Early Observations of the Type Ia Supernova iPTF 16abc: A Case of Interaction with Nearby, Unbound Material and/or Strong Ejecta Mixing
A. A. Miller, Y. Cao, A. L. Piro, N. Blagorodnova, B. D. Bue, S. B. Cenko, S. Dhawan, R. Ferretti, O. D. Fox, C. Fremling, A. Goobar, D. A. Howell, G. Hosseinzadeh, M. M. Kasliwal, R. R. Laher, R. Lunnan, F. J. Masci, C. McCully, P. E. Nugent, J. Sollerman, F. Taddia, S. R. Kulkarni
DDraft version October 16, 2018
Typeset using L A TEX twocolumn style in AASTeX61
EARLY OBSERVATIONS OF THE TYPE IA SUPERNOVA iPTF 16abc:A CASE OF INTERACTION WITH NEARBY, UNBOUND MATERIAL AND/OR STRONG EJECTA MIXING
A. A. Miller,
1, 2
Y. Cao, A. L. Piro, N. Blagorodnova, B. D. Bue, S. B. Cenko,
7, 8
S. Dhawan, R. Ferretti, O. D. Fox, C. Fremling, A. Goobar, D. A. Howell,
12, 13
G. Hosseinzadeh,
12, 13
M. M. Kasliwal, R. R. Laher, R. Lunnan, F. J. Masci, C. McCully,
12, 13
P. E. Nugent,
15, 16
J. Sollerman, F. Taddia, and S. R. Kulkarni Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy,Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA The Adler Planetarium, Chicago, IL 60605, USA eScience Institute and Astronomy Department, University of Washington, Seattle, WA 98195 The Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MD 20771, USA Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA The Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden Las Cumbres Observatory, Goleta, CA 93117, USA Physics Department, University of California, Santa Barbara, CA 93106, USA Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA University of California – Berkeley, Berkeley, CA 94720, USA (Received 2017 August 23; Revised 2017 November 23; Accepted 2017 December 5; Published 2018 January 11)
Submitted to ApJABSTRACTEarly observations of Type Ia supernovae (SNe Ia) provide a unique probe of their progenitor systems and explosionphysics. Here we report the intermediate Palomar Transient Factory (iPTF) discovery of an extraordinarily youngSN Ia, iPTF 16abc. By fitting a power law to our early light curve, we infer that first light for the SN, that is whenthe SN could have first been detected by our survey, occurred only 0 . ± . . days before our first detection. In the ∼
24 hr after discovery, iPTF 16abc rose by ∼ (cid:38) II absorption, which disappears after ∼ B − V ) colors of iPTF 16abc are blue and nearly constant in the days after explosion. We show that our early observationsof iPTF 16abc cannot be explained by either SN shock breakout and the associated, subsequent cooling or the SNejecta colliding with a stellar companion. Instead, we argue that the early characteristics of iPTF 16abc, including(i) the rapid, near-linear rise, (ii) the nonevolving blue colors, and (iii) the strong C II absorption, are the result ofeither ejecta interaction with nearby, unbound material or vigorous mixing of radioactive Ni in the SN ejecta, or acombination of the two. In the next few years, dozens of very young normal
SNe Ia will be discovered, and observationssimilar to those presented here will constrain the white dwarf explosion mechanism.
Corresponding author: A. A. [email protected] a r X i v : . [ a s t r o - ph . H E ] M a y Miller et al.
Keywords: methods: observational — supernovae: general — supernovae: individual (iPTF 16abc;SN 2011fe) — surveys INTRODUCTIONAlthough Type Ia supernovae (SNe Ia) have been ex-tensively used as standardizable candles, their progeni-tor systems and explosion physics are still debated (see arecent review by Maoz et al. 2014). Extremely detailedobservations in the hours to days after explosion providea promising avenue to further constrain this problem.While the shock breakout of an SN Ia occurs on a sub-second timescale, the subsequent quasi-adiabatic expan-sion and cooling of the unbound ejecta produces ther-mal emission that can be used to infer the radius ofthe exploding star (Piro et al. 2010; Rabinak & Wax-man 2011). Comparing models of this cooling emissionto the earliest-phase data of SN 2011fe, Bloom et al.(2012) concluded that the explosion came from a starwith R ∗ (cid:46) . R (cid:12) , where R (cid:12) is the solar radius. Com-bining the radius constraint with the measured ejectamass, Bloom et al. derive the mean density of the pro-genitor star, confirming that at least some Type Ia SNecome from compact and degenerate stars.Early-phase observations of SNe Ia from a white dwarf(WD)+nondegenerate binary may detect excess emis-sion, relative to most SNe Ia, due to the collision of theSN ejecta with the nondegenerate companion (Whelan& Iben 1973; Kasen 2010). This excess emission wasfirst detected in iPTF 14atg (Cao et al. 2015), a low-velocity SN Ia with a significant and declining ultraviolet(UV) pulse detected within a few days of the SN explo-sion. This UV pulse is best interpreted as an SN ejecta–companion collision (but see also Kromer et al. 2016;Noebauer et al. 2017). While such emission requiresa favorable geometric alignment and is only expectedin (cid:46)
10% of SNe Ia (Kasen 2010), many studies havesearched for signatures of an ejecta–companion interac-tion, typically resulting in nondetections (e.g., Haydenet al. 2010a; Bianco et al. 2011; Foley et al. 2012; Bloomet al. 2012; Olling et al. 2015; Zheng et al. 2013; Goobaret al. 2015; Shappee et al. 2016b; Im et al. 2015). Possi-ble exceptions include SN 2012cg, which exhibited excessblue emission in its early-phase light curve (Marion et al.2016; though for an interpretation that does not invokeejecta–companion interaction, see Shappee et al. 2016a),and SN 2017cbv, which shows a clearly resolved “bump”in the early
U Bg light curves (Hosseinzadeh et al. 2017).Interaction is not limited to systems with a nondegen-erate companion, however, as WDs enshrouded in dif-fuse material following a binary merger (e.g., Levanon et al. 2015) or expanded owing to a pre-explosion pulsa-tion can give rise to ejecta-interaction signatures (e.g.,Dessart et al. 2014). Models of this scenario naturallyproduce C II absorption that is comparable in strengthto Si II in the days after explosion (Dessart et al. 2014),as was observed in SN 2013dy (Zheng et al. 2013) andSN 2017cbv (Hosseinzadeh et al. 2017).The vast majority of SNe Ia are observed to be pow-ered purely by the radioactive decay of Ni. While thedetection of SN shock cooling or ejecta interaction israre, the level of Ni mixing in the SN ejecta can fun-damentally alter the appearance of the SN shortly afterexplosion (e.g., Dessart et al. 2014; Piro & Morozova2016; Noebauer et al. 2017).SNe Ia experience a dark phase after the SN shockbreakout but before radioactive energy diffuses into thephotosphere (Piro & Nakar 2014). The duration of thisdark phase is set by how the newly synthesized Ni ismixed and deposited into different layers of the ejecta.Strong mixing leads to a short, or nonexistent, darkphase, because the radioactive γ -rays rapidly diffuse tothe photosphere. This also leads to larger luminositiesand bluer optical colors at early times. Even with vig-orous mixing it is difficult at very early times, (cid:28) Ni has not had sufficient time to ra-dioactively decay to Co. If the mixing is weak and the Ni is confined to the innermost layers of the ejecta, thedark phase can last several days. Weak mixing resultsin redder colors and a more moderate rise in luminosity(Dessart et al. 2014; Piro & Morozova 2016). Thus, theearly light curves of even nonexotic SNe Ia convey in-formation about their progenitor systems and explosionmechanisms by constraining the distribution of Ni.Noebauer et al. (2017) demonstrate that disambiguat-ing between these different scenarios via optical photom-etry alone is challenging. Noebauer et al. further showthat estimates of the time of explosion, which are crit-ical for comparing models with observations, are oftenincorrect by as much as ∼ PTF 16abc .
