paper started out as a fishing expedition. For the last few years, I
Hubble Space Telescope
they are >500 days past maximum light.
In parallel, Adam
has been using
HST to calibrate the SN Ia distance ladder by
observing variable stars in nearby SN
Most of the
in these galaxies were years old, but some happened to still be visible. Since he knew
about my interest in late-time observations of SNe Ia, he invited me
to see what I could do with one
of the SNe in his data, SN
data for SN 2017erp would only range between ~200 and 500 days, but
of my previous observations were in the optical, so I was curious to
see whether the SN would behave similarly in the near-infrared F160W
It did not. Instead of declining in brightness, the light curve stayed pretty flat for nearly 300 days. I collected four more SNe Ia from Adam’s various HST programs (Figure 1). One of them - SN 2018gv, observed at similar phases as SN 2017erp - showed the same F160W plateau.
on sparse data, three
studies also noted
of a plateau.
However, each of those SNe had been observed at different phases, and
none of the studies had compared their observations to previous ones.
SNe 2017erp and 2018gv spanned the entire length of the plateau.
Taken together with the other three SNe from our study, and as many
SNe as I could find in the literature, the full story of the plateau
SNe Ia transition onto a plateau phase in the near-infrared at ~150
days past maximum. The plateau lasts for nearly a year and, at ~500
days, the SNe fall off the plateau into a second decline phase.
discovering the plateau was a fishing expedition, analyzing it
was a collaborative effort. Russell Ryan reduced
an HST spectrum of SN 2017erp at ~600 days that,
compared to similar
spectra of the
SN acquired by Kate Maguire, indicated
that the transition off the plateau could be caused by
a change in the ionization state of the SN ejecta (Figure
Matt Nicholl reduced spectra of SN 2014J that showed that the plateau
was not observed in the K band. Arturo Avelino measured the light
curve properties of the SNe in our sample, from which we noticed
correlations between the features of the plateau and the luminosities
of the SNe. And Luke Shingles, Ivo Seitenzahl, and Robert Fisher
provided the theoretical analysis of the plateau, noting that it
could be due to scattering of photons from the UV to longer
Do all types of SNe Ia exhibit this plateau? Are SNe Ia distributed
along the plateau in a continuous fashion or are there “bright”
and “dim” plateau SNe? Are there other possible explanations for
the existence of the plateau? We hope that these, and many other
questions, will be sparked by our paper.
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