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PTF 11kx: A Type Ia with a Symbiotic Progenitor B. Dilday et al. Science 337, 942 (2012); DOI: 10.1126/science.1219164

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two previous SNe Ia (SN 2006X and SN 2007le) and has been explained as the photoionization of circumstellar material (CSM) by the SN light, PTF 11kx: A with followed by recombination that is detected through the increased presence of neutral sodium (9, 11). a Symbiotic Nova Progenitor A statistical study using high-resolution spectra of 35 SNe Ia has shown that at least 20% of B. Dilday,1,2* D. A. Howell,1,2 S. B. Cenko,3 J. M. Silverman,3 P. E. Nugent,3,4 M. Sullivan,5 those with spiral host have CSM revealed S. Ben-Ami,6 L. Bildsten,2,7 M. Bolte,8 M. Endl,9 A. V. Filippenko,3 O. Gnat,10 A. Horesh,12 through narrow Na I D absorption lines (10). E. Hsiao,4,11 M. M. Kasliwal,12,13 D. Kirkman,14 K. Maguire,5 G. W. Marcy,3 K. Moore,2 Y. Pan,5 However, narrow lines of circumstellar Fe II, J. T. Parrent,1,15 P. Podsiadlowski,5 R. M. Quimby,16 A. Sternberg,17 N. Suzuki,4 D. R. Tytler,14 Ti II, and He I have not been seen previously in a D. Xu,6 J. S. Bloom,3 A. Gal-Yam,18 I. M. Hook,5 S. R. Kulkarni,12 N. M. Law,19 E. O. Ofek,18 SN Ia. He I l5876 strengthens over time, which D. Polishook,20 D. Poznanski21 can be explained by photoionization and recom- bination, if sufficient far-ultraviolet (UV) photons There is a consensus that type Ia supernovae (SNe Ia) arise from the thermonuclear explosion of white are generated in the early phases of the SN. SNe dwarf that accrete matter from a binary companion. However, direct observation of SN Ia progenitors Ia are expected to produce only weak emission in is lacking, and the precise nature of the binary companion remains uncertain. A temporal series of the far-UV, but interaction of the SN ejecta with high-resolution optical spectra of the SN Ia PTF 11kx reveals a complex circumstellar environment that an extended progenitor such as a provides an unprecedentedly detailed view of the progenitor system. Multiple shells of circumstellar material can produce an excess of x-ray and UV photons are detected, and the SN ejecta are seen to interact with circumstellar material starting 59 days after (12). Lower excitation states of Fe II ll4923, the explosion. These features are best described by a symbiotic nova progenitor, similar to RS Ophiuchi. 5018, and 5169 decrease in strength in time, whereas higher excitation states, such as Fe II upernova PTF 11kx was discovered 16 thus, this combination of absorption features l5316, remain constant. A possible explanation January 2011 (UT) by the Palomar Transient indicated that PTF 11kx may have affected its for this is that the excitation energy of Fe II SFactory (PTF) at a cosmological redshift surroundings, revealing evidence of its circumstellar l5316 (3.153 eV) corresponds to the saturated of z = 0.04660 T 0.00001 (1). This corresponds environment and progenitor system. Supernovae Ia Ca II K line, and thus the population of this level to a distance of ∼207 Mpc. The ini- (SNe Ia) are known to exhibit photometric (3, 4) is unaffected by the SN light. Because SNe Ia are on March 4, 2013 tial spectrum of PTF 11kx, taken 26 January and spectroscopic (5) diversity that is correlated weak sources in the UV, the distance over which 2011, showed saturated absorption in the Ca II with intrinsic brightness, such that bright (faint) they can ionize gas is limited, and hence the H and K lines, along with weak absorption in SNe Ia have relatively broad (narrow) light curves, observed photoionization indicates that the ma- the Na I D lines (Fig. 1). In interstellar gas, the high (low) photospheric temperature, and weak terial is circumstellar (9, 11). An alternative ex- Na I D lines are usually of strength comparable (strong) Si II l6150 absorption. Aside from the planation, as the projection effect of different to (or greater than) that of the Ca II lines (2); saturated Ca II absorption, the initial spectrum interstellar clouds, was proposed for the time- and a subsequent temporal series of spectra variable Na I D lines in SN 2006X (13); however, 6 7 1 show PTF 11kx to resemble SN 1999aa ( , ), interaction with the CSM in PTF 11kx demon- Las Cumbres Observatory Global Telescope Network, 6740 www.sciencemag.org 2 a broad/bright SN Ia (Fig. 1). This suggests that Cortona Drive, Suite 102, Goleta, CA 93117, USA. Department strates conclusively that CSM is present. There of Physics, University of California, Santa Barbara, Broida