2015/08/17

On the reddening law observed
for Type Ia Supernovae
Takaya Nozawa(National Astronomical Observatory of Japan)

1-1. Extinction law towards Type Ia SNe○ Type Ia supernovae (SNe Ia)

light curveof SNe Ia‐ thermonuclear explosion of a white dwarf (WD)- progenitor system: (WD+MS) or (WD+WD)?
‐ discovered in all types of galaxies- star-forming, elliptical, irregular, etc …
‐ used as cosmic standard candles

Phillips+1993
Riess+1996

MB = mB - 5 log10(DL) - AB - 5➜ Rv = 1.0 ~ 2.5 （RV = AV/(AB – AV)）to minimize the dispersion of Hubble diagram(e.g., Tripp+1998; Conley+2007; Phillips+2013)

cf. Rv = 3.1 for the average extinction
curvei n the Milky-Way (MW)Astier+2006

1-2. Other examples of reddening for SNe Ia○ Other examples of Rv for SNe Ia‐ average of ensembles of SNe Ia
Rv = 1.0-2.3‐ from obtained colors of SNe Ia innear-UV to near-infrared (NIR)
Rv ~ 3.2 (Folatelli+2010)
Rv = 1.5-2.2(e.g., Elisa-Rosa+2008; Kriscinuas+2007)○ Extinction in nearby galaxies‐ M 31 (Andromeda Galaxy)
Rv = 2.1-3.1 (e.g., Melchior+2000; Dong+2014)‐ elliptical galaxies
Rv = 2.0-3.5 (Patil+2007)➜ Rv is moderately low or normal

Howell+2011

1-3. Peculiar extinction towards SN 2014J○ Type Ia SN 2014J‐ discovered in M 82 (D ~ 3.5±0.3 Mpc)
- closest SN Ia in the last thirty years- highly reddened (Av ~ 2.0 mag)‐ reddening law is reproduced by CCM
relation with Rv ~ 1.5(Ammanullah+2014; Foley+2014; Gao+2015)
Amanullah+2014

Gao+2015, Li’s talk1-4. How peculiar is SNe Ia extinction curves?
small size

〇 CCM relation(Cardelli, Clayton, Mathis 1989)

RV : ratio of total-toselective extinction
RV = Av / E(B - V)
= AV / (AB – AV)

Aλ/AV = a(x) + b(x) / Rv
where x = 1 / λlarge size
Data: Fitzpatrick& Massa (2007)

in our Galaxy
Rv = 2.2-5.5
RV,ave ~ 3.1‐steeper extinction curve (lower Rv) ➜ smaller grains‐flatter extinction curve (higher Rv) ➜ larger grains

2-1. Low Rv: interstellar or circumstellar origin?
○ Origin of low Rv observed for SNe Ia‐odd properties of interstellar dust
(e.g., Kawabata+2014; Foley+2014)‐multiple scattering by circumstellar dust
(Wang 2005; Goobar 2008; Amanullah & Goobar 2011)
Goobar (2008)

circumstellar
dust shell of

τv ~ 1

infraredabsorption

SN Ia
scattering

If there is a thick dust shell,
we must detect thermal dust
emission as infrared echoes

infraredabsorption

2-2. Near-infrared observations of SNe Ia○ Near-infrared (NIR) observations‐ no excess flux at JHK bands‐ IR echo model (thin shell approximation)
constrain the mass of dust for a given
position of the dust shell (Maeda, TN+2015)➜ conservative upper limits of opticaldepths in B band is τB < ~0.1

Maeda+2015○ Spitzer observations‐ no excess flux at 3.5/4.5 µm(Johansson+2015)‐ upper limit of dust mass: ~10-4 Msun
➜ optical depth τ << 1Johansson+2015

3-1. Dust model for Rv = 1.5 CCM curve〇 MRN dust model (Mathis, Rumpl, & Nordsieck 1977)- dust composition : silicate (MgFeSiO4) & graphite (C)- size distribution : power-law distributionn(a) ∝ a^{-q} with q=3.5, amax = 0.25 μm, amin = 0.005 μm

Rv = 1.5 curve ➜ amax = 0.085 µm, amin = 0.005 µm

3-2. Dust models for Rv = 1.0 and 2.0 curve

Rv = 1.0 curve ➜ amax = 0.055 µm, amin = 0.005 µm
Rv = 2.0 curve ➜ amax = 0.12 µm, amin = 0.005 µm
But, the values of Rv obtained from the MRN dust model arehigher than Rv used for the CCM relation
RV,CCM = 1.0 curve ➜ RV, dust = 1.5RV,CCM = 2.0 curve ➜ RV, dust = 2.1

3-4. Dependence on the power-law index‐ Decreasing the power-law index (steeper size distribution)
does not fit the CCM curve with a low Rv very well
➜ leading to a remarkable 2175A-bump and UV-dip➜ quite high Rv values obtained from the MRN dust model,
compared to the Rv used for the CCM relation

5. Summary of this talk1) Many studies (mainly SNe Ia cosmology) suggest that
the Rv values towards SNe Ia are very low (Rv ~ 1-2.5),
compared with Rv = 3.1 in our Galaxy2) Non-detection of IR echoes towards SNe Ia indicatesthat the low Rv is not due to the circumstellar dust
but due to the interstellar dust in the host galaxies3) The CCM curves with Rv = 1-2 can be reasonably fitted
by the MRN dust model (graphite/astronomical silicate) withamax = 0.05-0.15 µm (instead of amax = 0.24 µm for Rv = 3.1)