Supernovae, Gamma-Ray Bursts, and Stellar Rotation

Transcription

1 Supernovae, Gamma-Ray Bursts, and Stellar Rotation

2 When Massive Stars Die, How Do They Explode? Neutron Star + Neutrinos Neutron Star + Rotation Black Hole + Rotation Colgate and White (1966) Arnett Wilson Bethe Janka Herant Burrows Fryer Mezzacappa etc. Hoyle (1946) Fowler and Hoyle (1964) LeBlanc and Wilson (1970) Ostriker and Gunn (1971) Bisnovatyi-Kogan (1971) Meier Wheeler Usov Thompson etc Bodenheimer and Woosley (1983) Woosley (1993) MacFadyen and Woosley (1999) Narayan (2004) All of the above? M

3 The answer depends on the mass of the core of helium and heavy elements when the star dies and on its angular momentum distribution.

4 It is clear that very many massive stars, probably most, produce neutron stars when they die. A good model should explain: Supernova energies, light curves and spectra Neutron star masses Pulsar rotation rates Pulsar peculiar velocities Neutron star magnetic field strengths Nucleosynthesis Remnant morphology The standard neutron star neutrino model shows great promise for explaining all of these.

5 Scheck et al. (2004)

6 Current supernova models a quick survey: Burrows, Hayes and Fryxell (1995) Fryer (1999); Fryer and Warren (2002) Bruen et al (2001); Liebendorfer et al (2001) Burras et al (2003); Shenk et al. (2004) Fryer and Heger (2000) Akiyama et al (2003) Fryer and Warren (2003) Thompson, Quataert, and Burrows (2004) Healthy explosions without rotation or magnetic fields. Neutrino powered. Crude neutrino physics. 2D and 3D. Marginal failures. 2D with improved neutrino transport. Full sphere shows importance of dipole. Rotation alters convective flow in the neutrino-powered model and makes it weaker Very strong B-fields develop from differential rotation. Rotation powered explosions. B ~ gauss 3D models. Estimate B-field much smaller than Akiyama et al. Neutrino transport dominates B-fields more like Akiyama in realistic model but rotational energy dissipates in neutrinos. Rotation triggers neutrino powered explosion

7 Common theme: Need iron core rotation at death to correspond to a pulsar of < 5 ms period if rotation and B-fields are to matter. This is much faster than observed in common pulsars. A concern: If calculate the presupernova evolution with the same efficient magnetic field generating algorithms as used in some core collapse simulations, will it be rotating at all?

8 Hirishi, Meynet, & Maeder (2004) Heger, Langer, & Woosley (2002) with B-field torques

9 no mass loss or B-field with mass loss with mass loss and B fields These results are from Heger et al (2000) and in preparation.

10 Red Supergiants at Death He core 4 6 Heger, Woosley, & Spruit, in prep. for ApJ Spruit, (2001), A&A, 381, *1.3 for bug fix note models b d (with B-fields) and e (without) note dependence on mass

11 Aside: Note an interesting trend. Bigger stars are harder to explode because they are more tightly bound and have big iron cores. But they also rotate faster when they die. Do GRBs occur even in single stars (that lose their envelope) if the star is big enough?

12 Ways to make j bigger: Smaller magnetic torques during H and He burning Less mass loss (low Z?) Angle dependent mass loss? Larger helium cores evolve quicker Binary mergers Spruit (2001) is approximate M Z 1/2 How much j is saved? 40? M Effect depends on timing of the merger. Need to have sufficient number of sources. r rω Ω 6 GRBs may take special circumstances. Only a percent or so of massive star deaths need go this route even with beaming.