36 UTC at R.A. = 13 h m . s ,Dec. = +13 ◦ (cid:48) . (cid:48)(cid:48) g PTF -band mag-nitude of 21 . ± .
25 (Miller et al. 2016). The tran-sient is spatially coincident with a tidal tail of thegalaxy NGC 5221, which lies at a distance of ∼
100 Mpc.iPTF 16abc is not detected to a 5 σ limit of g PTF =21 . .
42, less than 1 day prior to discov-ery, and rose by ∼ Ni mixing or ejecta in-teraction with nearby, unbound material, or a combi-nation of the two. Alongside this paper, we have re-leased our open-source analysis and all of the data uti-lized in this study. These are available online at
GitHubhttps://github.com/adamamiller/iPTF16abc . OBSERVATIONSDuring the spring of 2016, the iPTF survey observedthe field of iPTF 16abc every night during dark timein either the g PTF or R PTF band. Survey observa-tions were conducted with the CFH12K camera (Rah-mer et al. 2008) on the Palomar Observatory 48-inchtelescope (P48; Law et al. 2009). Images were pro-cessed by the IPAC image-subtraction pipeline, whichsubtracts background galaxy light using deep pre-SNimages and performs forced point-spread function (PSF)photometry at the location of the SN (Masci et al. 2017).The photometry is then calibrated to the PTF photo-metric catalog (Ofek et al. 2012).After discovery, g (cid:48) -, r (cid:48) -, and i (cid:48) -band photometry wasobtained with the SED Machine (SEDm; Blagorodnovaet al. 2017) mounted on the Palomar Observatory 60-inch telescope (P60). We utilized the Fremling Auto-mated Pipeline ( FPipe ; Fremling et al. 2016) to subtractgalaxy light from the SEDm images using archival Sloan P48 observations of iPTF 16abc are reported in the g PTF and R PTF filters throughout, which are similar to the SDSS g (cid:48) andMould- R filters, respectively (see Ofek et al. 2012 for details onPTF calibration). The correction from the g PTF and R PTF filtersto SDSS g (cid:48) and r (cid:48) requires knowledge of the intrinsic source color(see Equations (1) and (2) in Ofek et al. 2012). The spectral di-versity of SNe Ia in the days after explosion is poorly constrained,and as a result the color terms for iPTF 16abc at these epochs areunknown. We proceed by assuming that the g PTF and R PTF cal-ibration is on the AB system, which strictly speaking is incorrect,but this does not fundamentally alter any of our conclusions. -2 -1 0 1 2 3 10 50 100 t t (d) g ( m a g ) P48/CFH12K g PTF
P60/SEDm g'LCO-1m/Sinistro g' -20 -18 -15 -10 0 10 50 100 t T B , max (d) Figure 1.
The g -band light curve of iPTF 16abc, with5 σ upper limits shown as downward-pointing arrows. Ob-servations from different telescopes are shown with differentsymbols. The lower axis shows time measured in rest-framedays relative to t (see § B -band maximum. Note that the horizontal axisis shown with a linear scale for − ≤ t − t ≤ t − t > Digital Sky Survey (SDSS) images as a reference. Thispipeline then performed forced-PSF photometry at thelocation of iPTF 16abc, which is calibrated to the SDSScatalog (Ahn et al. 2014).The Las Cumbres Observatory (LCO) 1 m tele-scope network obtained
BV g (cid:48) r (cid:48) i (cid:48) photometry. PSFphotometry was measured on these images using the lcogtsnpipe pipeline (Valenti et al. 2016). The BV magnitudes are calibrated to the Fourth USNO CCDAstrograph Catalog (Zacharias et al. 2013), and the g’r’i’ magnitudes are calibrated to SDSS Data Release6 (Adelman-McCarthy et al. 2008).The Reionization and Transients InfraRed (RATIR)camera on the autonomous 1.5 m Harold L. JohnsonTelescope (Butler et al. 2012; Watson et al. 2012) wasused to observe iPTF 16abc in the r (cid:48) i (cid:48) ZY JH filters. Bydesign, RATIR lacks a cold shutter, which means thatIR dark frames are not available. Laboratory testing,however, confirms that the dark current is negligible inboth IR detectors (Fox et al. 2012).The RATIR data were reduced, co-added, and ana-lyzed using standard CCD and IR processing techniquesin
IDL , Python , SExtractor (Bertin & Arnouts 1996),and
SWarp . Aperture photometry is obtained followingthe methods described in Littlejohns et al. (2014). The r (cid:48) i (cid:48) Z filters are calibrated to SDSS (Ahn et al. 2014),while the JH filters are calibrated to the Two MicronAll Sky Survey (Skrutskie et al. 2006). For the Y -band Miller et al. calibration, we used an empirical relation in terms of the J and H magnitudes derived from the United KingdomInfrared Telescope (UKIRT; Casali et al. 2007) WideField Camera observations (Hodgkin et al. 2009).The Swift satellite observed iPTF 16abc on 14epochs, beginning ∼
15 days pre-maximum light through ∼
22 days post-maximum. The SN flux is measured viaaperture photometry on Ultraviolet-Optical Telescope(UVOT) images via the usual procedures in
HEASoft ,including corrections for coincidence loss and apertureloss. The image counts are converted to physical fluxesusing the latest calibration (Breeveld et al. 2011). Thereare no pre-SN UVOT images at the SN location in the
Swift archive. Visual inspection of the UVOT imagessuggests negligible host galaxy contamination in ourUVOT flux measurements. No X-ray emission is de-tected from iPTF 16abc by the
Swift
X-ray Telescope(XRT).The g -band discovery and follow-up data of iPTF 16abcare illustrated in Figure 1. The photometry is shownin the AB system. As previously noted, the color termsnecessary to convert g PTF to the AB system are un-known and assumed to be zero.Spectroscopic observations of iPTF 16abc were takenwith a variety of telescopes and instruments over multi-ple epochs beginning ∼ ∼ B -band maximum. An observing log islisted in Table 1. The spectra were reduced using stan-dard procedures in IDL / Python / Matlab . The opticalspectral evolution of iPTF 16abc is illustrated in Figure2, which excludes high-resolution Very Large Telescope(VLT) spectra for clarity. HOST GALAXY, REDDENING, ANDCLASSIFICATION3.1.
