2012/08/07

What can the Interstellar
Extinction Curves Tell us
About?
Takaya Nozawa
Masataka Fukugita(Kavli IPMU, University of Tokyo)

1. Introduction
Extinction curve: wavelength-dependence ofinterstellar extinction caused by dust grains
・ necessary for correcting the SEDs of stars/galaxies➔ especially, extragalactic objects whose appearances
are disturbed by the Galactic interstellar extinction
・ depends on the physical and optical properties of dust
➔ provides information on the composition and size
distribution of interstellar dust on the line of sight➔ holds important clues to the origin and evolution
history of interstellar dust (e.g., Hirashita & Nozawa 2012)

2. Average interstellar extinction curves in MW
It seems the Galactic average
extinction curve is universalBV〇 CCM formula (Cardelli, Clayton & Mathis 1989)
・ Aλ/AV = a(x) + b(x) / RV, where x = 1 / λ・ RV : ratio of total-to-selective extinction
RV = AV / (AB – AV) cf. RV,ave = 3.1

3. Interstellar dust models in MW〇 MRN dust model (Mathis, Rumpl & Nordsieck 1977)
・ dust composition : silicate & graphite・ size distribution : power-law distributionn(a)da ∝ a^{-q}da with q = 3.5, 0.005 μm ≤ a ≤ 0.25 μm〇 WD01 model(Weingartner & Draine2001)

・ dust composition :silicate + graphite +PAHs

・ size distribution :power-law withexponential decay +lognormal
0.3 nm ≤ a ≤ 1 μm

4. Variety of interstellar extinction curves
・ There are a large variety of interstellar extinction curves
・ How much can the properties of dust grains be changed?
gray curves:
328 extinction curves
derived by Fitzpatrick& Massa (2007, FM07)
red bars:
1σ ranges including
224 data

Our goal is to find the dustblue
properties
bars:
that satisfy these extinction2σranges
ranges including
312 data

5-1. Comparison between FM07 and CCM89
UV-through-IR extinction curves

Close-up of IR extinction curves

black: 1σ range of the FM07 data
red: CCM curve with Rv = 2.75
blue: CCM curve with Rv = 3.60
green: extinction curve for thecase of Rv=3.1 by WD01
fully consistent in UV region

Results from CCM formula withRv = 2.75-3.60 are 0.02-0.06 maghigher than the 1σ range in JHK
WD01 model is based on result
by Fitzpatrick (1999), which issimilar to CCM curve w/ Rv=3.1

5-2. What is the difference in IR extinction?
NIR extinction is interpolated by power-law formula Aλ/Av ∝ λ^-αCCM ➔ α = 1.61

FM07 ➔ α = 1.84

CCM : α = 1.61
from Rieke & Lebofsky (1985)

FM07 : α = 1.84
from Martin & Whittet (1990)

α = 1.7 (He et al. 1995)
α = 1.8 (Froebrich et al. 2007)
α = 2.0 (Nishiyama et al. 2006)
α = 2.3 (Larson & Whittet 2005)
α = 2.6 (Gosling et al. 2009)

6. Dust model(spherical grain)

・ power-law size distribution (amin < a < amax)

amin = 0.005 um
q, amax : parameters (same for different grain species)fi,j ➔ a fraction of an element i locked up in a grain j－ graphite, glassy carbon, amorphous carbon－ astronomical silicate (MgFeSiO4), Mg2SiO4－ Fe, Fe3O4
to search for the combination of q and amax (and fi,j)that fulfill the observed extinction ranges

7. Illustration of contour plots
1σ range of FM07 dataContour plots for graphite

UVUB
JHK

The 1σ ranges from FM07 data
are classified into three groups
UV: UV bump (0.22 μm), FUV dip(0.16 μm), FUV rise (0.125 μm)
UB: U band and B band
JHK: J band, H band, K band

A contour plot is depicted foreach of the groups defined inthe left panel
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

8-1. Contour plots for fgra/fsil = 1.0
Case of 1σ data, fgra/fsil = 1.0

Case of 1σ data, fgra/fsil = 1.0

contour plots of amax and q that
fulfill the 1σ range of FM07 data
for fgra/fsil = 1.0 (Mgra/Msil = 0.78)
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

contour plots of amax and q that
fulfill the 1σ range of CCM result
for fgra/fsil = 1.0 (Mgra/Msil = 0.78)
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

8-2. Contour plots for fgra/fsil = 0.5
Case of 1σ data, fgra/fsil = 0.5

Case of 1σ data, fgra/fsil = 0.5

contour plots of amax and q that
fulfill the 1σ range of FM07 data
for fgra/fsil = 0.5 (Mgra/Msil = 0.39)
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

contour plots of amax and q that
fulfill the 1σ range of CCM result
for fgra/fsil = 0.5 (Mgra/Msil = 0.39)
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

8-3. Contour plots for fgra/fsil = 0.2
Case of 1σ data, fgra/fsil = 0.2

Case of 1σ data, fgra/fsil = 0.2

contour plots of amax and q that
fulfill the 1σ range of FM07 data
for fgra/fsil = 0.2 (Mgra/Msil = 0.16)
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

contour plots of amax and q that
fulfill the 1σ range of CCM result
for fgra/fsil = 0.2 (Mgra/Msil = 0.16)
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

8-4. Brief summary of our results

Almost all of the dust models considered here do nothave combinations of q and amax that meet extinctionranges when the CCM NIR extinction is considered9-1. Piled-up contour for graphite-astro.sil
Piled-up contour

Values of q and amax that meetthe 1σ range of FM07 data are
confined to be narrow ranges
3.2 < q < 3.7
0.19 um < amax < 0.34 um

9-2. Piled-up contour for carbon-astro.sil
30 % of C included in glas.car

30 % of C included in amor.car

70 % of C ➔ graphite
30 % of C ➔ glassy carbon

70 % of C ➔ graphite
30 % of C ➔ amorphous carbon

The thin lines are for graphiteastronomical silicate

The thin lines are for graphiteastronomical silicate

9-3. Piled-up contour for carbon-asil-Fe bearing
50 % of Fe included in Fe grain

50 % of Fe included in Fe3O4

50 % of Fe ➔ astron.silicate
50 % of Fe ➔ Fe grains

50 % of Fe ➔ astron.silicate
50 % of Fe ➔ Fe3O4 grains

The thin lines are for graphiteastronomical silicate

The thin lines are for graphiteastronomical silicate

9-4. Piled-up contour for graphite-Mg2SiO4Graphite-Mg2SiO4Graphite-Mg2SiO4-Fe

astronomical silicate (MgFeSiO4)
is replaced with Mg2SiO4

all of Fe atoms are locked up inFe grains

The thin lines are for graphiteastronomical silicate

The thin lines are for graphiteastronomical silicate

10. Summary
・ The observed ranges of NIR extinction from FM07
do not match with the results from the CCM formula➔ The average interstellar extinction curve is notnecessarily universal in NIR regions
・ For the power-law grain-size distribution－ The values of q and amax that satisfy the observed
1σ ranges of FM07 are confined to narrow ranges－ There is no combination of q and amax that satisfy
the observed ranges when CCM results are adopted－ For any combinations of grain species considered,the values of q and amax that meet the observed
extinction are distributed around 3.5 and 0.25 um