2016/11/14

Peculiar Extinction Laws observed
for 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 curves
at 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 SEDs 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
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

with Rv = 1.5 - Astier+2006
2.02-3. How peculiar is SNe Ia extinction curves?
in the MW
Rv ~ 2 - 6
Rv,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. Fitting to CCM curves with Rv = 1-2What 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

3-2. 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)

3-3. 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. Summary of this talk1) SNe Ia are good targets to derive theextinction curve in external galaxies

2) Many studies suggest that Rv values
toward SNe Ia are very low (Rv = 1-2),
compared with Rv = 3.1. in the MW

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