Lecture 2 Supernovae and Supernova Remnants

Transcription

1 Lecture 2 Supernovae and Supernova Remnants! The destiny of the stars! Explosive nucleosynthesis! Facts about SNe! Supernova remnants * Morphological classification * Evolutive stages! Emission of SNRs across the electromagnetic spectrum

2

3 Fate of the stars Stars with initial mass 0.25 < M < 8 M 0 Stars with initial mass M > 8 M o M final <1.4 M o, isolated white dwarf M final <1.4 M o, with binary companion Type Ia Supernova Up to 20-30% of mass loss 1.4<M<3 M o Type Ib/c, II Supernova M>3 M o Neutron star Black hole

4 Relative sizes Earth White Dwarf Neutron Star

5 Supernovae and their remnants play a key role in the galactic ecology. They: " release the nucleosynthesis products " mix, process and redistribute the matter in the host galaxies, violently merging stellar material with gas and dust " accelerate particles to relativistic velocities and (maybe) give origin to cosmic rays "compress surrounding clouds and (maybe) initiate new cycles of stars

6 Facts about SN explosions Initial energy released by normal SNe (Type I and II): erg. Though observed (Pastorello 2004, MNRAS 347, 74) and (Nomoto 2001, SN and GRB, Cambridge, p.144) 1% of this energy goes as mechanical energy into the ISM After 100 yrs the original ergs are distributed within 1 pc 3 This is 2 x 10 7 ev/cm 3, 10 7 times larger than typical interstellar star light and cosmic ray density One to several solar masses are ejected at speed of 5000 to km/s

7 In the Universe there are approximately 8 new SNe exploding per second. In the next hour there will be almost 30 thousand new SNe.

8 Explosive nucleosynthesis The explosion is a powerful source of neutrons: neutrons/cm 2 sec With enough neutrons more massive nuclei can be built n + (A,Z) # (A+1, Z) # (A+1, Z+1) + e + neutrino n + (A+1, Z+1) # (A+2, Z+1) #(A+2, Z+2) + e + neutrino And so on

9 The addition of neutrons can be Fast r-process (fraction of seconds) Slow s-process (few seconds) There are also p-process Low neutrons fluxes produce elements up to Iodine (atomic mass 130). Large neutrons fluxes can produce actinids (Thorium, Uranium)

10 PRIMORDIAL NUCLEOSYNTHESIS STELLAR NUCLEOSYNTHESIS EXPLOSIVE NUCLEOSYNTHESIS

11 Calculated nucleosynthesis products in solar masses (Tsujimoto et al. 1995) Atom SNIa (Mo) CC SN Oxygen Neon Magnesium Silicon Sulphur Calcium Iron Nickel (25M o progenitor)

12 The remnants of the explosion After the explosion of a SN it is expected to find: ejected stellar debris a shell of shocked ISM and swept-up material a central compact core (neutron star or black hole) a synchrotron nebula around the central neutron star thermal X-ray emission from the hot interior optical filaments from stellar ejecta and from interaction between the SN shock and the surrounding clouds Their physical sizes vary from a few to a hundred pc

13 Standard picture of the evolutionary cycle of SNRs Explosion releases ~10 51 ergs of energy into the ISM in the form of a rapid blast wave accompanied by slower moving ejecta. The evolution can be described following 4 different stages: $ Free expansion (expanding into CSM, less than years) $ Adiabatic or Sedov-Taylor phase (about 20,000 years) $ Radiative or snow-plow phase (0.5 - few 10 5 yr) $ and then merge with the ISM

14 The onset and end of each phase is strongly dependent on the environs, and more than one phase can co-exist in a SNR.

15 Phase I: Free expansion In this phase the dynamics of the SNR is governed by the undecelerated expansion of the shock wave R Qt This phase lasts a few hundred years (for E 0 ~10 51 erg, M ej ~ 1 M o and n IS ~1 at/cm 2) It is usually assumed that the CSM (where the initial expansion occurs) has a power-law density profile n Q r- 5-7

16 Phase II: Adiabatic or Sedov-Taylor As the SNR evolves, the blast wave sweeps up ISM material forming a shell of gas at ~ K The shell surrounds a tenuous, hot interior, and a reverse shock propagates into the slower moving ejecta

17 The name adiabatic is because energy losses due to radiation are not important. That is, the cooling time for radiative processes is much longer than the characteristic expansion time (~ R / v). The dynamics of the remnants expansion can be treated as an explosion that releases energy but not mass, evolving into an ideally homogeneous, monoatomic, nonrelativistic gas. This phase can last several thousand years.

