A Basic Course on Supernova Remnants

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A Basic Course on Supernova Remnants. Lecture #1 How do they look and how are observed? Hydrodynamic evolution on shell-type SNRs Lecture #2 Microphysics in SNRs - shock acceleration Non-thermal emission from SNRs. Order-of-magnitude estimates. SN explosion Mechanical energy:
A Basic Course onSupernova Remnants
  • Lecture #1
  • How do they look and how are observed?
  • Hydrodynamic evolution on shell-type SNRs
  • Lecture #2
  • Microphysics in SNRs - shock acceleration
  • Non-thermal emission from SNRs
  • Order-of-magnitude estimates
  • SN explosion
  • Mechanical energy:
  • Ejected mass:
  • Ambient medium
  • Density: Mej~Mswept when:
  • SIZE:
  • AGE:
  • Tycho – SN 1572“Classical” Radio SNRs
  • Spectacular shell-like morphologies
  • compared to optical
  • spectral index
  • polarization
  • BUT
  • Poor diagnostics on the physics
  • featureless spectra (synchrotron emission)
  • acceleration efficiencies ?
  • 90cm Survey4.5 < l < 22.0 deg(35 new SNRs found;Brogan et al. 2006)Blue: VLA 90cm Green: Bonn 11cmRed: MSX 8 mm
  • Radio traces both thermal and non-thermal emission
  • Mid-infrared traces primarily warm thermal dust emission
  • A view of Galactic PlaneSNRs in the X-ray window
  • Probably the “best” spectral range to observe
  • Thermal:
  • measurement of ambient density
  • Non-Thermal:
  • Synchrotron emission from electrons close to maximum energy (synchrotron cutoff)
  • Cassiopeia AX-ray spectral analysis
  • Lower resolution data
  • Either fit with a thermal model
  • Temperature
  • Density
  • Possible deviations from ionization eq.
  • Possible lines
  • Or a non-thermal one (power-law)
  • Plus estimate of thephotoel. Absorption
  • SNR N132D with BeppoSAXHigher resolution data
  • Abundances of elements
  • Line-ratio spectroscopy
  • N132D as seen with XMM-Newton(Behar et al. 2001)
  • Plus mapping in individual lines
  • Thermal vs. Non-ThermalCas A, with ChandraSN 1006, with ChandraShell-type SNR evolutiona “classical” (and incorrect) scenarioIsotropic explosion and further evolutionHomogeneous ambient mediumThree phases:
  • Linear expansion
  • Adiabatic expansion
  • Radiative expansion
  • Goal: simple description of these phasesIsotropic(but CSM)HomogeneousLinearAdiabaticRadiativeForward shockDensityReverse shockRadiusForward and reverse shocks
  • Forward Shock: into the CSM/ISM(fast)
  • Reverse Shock: into the Ejecta (slow)
  • rVshockStrong shockIfBasic concepts of shocks
  • Hydrodynamic (MHD) discontinuities
  • Quantities conserved across the shock
  • Mass
  • Momentum
  • Energy
  • Entropy
  • Jump conditions(Rankine-Hugoniot)
  • Independent of the detailed physics
  • Dimensional analysisand Self-similar models
  • Dimensionality of a quantity:
  • Dimensional constants of a problem
  • If only two, such that M can be eliminated, THEN expansion law follows immediately!
  • Reduced, dimensionless diff. equations
  • Partial differential equations (in r and t) then transform into total differential equations (in a self-similar coordinate).
  • Log(ρ)COREENVELOPELog(r)Early evolution
  • Linear expansion only if ejecta behave as a “piston”
  • Ejecta with and
  • (Valid for the outerpart of the ejecta)
  • Ambient medium with and
  • (s=0 for ISM; s=2 for wind material)(n > 5)(s < 3)Dimensional parameters and
  • Expansion law:
  • Evidence of deceleration in SNe
  • VLBI mapping (SN 1993J)
  • Decelerated shock
  • For an r-2 ambient profileejecta profile is derived
  • Self-similar models(Chevalier 1982)
  • Radial profiles
  • Ambient medium
  • Forward shock
  • Contact discontinuity
  • Reverse shock
  • Expanding ejecta
  • Approximation: pressure ~ equilibration
  • Pressure increases outwards (deceleration)
  • Conservation of entropy
  • Stability criterion (against convection) P and S gradients must be opposite
  • ns < 9 -> SFS, SRS decrease with timeand viceversa for ns < 9Always unstable regionfactor ~ 3n=7, s=2n=12, s=0Linear analysis of the instabilities+ numerical simulations(Chevalier et al. 1992) (Blondin & Ellison 2001) 1-D results, inspherical symmetry are not adequateThe case of SN 1006
  • Thermal + non-thermalemission in X-rays
  • (Cassam-Chenai et al. 2008)FS from Ha + Non-thermal X-raysCD from 0.5-0.8 keV Oxygen band (thermal emission from the ejecta) (Miceli et al. 2009)Why is it so important?