Host Galaxy iPTF 16abc is spatially coincident with a tidal tailof galaxy NGC 5221. Theureau et al. (2007) deriveda distance modulus of 35 . ± . § . Reddening
A detailed study of the reddening toward iPTF 16abcis presented in a companion paper (Ferretti et al.2017). Briefly, the foreground Galactic extinction to-ward iPTF 16abc is E ( B − V ) = 0 . Table 1.
Spectroscopic observations of iPTF 16abc
Observation SN RangeMJD Phase Telescope Instrument (˚A)57 , . − . , . − . , . − . , . − . , . − . , . − . , . − . , . − . , . − . , . − . , .
32 +3 . , .
00 +6 . , .
27 +8 . , .
42 +18 . , .
03 +20 . , .
40 +29 . , .
41 +41 . , .
40 +49 . , .
38 +61 . https://lco.global/observatory/instruments/floyds Ca II H+K and Na I D doublets. Despite large equiva-lent widths (EWs) for these lines, implying significantextinction (e.g., Poznanski et al. 2012, Ferretti et al. findevidence for only a small amount of extinction. The em-pirical relation between the EW of Na I D and extinc-tion has a large scatter, and Phillips et al. (2013) haveshown that Na I D absorption is a poor tracer of red-dening in SNe Ia. Thus, we adopt E ( B − V ) = 0 .
05 magas the local extinction for iPTF 16abc (Ferretti et al.2017). For the remainder of our analysis we assume atotal, Galactic+host galaxy, line-of-sight extinction of E ( B − V ) = 0 .
08 mag.
PTF 16abc obs (Å) f + O ff s e t Figure 2.
Observed spectral sequence of iPTF 16abc. The spectra are normalized by their median flux between 6000 and7000 ˚A. The phase of each spectrum relative to the time of B -band maximum is shown. Telluric absorption bands are grayedout. Line identifications are provided for the spectral features discussed in the text. For clarity, high-resolution spectra obtainedwith the VLT have been omitted (see Ferretti et al. 2017, for a detailed discussion of these spectra). Classification
Using the SuperNova IDentification (
SNID ; Blondin &Tonry 2007) package, we find that the low-resolutionspectrum of iPTF 16abc at +18 . II and S II , can be easily identified iniPTF 16abc (Figure 2). From the +3 . − ± − ± II λλ II λ II velocity, v Si II λ , to Miller et al. a fixed grid at 1 day intervals. The curves are definedby the median v Si II λ at each point on the grid withat least three SNe (this prevents just one or two SNefrom defining the evolution of an entire subclass). Thevelocity evolution of iPTF 16abc is most reminiscent ofthe shallow-silicon subclass.
15 10 5 0 5 10 15 20 t T B , max (d) v S i II ( k m s ) iPTF16abcCore NormalBroad LineShallow SiliconCool Figure 3.
Velocity evolution of Si II , v Si II λ , foriPTF 16abc compared to the median evolution of the fourspectroscopic subclasses defined in Branch et al. (2006), us-ing data from Blondin et al. (2012). Typical uncertaintiesfor iPTF 16abc are ∼ − before T B, max , when theSi II λ ∼
300 km s − after T B, max .For the median curves, the typical scatter, determined viathe interquartile range of the sample, is ∼
700 km s − , around T B, max . At early times, core-normal and broad-line SNe havesignificantly faster Si II than iPTF 16abc, while the decliningtrend of cool SNe does not match iPTF 16abc. To determine the brightness and time of B -band maxi-mum for iPTF 16abc, we fit the P60 light curves with the sncosmo software package. This fit includes a
SALT2 template (Guy et al. 2007) that has been corrected forextinction using the Fitzpatrick (1999) reddening law, R V = 3 .
1, and E ( B − V ) = 0 .
08 mag.We determine the time of rest-frame B -band maxi-mum to be MJD max = 57499 . ± .
23, the coefficient ofthe zeroth principle component x = 0 . ± . x = 0 . ± .
15, and the color term c = 0 . ± . m ∗ B = 15 . ± .
04 mag in the SN rest frame. Inthe following sections, we adopt MJD max = 57499 .
54 asthe time of B -band maximum, T B, max , and phase t = 0.We measure the (pseudo-)bolometric luminosity, L UVOIR , of iPTF 16abc at peak via trapezoidal inte-gration of the reddening-corrected flux from the UV, sncosmo is available at https://sncosmo.readthedocs.io . optical, and near-IR (UVOIR) filters. The light curvesin the individual filters are interpolated so that L UVOIR is evaluated at common epochs in each filter. Fromthis integration, we measure a maximum luminosity L max = 1 . ± . × erg s − for iPTF 16abc. Thisvalue is consistent with the normal SNe Ia studied inDhawan et al. (2016). Following Arnett’s rule (Arnett1982; Arnett et al. 1985), the mass of Ni synthesizedin the explosion can be derived from L max . Assuming arise time of 19 ± M Ni = 0 . ± . M (cid:12) . After establishing iPTF 16abc as a normal SN Ia, weuse the latest calibration (Betoule et al. 2014) of thePhillips relation (Phillips 1993) using m ∗ B , x and c toderive a distance modulus µ = 34 . ± .
10 mag to theSN, provided that the host galaxy of iPTF 16abc has astellar mass < M (cid:12) . A more massive host galaxywould result in a larger inferred distance modulus thatis nevertheless consistent within the uncertainties. Forthe following analysis we adopt a distance modulus µ =34 . ± .
10 mag for iPTF 16abc. EARLY OBSERVATIONSHere we consider our suite of early observations ofiPTF 16abcand compare our findings with SN 2011fe, awell-studied, nearby SN that was discovered shortly af-ter explosion (Nugent et al. 2011; Bloom et al. 2012;Piro & Nakar 2014).4.1.
