Spectroscopy of the Type Ia supernova 2011fe past 1000 days
S. Taubenberger, N. Elias-Rosa, W. E. Kerzendorf, S. Hachinger, J. Spyromilio, C. Fransson, M. Kromer, A. J. Ruiter, I. R. Seitenzahl, S. Benetti, E. Cappellaro, A. Pastorello, M. Turatto, A. Marchetti
aa r X i v : . [ a s t r o - ph . S R ] D ec Mon. Not. R. Astron. Soc. , 1–6 (2014) Printed 13 July 2018 (MN L A TEX style file v2.2)
Spectroscopy of the Type Ia supernova 2011fe past 1000 days
S. Taubenberger , , N. Elias-Rosa , , W. E. Kerzendorf , S. Hachinger , J. Spyromilio ,C. Fransson , M. Kromer , A. J. Ruiter , , I. R. Seitenzahl , , S. Benetti , E. Cappellaro ,A. Pastorello , M. Turatto & A. Marchetti European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany Max-Planck-Institut f¨ur Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy Institut de Ci`encies de l’Espai (CSIC-IEEC), Campus UAB, Torre C5, 2a planta, 08193 Barcelona, Spain Department of Astronomy and Astrophysics, University of Toronto, 50 Saint George Street, Toronto, ON M5S 3H4, Canada Universit¨at W¨urzburg, Lehrstuhl f¨ur Astronomie / Lehrstuhl f¨ur Mathematik IX, Emil-Fischer-Str. 31 / 30, 97074 W¨urzburg, Germany The Oskar Klein Centre, Department of Astronomy, Stockholm University, Albanova, 10691 Stockholm, Sweden Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611, Australia ARC Centre of Excellence for All-sky Astrophysics (CAASTRO) INAF - IASF Milano, Via E. Bassini 15, 20133 Milano, Italy
Accepted 2014 December 13. Received 2014 December 10; in original form 2014 November 26
ABSTRACT
In this letter we present an optical spectrum of SN 2011fe taken 1034 d after the explosion,several hundred days later than any other spectrum of a Type Ia supernova (disregarding light-echo spectra and local-group remnants). The spectrum is still dominated by broad emissionfeatures, with no trace of a light echo or interaction of the supernova ejecta with surroundinginterstellar material. Comparing this extremely late spectrum to an earlier one taken 331 d af-ter the explosion, we find that the most prominent feature at 331 d – [Fe
III ] emission around4700 ˚A – has entirely faded away, suggesting a significant change in the ionisation state.Instead, [Fe II ] lines are probably responsible for most of the emission at 1034 d. An emis-sion feature at 6300–6400 ˚A has newly developed at 1034 d, which we tentatively identifywith Fe I λ , [Fe I ] λλ , or [O I ] λλ , . Interestingly, the features in the1034-d spectrum seem to be collectively redshifted, a phenomenon that we currently have noconvincing explanation for. We discuss the implications of our findings for explosion models,but conclude that sophisticated spectral modelling is required for any firm statement. Key words: supernovae: general – supernovae: individual: SN 2011fe – line: identification
For several years after the explosion, the luminosity of Type Iasupernovae (SNe Ia) is powered by the decay of radioactive nu-clei synthesised in the explosion. At the beginning, the ejectaare still optically thick, and the radiation is released on photon-diffusion time scales. About 100–200 d later, the ejecta have ex-panded enough to become transparent for optical photons. Duringthe now-commencing nebular phase, the radioactive-heating andradiative-cooling rates are similar, making the bolometric luminos-ity evolution a good tracer of the radioactive energy deposition.Cooling during the nebular phase is mostly accomplished byforbidden-line emission: low-lying levels