by models: SNe II
withlargerFeii5018ÅpEWstendtobefoundinenvironments
ofhigheroxygenabundance.Theseobservationalresultshence
agreewith modelpredictions,andmotivatefurtherworkto use
SNe II asenvironmentmetallicity indicators.We also notethat
theRMSerrorsontheM13N2andO3N2diagnostics(asplot-
tedinFigs.8and 9),appeartobelargewhencomparedtothe
spread of values in our plots. Given that we do find evidence
forcorrelation,thissuggeststhetrueprecisionofthosediagnos-
ticsforpredictingHii-regionabundanceisbetterthanthevalues
givenbyM13.
InFig.10weover-plotSNIImodelsatfourdifferentmetal-
licities,togetherwiththeadditionaltwomodelsatsolarmetallic-
Fig.7.M13N2andO3N2abundancedistributions,togetherwiththeir itybutwithdifferentpre-SNradii(D14),ontotheobservedFeii
meanvalues(dashedlines).
5018Å pEW vs. Hii-region abundance plot. The modelmetal-
licitiesareconvertedfromfractionalsolartooxygenabundance
totalof82eventswhichhaveSN IIFeii 5018Å pEWsandN2 using a solar value of 8.69 dex (Asplundetal. 2009). One can
oxygen abundance measurements, we find r = 0.34±0.09, and see thatthe modelsproducea muchsteeper trendthan thatob-
a chance probabilityof finding a correlation of ≤2.4 %. Statis- served.Wealsoseethatmodelswiththesameprogenitormetal-
tics using only the observed distribution give: r = 0.34, p = licitybutwith differentpre-SNradiiproducea rangeofalmost
0.18%.Hence,usingN2wefindamoderatestrengthcorrelation 10Å in Feii 5018Å pEWs. This uncertainty can be reduced
between SN pEW and host Hii-region abundance. It is clearly with a slight time shift (of pEW measurements)or by compar-
observedthatthereis a lackof SNe II with highpEWand low ing at a givencolour(see below for additionalanalysis). How-
abundanceatthebottomrightofFig.8.TherearefewerSNen- ever,ingeneral,SNIIobservationsfavourrelativelylowprogen-
vironmentswherewe wereable toalso measure(inadditionto itorradii(Dessartetal.2013;González-Gaitánetal.2015),and
Articlenumber,page7of21
A&Aproofs:manuscriptno.SNII_Z_EWs_refv_proof
explosions similar to that of the large radius model are proba- This epoch was chosen as it corresponds to when the vast
bly rare in nature.Thereforethe actualuncertaintyin observed majorityofSNeIIarearoundhalfwaythroughthephotospheric
SNe II is probably less than that represented by this range of phase of their evolution. SNe II can show significant tempera-
models.AnotherinterestingobservationfromFig.10isthelack turevariationsatearlytimesandonedoesnotwanttomeasure
of any SN close to the tenth solar model. There is also a lack pEWs when temperature differences could be a significant
ofSNeIIatsuper-solarvalues.Wediscussthis(possiblysmall) factor controlling their strength. Fig. 4 also shows that pEWs
rangeofmetallicitiesprobedbyobservationsbelow. have a rapid increase at early times after first appearing at
∼15 days, and therefore one wants to avoid this region where
non-metallicitysystematicsmaydominatedifferencesinpEWs.
4.3.1. Sub-samples
At much later than 50d some SNe II already start to transition
In Figs. 8 and 9 all SNe II were included irrespective of their from the photospheric phase to radioactively powered epochs.
light-curve or spectral properties. This means that we include Here, pEWs may start to be affected by mixing of He-core
SNe IIwitha widerangeofabsolutemagnitudes,‘plateau’de- material (in addition to the fact that spectral observations
clinerates(s ),andopticallythickphasedurations(OPTd)(and becomemoresparse).We nowtestwhetherthisselectedepoch
2
other SN parameters which differ from one event to the next). isthemostappropriatetomeasurepEWs.
