2016/12/13

Properties of Dust Responsible for Extinction
Laws toward Type Ia Supernovae- Ia型超新星の減光則を担うダストの性質 野沢 貴也（Takaya Nozawa）
国立天文台 理論研究部(National Astronomical Observatory of Japan)- Nozawa, T. 2016, PSS (special issue for Cosmic Dust VIII), 133, 26- Nagao, T., Maeda, K., Nozawa, T. 2016, ApJ, 823, 104- Maeda, K., Nozawa, T., Nagao, T., Motohara, K. 2015, MNRAS, 452, 3281

1-1. Extinction curves in the Milky Way

Data: Fitzpatrick & Massa (2007)

in the MW
Rv = 2.2―6
Rv,ave = 3.1〇 CCM relation (Cardelli, Clayton, Mathis 1989)➜ describes the variety of extinction curves in the MW throughtotal-to-selective extinction ratio Rv = Av/E(B-V) = AV/(AB–AV)

1-2. Deriving extinction curves in the MW
Pair method
I0

I0 exp(-τext)

I0
same spectral type
○ Light sources: OB stars (or RGs)
- luminous (~ 105 Lsun)➜ we can see a large volume- UV (or IR) bright➜ variation of extinction curves
at UV wavelengths

1-3. Applicability of OB stars in pair method‐ OB stars can be used only for MW, LMC, SMC, (M31)➜ too faint to be observed in external galaxies
Extinction curves in external galaxies are poorly known‐ QSOs and GRB afterglows➜ good light sources to extract the extinction curves at z = 0-6.5

However …
- intrinsic spectral energy distributions are not always established- local dust may also contribute the observed extinction

Gallerani+2010

2-1. Type Ia SNe as standard light sources
light curveof SNe Ia○ Type Ia supernovae (SNe Ia)‐ thermonuclear explosion of a WD

‐ highly luminous (Lpeak ~ 3x109 Lsun)➜ homogeneous peak luminosity➜ used as standard candles‐ intrinsic opt/IR spectral established➜ SN 2011fe as an unreddened template‐ discovered in all types of galaxies➜ star-forming, elliptical, spiral etc…

Phillips+1993
Riess+1996

SN 2011fe(Nugent+2011)

good targets to probe the extinction(dust) properties in external galaxies

2-2. Extinction laws measured for SNe Ia○ Many samples of SNe Ia
MB = mB - 5 log10SN(DL2014J) + 5 - AB
Foley+2014➜ Rv = 1.0 - 2.5

to minimize
Rv ~the1.5dispersion of
Hubble diagram(e.g., Tripp+1998; Conley+2007; Phillips+2013)- less reddened: Av < ~ 1.0 mag○ Individual (nearby) SNe Ia- highly reddened (Av > ~2.0 mag)- reliable extinction curves
Howell+2011

Rv values measured for SNe Ia
are unusually low (Rv ~ 1.0-2.5)➜ described by CCM curves
Astier+2006

with Rv = 1.5 - 2.0

2-3. How peculiar is SNe Ia extinction curves?
in the MW
Rv = 2.2 －6
Rv,ave = 3.1

smaller size

〇 CCM relation(Cardelli, Clayton, Mathis 1989)

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

Aλ/AV = a(x) + b(x) / Rv
where x = 1 / λlarger size
Data: Fitzpatrick& Massa (2007)‐steeper extinction curve (lower Rv) ➜ smaller grains‐flatter extinction curve (higher Rv) ➜ larger grains13-1. What is the cause for unusually low Rv?
(1) Unique properties or effects ofcircumstellar dust around SN Ia(2) Peculiar properties of interstellar
dust in host galaxies of SN Ia(3) Non-standard properties ofextragalactic dust ➜ unlikely(4) Something is wrong in deriving the extinction laws
toward SNe Ia