that are still populated atsuch late epochs often have no permitted transition to the groundstate, and collisional de-excitation is strongly suppressed owing tothe low density. The dominant coolants in nebular SNe Ia are iron-group elements, reflecting the composition of the inner ejecta. Inparticular, optical spectra of SNe Ia around one year after the ex-plosion show a characteristic pattern of [Fe II ] and [Fe III ] lines. The past decade has led to a wealth of high-quality late-time spectra of SNe Ia. This made it possible to study nucle-osynthesis and geometry effects in SNe Ia in unprecedented detail(e.g. Kozma et al. 2005; Maeda et al. 2010b,a; Mazzali et al. 2011;Blondin et al. 2012; Silverman et al. 2013; Taubenberger et al.2013a). However, all these studies concentrated on epochs between200 and 400 d after the explosion. Beyond 400–500 d, our knowl-edge on the spectroscopic evolution of SNe Ia is limited. When-ever a SN Ia was bright enough to perform spectroscopy at suchlate phases, as e.g. in the cases of SNe 1991T or 1998bu, it wasdominated by a light echo (Schmidt et al. 1994; Cappellaro et al.2001, respectively), prohibiting the study of actual late-time emis-sion from the SN ejecta. The few model calculations that exist for those phases also A spectrum of SN 1972E obtained ∼
700 d after maximum light(Kirshner & Oke 1975) suffers from low resolution, poor signal-to-noiseratio (S/N) and limited wavelength coverage. A spectrum of SN 2005cfc (cid:13)
Taubenberger et al. suffer from numerous uncertainties. To capture the relevant nebu-lar physics (e.g. McCray 1993; Fransson 1994), non-thermal pro-cesses have to be accurately modelled, which is sometimes impos-sible due to missing or inaccurate atomic data. Moreover, owingto the low ejecta densities, which lead to increased recombina-tion time scales, departures from steady state arise. As a conse-quence, ionisation freeze-out may occur (Fransson & Kozma 1993;Fransson et al. 1996). For this reason, a fully time-dependent treat-ment becomes essential after ∼
500 d (Sollerman et al. 2004) to pre-dict the correct ionisation state of the ejecta. Spectra observed atepochs > d could greatly help to assess the correctness ofmodel calculations, and hence provide a big step forward in ourunderstanding of both nebular physics and SN Ia explosions.With SN 2011fe (Nugent et al. 2011) this goal is now for thefirst time in reach. Its proximity ( d = 6 . Mpc; Shappee & Stanek2011), low dust extinction and relatively uncrowded environmentmake SN 2011fe the ideal object to push observations to new lim-its. Kerzendorf et al. (2014) recently reported multi-band opticalphotometry of SN 2011fe between 900 and 950 d after the explo-sion, concluding that the light-curve decline is consistent with ra-dioactive decay, and that there is no evidence for positron escape,an infrared catastrophe (IRC; Axelrod 1980), dust formation, or alight echo. The derived colours were still remarkably blue. Here,we present a spectrum of SN 2011fe taken ∼
100 d later – the firstnebular spectrum of a SN Ia ever obtained at more than 1000 d afterits explosion – and compare it with a spectrum taken after ∼ A spectrogram of SN 2011fe was obtained on 2012 July 20.02 (UTdates are used throughout this letter), 331 rest-frame days after itsinferred explosion on 2011 August 23.687 (Nugent et al. 2011),with the OSIRIS spectrograph at the Gran Telescopio Canarias(GTC). Two grisms (R1000B and R1000R) were used, with an ex-posure time of 300 s for each grism, and a 1.0-arcsec slit alignedalong the parallactic angle. Basic CCD reductions and a variance-weighted extraction of the spectra were carried out within
