Spectropolarimetry of SNe Ia: From Geometry to Physics

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Spectropolarimetry of SNe IaFrom Geometry to
Physics

• Dietrich Baade (ESO, Garching)
• Lifan Wang (AM University, College Station /
  Purple Mountain Obs., Nanjing)
• Peter Höflich (Florida State University,
  Tallahassee)
• Ferdinando Patat (ESO, Garching)
• Craig Wheeler (University of Texas, Austin)
• Alejandro Clocchiatti (Universidad Catolica,
  Santiago)

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Polarization results from asymmetry
Distorted/Decentered Photosphere Continuum/Line
Polarization
Point-symmetric Photosphere Spectral-Line
Polarization Only
Photosphere
Ca II
e-
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e-
e-
e-
e-
e-
e-
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Line forming, scattering atmosphere
Asymmetry does not need to be

• at source (photosphere) level intervening
  scattering screen possible

• in geometrical shape may also be in
  irradiation

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The Concept of Dominant Orthogonal Axes
Rule of thumb 1 polarization corresponds to
10-15 asymmetry (if oblate)
(N)
I90
U
Large-scale asymmetry only
I135
I45
Dominant axis
I0
(E)
Q
Q (I0-I90)/(I0I90) U (I45-I135)/(I45I135)
I90(N)
Orthogonal axis
U
I135
I45
I0(E)
Q
Additional unaligned smaller-scale structures
Polarimetry offers spatial resolution of order
2-3 (at any distance!)
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Typical Polarization Properties of SNe Ia

• Virtually all SNe Ia are intrinsically polarized
• Degree of polarization decreases with time
• vanishes one week after maximum
• ?cont const
• Pcont lt 0.5
• ? ? V 0.05 mag (if oblate,
• in any event vanishing one week after
  maximum)
• Spectral features lead their own lives

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1st example SN 2001el (Wang et al. 2003)
Day -4
Day 19
Day 19
Day -4
ISP
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SN 2001el (contd) Si II 6355 at 5 different
epochs
Day 39
Day -4
Day 19
Day 10
Day 2
Color shows velocity
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2nd example SN 2004dt(Wang et al. 2006)
A high-velocity SN (with considerable velocity
spread)
Structured envelope
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SN 2004 dt Velocity vs. Polarization Profiles
Si II, Mg II lines show significant polarization.
Peak blueshifted by 4000 km/sec
O I does NOT.
And yet, all these lines have about the same
velocity profile (i.e. same radial
distribution) ? Several non-global structures
Day -8
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SN 2004dt (contd)
On smaller scales (i.e. irrespective of any
global asymmetry) The ejecta are lumpy in Si II
6355 and OI 7774, but not in Mg II 4481 (The
chi2/Dof suggest good fits by a straight line
for the Mg II 4481 line ? dominant axis)
km/s
Day -8
10
Observations of a polarized standard star confirm
that the effect is not instrumental or due to the
error analysis.
km/s
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SN 2004dt (contd) an integrated polarization
view
Large line polarizations consistent with
deflagration
Decrease of continuum polarization hinting at
off-center explosion??
Low continuum polarization more characteristic of
detonation
Day -8
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Statistics(Wang et al. 2006)
Loose correlation between ?m15 and degree of
polarization across Si II 6355 line at day -5
Slower (brighter) SNe seem to be less polarized,
i.e. more spherical (in Si II 6355) No
projection effect ? Lumpy structure, not
overall asymmetry
The dotted lines show the 1-srange expected from
clumpy ejecta with the fitted mean degree of
polarization (solid line). The 91bg-like SN
1999by and the extremely highly polarized SN
2004dt were excluded from the fit.
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Evidence of Circumstellar Matter? SN 2001el
(Wang et al. 2003) Early, transient absorption
feature _at_800nm Interpreted as high-velocity CaII
IR triplet Higher, not overlapping velocity
? spatial (radial) separation Higher
polarization ? different
structure/shape Different polarization angle
? different orientation Obscuration
(vanishing with expansion) of photosphere by an
outer Ca-rich torus is one possible explanation
? Relic of accretion disk?
Si II 635.5 nm
800 nm (Ca II)
ISP
Day -4
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Spectropolarimetry
is 3D spectroscopy at low spatial resolution
but any distance. confirms that the
photospheres of SNe Ia are roughly
spherical early on, nearly perfectly so
one week after maximum. ... finds ubiquitous
sub-structures with distinct geometric,
dynamical, and chemical properties. may help
to discriminate between different explosion
models. can reveal circumstellar matter and
structures (e.g., due to binarity).
requires close interaction with theory to take
full effect.