Time of First Light from the Early Light Curve
The time of first light for SNe is usually estimatedby extrapolating early-phase light curves to determinewhen the SN flux is equal to 0. Assuming an ideal,expanding fireball with constant temperature, Arnett(1982) derives that f ∝ t , where f is the SN flux and t is the time since explosion. Despite these simplifiedassumptions, multiple studies have found that the earlyemission from Type Ia SNe can be described as a powerlaw in time, with power-law index consistent with 2, i.e. f ∝ t (e.g., Conley et al. 2006; Hayden et al. 2010b;Ganeshalingam et al. 2011). We model the early flux from iPTF 16abc as a powerlaw: f ( t ) = 0 , when t ≤ t ∝ ( t − t ) α , when t > t , (1) A 17.9 day rise time ( § M Ni . This µ is consistent with the z SN , H = 73 km s − Mpc − ,and Virgo-infall-corrected distance (Mould et al. 2000). Many of these studies sample SN Ia light curves at phasescloser to T B, max than our initial observations of iPTF 16abc. PTF 16abc t is the time of first light, α is the power-lawindex, and t is measured in the SN rest frame. We allow α to vary to find the best match to the data, and we latershow that α = 2 is not compatible with the observations.To determine t and α we fit the earliest observations ofiPTF 16abc. Due to slight variations in the passbands,the model is fit only to the relative g PTF -band flux. g PTF is the only filter with observations prior to first light, anecessity for constraining t .To determine the best-fit parameters, we search alarge grid over t , α , and the proportionality constantand minimize χ . The modeling results show that theSN flux rises approximately linearly between t = −
18 dand t = −
15 d. Figure 4 shows the best-fit resultand the joint marginal distribution of t and α . Fromthe best-fit model we obtain α = 0 . ± . . and t = − . ± . . days, where the uncertainties represent themarginalized 95% confidence intervals. Our first detec-tion of iPTF 16abc occurred ∼ .
15 d after t . In theanalysis that follows, the precise values of the best-fitparameters are not important. The critical finding hereis that α ≈ t ≈ −
18 d.Figure 4 also shows the best-fit model while fixing α =2. The f ∝ t model does not match the observations.Formally, for the α = 2 model χ = 63 . ν = 15degrees of freedom (dof), while χ = 10 . ν = 14dof for the α = 0 .
98 model.As previously noted, a precise determination of therise time, t rise , of SNe Ia is challenging, as there maybe a dark phase following explosion (Piro & Nakar2014). Nevertheless, to be consistent with previousstudies (e.g., Ganeshalingam et al. 2011), here we find t rise = 17 . ± . . days based on our fit for t . We cau-tion, however, that t corresponds to the time wheniPTF 16abc was first detectable by P48 and not the timeof explosion.Unlike iPTF 16abc, the early emission from SN 2011feis well fit by an f ∝ t model (Nugent et al. 2011). Thus,the near-linear flux evolution observed in iPTF 16abcis distinct compared to SN 2011fe. To our knowledgethis behavior has only been observed in two other SNe(SN 2013dy and SN 2014J; Zheng et al. 2013, 2014; Goo-bar et al. 2015). Any model to explain the observationsof iPTF 16abc must account for this near-linear rise inthe days after first light.As a brief aside, we note that simulations presented inNoebauer et al. (2017) show that SN Ia explosion modelsdo not evolve as a power law in time. Noebauer et al.demonstrate that f ∝ t α fits to simulated light curvesresult in glaring errors to the estimated explosion times.While caution is advised in Noebauer et al. (2017), we f g P T F = 0.98= 2
20 19 18 17 16 15 t T
B, max (d) ff = . t (d) Figure 4.
Best-fit f ∝ t α model to describe the early fluxfrom iPTF 16abc in the g PTF band.
Top : The relative flux, f g PTF (shown as green circles), is measured via forced-PSFphotometry. The model flux, adopting best-fit parameters α = 0 .
98 and t = − .
91 d, is shown as a thick dashed line.Also shown is the best-fit model after fixing α = 2 (thindot-dashed line). The inset shows the joint distribution of t and α for the best-fit power-law model. The solid contoursrepresent the 68% and 99 .
7% confidence levels.
Bottom : Theobservations and models following subtraction of the best-fitpower-law model, f α =0 . . The t . model provides a muchbetter fit to the observations than the t model. note that our primary aim with the power-law fit is tocharacterize α for iPTF 16abc compared to SN 2011fe.4.2. Time of Explosion from the Photospheric Velocity
The time of explosion t exp is not equal to t (seeabove); thus, Piro & Nakar (2014) suggest that measure-ments of the photospheric velocity can determine t exp given that the ejecta begin expanding from the momentof explosion. Assuming a constant opacity in the ejecta,Piro & Nakar find that the photospheric velocity evolvesas v ph ∝ ( t − t exp ) − . . Numerical experiments by Piro& Morozova (2016) find that the constant-opacity as-sumption strongly depends on the amount of Ni mix-ing in the SN ejecta. As a result, the adoption of a t − . power-law model may not be valid for all SNe Ia. Never-theless, we proceed on the assumption that iPTF 16abcexperienced strong Ni mixing (see § t − . power law. We do this in part to compare with previousstudies, though we caution that the inferred value of t exp is subject to uncertainties related to ejecta mixing.While the photospheric velocity is not easy to mea-sure, line velocities of Si II or Ca II can be used asa proxy (Piro & Nakar 2014; Shappee et al. 2016b). Inthe case of iPTF 16abc, the Ca II IR triplet is very weak,likely due to high temperatures in the ejecta. Thus, we
Miller et al. determine the photospheric velocity from the Si II λ II , and that the C II λ II line (see Figures 2 and 6).Consequently, we model the observed spectra between5900 and 6500 ˚A (rest frame) as the combination of twoGaussian kernels plus a linear baseline, which accountsfor Si II , C II , and the continuum, respectively. Theexpansion velocity of Si II is measured by the centralwavelength of the Si II Gaussian kernel.We fit the measured velocities of Si II λ v ph ∝ ( t − t exp ) − . model by minimizing the χ valueand find the best-fit explosion time relative to T B, max in the SN rest frame to be t exp = − . ± . . days,where the uncertainties represent the 95% confidenceinterval (Figure 5). Following the analysis in Piro &Nakar (2014), we additionally alter the power-law in-dex to − .
20 and − .
24 to examine the sensitivity ofthe result on the assumed value of − .