IRAF .The wavelength calibration was accomplished using arc-lamp ex-posures and checked against night-sky lines. A spectrophotometricstandard star observed during the same night as the SN was usedfor flux calibration and telluric-feature removal.SN 2011fe was again targeted on 2014 June 23.22, 1034 rest-frame days after the explosion, when the SN had faded to i ′ ∼ .The observations were carried out at the Large Binocular Telescope(LBT), equipped with the MODS1 dual-beam spectrograph, dur-ing Italian / INAF time. Three exposures of 3600 s each were takenin good seeing conditions (0.6 to 1.0 arcsec FWHM) through a1.0-arcsec slit, aligned along the mean parallactic angle over thetime of the observations (see Fig. 1). A dichroic split the lightbeam at 575 nm, and the G400L and G670L gratings were usedas dispersers for the blue and red channel, respectively. The datawere pre-reduced using the modsCCDRed package , and the ex-traction and calibration of the spectra followed the same scheme taken 614 d after maximum light (Wang et al. 2009) turns out to be that ofan M-type star upon closer inspection. IRAF is distributed by the National Optical Astronomy Observatory,which is operated by the Association of Universities for Research in Astron-omy under cooperative agreement with the National Science Foundation. Figure 1. × -arcsec section of an i ′ -band image, taken with theGemini-N Telescope + GMOS-N four days after our LBT spectrum. Northis up, east to the left. The width and orientation of the 1-arcsec slit used forthe LBT observations are indicated. as described for the GTC data. Eventually, the individual medium-resolution spectra were combined and rebinned to 5- ˚A bins. In Fig. 2 our nebular spectra of SN 2011fe are presented. At +331dthe line identification is relatively straightforward and follows ear-lier work in this area (for example, see Maeda et al. 2010b). Thestrong emissions between 4000 and 5500 ˚A are well fitted by non-LTE excitation of iron by a ∼ III ],with only a small contribution from [Fe II ]. The 5250- ˚A feature isa blend of [Fe III ] and [Fe II ]. Other features, notably the [Fe II ] λ / [Ni II ] λ blend around 7200 ˚A, are marked in Fig. 2.By day 1034 dramatic changes have occurred. The formerlyvery prominent [Fe III ] line at 4700 ˚A has disappeared, while the4400- and 5300- ˚A features have preserved their line ratios and nowdominate the flux in the optical regime. The 7200- ˚A feature hasweakened relative to the strong emission in the 5000- ˚A region. Thecomparison of the 1034- and 331-d spectra naturally leads to theconclusion that the ionisation structure of the ejecta has changedand only little Fe
III is present.However, the spectrum is difficult to reconcile with thermalexcitation. The 7200- ˚A feature has a contribution from Fe II mul-tiplet 14F (a F – a G) with an upper level at ∼ II multiplets [e.g. 6F(a D – b F), 18F (a F – b P), 19F (a F – a H)] with upper lev-els near 2.5 eV. Only a hot ( ∼ I multiplets 2F (a D –a P) and 3F (a D – a P2) to the 5300- ˚A feature, and by multi-plets 4F (a D – b F2) and 6F (a D – b P) to the 4400- ˚A feature.Again, the observed ratio of the 4400- to the 5300- ˚A feature is notin agreement with thermal excitation at any realistic temperature. c (cid:13) , 1–6 pectroscopy of SN 2011fe past 1000 days Figure 2.