ThemodelsofD14allproduceSNlight-curvesandspectratyp- pEW measurements are interpolated to: 30, 40, 60 and 70
ical of ‘normal’SNe IIP. In addition,those figuresincludedall days post explosion. These values are then correlated against
SN II abundance measurements, including events with abun- host Hii-region abundances on the N2 scale (used to ensure
dancesestimatedfroma regionoftheirhostgalaxiesatsignifi- sufficientstatisticsforvalidcomparisons)andthestrengthofthe
cantdistancesfromexplosionsites.Heretheseissuesarefurther correlationsarecompared.Inaddition,inplaceoftheexplosion
investigated. epoch, pEWs are estimated with respect to ttran: the epoch of
Firstweconstructasub-sampleofeventswhereabundance transition from the initial s1 decline to the slower ‘plateau’ s2
estimationswerecarriedoutlessthan2kpcawayfromtheexplo- phase (see A14 for details of those measurements).We choose
sionsites5.Whenthisisachievedweareleftwith56SNeIIwith theepochttran+20days,whichroughlycoincideswith50d,but
measurements on the N2 scale and 32 on the O3N2 scale. We varies(approximately±10–15days)betweenSNeII.Theresults
againtestforcorrelationusingMonteCarlobootstrappingwith ofthesecomparisonsarepresentedinTable1.Intermsoftime
replacements,andinthecaseofN2acorrelationcoefficientrof postexplosionweseethatthechoiceof50daysisinfactvalid.
0.42±0.10isfound,givingachanceprobability(N=56)of≤1.6 Itsisalsoobservedthat50ddoesjustaswellasttran+20days.A
%.UsingO3N2weobtainr=0.55±0.10anda pvalueof≤1.0 time epoch with respect to OPTd is also investigated(OPTd is
%(N=32).Thelevelofcorrelationthusincreaseswhenweonly the OPtically Thickphase time duration,from explosionto the
include abundance measurements closer to SN explosion sites. end of the ‘plateau’, A14). pEWs are interpolatedto OPTd–30
Thisisto be expected,asSNe II haverelativelyshortlifetimes days(againtocoincideonaveragewitht= ∼50d),andwerun
and are thereforenotexpectedto movesignificantlyfrom their our correlation tests for both this new OPTd sample, and the
birth sites. As one moves away from exact explosion sites en- sameSNeIIbutwithpEWsat50d.Valuesareagaindisplayed
vironmentmetallicitybecomeslessrepresentativeofprogenitor in Table 1, and it is found that using OPTd as the time epoch
abundanceduetospatialmetallicitychangesfromoneregionof is no better than using the explosion epoch. In conclusion the
agalaxytoanother,especiallyintermsofincreasing/decreasing choiceof50dforpEWmeasurementsappearstoberobust.
galacto-centricoffset.
Toproduceasub-sampleof‘normal’SNeIIPcutsaremade Now,inplaceoftime,weinvestigatewhetherastrongercor-
tooursampleintermsoflight-curvemorphologies.SNeIIwith relation exists if pEWs are measured at a colour epoch. As al-
s values≥1.5magper100days,and/orOPTdvalues≤70days ready seen in the D14 spectral models: differences in pre-SN
2
areremovedfromthesample.Usingthesecutsa‘GoldIIP’sam- properties can significantly affect the strength of spectral lines
pleof22SNeIIisformed,whichwouldgenerallybeconsidered (seeFigs.4,10).Togetherwiththemetalabundancewithinthe
typicalSNe IIPby the community.Testing for correlation,this ejecta,theothermaincontributortolineappearanceandsubse-
sub-sample has an r value = 0.68±0.10 and a p value = ≤0.5 quentstrengthisthetemperature/ionisationofthelineformation
%. This correlation is presented in Fig. 11, and shows the in- region.Models correspondingto differentprogenitorradii pro-
crease in strength of correlation as compared to the full O3N2 duceSNe IIwith distincttemperatureevolution.Therefore,ex-
sampleinFig.9 (characterisedbyr =0.50±0.10).Thefastde- aminingobservationalpEWsataconsistentcolour(i.e.temper-
clining (s ≥ 1.5 mag per 100 days) sample presents a lower ature)maybeofinterest.Fig.12presentstheevolutionofpEWs
2
level of correlation (than that of the ‘normal’ SNe IIP): for 15 withSNcolour(usingthesampledefinedbelow).Thisconfirms
SNe, r = 0.57±0.18and p = ≤4.6 %. Both the ‘Gold IIP’ and the behaviour discussed above: pEWs increase with increasing
fast-decliningsamplesshowthe samelineartrendswithintheir SNcolour.
