2016/09/16

Properties of Interstellar Dust as Probed
by Extinction Laws toward Type Ia
Supernovae

Takaya Nozawa(National Astronomical Observatory of Japan)
SN 2014J in M82 © NASA/ESA Foley & McCully

To be appeared in Planetary and Space Science
arXiv: 1608.06689, doi: 10.1016/j.pss.2016.08.006

1-1. 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 curvesat UV wavelengths

1-2. Applicability of OB stars in pair method‐ OB stars can be used only for MW, LMC, SMC, (M31)➜ OB stars are 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 properties are not always established- local dust may also contribute the extinction

Type Ia supernovae (SNe Ia ) could be a good target toderive the extinction curve in external galaxies

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 (L peak ~ 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…

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

Phillips+1993
Riess+1996

SN 2011fe(Nugent+2011)

2-2. Extinction laws measured for SNe Ia○ Many samples of SNe Ia
MB = mB - 5 log10(DL) + 5 - AB

➜ Rv = 1.0 - 2.5
Rv ~the1.5dispersion ofto minimize
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2-2. Extinction laws measured for SNe Ia○ Many samples of SNe Ia
SN 2014J

MB = mB - 5 log10Foley+2014(DL) + 5 - AB

➜ Rv = 1.0 - 2.5
Rv ~the1.5dispersion ofto minimize
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 - 6Rv,ave ~ 3.1

smaller size

〇 CCM relation(Cardelli, Clayton, Mathis 1989)

RV : total-to-selectiveextinction 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 grains

3-1. What is the cause for unusually low Rv?
○ Non-standard properties of interstellar dust in hostgalaxies of SNe Ia○ Different properties of extragalactic dust ➜ unlikely○ Unique properties or effects of circumstellar dust
around SNe Ia○ 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 ofτv ~ 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
circumstellar
dust shell of

τv ~ 1absorption

infraredMaeda, TN, Motohara (2015)
SN Ia
scattering

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?
○ Non-standard properties of interstellar dust in hostgalaxies of SNe Ia○ Different properties of extragalactic dust ➜ unlikely○ Unique properties or effects of circumstellar dust
around SNe Ia➜ unlikely○ 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 M100SN 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 MW

2) 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