Nebular spectra of SN 2011fe, along with an attempted line iden-tification. A smoothed version (with a boxcar of ∼ − ; solid redline) is overlaid on the 1034-d spectrum to facilitate the assessment of lineprofiles. The inset enlarges the region of the possible Fe I or [O I ] feature. Recombination or non-thermal excitation processes must thereforeprevail.The series of cobalt lines between 5800 and 6700 ˚A, stillfairly prominent in the 331-d spectrum, is no longer detected at1034 d. Given that most of the cobalt in SNe Ia is Co, whichdecays with a half-life of 77 d, the observed fading of the cobaltlines is expected. However, in the same region a new, broad emis-sion feature has emerged (see inset of Fig. 2). It is centred around6360 ˚A, and might be identified with some combination of Fe I λ , [Fe I ] λλ , and [O I ] λλ , . Given thedominance of [Fe II ] lines in the spectrum and our proposal that[Fe I ] contributes in the 4000–5500- ˚A range, the identification withFe I lines may appear more natural. On the other hand, if [O I ] canbe confirmed through spectral modelling, this would be the thirddetection of [O I ] λλ , in the nebular spectrum of a ther-monuclear SN, after SN 1937C (Minkowski 1939) and SN 2010lp(Taubenberger et al. 2013b), though this time only at a much laterepoch.There is no sign of narrow or intermediate-width emissionlines that might hint at interaction with interstellar material (ISM).The transition of SN 2011fe into the remnant phase, when the emis-sion becomes dominated by the shock originating from ejecta-ISMcollisions, has not yet started. In particular, no narrow H α line isdetected. Such a line might be expected in nebular spectra of single-degenerate explosions, where H-rich material is stripped from anon-degenerate companion star upon the impact of the SN ejecta(Marietta et al. 2000; Pakmor et al. 2008; Liu et al. 2012). Thus far, H α arising from stripped material has never been detected in neb-ular SN Ia spectra (Mattila et al. 2005; Leonard 2007). With our1034-d spectrum, we now extend the series of non-detections tomuch later epochs. Though the presence of a very weak H α linecannot be excluded because of S/N limitations, our non-detectionis in line with the absence of narrow H α in our 331-d spectrumand in an even higher-S/N spectrum of SN 2011fe taken 275 d after B -band maximum (Shappee et al. 2013).Kerzendorf et al. (2014) discussed the possibility of a lightecho, as in their photometry taken about 950 d after the explosionSN 2011fe appeared bright and blue. They argued, however, thata strong light echo – as e.g. observed in SNe 1991T and 1998bu(Schmidt et al. 1994; Cappellaro et al. 2001, respectively) – wasunlikely, since the observed colours did not agree with those ofSNe Ia around maximum light, as one would expect for a lightecho. With our 1034-d spectrum we can now finally rule out thepossibility of a light echo, since no (pseudo-) continuum or P-Cygnifeatures are detected. Once a SN has turned transparent to optical photons, emission-line profiles probe the underlying emissivity distribution in theejecta. The latter depends on the relative location of coolants andradioactive material, and on the mean free paths of γ -rays andpositrons. During the epochs under consideration in this letter( > d after the explosion), the ejecta are largely transparent to γ -rays, and the energy deposition is dominated by positrons andelectrons (Milne et al. 2001; Seitenzahl et al. 2009). Studies of thelate-time bolometric luminosity of SNe Ia (Cappellaro et al. 1997;Leloudas et al. 2009; Kerzendorf et al. 2014) have argued for al-most complete positron trapping even as late as 900 d, suggesting arather short mean free path of positrons, and a nearly in-situ deposi-tion of the radioactive-decay energy during the positron-dominatedphase.Detailed studies of nebular optical and infrared emission-lineprofiles in SNe Ia have been carried out in the past (Mazzali et al.1998; Motohara et al. 2006; Gerardy et al. 2007; Maeda et al.2010b,a). An interesting result of such studies was that certainemission lines (e.g. [Fe II ] λ and [Ni II ] λ ; Maeda et al.2010b) showed significant blue- or redshifts in different SNe,which was interpreted as a viewing-angle effect. Maeda et al.(2010a), finally, found a correlation between the post-maximum ve-locity gradient in Si II λ (Benetti et al. 2005) and the shift ofnebular [Fe II ] λ and [Ni II ] λ lines. In our 331 d spec-trum of SN 2011fe we find both lines to be blueshifted, the [Fe II ] λ line by almost 1000 km s − , the [Ni II ] λ line by 1100km s − . This meets the expectations, given that SN 2011fe is alow-velocity-gradient SN in the Benetti et al. (2005) classificationscheme (e.g. Pereira et al. 2013).In the 1034-d spectrum the S/N is insufficient to directly mea-sure accurate positions for all except the two strongest emissionlines. However, as noted in the previous section, the 331-d and1034-d spectra appear to have several [Fe II ] features in common,and so a superposition of the two spectra (Fig. 3, top panels) canbe instructive to determine changes in the line profiles or positions.From Fig. 3 it is immediately evident that there seems to be a globaloffset in several (if not all) common features, in the sense that the1034-d spectrum appears globally redshifted with respect to the331-d spectrum. The shift amounts to 4200 km s − , as inferredfrom a cross-correlation of the peaks in the 4000–5500 ˚A region,with an estimated uncertainty of ± km s − . That this shift is c (cid:13) , 1–6 Taubenberger et al.
