Destruction of Dust by Supernova Shocks
Kazik Borkowski
North Carolina State University
ULIRG Workshop Cornell, June 2006
Dust Destruction Mechanisms
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Early adiabatic stages of supernova remnant evolution – sputtering by H and He in hot X­ray emitting plasmas heated by fast shocks.
Late radiative stages of supernova remnant evolution – mutual collisions between grains accelerated by the betatron process in slow radiative blast waves (grain vaporization, cratering and shattering).
Grain Sputtering
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Grains are destroyed by sputtering by H and He in cosmic abundance plasmas.
Sputtering cross sections from Bianchi & Ferrara (2005) for graphite (green) and silicates (blue)
“Nonthermal” sputtering (not included above) particularly important at low T.
Enhanced Sputtering of Small Grains
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Jurac, Johnson, & Dunn (1998) ●
Very important in SNRs
Enhanced Sputtering Rates
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Tp= 1 keV (blue), Tp= 10 keV (green)
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Flattening of curves at small grain radii is an artifact.
Grain Size Distribution
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Provisional dust model of Weingartner & Draine (2001).
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Separate carbonaceous (green) and silicate (blue) grain populations.
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Most dust mass in silicates.
Grain Size Distribution after Sputtering
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shock sputtering age – 5000 yr/cm3, Tp = 1 keV
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green (light blue) – pre­(post­)shock silicates, blue(red) ­ graphite
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small grains are preferentially destroyed
Grain Emission
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Cabs depends on grain material.
Absorption is generally only weakly dependent on grain temperature.
Cabs is not constant in IR – grains do not radiate as blackbodies.
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Cabs is approximately proportional to grain volume.
Grain Heating
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Grains are heated in collisions with electrons (and also ions in very fast shocks), which deposit their kinetic energy in grains.
Particles can reflect from the grain surfaces, and they may penetrate grains and escape on their back sides. Hence the presence of factor h (less than unity).
Steady State
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If energy absorbed in a single collision, Eabs, is much less that the grain enthalpy U(T), then grain temperature Tss does not vary with time (steady state). This is always true for sufficiently large grains, because U is proportional to the number of atom within the grain.
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Smaller grains are hotter than larger grains, because the cooling/heating ratio increases with grain radius.
In view of a large range in grain radii in the ISM, modeling of IR spectra with a single grain temperature can be misleading.
Stochastic Heating
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If Eabs is comparable or larger than U(Tss), then steady state does not hold.
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“Flickering” grains with time­varying temperature must be considered.
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Small grains dominate short­wavelength emission.
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Dust emission in the IRAC bands in SNRs must be produced by “flickering” PAH grains except for very dense gas.
Stochastic heating important even at longer wavelengths of the MIPS 24 micron band.
IR spectra are sensitive to the grain size distribution.
A Sample Spectrum
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np = 5 cm­3, T = 1 keV, shock model shown previously
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blue ­ graphite, green – silicates, red ­ total
Imaging Survey of Supernova Remnants in Magellanic Clouds
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IRAC and MIPS Imaging from 3.6 to 160 microns
Main goals: (1) examine how interstellar dust is destroyed, (2) find how much dust forms in supernova explosions
SNRs with ejecta imaged in all IRAC and MIPS bands
SNRs with detectable X­ray emission imaged in the MIPS 24 and 70 micron bands
Remaining SNRs imaged in the MIPS 24 micron band
Type Ia SNRs DEM L71 and 0548­70.4
MIPS 24 microns MIPS 70 microns
H
Chandra X­rays
Type Ia SNRs 0509­67.5 and 0519­69.0
MIPS 24 microns
H
Chandra X­rays
Shock Model for DEM L71: Input
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We derived plasma parameters by modeling optical and X­ray spectra.
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Te = 0.65 keV, Ti = 1.1 keV, ne = 2.7 cm­3 , t = 4400 yr.
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plane­shock shock sputtering age sputter = npt/3.
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The preshock grain size distributions of carbonaceous and silicate grains from Weingartner & Draine (2001).
Shock Model for DEM L71: Results
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Observed 70/24 micron band ratio = 5.1
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Model 70/24 micron band ratio without sputtering = 2.3
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Model 70/24 micron band ratio with sputtering = 5.1
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Destruction of small dust grains – 35% of dust has been destroyed.
Dust mass = 0.034 solar masses (obtained by matching model 24 micron flux to the DEM L71 24 micron flux).
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Inferred preshock dust/gas mass ratio = 0.00042
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Expected preshock dust/gas mass ratio = 0.0025
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We find less dust than expected.
Dust/Gas Mass Ratio in LMC SNRs
Grain­Grain Collisions
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Observational evidence exists for destruction of grains in radiative shocks.
Modeling done by Tielens et al. (1994), Borkowski & Dwek (1995), Jones, Tielens, & Hollenbach (1996), Slavin, Jones, & Tielens (2004).
Cratering, shattering, and vaporization rates are based on extrapolation from macroscopic bodies.
Conclusions
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Collisionally­heated dust is present in X­ray bright SNRs.
It is most easily detected in the MIPS 24 micron band.
Dust is a useful density diagnostic of X­ray emitting plasmas.
Dust destruction rates can be deduced by measuring and modeling IRAC and MIPS band ratios.
Dust is less abundant in SNRs than expected – dust destruction rates underestimated?