# Presentation: 408281

## Spectropolarimetry of SNe Ia:From Geometry to Physics

_**Spectro**_**polarimetry of SNe Ia:****From Geometry to Physics**

**Dietrich Baade (ESO, Garching)**
**Lifan Wang (A&M University, College Station / Purple Mountain Obs., Nanjing)**
**Peter H****öflich (Florida State University, Tallahassee)**
**Ferdinando Patat (ESO, Garching)**
**Craig Wheeler (University of Texas, Austin)**
**Alejandro Clocchiatti (Universidad Catolica, Santiago)**

## Polarization results from asymmetry

**Photosphere**

**Line forming, scattering atmosphere**

- e-

- e-

- e-

- e-

- e-

- e-

- e-

- e-

- e-

- e-

- e-

- e-

- e-

- Ca II

**Distorted/Decentered Photosphere:**
**Continuum/Line Polarization**

**Point-symmetric Photosphere: **
**Spectral-Line Polarization Only**

**Asymmetry does not need to be **

**  ****at source (photosphere) level: intervening scattering screen possible **

**  ****in geometrical shape: may also be in irradiation**** **

## The Concept of Dominant + Orthogonal Axes

- Q

- U

- Q

- U

- (N)

- (E)

- Q = (I0-I90)/(I0+I90)
- U = (I45-I135)/(I45+I135)

- I0

- I135

- I90

- I45

- I0(E)

- I45

- I90(N)

- I135

**Large-scale asymmetry only**

**Additional unaligned smaller-scale structures**

**Polarimetry offers spatial resolution of order 2-3 (at any distance!)**

**Rule of thumb: 1% polarization corresponds to 10-15% asymmetry (if oblate)**

**Dominant axis**

**Orthogonal axis**

## Typical Polarization Properties of SNe Ia:

**Virtually all SNe Ia are intrinsically polarized**
**Degree of polarization decreases with time; **
**    ****vanishes ~one week after maximum**
**θ****cont**** ~const**
** ****P****cont**** < 0.5 %**
** ****►****  ****Δ**** V ~0.05 mag (if oblate, **
**     ****in any event vanishing ~one week after maximum)**
**Spectral features lead their own lives**

## 1st example: SN 2001el (Wang et al. 2003)

**1****st**** example: SN 2001el**** (Wang et al. 2003)**

- Day -4

**Day -4**

**Day 19**

**ISP**

## SN 2001el (cont’d): Si II 6355 at 5 different epochs

**Day -4**

**Day 19**

**Day 10**

**Day 2**

**Day 39**

**Color shows velocity**

## 2nd example: SN 2004dt(Wang et al. 2006)

**A high-velocity SN **
**(with considerable velocity spread)**

**Structured envelope**

## SN 2004 dt: Velocity vs. Polarization Profiles

- Peak blueshifted
- by 4000 km/sec

**Si II, Mg II lines show**
**significant polarization.**

**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**

## SN 2004dt (cont’d) 

**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 chi^2/Dof suggest**
**good fits by a straight line for the Mg II 4481 **
**line  ****► dominant axis****)**

**SN 2004dt**** ****(cont’d)**** **

**km/s**

**Day -8**

## Observations of a polarized standard star confirm that the effect is not instrumental or due to the error analysis. 

**Observations of a polarized standard star confirm that the effect is not instrumental or due to the error analysis.**** **

**km/s**

## SN 2004dt (cont’d):  an integrated polarization view

**SN 2004dt (cont’d):  an integrated polarization view**

| 
Feature | Pol. Degree |
| --- | --- |
| Si II 6355 | 2.5% |
| Si III | 0.2% |
| Mg II 4481 | 1.2% |
| Ca II IR Triplet | 1.5% |
| S II  | 0.4% |
| OI 7774 | -0.1% |
| Continuum | ~-0.3% |

**Large line polarizations **
**consistent with deflagration**

**Low continuum polarization**
**more characteristic of detonation**

**Decrease of continuum polarization hinting at off-center explosion??**

**Day -8**

## Statistics:(Wang et al. 2006)

**Slower (brighter) SNe seem to be less polarized, i.e. more spherical  **
**(in Si II 6355)**
**No projection effect  **
**► ****Lumpy structure, **
**   ****not overall asymmetry **

**Loose correlation between **
**Δ****m****15**** and degree of polarization across **
**Si II 6355 line at day -5**

**The dotted lines show the **
**1-****σ****range 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.]**

## Evidence of Circumstellar Matter?

_**Evidence of Circumstellar Matter?**_
_**SN 2001el**_** (Wang et al. 2003)**
**Early, transient absorption feature @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**

## 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. **