18 The expansion relation is: R s = 1.17 (E 51 / n 1 ) 1/5 t 2/5 pc v S = 0.4 R s / t km/s Ts = 1.8 x 10 5 (Rs / t) 2 kev shell width!r = Rs/12 For strong shock conditions and ideal gas behavior: n 2 = 4 n 1 v2 = ¾ v1 (1 for undisturbed ISM, 2 post shock gas) For ideal gas with adiabatic index! = 5/3

19 Outer shock Stellar ejecta Reverse shock Shocked ISM Contact discontinuity The contact discontinuity is a surface of equal pressure separating stellar ejecta from shocked gas

20 The reverse shock develops when the pressure behind the shocked region (freely expanding ejecta) is lower than the pressure of the shock heated gas. The reverse shock initially moves in the same direction as the forward shock but at smaller velocity. The name derives from the reverse motion relative to the forward shock. When the mass of the swept up ISM is several times the ejected mass, it will also move backward with respect to an outside observer The contact discontinuity separates the shocked circumstellar matter from shocked ejecta.

21 Blast wave: in radio Stellar ejecta: optical X-rays Shocked gas: in radio, optical?, X-ray?

22 SNR Approx. age Shock velocity Kepler Tycho SN1006 RCW 86 Cygnus Loop CTB yr 1500 km/s 440 yr 2400 km/s 1000 yr 1900 km/s 1800 yr 600 km/s 8000 (20000) yr km/s yr 200 km/s

23 Phase III: Radiative As the remnant expands and cools, radiative cooling losses become important, and the shock speed decreases as the remnant cools further. Material inside the shock front form a dense shell. When the shock becomes radiative,, the compression changes from! 4 to ~ 100 and the magnetic pressure dominates the postshock gas (rather than gas pressure).

24 When the shock expansion velocity falls to a few tens of km/s, the SNR looses its identity and merges with the surrounding ISM. TOTAL LIFETIME OF A SNR ~ 10 5 yrs

25 IMPORTANT This simplified scheme is only valid for shell-type SNRs,, with no interior NS refreshing the radiation by injecting newly accelerated particles. Spherical expansion was proposed for cases of evolution into homogeneous ISM. Also asymmetric explosions will drastically change the evolution. Several dynamic phases can coexist in the same SNR.

26 Although the optical Crab nebula was identified by Lundmark back in 1921 (PASP 33, 234) as the remnant of the historical SN seen in 1054, it was only after the development of radio astronomy (1950s) when many more SNRs were identified in our Galaxy. Almost all Galactic SNRs emit in radio, ~ 30% in X-rays and ~ 20% in optical wavelengths There are 265 SNRs positively identified in our Galaxy (year 2006) ~ 40 in the LMC and ~ 20 in the SMC ~98 optical candidates in M33, of which 53 were detected in radio

27 Optical SNRs in the LMC

28 Supernova Remnants across the electromagnetic spectrum

29 Radio continuum emission from SNRs It is non-thermal, of synchrotron origin If there is an extended source with non-thermal spectrum: it is a SNR Thermal absorption SNR synchrotron emission Thermal emission S Qn " Where S is the observed flux density at the frequency " and " the radio spectral index

30 To generate synchrotron emission we need accelerated particles and magnetic field

31 Source of relativistic particles: original SN event a pulsar within the remnant ambient cosmic rays particles accelerated at the Rayleigh-Taylor unstable contact discontinuity Source of magnetic field: generated by the neutron star swept up along with ambient ISM turbulently amplified in the shock or at the contact discontinuity

32 For Crab-like remnants, a pulsar is a reasonable source of particles and field. For old shell-type remnants, field and particles can be locally generated or taken from the ambient medium. Young remnants, however, are too bright to be explained in this way. For example, Tycho s mean emissivity is 10 5 times larger than the ambient galactic value. Too large to be accounted for by a factor of 4 compression.