22. We findthat this variation in the power-law index results in achange of t exp of ≈ ± v ph ∝ t − . , we adopt t exp = − . ± . t exp and t (Figure 5),we find that t (cid:46) t exp . Since physical causality re-quires t exp ≤ t , we draw the qualitative conclusion that t (cid:39) t exp , which is consistent to within the uncertain-ties. This derivation of t exp relies on the assumption v ph ∝ t − . , which may not be valid for all SNe Ia.4.3. Strong and Short-lived Carbon Features
The early spectra of iPTF 16abc exhibit unusuallystrong C II λλ II λ II λ t ≈ −
15 d. The strength of the C II lines declines withtime, and by t ≈ −
10 days C II is no longer detectable.Similar to our analysis of the Si II λ II λλ II with that of Si II in the right panel of Figure 5, which also shows thepEWs of these lines. These measurements confirm thequalitative analysis from Figure 6: namely, the strengthof C II λ II λ t ≈ −
16 dbefore decreasing and eventually disappearing around t = −
10 days.The detection of C II in SN Ia spectra is relativelyrare, as it requires both unburned carbon, which is likelyonly present in the outermost layers of the ejecta, andnonlocal thermal equilibrium effects in order to excite the ionized carbon (e.g., Thomas et al. 2007). Spec-tra obtained around or after T B, max rarely show C II asthe photosphere has receded from the outermost ejecta,while pre-maximum spectra show evidence for weak C II absorption in ∼ II features like iPTF 16abc (Zheng et al.2013; Hosseinzadeh et al. 2017). As a counterexam-ple, SN 2011fe only exhibited weak C II features in itsfirst spectra (Parrent et al. 2012). Thus, models ofiPTF 16abc must explain the strong C II absorption ob-served shortly after explosion.4.4. Blue Optical Colors Shortly after Explosion
Multiband observations of iPTF 16abc began ∼ ∼ t . In Figure 7 we comparethe ( B − V ) color evolution of iPTF 16abc to observa-tions of SN 2011fe (Zhang et al. 2016). For both SNe thecolors have been corrected for the total inferred redden-ing along the line of sight. Interestingly, iPTF 16abc hasa nearly flat color evolution up to t ≈ −
10 days, whileSN 2011fe initially exhibits red colors before evolving tothe blue.Roughly 16 days prior to T B, max , the ( B − V ) color of iPTF 16abc is ∼ B − V ) colors that are significantly bluer than SN 2011fe atvery early epochs. While there are many factors thatcontribute to the early optical colors of SNe Ia (see § − optical colorsof iPTF 16abc, SN 2012cg, and SN 2017cbv are signifi-cantly redder at these early epochs than the UV − opticalcolors of iPTF 14atg (Cao et al. 2015), the most likelycandidate for SN ejecta–companion interaction. MODELING THE EARLY EVOLUTION OFiPTF 16abcRelative to the nearby, normal SN 2011fe, we haveidentified several distinct characteristics of the early evo-lution of iPTF 16abc, including (i) a near-linear photo-metric rise; (ii) a qualitatively short, or possibly absent,dark phase, assuming v ph ∝ t − . ; (iii) the presence ofstrong C II absorption; and (iv) blue and nearly con-stant ( B − V ) color in the week after explosion. While PTF 16abc t T B , max (d) v ( t t exp ) v ( t t exp ) v ( t t exp )
15 14 13 12 11 10 9 8 7 t T B , max (d) v ( k m s ) Si II 6355C II 6580C II 7234
15 14 13 12 11 t T B ,max (d) p E W ( Å ) Figure 5.
Constraints on t exp from the velocity evolution of Si II . Left panel: the dashed, solid and dot-dashed curves show χ for fitting power laws with indices − . − .
22, and − .
24, respectively. The blue vertical line and the orange shadedregion indicate t and its 95% confidence interval from Section 4.1, respectively. Right panel:
Observed Si II λ − .
22 (dashed line). For comparison, the measured velocitiesof C II λλ Right inset : vvolution of thepseudo-equivalent width of Si II λ II λλ ∼ f + O ff s e t Si II C II
C II rest (Å)
Figure 6.
Evolution of the C II features observed in theearly spectra of iPTF 16abc. The raw spectra are shown inorange, while the solid blue lines show the best-fit models(see text for further details). The dark gray vertical linesshow the measured line centers and clearly show the declinein the photosphere velocity in the ∼ II λ − . T B, max is labeled. most SNe Ia are powered purely by radioactive decay,the observed radiation shortly after explosion can alsoinclude contributions from SN shock cooling or the col-lision of the SN ejecta with a nondegenerate companionor nearby, unbound material. Here we consider thesescenarios as possible explanations for the early behaviorof iPTF 16abc.
17 16 15 14 13 12 11 10 t T B , max (d) ( B V ) ( m a g ) iPTF 16abcSN 2011fe Figure 7. ( B − V ) color evolution of iPTF 16abc (squares)compared to SN 2011fe (stars). The B and V photometryare calibrated on the Vega system, and have been correctedfor extinction. The data for SN 2011fe are from Zhang et al.(2016). SN Shock Cooling
The shock breakout of an SN Ia lasts for a fractionof a second due to compact size of the exploding star.Emission from the subsequent cooling phase may last forseveral days, however (e.g., Piro et al. 2010). Followingthe analysis of Bloom et al. (2012) for SN 2011fe, wecompare the early-phase g PTF light curve of iPTF 16abcwith two shock cooling models (Piro et al. 2010; Ra-binak & Waxman 2011). From this analysis, we con-strain the iPTF 16abc progenitor radius to be < R (cid:12) .Our observations of iPTF 16abc cannot place tight con-straints on the size of its progenitor. Indeed, for a typical0 Miller et al.
WD radius, such as that inferred for SN 2011fe ( (cid:46) . . R (cid:12) ; Bloom et al. 2012; Piro & Nakar 2014), theexpected emission from shock cooling is ∼ g PTF detection limit at this distance.Thus, we conclude that shock cooling does not con-tribute to the early emission detected from iPTF 16abc.5.2.
SN–Companion Collision
The detection of emission from the collision of the SNejecta with a nondegenerate companion requires a favor-able orbital alignment relative to the line of sight. Thus,from geometric considerations alone the probability ofdetecting ejecta–companion interaction is low, ∼ g -bandemission is expected to be weak.To examine the possibility of a SN–companion signa-ture in the early light curve of iPTF 16abc, we employthe Kasen (2010) model and assume canonical values forthe ejecta mass, 1 . M (cid:12) , expansion velocity, 10 km s − ,and a constant opacity, 0 . g − . We calculate theexpected g PTF brightness of an ejecta–companion colli-sion at the distance of iPTF 16abc behind a total red-dening of E ( B − V ) = 0 .
08 mag using the parameterizedequations in Brown et al. (2012). If we assume that thebinary is aligned with the optimal orientation relative tothe line of sight, a binary separation of a ≈ × cmis needed to explain the initial detection of iPTF 16abc,as shown in Figure 8. The minimum binary separa-tion capable of explaining the observed brightness at theepoch of discovery is a ≈ cm. Figure 8 shows thatsuch models peak at g PTF ≈ . t exp ≈ t − . g PTF evolution for t > t + 0 . ∼
24 hr after discovery.Figure 8 additionally shows that a companion at a ≈ × cm provides a good match to the ini-tial optical rise, if t exp ≈ t + 0 . a (cid:38) cm, which can explain the initial g PTF rise,significantly overpredict the observed UV flux, however.There is no choice of a capable of replicating the earlyrise of iPTF 16abc without also overpredicting the ob-served UV flux.The challenges associated with each of the previouslyconsidered models lead us to conclude that the earlyevolution of iPTF 16abc cannot be explained via ejecta– t t (d) m a g ( o b s e r v e d ) a = 1 × 10 cm; t exp = t a = 3 × 10 cm; t exp = t a = 18 × 10 cm; t exp = t + 0.1 g PTF
UVW Figure 8.