13 nb. B r << B φ Torque B r B φ

14 Collapsar Basics Wolf-Rayet Star no hydrogen envelope about 1 solar radius. Collapse time scale tens of seconds Rapid rotation j ~ erg s Black hole ~ 3 solar masses accretes several solar masses

15 The star collapses and forms a disk (log j > 16.5) In the vicinity of the rotational axis of the black hole, by a variety of possible processes, energy is deposited. 7.6 s after core collapse; high viscosity case. Note offset from t = 0

16 The alternative: The ms Magnetar Model: Wheeler, Yi, Hoeflich, and Wang (2001) Usov (1992, 1994, 1999) Thompson (2003) Assuming the emission of high amplitude ultra-relativistic MHD waves, one has a radiated power P ~ 6 x 10 (1 ms/p) (B/10 gauss) erg s and a total rotational kinetic energy E rot ~ 10 (1 ms/p) (10 km/r) erg Note that this begins immediately after collapse. (If it did not, a black hole would form)

17 How Do We Tell the Difference? 56 Ni mass needs large high initial power and solid angle or convection through a disk Explosion energy (Γ > 2) Collapsar can stay on longer and provide more power Supernova morphology Magnetar may give a more symmetric explosion or equatorial? Are supernovae and GRBs a continuous class? Magnetars might be expected to be continuous. Black hole formation may be qualitatively different Beaming may be different in the two models

18 The passage of a jet through a massive star will always lead to an explosion. All GRBs from massive stars will be accompanied by a supernova. The real issue is the mass of 56 Ni.

19 56 Ni An Important Diagnostic A Type Ib/c supernova is visible only because of the radioactivity it makes. The luminosity at peak measures directly the mass of 56 Ni made in the explosion. For 1998bw, this was near 0.5 solar masses. 56 Ni is only made when matter with equal numbers of neutrons and protons (e.g., 12 C, 16 O, 28 Si) is heated above 5 x 10 9 K. A narrow jet alone will not make enough 56 Ni. Probably the same criticism applies to a thin equatorial fan In the magnetar model the 56 Ni must be made by a shock. (quasi-spherical) In the collapsar it is made by the disk wind.

20 Except near the "mass cut", the shock temperature to which the explosive nucleosynthesis is most sensitive is given very well by 4 π R at Explosion energy erg

21 Ni ESiB Si O EOB ENeB Ejected without much modification

22 Conditions for explosive burning: To nse T > 5 R < er g erg But the shock travels at ~c/20, so must deposit this energy well within 1 second. Otherwise reach too low a temperature to make enough 56 Ni.

23 But the jet must be maintained at least 10 seconds (and better yet, 30) in order that relativistic matter escape the star. The collapsar satisfies these twin constraints of 56 Ni production and long time scale by a) staying on for a hydrodynamic time scale for the whole star and b) making the 56 Ni by advection through a disk, not by a spherical shock. Can the magnetar model do it? 1 16 E52 with t in sec (B = 10 gauss; P o = 1 ms) t + 1 Emit about erg first two seconds and erg after 10 s.

24 If energy is not maintained at the base for a jet crossing time the jet dies.

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26 de rot So if 10 ms ~ 10 gauss dt E 2 rot P Pulsar emits most of erg in first couple of seconds and after 10 s has only erg to provide to the releativistic ejecta. But Berger says E rel > 5 x erg. B

27 How many GRBs are there? (How many could there be given GRB/SN ratio ~ 5%?) Are SN and GRBs a continuous Class? Soderberg et al (2004)

28 Why energy in relativistic ejecta probably is not constant: Variable stellar mass at death Variable distribution of angular momentum Variable interaction of jet with star Which in turn may be affected by metallicity, stellar radius, binary evolution, mass, IMF, etc.

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30 Magnetar Model Collapsar Model? may be more isotropic SN Bang - Whoosh Whoosh - Spray

31 Gravitational Radiation and the Ten Second Gap When the (spherical) star collapses in the collapsar model the jet does not start immediately. There should exist a transition object (proto-neutron star?) endowed with more mass and rotation than allowed for an axially symmetric model.

32 Observational Diagnostics To Distinguish Central Engines Range of supernova properties? Is there a continuum of events ranging down to ordinary SNae? What is the energy in relativistic ejecta (Γ > 2)? Much more than erg may be difficult to accommodate in the magnetar model. How many GRBs (or XRFs) are there? Supernova morphology and better models Gravitational radiation

33 Key Theoretical Issues What is the range of angular momenta in core collapse supernovae? Can ms magnetars provide sufficient beaming, relativistic ejecta energy, and 56 Ni mass? Do all core collapses above a certain mass (and below a certain metallicity) make GRBs? What determines the partition between Γ > 200, Γ > 2, Γ ~ 1, and how do things vary with angle?