  • RFS/RCD ratios in the range 1.05-1.12
  • Models instead require RFS/RCD > 1.16
  • ARGUMENT TAKEN AS A PROOF FOR EFFICIENT PARTICLE ACCELERATION (Decouchelle et al. 2000; Ellison et al. 2004)
  • Alternatively, effectdue to mixing triggeredby strong instabilities
  • (Although Miceli et al. 3-Dsimulation seems still tofind such discrepancy)Acceleration as an energy sink
  • Analysis of all the effects of efficient particle acceleration is a complex task
  • Approximate modelsshow that distancebetween RS, CD, FSbecome significantlylower(Decourchelle et al. 2000)
  • Large compressionfactor - Low effectiveLorentz factor
  • FSDeceleration factorRS1-D HD simulation by BlondinEnd of the self-similar phase
  • Reverse shock has reached the core region of the ejecta (constant density)
  • Reverse shock moves faster inwards and finally reachesthe center.
  • See Truelove & McKee1999 for a semi-analytictreatment of this phaseThe Sedov-Taylor solution
  • After the reverse shock has reached the center
  • Middle-age SNRs
  • swept-up mass >> mass of ejecta
  • radiative losses are still negligible
  • Dimensional parameters of the problem
  • Evolution:
  • Self-similar, analytic solution (Sedov,1959)
  • Shocked ISMISMBlast waveThe Sedov profiles
  • Most of the mass is confined in a “thin” shell
  • Kinetic energy is also confined in that shell
  • Most of the internal energy in the “cavity”
  • Thin-layer approximation
  • Layer thickness
  • Total energy
  • Dynamics
  • Correct value:1.15 !!!from spectral fitsWhat can be measured (X-rays)… if in the Sedov phaseDeceleration parameterTycho SNR (SN 1572) Dec.Par. = 0.47SN 1006 Dec.Par. = 0.34Testing the Sedov expansionRequired:
  • RSNR/D(angular size)
  • t(reliable only for historical SNRs)
  • Vexp/D(expansion rate, measurable only in young SNRs)
  • Other ways to “measure”the shock speed
  • Radial velocities from high-res spectra(in optical, but now feasible also in X-rays)
  • Electron temperature, from modeling the (thermal) X-ray spectrum
  • Modeling the Balmer line profile in non-radiative shocks
  • End of the Sedov phase
  • Sedov in numbers:
  • When forward shock becomes radiative: with
  • Numerically:
  • Internal energyKinetic energyBeyond the Sedov phase
  • When t > ttr, energy no longer conserved.What is left?
  • “Momentum-conservingsnowplow” (Oort 1951)
  • WRONG !! Rarefied gas in the inner regions
  • “Pressure-driven snowplow” (McKee & Ostriker 1977)
  • 2/52/7=0.291/4=0.25Numerical results(Blondin et al 1998)0.33ttrBlondin et al 1998An analytic model
  • Thin shell approximation
  • Analytic solution
  • H either positive (fast branch) limit case: Oort or negative (slow branch) limit case: McKee & OstrikerH,K from initial conditionsBandiera & Petruk 2004Inhomogenous ambient medium
  • Circumstellar bubble (ρ~ r -2)
  • evacuated region around the star
  • SNR may look older than it really is
  • Large-scale inhomogeneities
  • ISM density gradients
  • Small-scale inhomogeneities
  • Quasi-stationary clumps (in optical) in young SNRs (engulfed by secondary shocks)
  • Thermal filled-center SNRs as possibly due to the presence of a clumpy medium
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