Comparison of SN ejecta–companion interactionmodels with early observations of iPTF 16abc. g PTF detec-tions and 3 σ upper limits are shown as green circles anddownward-pointing arrows, respectively. Swift / UV W g PTF (green) and
UV W a , and time of explosion, t exp , as labeled in the legend.While models with a (cid:38) cm can explain the early op-tical rise, they greatly overpredict the UV flux. Models with a ≈ cm can explain the initial detection of iPTF 16abc,but they fail to replicate the ∼ ∼
24 hr afterexplosion. companion interaction. We cannot, however, excludethe presence of a red giant, or other nondegenerate, com-panion as our calculations have assumed that the binaryis aligned with the optimal geometry relative to the lineof sight. If the geometry is not favorable, then it is pos-sible that signatures from interaction with a companionare not visible.5.3.
Sub-Chandrasekhar Detonations and PureDeflagrations
In Noebauer et al. (2017) the early photometric evo-lution of SNe Ia is explored via a variety of explo-sion models and detailed radiative transfer calcula-tions. Specifically, Noebauer et al. (2017) examinetwo Chandrasekhar-mass ( M Ch ) explosions and com-pare their evolution to sub-Chandrasekhar detonationsand pure deflagrations. The M Ch explosions includethe “W7” carbon-deflagration model of Nomoto et al.(1984) and the “N100” delayed-detonation model fromSeitenzahl et al. (2013). The sub-Chandrasekhar mod-els include a violent WD-WD merger, which triggersa carbon detonation in the more massive WD, a cen- PTF 16abc M V ( A B m a g ) LCO-1m/Sinistro VP48/CHFT12K g t t (d) ( B V ) ( A B m a g ) Merger 2.3 dN100 0.7 dW7 1.6 d SubChDet 1.4 dSubChDoubleDet + 0.3 dN5def 0.9 d0 1 2 3 4 5 6 7 80.60.40.20.00.20.40.6
Figure 9.
Comparison of the models from Noebauer et al.(2017) to the iPTF 16abc V -band light curve ( top ) and the( B − V ) color curve ( bottom ). The light curve has been cor-rected for the distance modulus to iPTF 16abc ( § g PTF observations are shown,though we caution that iPTF 16abc may exhibit significantcolor evolution at this phase, in which case g PTF would bea poor proxy for V . Each model light curve is translated tomatch the LCO observations ∼ t given the un-certain time of explosion. Translational offsets are listed inthe bottom panel legend, which shows the models in order ofdecreasing Ni from top to bottom, then left to right. Theinset in the top panel shows the residuals relative to the W7model. trally ignited sub-Chandrasehkar detonation, and a sub-Chandrasehkar double-detonation explosion, in whichan He-surface-layer detonation triggers a carbon deto-nation in the core. Noebauer et al. (2017) note that,of these last two sub-Chandrasehkar models, the latterprovides the more realistic scenario. Finally, Noebaueret al. also examine the “N5def” and “N1600Cdef” puredeflagration explosions from Fink et al. (2014). The“N5def” model is particularly unique in that the ex-plosion does not fully unbind the WD, meaning that,unlike with typical SNe Ia, a remnant remains. In Figure 9 we compare photometric observationsof iPTF 16abc to the models presented in Noebaueret al. (2017). Interestingly, the sub-Chandrasekhardouble-detonation model (subChDoubleDet) replicatesthe early wiggle in the V -band light curve. However,this match requires an explosion after our initial de-tection of iPTF 16abcand predicts extreme color evolu-tion that is not observed. Thus, the subChDoubleDetmodel is incompatible with the observations. The sub-Chandrasekhar detonation (subChDet) provides a bet-ter match to the observations, though this model is notfavored as a particularly realistic scenario (see above).Of the sub-Chandrasekhar models, the violent mergermodel (Merger) provides the best match to the obser-vations, including the early rise and color evolution. Indetail, however, this model does not match the earlywiggles in the light curve, has consistently redder colorsthan iPTF 16abc, and requires t exp ≈ . t . As such, we postulate that iPTF 16abc is not theresult of a violent WD–WD merger.For clarity, of the two pure deflagration models onlyN5def is shown; however, the evolution of N1600Cdef isvery similar. While the pure deflagration models pro-duce the bluest colors at early times, they are under-luminous at times > t + 4 days and already rapidlyevolving toward the red at ∼ t + 7 (iPTF 16abc exhibitsa nearly constant ( B − V ) color for ∼
19 days after t ).Thus, we conclude that iPTF 16abc is not compatiblewith pure deflagrations.Of the M Ch models, the W7 model better matchesthe observations, as the N100 model features a fasterrise and higher luminosity than what is observed. Weexplore delayed-detonation models in further detail be-low.5.4. Interaction with Nearby, Unbound Material
To model SN 2011fe, Dessart et al. (2014) examinedpulsational delayed-detonation (PDD) models as an ex-planation for some SNe Ia. Briefly, PDD models dif-fer from “standard” delayed-detonation (DD) models inthat the expansion of the WD during the initial defla-gration phase leads to the release of unbound material.Following this pulsation, the bound material contracts,eventually triggering a subsequent detonation. An im-portant consequence of this progression for PDD modelsis that the unbound material expands and avoids burn-ing, unlike DD models that typically leave no unburnt Available at https://hesma.h-its.org/ . Dessart et al. (2014) note that the deflagration and detonationin their PDD models are artificially triggered. Miller et al. M g ( A B m a g ) P48/CFH12K gP60/SEDm g'LCO-1m/Sinistro g' t t (d) ( B V ) ( A B m a g ) DDC0 1.2 dPDDEL1n 0.9 dDDC6 1.7 dPDDEL7n 1.3 dPDDEL4n 1.5 d
Figure 10.
Same as Figure 9, but featuring the g -bandlight curve and the DD and PDD models from Dessart et al.(2014). The bottom panel legend lists the models in orderof decreasing M Ni from top to bottom. DD models (labeledas DDC to match the nomenclature of Dessart et al. 2014)are shown as dot-dashed lines, while PDD models (labeledas PDDEL) are shown as dashed lines. The inset in the toppanel shows the residuals relative to the PDDEL7n model.The PDD models provide a better match to the observations. material. This results in significantly more carbon inthe outer layers of the SN ejecta (Dessart et al. 2014).Dessart et al. (2014) find that DD models are univer-sally faint and red at early times, ∼ Ni yield DD and PDD modelspresented in Dessart et al. (2014). DD and PDD models Available at . with M Ni /M (cid:12) (cid:46) . g -band flux in the days after explosion. Further-more, unlike the DD models, the PDD models exhibitstrong C II lines that gradually disappear in the ∼ Ni mixing, which Dessart et al. only explore for DDmodels) may better match iPTF 16abc.5.5.