Hall, insights into the progenitor of PTF 11kx may be are other narrow sodium features at different rel- Mail Code 9530, Santa Barbara, CA 93106–9530, USA. applicable to SNe Ia generally, rather than to only ative velocities that do not vary with time, which 3Department of Astronomy, University of California, Berkeley, a subset of peculiar objects. A complete sample must originate at a larger distance and are most – 4 CA 94720 3411, USA. Lawrence Berkeley National Labora- of nearby SNe Ia shows that the subclass similar likely interstellar (Fig. 2). tory, Mail Stop 50B-4206, 1 Cyclotron Road, Berkeley, CA ≥ 8 94720, USA. 5Department of Physics (Astrophysics), University to SN 1999aa comprises 9% of all SNe Ia ( ). The hydrogen Balmer series is clearly detected –1 ofOxford,KebleRoad,OxfordOX13RH,UK.6Department of We obtained high-resolution spectra (R = in absorption at ∼65 km s and is consistent with Particle Physics and Astrophysics, Weizmann Institute of l/Dl ≈ 48,000) of PTF 11kx with the Keck I being at the same velocity as the Fe II, Ti II, Na I, 7 Downloaded from Science, Rehovot 76100, Israel. Kavli Institute for Theoretical High Resolution Echelle Spectrometer (HIRES) and He I lines (Fig. 2). The Ha and Hb lines Physics, University of California, Santa Barbara, CA 93106, − USA. 8University of California Observatories, Lick Observatory, instrument at 1, +9, +20, and +44 days, rel- show narrow P-Cygni profiles, which are char- University of California, Santa Cruz, CA 95064, USA. 9McDonald ative to B-band maximum light in the observer acteristic of an expanding shell of radiating ma- Observatory, University of Texas at Austin, Austin, TX 78712, frame. Low-resolution spectra were obtained on 14 terial. The presence of circumstellar hydrogen 10 USA. Racah Institute of Physics, Hebrew University of Jeru- epochs between −3 and +130 days and show the has long been identified as one of the character- salem, 91904, Israel. 11Carnegie Institution of Washington, Las Campanas Observatory, Colina El Pino, Casilla 601, Chile. composition and evolution of features that are istics expected in the single-degenerate SN Ia pro- 12Cahill Center for Astrophysics, California Institute of Tech- unique to SNe Ia (Fig. 1). The SN system is genitor model (14, 15) but has only been detected nology, Pasadena, CA 91125, USA. 13Observatories of the blueshifted from the cosmological red- in at most two other cases, SN 2002ic (16)andSN Carnegie Institution for Science, 813 Santa Barbara Street, ∼ –1 17 14 shift by 100 km s , as determined from the 2005gj ( ), and only at levels much stronger than Pasadena, CA 91101, USA. Center for Astrophysics and strongest narrow interstellar Na D absorption line Space Sciences, University of California San Diego, La Jolla, CA in the early epochs of PTF 11kx. 92093–0424, USA. 15Department of Physics and Astronomy, (9, 10). An assumption that emission lines from a These narrow features of H, He, Fe, Ti, and Na Dartmouth College, Hanover, NH, USA. 16Institute for the nearby H II region evident in the two-dimensional arise from material that is distinct from the stron- Physics and Mathematics of the Universe, University of Tokyo, spectra are indicative of the progenitor velocity ger Ca absorption, which, owing to its more blue- 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8583, Japan. gives a consistent result. shifted absorption minimum of ∼100 km s–1,must 17Minerva Fellow, Max Planck Institute for Astrophysics, Karl Schwarzschild Strasse 1, D-85741 Garching, Germany. 18Benoziyo The high-resolution spectra reveal narrow ab- be at a higher velocity. In fact, at later epochs, Center for Astrophysics, Weizmann Institute of Science, Rehovot sorption lines of Na I, Fe II, Ti II, and He I (Fig. 2). emission lines of Ca and H emerge, marking the – 76100, Israel. 19University of Toronto, 50 St. George Street, All are blueshifted by a velocity of ∼65 km s 1 rel- onset of interaction between SN ejecta and CSM. 20 Toronto M5S 3H4, Ontario, Canada. Department of Earth, ative to the progenitor system, with a velocity This transition of circumstellar lines from absorp- Atmospheric, and Planetary Sciences, Massachusetts Institute ∼ –1 of Technology, Cambridge, MA 02139, USA. 21School of Phys- dispersion of 10 km s , and are consistent with tiontoemissionhasnotpreviouslybeenobserved ics and Astronomy, Tel-Aviv University, Tel-Aviv 69978, Israel. arising in the same cloud or shell. The sodium in a SN Ia and shows definitively the presence of *To whom correspondence should be addressed. E-mail: lines increase in depth over time, a feature that CSM. Narrow absorption components can be seen [email protected] has been resolved in high-resolution spectra of atop the broader emission (full width at half