errors.Thissuggeststhat‘normal’SNeIIParebettermetallicity Amajorissuewithanycolouranalysisiscorrectionforex-
indicatorsthantheirfasterdecliningcounterparts. tinction in the line-of-sight within host galaxies. However, as
discussedindetailinFaranetal.(2014b),withoutdetailedmod-
ellingandearly-timedata(seee.g.Dessartetal.2008),thereis
4.3.2. TheepochofpEWmeasurements
nocurrentsatisfactorymethodforaccuratelycorrectingSNe II
for host galaxyreddening.This is particularlypertinentfor the
AbovewepresentedthepEWdistributionandthencorrelations
with Hii-region abundances using Feii 5018Å pEWs at 50d. current study where our goal is to obtain differences in intrin-
siccolours(i.e.assumingSNeIIhavesimilarcoloursduringthe
5 Thislimitissomewhatarbitrary,howeveritremovescaseswhereex- plateauwouldsimplyleadustouseatimeepoch).Anothercom-
tractionsareatasignificantdistancefromexplosionsites,whilemain- plicationisthattheCSPandpreviousCATSetal.samplesdonot
tainingasufficientnumberofeventstoenableastatisticallysignificant havethesamefilterobservations.
analysis. To continue the investigation we proceed in the following
Articlenumber,page8of21
Andersonetal.:SNeIIasmetallicitytracers
Fig.9.SameasFig.8,butnowfortheO3N2diagnosticontheM13scale.Thedashedlineindicatesthemeanbestfittothedata.Errorbarson
individual measurementsarethestatisticalerrorsfromlinefluxmeasurements. ThelargeblackerrorbargivestheO3N2diagnosticerrorfrom
M13.
manner.Tobegin,allSNeIIhavinghostgalaxyA valuespub- forcorrelationwefind,forN=17,r=0.69±0.12andp≤1.7%,
V
lished in A14 higher than 0.1 magnitude, or where no A es- while forthe same SNe II butwith pEWs measuredat 50d we
V
timate was possible because either the pEW of narrow ISM findr =0.61±0.16and p≤7.0%. Thissuggeststhatinusinga
sodium lines was higher than 1Å, or where the upper limit of colourepochinplaceoftime,onefurtherremovesSNsystemat-
this quantity was higher than 1Å, are removed from the sam- icsandstrengthensthecaseforSNeIItobeusedasmetallicity
ple.Wethenassumethattherestofthesampleiseffectivelyfree indicators.
of significant host galaxy reddening. For all SNe II within the
CSPsamplewecreateV −icolourcurves,whilefortheCATS
4.4.CorrelationsbetweenpEWsandotherSNIIparameters
et al. samples V −I colourcurves(bothcorrected for MW ex-
tinction) are produced6. Low-orderpolynomialsare fit to these
Here we correlate pEWs at 50d with the SN II light-curve
colour curvesand we then interpolate to measure the colour at
and spectral parameters presented by A14 and Gutiérrezetal.
50d.Foreachsample(CSP/CATSetal.)amean50dayV−i/V−I
(2014). The statistical significance of correlations between
colouriscalculated.ForeachindividualSNweproceedtomea-
pEWsandtheseparametersarelistedinTable2.A fulldiscus-
sure a pEW for Feii 5018Å at the correspondingmean colour
sion of these correlations, together with figures and discussion
epoch.Thismethodnegatesanyneed to converti-bandmagni-
oftheirimplicationsforourunderstandingofSNeIIexplosions
tudesinto I, or vice versa,and is valid if it is assumedthat the
and progenitors will be left for a future publication (Gutíerrez
CSPandCATSetal.samplesaredrawnfromthesameunderly-
etal.inpreparation).However,therearesomeinterestingtrends
ingSNIIdistribution.
seeninTable2andwebrieflydiscussthosenow.
InFig.13weplotFeii5018ÅpEWsatthesecolourepochs The absolute magnitude at maximum light, M , of SN II
against host Hii-region abundance on the O3N2 scale. Testing isfoundtostronglycorrelatewiththeFeii5018ÅmpaExWat50d.
Indeed, while the pEW also shows correlation with M and
6 Note,CATSetal.photometrywaspublishedinGalbanyetal.2016, end
M (all in the sense that brighter SNe II have lower pEWs at
whileCSPopticalphotometrywillbepublishedinAndersonetal.(in tail
preparation), and a full colour analysis of the latter concentrating on a consistenttime epoch),the strengthof the correlationis less.
hostgalaxyextinctionwillbeprovidedindeJaegeretal.(inprepara- This is in agreement with A14, where Mmax was shown to be
tion). a more important parameter in understanding the diversity of
Articlenumber,page9of21
A&Aproofs:manuscriptno.SNII_Z_EWs_refv_proof
Table1.EpochofpEWmeasurements:testingthelevelofcorrelationbetweenFeii5018ÅpEWsandhostHii-regionabundanceswhendifferent
epochs for pEWmeasurements areused. Inthefirstcolumn theepoch islisted,followed bythenumber of SNeIIavailableincolumn 2.The
Pearson’smeanrvalueisthengiventogetherwiththestandarddeviation.Finallyincolumn4thechanceprobabilityoffindingacorrelationis
listed.