Strong Ni Mixing in the SN Ejecta
Having examined other possibilities, we now considerwhether the early evolution of iPTF 16abc can be ex-plained simply by invoking strong mixing in the SNejecta. Strong mixing leads to a faster initial rise, aswell as a more rapid evolution toward blue colors.Figure 11 compares the models from Piro & Morozova(2016) to iPTF 16abc. The Piro & Morozova models em-ploy a piston-driven explosion to explode a single WDprogenitor model. As the piston explosion does not re-sult in any nucleosynthesis, the distribution of Ni inthe ejecta must be prescribed by hand, which enables astudy of the effects of mixing on the resulting SN emis-sion. Each model employs a fixed 0 . M (cid:12) of Ni thathas been distributed throughout the ejecta via boxcaraveraging (see their Figure 1). The resulting light curvesare synthesized using the SuperNova Explosion Code(
SNEC ; Morozova et al. 2015), as shown in Figure 11.Broadly speaking, the results can be summarized as fol-lows: SNe with strong mixing exhibit a rapid rise andquickly develop blue colors, whereas models where the Ni is confined to the innermost layers of the ejectaremain faint for days after explosion and feature a grad-ual color evolution from the red to the blue. The modelwith the strongest mixing (dark long-dashed line in Fig-ure 11) best matches the observations of iPTF 16abc.This is the only model we have found that exhibits aflat ( B − V ) color evolution in the days after explosion;however, in detail this model is too red relative to theobservations of iPTF 16abc. PTF 16abc M g ( A B m a g ) P48/CFH12K gP60/SEDm g'LCO-1m/Sinistro g' t t (d) ( B V ) ( A B m a g ) M d M d M d M d M d M d M d Figure 11.
Same as Figure 9, but featuring the g -bandlight curve and models from Piro & Morozova (2016). Theamount of Ni mixing in the SN ejecta increases from thelight, short-dashed lines to the dark, long-dashed lines. Un-like Figure 10, each model features the same M Ni , while themodel names reflect the boxcar widths used to approximatethe effects of mixing in the ejecta (see Piro & Morozova2016). The top panel inset shows the residuals relative tothe 0 . M (cid:12) model. The observations are best matched bythe 0 . M (cid:12) model, i.e. the model with the most significantmixing. While the models from Piro & Morozova (2016) pro-vide a good match to the optical photometric evolutionof iPTF 16abc, they consistently overpredict the flux inthe UV. They also overpredict the photospheric velocityof iPTF 16abc by ∼ − . Furthermore, thesimple gray opacities in SNEC likely produce a fasterrise and bluer colors than the more detailed treatmentsemployed in Dessart et al. (2014) and Noebauer et al.(2017).Both the PDD models and ejecta-mixing modelsshow discrepancies with some early observations ofiPTF 16abc. Nevertheless, we conclude that one, orboth, of these scenarios, which feature qualitativelysimilar predictions, is the most likely explanation foriPTF 16abc. Indeed, it may be the case that the typical sequence of photometric and spectroscopic observationsof young SNe Ia can never distinguish between thesetwo possibilities (Noebauer et al. 2017). THE EMERGING SAMPLE OF YOUNG SNe IaThe proliferation of high-cadence, time-domain sur-veys has led to several SNe Ia being discovered within ∼ § reveals considerable diversity. In other words, at earlytimes SN 2011fe may not be the norm.For SN 2011fe the initial rise is well described by a t power law, the ( B − V ) colors evolve from the red to theblue in the ∼ II presentin the initial spectra is weak (Nugent et al. 2011; Parrentet al. 2012; Zhang et al. 2016). In contrast, iPTF 16abcexhibits a near-linear rise in flux, the ( B − V ) colorsare blue and roughly constant, and the C II absorp-tion is strong. Examining just these three qualitativefeatures, SN 2009ig is well matched to SN 2011fe (Fo-ley et al. 2012), while SN 2013dy (Zheng et al. 2013),SN 2017cbv (Hosseinzadeh et al. 2017), and iPTF 16abcall bear a striking resemblance. SN 2012cg, on the otherhand, is intermediate to these two groups, with weakC II and a relatively shallow early rise, like SN 2011fe,but blue ( B − V ) colors, like iPTF 16abc (Silvermanet al. 2012; Marion et al. 2016). SN 2014J is intermedi-ate in the other direction in that it exhibits a near-linearrise (Zheng et al. 2014; Goobar et al. 2015), but the colorevolution is very similar to that of SN 2011fe (Amanullahet al. 2014). That these early observations cannot beeasily separated into two distinct groups suggests that This definition excludes iPTF 14atg, which was shown to besubluminous with SN 2002es-like spectra (Cao et al. 2015). C II is not detected in the spectra of SN 2014J (Goobar et al.2014; Zheng et al. 2014), though the earliest spectra of SN 2014Jwere obtained at a much later phase than the other SNe discussedhere. Marion et al. (2015) find evidence for C I in the NIR spec- Miller et al. it is unlikely that a single physical mechanism drives thediversity of SNe Ia at early times.In the case of SN 2012cg and SN 2017cbv it has beenargued that the early blue optical colors are indicativeof interaction between the SN ejecta and a binary com-panion (Marion et al. 2016; Hosseinzadeh et al. 2017).SN 2017cbv is particularly remarkable in that the ob-servations presented in Hosseinzadeh et al. (2017) showa clearly resolved bump in the U , B , and g (cid:48) bands inthe ∼ § − optical colors aresignificantly redder than those observed in iPTF 14atg.It is argued in Hosseinzadeh et al. (2017) that severalmodel assumptions, including (i) ideal blackbody emis-sion, (ii) a constant opacity, (iii) a simple power-law den-sity profile for the ejecta, and (iv) spherical symmetry,may be incorrect, which could reconcile the discrepancywith the UV observations. Above, we argued for inter-action with diffuse, unbound material and strong Nimixing as a possible explanation for iPTF 16abc, andindeed Hosseinzadeh et al. (2017) consider these possi-bilities for SN 2017cbv as well. Separately, several argu-ments against companion interaction for SN 2012cg arepresented in Shappee et al. (2016a).Ultimately, there are arguments in favor of and againsteach of the possibilities to model the early emission fromSNe Ia. Moving forward, more detailed models and sim-ulations are needed to properly explain the observed di-versity. No matter the correct explanation for the earlybehavior of SN 2013dy, SN 2017cbv, and iPTF 16abc,the strong similarities between these events suggest thatthey may reflect a common physical origin. CONCLUSIONWe have presented observations of the extraordinarilyearly discovery of the normal SN Ia iPTF 16abc. Ourfast-response follow-up campaign allowed us to draw thefollowing conclusions:1. Extrapolation of the early light curve shows thatthe initial detection of iPTF 16abc occurred only0 . ± . . days after the time of first light, t . tra of SN 2014J, but this detection cannot constrain the relativestrength of C II and Si II shortly after explosion.