33 A compression ratio of 4, if applied to both field and electrons, can boost the emissivity by a factor of ~ 50. Conclusion: particles and/or magnetic field must have a different origin We need turbulent acceleration at the unstable contact discontinuity and at the outer shock. First and second order Fermi acceleration

34 In first-order Fermi mechanism the turbulent processes occur behind the shock (diffusive shock acceleration): produces smooth edges Second-order Fermi acceleration requires random scattering centers: produces irregular radio shells

35 What can radio observations of SNRs tell us? Morphology: size, shape, asymmetries, brightness distribution Radio spectral index " (S # $# " ) measures the energy spectrum N(E) $ E -! of the relativistic GeV electrons accelerated in SNR shocks (where %=1-2"). Spectrum is the only way to investigate particle acceleration Polarization: provides information about orientation of Electric field. Faraday rotation and depolarization along the line of sight can seriously compromise the true knowledge of the B field orientation

36 Radio SNRs come in three basic models Shell - type: where the relativistic electrons are accelerated at the shock front. About 85% of the Galactic SNRs belong to this class. The radio emission is weakly polarized, between 3 and 5 %. The spectral index " = -0.55, though it varies -0.3 &" & -0.8 Typical examples of this class are Cas A and Tycho's, SNR.

37 SN 1006 Gamma Cygni

38 Crab-like, filled-center or plerions: About 4% of the catalogued Galactic SNRs belong to this class. The radio brightness is centrally concentrated. The accelerated particles and magnetic fields responsible for the synchrotron emission are injected by a central neutron star (sometimes undetected) generated in the supernova event. They are characterized by a flat non-thermal spectrum -0.3&"& 0 and high fractional linear polarization (20 to 30 %).

39 Composites: which include a shell plus a central component (in radio or thermal composites, when he internal nebula is only seen in X-rays. The central nebula is polarized and has a flat radio spectral index, while the shell has a steeper " and is less or not polarized. The represent ~12% of the Galactic SNRs

40 Although they can exhibit a large variety of morphologies

41 W44: core-collapse SN from a progenitor with mass between 8 and 20 M o. Age ~20000yr (during MS, the progenitor would have been between B4 and O8) Castelletti et al. 2006

42 Radio polarization Young remnants have low polarized fractions and their ordered components are dominantly radial in orientation, while older remnants have more varied geometries with occasionally much higher polarized fractions and tangential orientation.

43 VLA radio image of Tycho SNR Shock front Fluid instabilities Rayleigh Taylor instabilities are very important because VLA radio they produce turbulence and also enhance the magnetic image field, producing important synchrotron radiation

44 Infrared Continuum radiation Thermal:shock shock-heatedheated circumstellar and interstellar dust swept-up at the boundary of the SNR. The main mechanism to heat the dust is through collisions in the X-ray emitting material (good spatial correlation observed in several SNRs among the infrared radiation and the emission in others spectral regimes, e.g. Tycho, Cas A, Cygnus Loop,, IC443, Kepler). Non thermal:synchrotron

45 Infrared lines Shocked gas cools through atomic and molecular lines, many of which emit in the near and mid-infrared infrared region of the spectrum. The dominant coolant for shocked molecular gas over a wide range of densities is H 2 line emission. Its detection is a useful tool to trace regions of interaction between molecular clouds and SNRs. Infrared atomic fine-structure lines, such as CII, NII, NIII, OI, OIV, FeII, arise from fast shocks cooling into moderately dense gas (e.g( e.g km/s shocks into gas with density cm -3 )

46 Infrared observations towards the very young SNRs offer the possibility of detecting dust that has been formed from metalenriched ejecta of the SN itself before it is dispersed and mixed into the general ISM. Cas A # = 160 µm Krause y col. 2004, Nature 432,596 Spitzer image, Ennis et al. 2006