Figure 3.
Top left panel:
Superposition of the 1034-d and 331-d spectra of SN 2011fe, scaled arbitrarily in flux. A wavelength offset seems to be present in allfeatures that can be identified in both spectra.
Top right panel:
The match of features is substantially improved if a wavelength shift corresponding to − km s − is applied. Bottom panel:
No offset is present in the night-sky spectra. real is demonstrated in the bottom panel of Fig. 3, where no offsetis seen in the night-sky spectra of the two observations.Following the paradigm that nebular spectra probe the ejectageometry, the observed shift might be understood if the emittingregions of the ejecta were different at 331 and 1034 d. At 331 d theenergy deposition is dominated by positrons from Co decay, at1034 d by electrons and X-rays from Co decay (Seitenzahl et al.2009; R¨opke et al. 2012). A spatial separation of Co and Cocould thus lead to the observed effect. However, explosion models(e.g. Seitenzahl et al. 2013) suggest that Co and Co are syn-thesised co-spatially, making this explanation unlikely. The same istrue for the attempt to explain the observed time evolution by resid-ual opacity in the core of the ejecta at 331 d. First, a sufficientlylarge optical depth to produce line shifts of 4000 km s − almostone year after the explosion is unreasonable. Second, the lines inthe 331-d spectrum are close to their rest-frame position, so that wedo not have to explain a blueshift at 331 d, but a redshift at 1034 d.Since the affected features are blends (actually often multi-plets), changes in the ionisation and excitation conditions mightlead to a strengthening or weakening of individual constituents, re-sulting in an effective shift of the entire blend. Also, if there is in-deed a significant contribution by lines from Fe I at 1034 d, wave-length shifts compared to the 331-d spectrum are to be expected.However, it appears rather unlikely that any of these effects leadsto the same offset in all features throughout the spectrum, thoughultimately this has to be verified by accurate spectral modelling. Comparing the 1034-d spectrum of SN 2011fe to synthetic nebu-lar spectra would be extremely worthwhile to infer details aboutthe composition, ionisation and excitation state of the ejecta. Un-fortunately, not many model calculations with at least broad-bandspectral information have ever been performed for SNe Ia at sucha late phase. One of the most extended non-grey time-dependentmodel calculations for SNe Ia to date is that by Leloudas et al.(2009). Their synthetic
UBVRIJHK -band light curves of theW7 model (Nomoto et al. 1984) show a rapid decline starting at ∼
500 d, which can be attributed to the onset of an IRC (Axelrod1980). Such an IRC occurs when the temperature drops below ∼ , or if the observed lines areactually recombination lines and hence not excited thermally.As already mentioned in Section 3.1, the 1034-d spectrum The critical temperature for an IRC is density-dependent; local densityenhancements in form of clumping may significantly postpone the IRC.c (cid:13) , 1–6 pectroscopy of SN 2011fe past 1000 days of SN 2011fe shows a broad (FWHM ∼
12 000 km s − ) emis-sion feature centred at ∼ I λ , [Fe I ] λλ , and [O I ] λλ , are possible identifications.Interpreted as [O I ], the feature would be redshifted by ∼ − . If this identification is correct, it has important conse-quences for the preferred explosion scenario for SN 2011fe in par-ticular and – given the conception of SN 2011fe as a perfectly‘normal’ SN Ia – for the entire SN Ia class. To produce late-time[O I ] emission with the given line profile, oxygen has to be presentin the inner part of the ejecta (the line profile is not flat-topped,which