2. We find no evidence for detectable signatures ofSN shock cooling or the collision of the SN ejectawith a non-degnerate binary companion.3. Assuming that v ph ∝ t − . , then t exp ≈ t . Ashort dark phase, as this implies, is likely the resultof either strong Ni mixing or interaction of theSN ejecta with nearby, unbound material.4. The strong and short-lived carbon features seen inthe earliest spectra of iPTF 16abc can only be ex-plained if there is incomplete burning. The pul-sational delayed-detonation models presented inDessart et al. (2014) produce C II absorption thatis as strong as Si II at very early phases.5. In contrast to SN 2011fe, ( B − V ) is ∼ t ≈ −
16 days. Further-more, the ( B − V ) colors of iPTF 16abc show noevolution over the first ∼ Ni in the SN ejecta.The PDD models from Dessart et al. (2014) are par-ticularly attractive for explaining iPTF 16abc, becausethey produce strong C II absorption at early times. Inthe future, it would be useful to investigate more de-tailed PDD models that incorporate strong Ni mixingto see whether they better replicate the observations ofiPTF 16abc, as it is otherwise difficult to distinguish be-tween these two scenarios.Extremely early observations of young SNe provide a“smoking gun” to probe the mixing level in the ejecta,which, in turn, is a result of the explosion mechanism.Wide-field, high-cadence surveys, such as the ZwickyTransient Facility (Bellm 2016) and ATLAS (Tonry2011, 2013), will discover a large number of very youngSNe over the next few years, allowing us to extend ourstudies beyond single objects. While the sample of ex-tremely young SNe Ia will grow by more than an or-der of magnitude, the detection of shock breakout cool-ing and ejecta–companion interaction will prove chal-lenging. Given the diminutive size of WDs, the ther-mal emission following shock breakout can only be de-tected to ∼
10 Mpc on 1 m class telescopes. Further-more, only ∼
10% of single-degenerate progenitors are
PTF 16abc II in SNe. Similarly, it is our pleasure to buya beer for R. Amanullah and U. Feindt for discussionsregarding SALT2.This paper utilizes the LCO infrastructure for rapidand regular monitoring of SNe, and we thank S. Valentiand I. Arcavi for their development efforts. Figure 3would not have been possible without S. Blondin gener-ously sharing the data from Blondin et al. (2012).Much of the analysis presented herein would not havebeen possible without the help of several observers.We thank M. West for taking the first spectrum ofiPTF 16abc as a ToO on the DCT. We also thankP. GuhaThakurta, E. C. Cunningham, K. A. Plant,H. Jang, and J. Torres for executing a Keck ToO aspart of the UC/Caltech partnership, and also the Gem-ini service observers for executing our ToO observations.Additionally, J. Cohen, N. Suzuki, V. Ravi, R. Walters,A. Ho, H. Vedanthamand, K. De, and L. Yan helpedobtain data for this paper. AAM is funded by the Large Synoptic Survey Tele-scope Corporation in support of the Data Science Fel-lowship Program. YC acknowledges support from apostdoctoral fellowship at the eScience Institute, Uni-versity of Washington.DAH, CM, and GH are supported by NSF-1313484.The Intermediate Palomar Transient Factory projectis a scientific collaboration among the California Insti-tute of Technology, Los Alamos National Laboratory,the University of Wisconsin, Milwaukee, the Oskar KleinCenter, the Weizmann Institute of Science, the TANGOProgram of the University System of Taiwan, and theKavli Institute for the Physics and Mathematics of theUniverse. This work was supported by the GROWTHproject funded by the National Science Foundation un-der Grant No 1545949. Part of this research was carriedout at the Jet Propulsion Laboratory, California Insti-tute of Technology, under a contract with the NASA.This work makes use of observations from the LCO net-work. These results made use of the Discovery ChannelTelescope at Lowell Observatory. Lowell is a private,nonprofit institution dedicated to astrophysical researchand public appreciation of astronomy and operates theDCT in partnership with Boston University, the Uni-versity of Maryland, the University of Toledo, NorthernArizona University, and Yale University. The upgrade ofthe DeVeny optical spectrograph has been funded by agenerous grant from John and Ginger Giovale. Based onobservations made with the Nordic Optical Telescope,operated by the Nordic Optical Telescope Scientific As-sociation at the Observatorio del Roque de los Mucha-chos, La Palma, Spain, of the Instituto de Astrof´ısica deCanarias. Facility:
DCT, Gemini:Gillett, Hale, Keck:I, Keck:II,LCOGT, PO:1.2 m, PO:1.5 m, NOT, VLT,
Swift ,OANSPM:HJT
Software:
PTFIDE (Masci et al. 2017),
FPipe , SALT2 , SNID , lcogtsnpipe , UVOTSOURCE .APPENDIX A. PHOTOMETRIC LIGHT CURVESThe full photometric light curves of iPTF 16abc are shown in Figure 12.REFERENCES
Adelman-McCarthy, J. K., Ag¨ueros, M. A., Allam, S. S.,et al. 2008, ApJS, 175, 297Ahn, C. P., Alexandroff, R., Allende Prieto, C., et al. 2014,ApJS, 211, 17 Amanullah, R., Goobar, A., Johansson, J., et al. 2014,ApJL, 788, L21Arnett, W. D. 1982, ApJ, 253, 785Arnett, W. D., Branch, D., & Wheeler, J. C. 1985, Nature,314, 337 Miller et al.
20 0 20 40 60 80 100 120 t T B , max (d) m a g ( o b s e r v e d ) P48/CFH12K gP48/CFH12K R+2P60/SEDm gP60/SEDm r+2P60/SEDm i+3LCO-1m/Sinistro B 1LCO-1m/Sinistro V+1 LCO-1m/Sinistro gLCO-1m/Sinistro r+2LCO-1m/Sinistro i+3Swift/UVOT B 1Swift/UVOT V+1SPM-1.5m/RATIR r+2SPM-1.5m/RATIR i+3
15 10 5 0 5 10 15 20 25 30 35 40 t T B , max (d) m a g ( o b s e r v e d ) Swift/UVOT UVW2 5Swift/UVOT UVM2 5Swift/UVOT UVW1 1Swift/UVOT U+2 SPM-1.5m/RATIR Z+4SPM-1.5m/RATIR Y+5SPM-1.5m/RATIR J+6SPM-1.5m/RATIR H+7
Figure 12.
UV, optical, and NIR light curves for iPTF 16abc.
Left : BV gri light curves from the P48, P60, LCO-1m,
Swift ,and SPM-1.5m telescopes. The solid lines represent the best-fit model from SALT2 (see § Right : UV and NIR light curvesfrom the