3-2. Multiple scattering scenario by local dust
○ Multiple scattering scenario‐multiple scattering by circumstellar dust steepens extinction curves(Wang 2005; Goobar 2008; Amanullah & Goobar 2011)

circumstellar
dust shell of

τv ~ 1scattering

Goobar (2008)
SN Ia
scatteringscatteringradiative transfer calculations

3-3. Concern for multiple scattering scenario○ Goobar (2008)- LMC dust (WD01) : ω(B)/ω(V) < 1 ➜ steepen- MW dust (WD01) : ω(B)/ω(V) < 1 ➜steepen- SMC dust (WD01) : ω(B)/ω(V) > 1 ➜ flatten
Nagao, Maeda, TN (2016)○ Nagao, Maeda, TN (2016)- LMC dust (MRN77) : ω(B)/ω(V) > 1 ➜ flatten- MW dust (Pei92) : ω(B)/ω(V) > 1 ➜ flatten- SMC dust (Pei92) : ω(B)/ω(V) > 1 ➜ flatten

highly model-dependent !!
Multiple scattering does not always steepen the extinction curves

3-4. Problem on multiple-scattering scenario○ No excess of NIR emission- conservative upper limits of optical depth
in B band is τB < ~0.1
- no massive dusty envelope around SNe Ia
Maeda, TN, Nagao,
Motohara (2015)

circumstellar
dust shell of

τv ~ 1absorption

infrared

SN Iascattering

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

infraredabsorption

3-5. What is the cause for unusually low Rv?
(1) Unique properties or effects ofcircumstellar dust ➜ unlikely(2) Peculiar properties of interstellar
dust in host galaxies of SN Ia(3) Non-standard properties ofextragalactic dust ➜ unlikely(4) Something is wrong in deriving the extinction laws
toward SNe Ia

4-1. Fitting to CCM curves with Rv = 1-2
What properties of dust cause steep extinction curves?
○ Data on extinction curves to be fitted
CCM extinction curves with Rv = 2.0, 1.5,
1.0 at representative photometric bands

Nozawa (2016)○ Interstellar dust model(e.g., Mathis+1977, Draine & Lee 1984)- graphite & astronomical silicate- power-law grain size distribution- Model 1 (simplest model)
same size distribution with q = -3.5 andamin = 0.005 µm for two grain species
parameters:- amax (upper cutoff radius)- f_gs (graphite-to-silicate mass ratio)

amax = 0.24 µm
f_gs = 0.57

black dots: data of extinction Aλi/Av
derived from the CCM formula atphotometric bands

4-2. Results of fitting calculations(b) Rv = 1.5(a) Rv = 2.0

amax = 0.13 µm
f_gs = 0.46

amax = 0.094 µm
f_gs = 0.49
Nozawa (2016)(c) Rv = 1.0- steep extinction curves with
Rv=1-2 can be described by
the power-law grain model- f_gs = 0.45-0.6➜ Mgra/Mtotal = 0.3-0.4
amax = 0.057 µm
f_gs = 0.60

cf. f_gs = 0.3-0.7 in the MW(Nozawa & Fukugita 2013)

4-3. Unusual dust properties: selection bias?
SN 2011iv in NGC1404© S. Kohle

SN 2014J in M82© NASA/ESA Foley&McCully

SNe Ia appear in any type of galaxies!

SN 2006X in M100

SN 2003cg in NGC3169©ESO, Igor Chkalin

4-4. Selection effects of sightlines?
In the MWOB starIn the external galaxies

GCGCSN Ia

Reddened SNe Ia:high Av but small Rv !

5. Summary of this talk1) Many studies suggest that Rv values
toward SNe Ia are very low (Rv = 1-2),compared with Rv = 3.1 in the MW2) Non-detection of IR echoes toward SNe Ia
indicates that the low Rv is not caused bycircumstellar dust but by interstellar dustin the host galaxies

3) The CCM curves with Rv = 1-2 can benicely fitted by power-law grain size
distributions with amax = 0.05-0.15 µm