942 24 AUGUST 2012 VOL 337 SCIENCE www.sciencemag.org REPORTS maximum ≈ 1000 km s–1) (Fig. 3), indicating determined to be ∼5×1018 cm−2 (18). Assuming shell of CSM) (Fig. 4), but this is no longer evident that there are two regimes of material, with the that the Ca is part of a spherical shell of material after day +56. At day +39, Ha also begins to faster-moving H and Ca interior to the slower- with uniform density would imply a total mass in exhibit a broad emission component. Assuming a moving shell (Fig. 4.) the CSM [∼5.3 times the mass of the (M⊙)] rise time of ∼20 days and an ejecta velocity of The saturation of the Ca II K line near max- that is incompatible with the photometric behavior 25,000 km s–1 would imply that the location of the imum light requires that no part of the SN pho- of the SN (18). Therefore, the material must be interaction is ∼1016 cm. The temporal evolution tosphere is uncovered by the absorbing material; nonuniform. A small range of viewing angles that oftheCaIIKandHa lines are shown in Fig. 3. this constrains the distance of the absorbing cloud intersect such a dense cloud of absorbing material A model for the progenitor of PTF 11kx must from the progenitor to be further than the SN may partly explain the rare occurrence of variable thus explain the presence of multiple components ejecta and the size of the cloud to be as large as absorption features in SNe Ia. of CSM, fast-moving material interior to slower- the SN . Assuming typical values for The Ca II H and K lines detected in lower- moving material, velocities of the narrow ab- the ejecta expansion velocity of the outer layers resolution spectra confirm the disappearance of sorption components that are larger than typical of the exploding of 25,000 km s–1 and the absorption over ∼40 days and the emergence red giant winds, and a region evacuated of CSM, mean photospheric velocity of 10,000 km s–1, of a broad emission feature in the spectrum taken leading to a delay between the explosion and the these constraints are ∼4.3 × 1015 cm and ∼1.7 × +39 days after maximum (Fig. 1). The broad emis- emergence of broad components of Ca and H 1015 cm, respectively (Fig. 4). The column density sion feature initially exhibits a narrower absorption emission. A SN Ia occurring in a symbiotic nova of Ca II in the near-maximum-light spectra is component as well (possibly because of an outer system naturally provides an explanation for all of these features. In this model, onto a near-Chandrasekhar-mass white dwarf occurs through the wind from a red . The wind also deposits CSM into the system, which is concentrated in the orbital plane (19). Episodic thermonuclear runaways on the surface of the white dwarf cause nova events, which eject a −7 mass of ∼10 M⊙ at velocities of thousands of km s–1. This process results in expansion veloc- ities of CSM at ∼50 to 100 km s–1 concentrated in on March 4, 2013 the orbital plane, as is observed in high-resolution spectra of RS Ophiuchi (20). The delay in the interaction between SN ejecta and CSM, leading to the emergence of the broad Ha and Ca II emis- sion, is explained as being due to an evacuated region around the SN caused by the previous nova event (21) that will be present until the red giant wind has had time to replenish the local CSM. The system of features seen in the spectra of www.sciencemag.org PTF 11kx is inconsistent with expectations for double-degenerate SN Ia progenitors. Double- degenerate SN Ia progenitors first go through a common-envelope phase in which the outer layers of the secondary star are stripped, resulting in large amounts of CSM. However, it is generically ex- pected that the envelope material will have dis- Downloaded from sipated by the time of the SN Ia event (22). Recent modeling of the white dwarf merger process in double-degenerate SN models suggests that heat- ing in the can drive a wind, de- positing CSM into the system; however, because of the previous common-envelope phase, the pres- ence of hydrogen cannot be accounted for (23). Alternative models for SN 2002ic and SN 2005gj, which suggested that they were not SNe Ia, are also implausible for PTF 11kx (18). For epochs less than ∼20 days after maximum, the light curve of PTF 11kx resembles that of typical broad/bright SNe-Ia (18). Using the color law of the SiFTO (24) light-curve model, we esti- mate that PTF 11kx is only moderately extin- guished by dust (∼0.5 magnitudes in V-band), and that the absolute (MV ≈−19.3) is as expected for a SN 1999aa–like SN Ia. At epochs Fig. 1. Comparisonofspectra.AtemporalseriesofspectraofPTF11kxiscomparedwiththatofthe later than ∼20 days, the light curve appears brighter broad/bright SN 1999aa (7) and the CSM interaction SN Ia, SN 2002ic (16), all at a similar phase. The than the SN Ia templates, indicating that some location of Mg II, Fe II, S II, and Si II present in the SN ejecta are labeled. The presence of both Si II and S II brightening due to CSM interaction has begun. At is a feature seen uniquely in SNe of type Ia. an epoch of ∼+280 days the SN is at an absolute