47 Shock heated circumstellar and interstellar dust Kepler #=24 µm Williams y col ApJ 662,1013

48

49

50

51 Optical emission

52 Crab Nebula: dominant radiation synchrotron

53 Optical spectrum: Forbidden emission lines produced by a wide variety of elements [OII], [OIII], [SII] and [NII] and fainter lines of HeI, HeII,, [OI], [NI], [NIII], [FeII[ FeII], [FeIII[ FeIII], [CaII[ CaII] and [ArIII], as well as the hydrogen Balmer series. These lines arise from shocked interstellar material that is cooling radiatively. An important line discrimator useful to discover optically SNRs is [SII] / H" H > 0.45 (in general greater than 1) together with simultaneous detection of [OII] 3727A, [OIII] 5007 A and [OI]6300 A, which characterize emission from shock heated gas

54 G in OIII Fragments of ejecta in Cas A Winkler & Long (2006) AJ,132,360

55 A few remnants have the spectrum dominated by Balmer lines emission with little or no evidence of the forbidden lines. These Balmer-dominated shocks can be explained in terms of a high velocity non-radiative radiative, collisionless shock moving into partially neutral interstellar material. Such filaments define the current location of the blast wave and provide an important diagnostic of physical conditions in the forward shock (e.g.. in SN1006).

56

57 Other SNRs are O-rich because they have a higher O yield. The most prominent members of this group are Cas A, Puppis A and G The X-ray spectroscopy confirms that these remnants are also O-rich in this band. Possibly they are the product of the most massive stars, probable remnants of Type Ib/c Sne and the O-rich filaments are fragments of nearly pure ejecta that were launched from the core of the progenitor star during its explosion and that remain virtually incontaminated.

58 X-ray emission

59 Thermal and non-thermal X-ray emission Non-thermal spectrum: particles accelerated at the shock front Thermal spectrum: composition of ejecta and swept-up ISM

60 Shell thermal Optically thin plasma shock heated to T ~ 10 6 K non-thermal Synchrotron SN1006, RCW86, CasA, G347, G Plerionic omponent non-thermal Synchrotron feeded by a central

61

62 The X-ray spectrum is a combination of a thermal bremsstrahlung continuum and emission lines from highly ionized species such as C, N, O, Fe and Ne, Mg, Si, S, and Ca. The quantitative analysis of thermal X-ray spectrum provides immense detail regarding T, composition, distribution, and ionization state of material synthesized and ejected in SN explosions, as well as that for swept-up material from the circumstellar and interstellar medium.

63 Chandra X-ray image of Tycho SNR Outer shock Stellar ejecta Contact discontinuity Interface between stellar ejecta and swept up ISM

64 From Warren et al. 2005, ApJ 634, 376 Radio- Green contour indicates CD reverse shock blast wave (from radio) Fe K" K image with continuum subtracted.

65 Composite: thermal shell and internal synchrotron nebula

66 For filled-center SNRs in which the X-ray emission is nonthermal, its origin is synchrotron powered by a central pulsar. In the cases that the emission is thermal its origin has not yet been established. Several scenarios have been proposed to explain it, such as thermal conduction in the remnant interior (Cox et al. 1999), evaporation of clouds that are left relatively intact after the passage of the SNR blast wave (White & Long 1991) or projection effects (Petruk 2001).

67 Connection between SN type and SNR based on the X-ray data Thermonuclear explosion Gravitational collapse

68 Gamma-ray emission Nucleosynthesis products --> in very young SNRs Very high-energy emission

69 Explosive nucleosynthesis 44 Ti # 44 Sc # 44 Ca Line emission at 68, 78 and 1157 kev. Since the decay time of 44 Ti is about 86 years, this line may be detected from young (few centuries old) SNRs, (e.g. Cas A ). The production of this isotope has a large variation of yields depending on the mechanism of explosion, the progenitor mass and the mass cut, which defines the mass of the stellar remnant (Woosley & Weaver 1994, 1995).

70 Very high energy!-ray emission from SNRs Several mechanisms have been proposed for their origin: Non-thermal bremsstrahlung of electrons colliding with ambient gas Inverse Compton scattering of ambient photons, such as the cosmic microwave background and Decay of neutral pions created by the collision of energetic protons. In spite of the very good morphological and spectral detail provided by the H.E.S.S instrument and the new gamma-ray observatories, without additional constraints, it is difficult to differentiate between these mechanisms.

71 G Vela Jr

72