Superluminous supernova SN 2006gy as a result ofcommon envelope merger between a white dwarf and a
massive star
Jerkstrand, Maeda and Kawabata, Science 2020, Vol. 367, 6476
Anders Jerkstrand
Department of Astronomy, Stockholm University

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SN 2006gy - one of the brightest SNe ever seen
Smith 2007,2008,2010, Ofek 2008, Agnoletto 2009, Miller 2010, Fox 2015, +more

I Type IIn.
I Rise time 70d, peak mag -22,radiated energy ⇠ 1051 erg.
Significant extinction(EB V = 0.5 0.75).
I Interaction with a massive(⇠ 10 M ), slow-moving (⇠ 100km/s) CSM indicated fromnarrow H lines. This CSM ejected. 100y before the SN.
I A diverse set of models proposed:e.g. pulsational pair instability SN,
an LBV core-collapse soon after a
Eta-Carina like eruption,...
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A pulsational PISN?

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Ia-CSM? An early idea that was then forgotten

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Strange, unidentified emission lines seen at +394d
Kawabata et al 2009

Ca and Fe only other lines seen.

I Line widths ! 1500 km s 1 expansion.
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Identification (10y later) : Fe I Jerkstrand, Maeda & Kawabata 2020
Theseseenlines
beforehaveinnotany

SN 2006gy at +1y.
NLTE model of 0.5 M

beenSN.
Fe I at 5000 K.

Modelling of the lines constrains the iron mass to 0.3 < MFe < 2.1 M .
Also, the brightness of the spectrum at +1y matches the decay of 0.5 M
56 Ni.
I Pulsational PISN? No. These have M56 Ni = 0.
I Core-collapse SN? Nja... These have M56 Ni . 0.2 M . Also no O
lines seen in SN 2006gy.

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Back to Ofek’s idea..could SN 2006gy be the results of awhite dwarf merging with a massive star, ejecting acommon envelope, and then exploding as a Ia SN?
Type Ia SNe make the right amount of 56 Ni (0.30.7 M ).

Causally connects the CSM ejection with the SN explosion ! only
model scenario that can account for the inferred <100y synchronization.
Efficient CE ejection when compact objects spirals into RG/RSG
envelopes has been demonstrated - timescales of years/decades.

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Preparing for model comparisons : detailed data reduction

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Testing the idea: Spectrum of a decelerated Ia SN at +1yfits reasonably well.
Spectral simulations with the SUMO NLTE code. W7 ejecta model withscaled down velocities (factor 7), with a few solar masses CSM mixed in.

Fe I lines emerge.
No flux rescaling - a major strength of the model.
Physical conditions (temperature, ionization) satisfactory.
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Testing the idea: The light curve properties also work out
SNEC with a 2-parameter CSM (MCSM ,RCSM ).
The CSM mass controls both light curve duration and iron deceleration.

I Too large CSM masses give interaction for too long and decelerates theiron too much.
I Too small CSM masses give too fast rise and too bright peak, andinsufficient iron deceleration.
I A ⇠ 10 15 M CSM gives the right properties.
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Questions raised if WD-RG/RSG merger is the rightexplanation
1. How do you get a WD close to a RG or RSG star?
2. How do you get it to spiral in, eject virtually all the envelope, andmerge with the core of the other star?
3. How do you get it to explode?

Support in the binary stellar evolution and CE literature for (1) and (2), e.g.
Tutokov 1993, Tauris & Sennels 2000, Terman+1994,1995,
Sandquist+1998, Ablimit 2021.
Little known about (3) - major di↵erences if companion is AGB star (WDcore) or RSG (He core).

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Ia-CSM? An early idea that was then forgotten

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Ia-CSM? An early idea that was then forgotten

10 / 12Others?

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SummaryI A large iron reservoir (⇠ 0.5 M ) identified in the superluminous
Type IIn supernova SN 2006gy, almost certainly coming from 56 Ni. This
rules out several previous scenarios such as shell collisions (these are56 Ni-free).
I A model scenario of a white dwarf merging with a massive
companion and then exploding into the ejected CE can reproduce
both light curve and spectra well : only scenario that explains why a SNoccurs soon after (< 100y) a massive CSM creation.
I Best-fitting CSM mass is 10-15 M which would suggest a RSG rather
than a RG companion. But may be lower for sub-Chandra white dwarfs.
I If the scenario is correct, important new constraints on both CE andWD explosion physics.
Thank you!

12 / 12Fe mass modelling
105

Ionization degree x = nFeII/nFeI

104
103

UnclumpedIon(f=.b1)alanceCl uconmps tredain(f=t0. 1)

Matching 56Co
luminosity
at +394d102
clumped (f=0.1)
Al l101
100

Spectralconstraints,
unclumped (f=1)

10 1
10 2
0.01

owed

0.1

1

10Fe mass (M )12 / 12

WD-RSG binaries

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Plunge-in

I If the companion is massive enough (>5 times the WD mass), thesystem will never settle into RLOF accretion but the WD will plunge
into the companion.
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Explosion
1. Merger with a RG (AGB)
star. WD-RG CE merger likely
channel to produce WD-WD
close binaries (normal Iaprogenitors). With an AGB starcompanion another WD ready(! Super-Chandra merger
explosion). Some tension withestimated CSM mass in SN
2006gy.
2. Merger with a RSG.
Sub-Chandra double detonation
explosion as WD merges with He
core. No tension with estimatedCSM mass.
Ivanova 2013.

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