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Fig. 2. The temporal evolution of narrow absorption features in PTF 11kx. The panels show (a)NaID2,(b)FeII l5018, (c)FeIIl5316, (d)NaID1,(e)FeIIl5169, and (f)HeIl5876. Separate components of Na I D are labeled as A, B, and C. Component A strengthens with time and is interpreted as ionization from the SN followed by recombination and thus is from CSM. Components B and C are attributed to the interstellar medium. The velocities are given relative to component B, which is taken as indicative of the local velocity of the SN progenitor system. The lower excitation states of Fe II [(b) and (e)] are seen to vary with time, whereas the higher excitation Fe II line (c) appears to be constant in time. He I absorption emerges in the later spectra. The vertical dashed line marks the velocity of the CSM at −65 km s–1. The horizontal dashed lines mark 1 and 0 in normalized flux units. A cosmic ray in the day −1 spectrum near Na I D1 is marked by CR; it produces a spurious absorption feature near the C component. on March 4, 2013 Fig. 3. The temporal evolution of Ha and Ca II K features in PTF 11kx. (A)Low-resolutionHa,(B)high- A B resolution Ha,(C) low-resolution Ca II K, and (D) high-resolution Ca II K. The +9 and +20 day high- resolution Ca II spectra have been rebinned to 20 km s–1 precision for clarity of presentation. The vertical dashed line marks the velocity of the slower CSM at −65 km s–1. The large optical depth in the Ha line causes an apparent blueshift in the absorption mini- mum, relative to the −65 km s–1 expansion velocity www.sciencemag.org of the narrow lines shown in Fig. 2. The horizontal dashed lines mark 0 and 1 in normalized flux units and show that at early times the Ca II lines are sat- urated. Narrow absorption, indicated by an arrow, can be seen superposed on the broad emission in the day CD +44 spectrum (D). Narrow interstellar lines of Ca II, corresponding to components B and C in panels (a) and – (d)ofFig.1,canbeseenat0and+45kms 1.Several Downloaded from spectra in (A) and (C) appear to show narrow absorp- tion superposed on broad emission, although the ab- sorption is not always resolved. This indicates that, subsequent to the onset of CSM interaction, slower- moving material external to the inner shell of inter- acting material remains, possibly due to a decelerated nova shell.

magnitude of ∼−16.6, roughly 3 magnitudes brighter between broad/bright SNe Ia and SN 2002ic/SN from relatively young stellar populations (25), than expected, indicating that circumstellar interac- 2005gj, showing that SNe Ia exist with CSM and a complete understanding of SN Ia progen- tion is still ongoing. Circumstellar interaction, in- interaction that is weaker and begins much later itors must account for the occurrence of CSM in- cluding broad H emission and excess luminosity, after explosion. This supports and enhances the teraction preferentially in young progenitor systems was observed previously in SN 2002ic and SN interpretation that the earlier SNe with H emission (10). The host galaxy of PTF 11kx is a spiral galaxy 2005gj, which had the appearance of the extreme were SNe Ia, where the strength of the CSM with active and therefore has a broad/bright SN 1991T, superposed with features interaction depends on details of the progenitor substantial fraction of young stars. In contrast to from strong CSM interaction. The strength of the system, such as the mass-loss and accretion rates SN 2002ic/SN 2005gj, the host galaxy has typ- interaction casts some doubt on their identity as and the time since the last nova eruption. ical values for mass and , so there is no SNe Ia, which is not the case for PTF 11kx (Fig. 1). Measurements of the host-galaxy properties evidence that PTF 11kx must have come from an Indeed, PTF 11kx bridges the observational gap for SNe Ia suggest that broad/bright SNe Ia arise atypical .

