The Late Evolution and Explosion of Massive Stars With Low Metallicity

1. The Late Evolution and Explosion of Massive Stars With Low Metallicity Stan Woosley (UCSC) Alex Heger (LANL)

2. Evolution and Explosion • of Z = 0 Stars, 12 – 100 • Evolution and Explosion of Z = 0 stars, 140 – 300 • Gamma-Ray Bursts, Supernovae, and Rotation

3. The evolution of Z = 0 massive stars has been studied for many years, e.g. Ezer & Cameron (ApSS, 1971) pointed out that such stars would burn on the CNO cycle: Some generalities: k = 0 affects star formation, the IMF, the star’s pulsational stability, binary evolution, and the formation of red giants Hotter, bluer stars on MS than modern stars. Greatly decreased mass loss Different sort of final evolution – bigger stars, more tightly bound, more rapidly rotating(?)

4. With considerable uncertainty about the critical masses, one can delineate four kinds of deaths (neglecting rotation). He Core Main Seq. Mass Supernova Mechanism

5. Survey 1 (Heger & Woosley, in preparation) Big Bang initial composition, Fields (2002), 75% H, 25% He Evolved from main sequence to presupernova and then exploded with pistons near the edge of the iron core (S/NAk = 4.0) Each model exploded with a variety of energies from 0.3 to 10 x 1051 erg. 126 Models at least 500 supernovae

6. Use Kepler implicit hydrodynamics code • Arbitrary equation of state (electrons, pairs, degeneracy, etc.) • “Adaptive”nuclear reaction network. Nuclei included where flows indicate they are needed. Typically 900 isotopes • Explosions simulated using pistons and models mixed artificially • No mass loss • Approximate light curves calculated using single temperature radiative diffusion. Radioactive decay included.

7. Fe He Si H O

8. Overall, good agreement with solar abundances, but an appreciable odd-even effect. E.g. Na/Mg, Al/Mg, and P/Si and no A > 64.

9. Some general features: • Large odd-even effect • No synthesis above A about 64 (does not include neutrino powered wind or proton-rich bubble, which greatly affect, e.g., Sc, Zn; Pruet et al (2004), astroph 0409446) • Primary B and F from neutrino process • Above M ~ 50, primary N production • Nucleosynthesis sensitive to mixing and fall back

10. Integrating the yields of these models over a Salpeter IMF for various explosion energies, one obtains an approximation to the nucleosynthesis from the first generation of stars.

11. Si Zn Heger & Woosley (2004) Co Ca Ti Mg K Fe Sc Ni Al Cr 1052 erg 1.2 x1051 erg for Sc and Zn see also Pruet et al (astroph 0409446) Mn Na Data from Cayrel et al, A&A, 416, 1117, (2004)

12. In most cases, up to about 50 solar masses, the stars are blue supergiants when they die and their light curves are not exceptionally brilliant –much like SN 1987A

13. 0.3 x 1051 erg 0.6 0.9 1.2 1.5 1.8 2.4 3.0 5.0 10 Masses of 56Ni (solar masses): 0.048 0. 0.057 0.003 0.065 0.22 0.072 0.23 0.078 0.24 0.082 0.25 0.090 0.27 0.095 0.29 -- 0.34 -- 0.44

14. 0.3 x 1051 erg 0.6 0.9 1.2 1.5 1.8 2.4 3.0 5.0 10 Masses of 56Ni (solar masses): 0. 0. 0. -- 0. -- 0. 0.02 0.27 -- 0.28 0.40 0.31 0.42 0.33 0.44 0.37 0.49 0.45 0.59

16. Solar metallicity ; mass cut at Fe-core (after fall back)

17. PreSN Models Black hole formation may have been more frequent early in the universe

18. Gravitational Binding Energy of the Presupernova Star solar low Z This is just the binding energy outside the iron core. Bigger stars are more tightly bound and will be harder to explode. The effect is more pronounced in metal-deficient stars.

19. Summary M < 100 • Nucleosynthesis overall is reasonably consistent with what is seen in the most metal deficient stars in our Galaxy. No clear need for a separate (e.g. supermassive) component at the metallicities studied so far • Supernovae like 87A; brighter if much primary nitrogen is made • Efficient at making black holes for M above about 30 • May be more efficient at making GRBs in the collapsar model

20. Good: • Explosion mechanism well understood • Mass loss may be negligible • Initial composition well known • Pulsationally stable Many Studies in 1970s and 1980s Rakavy, Shaviv Fraley Barkat Arnett, Bond, Carr Ober, El Eid, Fricke Talbot Appenzeller ….