disfavours emission from a shell), which is fulfilled onlyfor a small subset of SN Ia explosion models (Taubenberger et al.2013b), notably violent mergers (Pakmor et al. 2012; Kromer et al.2013). However, Jerkstrand et al. (2011) found that at very latephases, when thermal collisional excitation of [O I ] λλ , was no longer possible, even in the arguably much more oxygen-rich SN 1987A the emission feature near 6300 ˚A was dominated byFe I recombination lines rather than [O I ]. Whether these conditionsare met in SN 2011fe at 1034 d, where we still observe relativelyhigh-excitation [Fe II ] lines, has to be verified by detailed simula-tions of the plasma state. We have presented two optical spectra of SN 2011fe taken 331 and1034 d after explosion. At 1034 d the emission still comes fromthe nebular SN ejecta, with no signs of a light echo or interac-tion with interstellar material. Nonetheless, strong changes haveoccurred compared to the early nebular phase. The most strikingof these is the complete fading of the 4700- ˚A [Fe
III ] emission –the by far most prominent feature at 331 d – which we attribute to adecrease of the ionisation state. [Fe II ] features, on the contrary, canstill be identified. A weak, broad emission feature is now presentat ∼ I , orto [O I ] λλ , , which would have important consequencesfor explosion scenarios.From the features that the 331-d and 1034-d spectra seem tohave in common (mostly [Fe II ] blends) a relative wavelength shiftcan be derived, in the sense that at 1034 d all features appear tobe redshifted by ∼ − compared to the earlier epoch. Wecurrently have no convincing explanation for this unexpected be-haviour, and it remains unclear whether the origin of this shift isgeometric, optical-depth-related, or a conspiracy of atomic physics.The detection of prominent emission lines in the 4000–5000 ˚Arange, combined with the late-time luminosity of SN 2011fe re-ported by Kerzendorf et al. (2014), seems to disfavour the idea thatan IRC has taken place in the bulk of the ejecta. In contrast, modelcalculations predict the onset of an IRC with strong observable con-sequences already at 500 d (Leloudas et al. 2009). Whether this dis-crepancy hints at an inadequacy of the W7 explosion model usedfor those calculations, or at shortcomings in the atomic data andnebular-physics treatment, has to be tested in future modelling ef-forts. The 1034-d spectrum of SN 2011fe presented here providesthe perfect benchmark for such modelling. ACKNOWLEDGEMENTS
Observations were carried out using the Gran Telescopio Canarias(GTC), installed in the Spanish Observatorio del Roque de losMuchachos, on the island of La Palma, and the Large Binocular Telescope (LBT) at Mt. Graham, AZ. The authors are grateful tothe LBT-Italy consortium for making these ground-breaking DDTobservations possible, and would like to thank the telescope oper-ators at LBT, and the support astronomers at GTC, for their com-mitment. We are also grateful to our referee, Jeffrey Silverman, forhis careful reading of the manuscript and his helpful comments.ST is supported by the Transregional Collaborative ResearchCentre TRR 33 ‘The Dark Universe’ of the DFG. NER acknowl-edges support from the European Union Seventh Framework Pro-gramme (FP7/2007-2013) under grant agreement n. 267251 (As-troFIt). SH is supported by an ARCHES award. Parts of this re-search were conducted by the Australian Research Council Cen-tre of Excellence for All-sky Astrophysics (CAASTRO), throughproject n. CE110001020, and by ARC Laureate Grant FL0992131.SB, EC, AP and MT are partially supported by PRIN-INAF 2011with the project ‘Transient Universe: from ESO Large to PESSTO’.
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