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10. A. Sternberg et al., Science 333, 856 (2011). 11. J. D. Simon et al., Astrophys. J. 702, 1157 (2009). 12. D. Kasen, Astrophys. J. 708, 1025 (2010). 13. N. N. Chugai, Astron. Lett. 34, 389 (2008). 14. I. Iben Jr., A. V. Tutukov, Astrophys. J. Suppl. Ser. 54, 335 (1984). 15. D. Branch, M. Livio, L. R. Yungelson, F. R. Boffi, E. Baron, Publ. Astron. Soc. Pac. 107, 1019 (1995). 16. M. Hamuy et al., Nature 424, 651 (2003). 17. G. Aldering et al., Astrophys. J. 650, 510 (2006). 18. Additional information is available in the supplementary materials on Science Online. 19. R. Walder, D. Folini, S. N. Shore, Astron. Astrophys. 484, L9 (2008). 20. F. Patat et al., Astron. Astrophys. 530, A63 (2011). 21. W. M. Wood-Vasey, J. L. Sokoloski, Astrophys. J. 645, L53 (2006). 22. N. N. Chugai, L. R. Yungelson, Astron. Lett. 30, 65 (2004). 23. K. J. Shen, L. Bildsten, D. Kasen, E. Quataert, Astrophys. J. 748, 35 (2012). 24. A. Conley et al., Astrophys. J. 681, 482 (2008). 25. M. Sullivan et al., Astrophys. J. 648, 868 (2006). 26. D. Maoz, K. Sharon, A. Gal-Yam, Astrophys. J. 722, 1879 (2010). 27. D. A. Howell et al., Nature 443, 308 (2006). 28. J. M. Silverman et al., Mon. Not. R. Astron. Soc. 410, 585 Fig. 4. Schematic showing the interpretation of the observations of PTF 11kx. In the symbiotic nova (2011). 29. B. E. Schaefer, A. Pagnotta, Nature 481, 164 (2012). progenitor system, a red giant wind deposits CSM into the system, which is concentrated in the orbital 30. W. Li et al., Nature 480, 348 (2011). –1 plane. Episodic nova events further shape the CSM, resulting in expansion velocities of ∼50 to 100 km s . 31. P. E. Nugent et al., Nature 480, 344 (2011). – There is an inner region of material containing at least H and Ca, moving at ∼100 km s 1, surrounded by 32. A. Horesh et al., Astrophys. J. 746, 21 (2012). moredistantmaterialcontainingH,Ca,Na,Fe,Ti,andHe,movingat∼65 km s–1.Thepresenceof 33. A. Chiotellis, K. M. Schure, J. Vink, Astron. Astrophys. 537, A139 (2012). hydrogen in the inner region is inferred from the onset of emission, concurrent with the onset of Ca II 34. Z. Han, P. Podsiadlowski, Mon. Not. R. Astron. Soc. 350, on March 4, 2013 emission. This geometry, and the delay in the onset of CSM emission, is consistent with a relatively recent 1301 (2004). nova whose ejecta are sweeping up the CSM and decelerating. The inner boundary of CSM is at a distance 35. G. Lü, C. Zhu, Z. Wang, N. Wang, Mon. Not. R. Astron. of ∼1016 cm, as determined from the onset of interaction. Soc. 396, 1086 (2009).