21. Problematic: • Their existence • Mixing between H envelope and He convective core makes primary nitrogen resulting in radical restructuring of the starSensitive to overshoot mixing, rotation, and zoning Determines whether star is BSG or RSG at death • Rotation (no observations for guidance) • Lack of opacity tables for CNO rich Fe deficient matter

23. shortly thereafter star becomes a red supergiant with R > 1014 cm. With rotation and standard overshoot and semiconevcetive parameter settings this happens for all the Z = 0 stars over 100 solar masses

24. Survey 2: Helium Cores: Full Stars: Nucleosynthesis and light curves

25. Initial mass: 150M After explosion

26. Initial mass: 150M Si O Mg S Ar Ca C Fe-deficient by 103

27. Initial mass: 250M

28. Initial mass: 250M Fe Si S Ar Ca Mg O C iron-rich

29. Si O C N

30. N?

31. N ?

32. Bright Supernovae at the edge of the Universe? Scannapieco et al (2005) astroph 0507182 • Explosion energy up to 1053erg(50-100x that of “normal” supernovae) • Up to 50 solar masses of radioactive 56Ni(50-100x that of “normal” supernovae)

33. Calculations by Sergei Blinnikov

35. 130 solar mass helium star Should the stars lose their hydrogen envelopes they could be even brighter (or fainter), 60 solar mass helium star

36. Scannapieco et al conclude that one should be able to limit the fraction of stars in the 140 – 260 solar mass range to less than 1% of the star forming mass density to redshift 2 using current ongoing searches. With JDAM, the limit might be pushed to z = 6 Note that the evolution of metallicity in the universe is not homogeneous. Pockets of low Z material might persist up to observable redshifts.

37. Temperature in 109 K just prior to black hole formation. about 90 solar masses quickly accretes into the black hole. radial velocity 6.5 s after black hole formation Pair-instability collapse for M ~ 300 solar masses (Fryer, Woosley, & Heger ApJ, 550, 372, 2001) Possible observational challenge: long time scale, soft spectrum. Is the envelope on or off?Does the star have enough rotation?

38. III. Gamma-Ray Bursts • GRBs (at least a lot of those of the long-soft variety) come from the deaths of massive stars • At least some of these eject about 0.5 solar masses of56Ni – an important diagnostic of the central engine • The supernovae may be, on the average, hyperenergetic (~1052 erg) and asymmetric. They are Type Ib/c. • Unlike ordinary supernovae, those that make GRBs eject an appreciable – and highly variable - fraction of their energy in relativistic ejecta (G > 200) • The fraction of all supernova-like events that make GRBs is small. Typical Ib/c supernovae have progenitor masses ~ 3 – 5 solar masses and do not make GRBs.

39. The GRB rate is a very small fraction of the total supernova rate Madau, della Valle, & Panagia, MNRAS, 1998 Supernova rate per 16 arc min squared per year ~20 This corresponds to an all sky supernova rate of 6 SN/sec For comparison the universal GRB rate is about 3 /day * 300 forbeaming or ~ 0.02 GRB/sec

40. Today, after times when over 150 GRB models could be “defended”, only two are left standing (for long-soft bursts): • The collapsar model • The millisecond magnetar model Both rely on the existence of situations where some fraction of massive stars die with an unsually large amount of rotation. The degree of rotation and the distribution of angular momentum is what distinguishes GRBs from ordinary supernovae.

41. Common theme (and a potential difficulty): Need iron core rotation at death to correspond to a pulsar of < 5 ms period if rotation and B-fields are to matter at all. Need a period of ~ 1 ms or less to make GRBs. This is much faster than observed in common pulsars. To make a disk around a 3 solar mass black hole need j ~ 5 x 1016 cm2 sec-1

42. Calculations agree that without magnetic torques it is easy to make GRBs This is plenty of angular momentum to make either a ms neutron star or a collapsar. Heger, Langer, & Woosley (2002)

43. Much of the spin down occurs as the star evolves from H depletion toHe ignition, i.e. as a RSG. Heger, Woosley, & Spruit (2004)

44. Good news for pulsars Bad news for GRBs! Heger, Woosley, & Spruit (2004) using magnetic torques as derived inSpruit (2002)

46. never a red giant vrot = 400 km/s He C,Ne He H C,Ne O Si O

47. PreSN He-depl GRB C-depl H 8 ms pulsar

49. Woosley & Heger astroph - 0508175 And so maybe …. GRBs come from single stars on the high- velocity tail of the rotational velocity distribution Such stars mix completely on the main sequence The WR mass loss rate is low (because of metallicity) See also Yoon and Langer astroph - 0508242

50. Effect of Mass Losson Burst Properties The wind mass required to decelerate a relativistic jet of equivalent isotropic energy E and Lorentz factor G is the mass loss rate times the time before the burst For typical GRB (equivalent isotropic) energies, E53 = 1 the relativistic jet with G ~ 100 gives up its energyat around 1015 cm.