# A year-long plateau in the late-time near-infrared light curves of type Ia supernovae

## Abstract

The light curves of type Ia supernovae are routinely used to constrain cosmology models. Driven by radioactive decay of 56Ni, the light curves steadily decline over time, but after 150 d post-explosion the near-infrared portion is poorly characterized. We report a year-long plateau in the near-infrared light curve at 150–500 d, followed by a second decline phase accompanied by a possible appearance of [Fe i] emission lines. This near-infrared plateau contrasts sharply with type IIP plateaux and requires a new physical mechanism. We suggest a masking of the ‘near-infrared catastrophe’—a predicted, yet unobserved, sharp light-curve decline—by scattering of ultraviolet photons to longer wavelengths. The transition off the plateau could be due to a change in the dominant ionization state of the supernova ejecta. Our results help explain the complex radiative transfer processes that take place in type Ia supernovae and enhance their use as ‘standard candles’.

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## Data availability

HST observations are available through MAST. Supplementary Tables 1 and 2 and the data for Figs. 2–5 (Supplementary Data 1, 2, 3 and 4) are provided as machine-readable tables. Any further data are available from the corresponding author on reasonable request.

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## Acknowledgements

We thank G. Brammer, D. Eisenstein, W. Kerzendorf, S. Sim and L. Strolger for helpful discussions and comments, S. Dhawan for sharing his spectra of SN 2014J and A. Calamida, W. Eck, M. Gennaro and W. Januszewski for supporting the HST programs used here. O.G. is supported by an NSF Astronomy and Astrophysics Fellowship under award AST-1602595. K.M. acknowledges support from H2020 through an ERC Starting Grant (758638). M.N. is supported by a Royal Astronomical Society Research Fellowship. R.F. acknowledges support from NASA ATP award 80NSSC18K1013. This work is based on data obtained with the NASA/ESA HST, all of which was obtained from the Mikulski Archive for Space Telescopes (MAST). Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. This work is based on data taken at the European Organization for Astronomical Research in the Southern Hemisphere, Chile, under programme IDs 0100.D-0242(A) and 0101.D-0443(A). The research has made use of NASA’s Astrophysics Data System and the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. The NIST databases were funded, in part, by NIST’s Standard Reference Data Program (SRDP) and by NIST’s Systems Integration for Manufacturing Applications (SIMA) Program. Finally, this work has made use of the Open Supernova Catalog78.

## Author information

O.G. planned the observations for HST programs GO–15686 and 15693, reduced the HST imaging data, performed the analysis and wrote the manuscript. K.M. obtained the ground-based spectra of SN 2017erp. M.N. reduced the Gemini spectra of SN 2014J. R.R. reduced the grism observation of SN 2017erp obtained through HST program GO–15686. A.A. measured Δm100(H) values for the correlation study. A.G.R. planned the HST observations for programs GO–12880, 15145 and 15640. I.R.S., R.F. and L.S. assisted with the theoretical analysis of the observations.

Correspondence to O. Graur.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Astronomy thanks Yi Yang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Captions for Supplementary Tables 1 and 2 and Supplementary Data 1–5.

### Supplementary Table 1

Machine-readable table presenting HST photometry of SNe 2012ht, 2013dy, 2017erp, 2018gv and 2019np.

### Supplementary Table 2

Machine-readable table presenting synthetic photometry of SN 2014J.

### Supplementary Data 1

Source data for Fig. 2.

### Supplementary Data 2

Source data for Fig. 3a.

### Supplementary Data 3

Source data for Fig. 3b.

### Supplementary Data 4

Source data for Fig. 4.

### Supplementary Data 5

Source data for Fig. 5: machine-readable table presenting the HST grism spectrum of SN 2017erp.

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