Acknowledgments: The Palomar Transient Factory project is a scientific collaboration between the California Institute of There are several indications that white dwarf tionally, even more extreme interaction of a SN Ia Technology, Columbia University, Las Cumbres Observatory, mergers are required to explain some SNe Ia, with CSM would not be distinguishable from a the Lawrence Berkeley National Laboratory, the National including the rate in old stellar populations (26) type IIn SN, and it has been speculated that some Energy Research Scientific Computing Center, the University and the existence of SNe Ia radiating at a lumi- SNeIInareinfactSNeIahiddenwithinvery of Oxford, and the Weizmann Institute of Science. The 16 National Energy Research Scientific Computing Center, www.sciencemag.org nosity that requires an exploding white dwarf strong CSM interaction ( ). Our data imply that supported by the Office of Science of the U.S. Department of above the (27, 28). The lack there is indeed a wide range of CSM interactions Energy, provided staff, computational resources, and data of a surviving companion near the remnant of a possible from SNe Ia and support this interpre- storage for this project. Some of the data presented herein SN Ia in the Large Magellanic Cloud has provided tation. If this is the case, then current estimates of were obtained at the W. M. Keck Observatory, which is evidence for a double-degenerate progenitor for the rate of SNe with progenitors similar to that of operated as a scientific partnership among the California 29 Institute of Technology, the University of California, and NASA; that particular case ( ). A symbiotic nova pro- PTF 11kx will be underestimated. Theoretical the observatory was made possible by the generous financial genitor has been ruled out for the nearby SN Ia expectations for the fraction of SNe Ia from the support of the W. M. Keck Foundation. The authors wish to PTF 11kly/SN 2011fe (30), which implies either symbiotic progenitor channel are ∼1 to 30% recognize and acknowledge the very important cultural role Downloaded from a main-sequence companion or a double-degenerate (34, 35), consistent with observational constraints. and reverence that the summit of Mauna Kea has always had 31 32 within the indigenous Hawaiian community. We are most progenitor ( , ). On the other hand, the detec- The solution to the long-standing puzzle of the fortunate to have the opportunity to conduct observations from tion of CSM, inferred through either temporal progenitors of SNe Ia is that there is no single this mountain. We are grateful to the staffs of the Lick, variability (9, 11) or statistical analysis of the ve- progenitor path. It remains to be determined in what Keck, and other observatories for their assistance. We thank locity (10) of narrow absorption features, has proportion different progenitor channels contrib- D. Kasen for insightful discussions on the analysis presented here, and we thank M. Auger for assistance in using his spectroscopic provided recent evidence in support of the single- ute to the total rate of SNe Ia, whether different reduction pipeline. The research of A.V.F.’sgroupwassupported degenerate progenitor scenario for some SNe Ia. channels result in different absolute by grants from the NSF, the TABASGO Foundation, Gary and The remnant of Kepler’s SN shows evidence for or color evolution, and how the potential change Cynthia Bengier, and the Richard and Rhoda Goldman Fund. interaction with a circumstellar shell that supports of the relative contributions of different channels M.M.K. acknowledges generous support from the Hubble Fellowship 33 and Carnegie-Princeton Fellowship. A.G.-Y. is supported by a single-degenerate progenitor for that SN Ia ( ). with look-back time may affect the use of SNe Ia grants from the Israel Science Foundation and the U.S.-Israel The system of features observed in PTF 11kx in accurately measuring cosmological expansion. Binational Foundation, an Award for Research Cooperation and supplies direct evidence for a single-degenerate High Excellence in Science from the Bundesministerium für Bildung progenitor with a red giant companion, and our References and Notes und Forschung, and the Lord Seiff of Brimpton Fund. The data data suggest a link between SNe Ia that show 1. H. Aihara et al., Astrophys. J. Suppl. Ser. 193, 29 (2011). presented in this paper are available from the Weizmann Interactive Supernova Data Repository (www.weizmann.ac.il/astrophysics/wiserep). weaker signatures of the circumstellar environ- 2. D. E. Welty, D. C. Morton, L. M. Hobbs, Astrophys. J. Suppl. Ser. 106, 533 (1996). Supplementary Materials ment (SN 2006X, SN 2007le, and Kepler's SN) Sov. Astron. 3. I. P. Pskovskii, 21, 675 (1977). www.sciencemag.org/cgi/content/full/337/6097/942/DC1 Astrophys. J. and those that show stronger signatures (SN 4. M. M. Phillips, 413, L105 (1993). Supplementary Text 2002ic and SN 2005gj). The fraction of SNe Ia 5. P. Nugent, M. Phillips, E. Baron, D. Branch, P. Hauschildt, Fig. S1 Astrophys. J. that exhibit prominent circumstellar interaction 455, L147 (1995). Tables S1 to S4 et al Astrophys. J. ∼ 6. W. Li ., 546, 734 (2001). References (36–57) near maximum light is 0.1 to 1%, but more 7. G. Garavini et al., Astron. J. 128, 387 (2004). subtle indications of a symbiotic progenitor could 8. W. Li et al., Mon. Not. R. Astron. Soc. 412, 1441 (2011). 16 January 2012; accepted 5 July 2012 be missed in typical SN observations (18). Addi- 9. F. Patat et al., Science 317, 924 (2007). 10.1126/science.1219164

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