Epoch NSNeII r p
Daysafterexplosionepoch
30 90 0.30±0.09 ≤4.7%
40 83 0.32±0.09 ≤3.6%
50 82 0.35±0.09 ≤1.8%
60 75 0.30±0.11 ≤10.3%
70 68 0.38±0.10 ≤2.1%
Withrespecttot
tran
+20 39 0.24±0.16 ≤63%
Samesamplebutat50d
50 39 0.30±0.15 ≤36%
WithrespecttoOPTd
–30 55 0.34±0.11 ≤9.1%
Samesamplebutat50d
50 55 0.35±0.11 ≤7.8
Table2.StatisticsofcorrelationsbetweenFeii5018ÅpEWsandSNIIlight-curveandspectralparameters.InthefirstcolumntheSNIIparameter
islisted.(M isthemaximumV-bandabsolutemagnitude;M themagnitudeattheendofthe‘plateau’,M themagnitudeatthestartof
max end tail
theradioactivetail;s theinitialdeclinefrommaximum;s thedeclinerateduringthe‘plateau’;s thedeclinerateoftheradioactivetail;56Ni
1 2 3
thesynthesised nickel mass, with56Ni*including upper limitcalculations; Pdthedurationfromtheinflectionpoint of s and s totheendof
1 2
the‘plateau’;OPTdthedurationfromexplosiontotheendofthe‘plateau’;a/etheratioofthepEWofabsorptiontoemissionofHα;andHα
vel
theFWHMvelocityofHα.)ThisisfollowedbythenumberofSNeIIincolumn2.ThePearson’smeanrvalueisthengiventogetherwiththe
standarddeviation.Finallyincolumn4thechanceprobabilityoffindingacorrelationislisted.
LC/specparameter NSNeII r p
M 77 0.64±0.09 ≤2.2×10−4%
max
M 79 0.47±0.11 ≤0.11%
end
M 29 0.48±0.14 ≤7.1%
tail
s 20 –0.48±0.15 ≤16%
1
s 77 –0.41±0.13 ≤1.4%
2
s 24 –0.50±0.24 ≤22%
3
56Ni 13 –0.46±0.23 ≤45%
56Ni* 36 –0.54±0.10 ≤0.72%
Pd 17 0.26±0.31 ≤100%
OPTd 24 0.12±0.14 ≤100%
a/e 43 0.60±0.10 ≤0.064%
Hα 24 –0.61±0.09 ≤0.92%
vel
SNe II than the end of ‘plateau’ magnitude M . The decline relation is found between abundance and any SN II parame-
end
rates, s , s and s , all show some degree of correlation with ter, except that with metal line pEWs. In Figs. 14, 15, 16 we
1 2 3
theFeii5018ÅpEWinthedirectionthatslowerdeclinershave show host Hii-region abundance plotted against Mmax, s2, and
higherpEWs.Interestingly,whenweincludeupperlimits,56Ni OPTd respectively. One can see that no trends appear. In Ta-
massesshowastrongcorrelationwithpEWs:SNeIIwhichpro- ble 3 the resultsof statistical tests for trendsbetweenthese pa-
duce more nickel have lower pEWs. The time duration OPTd, rametersandhostHii-regionabundance,togetherwithallother
i.e. the timebetweenexplosionandthe endof the‘plateau’to- SN II light-curveand spectral propertiespresented in A14 and
gether with Pd (t to end of ‘plateau’) show zero correlation Gutiérrezetal. (2014) are shown. In addition, we repeat the
tran
withpEWs.Finally,thespectralparametersa/e(theratioofthe statistics for the correlation between SN II Feii 5018Å pEWs
pEWofabsorptiontoemissionofHα)andtheFWHMvelocity and host Hii region abundance already presented above. It is
of Hα (see Gutiérrezetal. 2014 for more details) both show a clear that the only (thus far measured/presented)SN II param-
strong correlation with pEWs. SNe II with large pEWs at 50d eter which shows any correlation with environmentmetallicity
havelargera/evaluesandsmallervelocities. istheFeii5018ÅpEW.
TheseresultssuggestthatthediversityofSNIIpropertiesin
the currentsample doesnotstem from variationsin metallicity
4.5.TheinfluenceofmetallicityonSNIIdiversity –eithertherangeinmetallicityistoosmalltomatter(whichis
likely)orsomeotherstellarparameter(e.g.mainsequencemass)
WetestforcorrelationbetweenSNIIhostHii-regionabundance ismoreinfluential.
and light-curve and spectral parameters. No evidence for cor-
Articlenumber,page10of21