The evolution and collapse of BH forming stars

1. The evolution and collapse of BH forming stars • Formation scenarios – If we form them, they will form BHs. • Stellar evolution: current issues • Issues in collapse • Neutrinos from Massive Stars Chris Fryer (LANL/UA)

2. Formation Scenarios • First stars: Without metals, it is possible that the first generation of stars is more massive than later generations (how different is yet to be determined) • Collisions of stars can build up a massive star progenitor (e.g. Portegies-Zwart 2004, see next talk by Evghenii Gaburov’s talk) • Low metallicity scenarios discussed by Marta Volunteri • However such stars are formed, the fate depends only a few parameters – metals (and winds) and final entropy

3. Explosions from stellar collapse – the fate depends on the metallicity

4. The lower metallicity of the first star alters the winds and hence the fate of these stars (but this also depends on the stellar model).

5. Mass Loss depends upon prescription used in stellar model: At solar metallicity, a 120 Msun star may end its life as a 2-3 Msun star or a >30 Msun star (depending on code). Limongi & Chieffi Heger et al.

6. The lower metallicity of the first star alters the winds and hence the fate of these stars (but this also depends on the stellar model). Limongi & Chieffi mass-loss changes BH line and BH masses

7. Heger et al. now working on stars from 1000Msun up to 1 million Msun

8. Mixing in Stars • To construct a single-star progenitor for GRBs, Yoon et al. (2005,2006) “discovered a new rotationally induced mixing algorithm • Although this mechanism increases the number of stellar-mass black holes, it tends to make smaller-mass black holes

9. Stellar Evolution Issues • The amount of mass-loss is still poorly determined and different groups get different answers. For many models, the mass-loss has been scaled to match Wolf-Rayet observations neglecting binary effects. Much of our intuition may be flawed. • The treatment for convection is the source of many numerical errors and is still poorly understood.

10. Modeling Collapse • The Herant et al. (1994) convective engine seems to work. Most groups produce explosions in normal-massed (12-20 solar mass) when they model the instabilities above the proto-neutron star. • But this engine (and the intuition we have gained from it) is not valid for massive stars. Basic SN engine: the core collapses and bounces; convection above the PNS (perhaps SASI) revives explosion

11. Neutrino-Driven Supernova Mechanism: Convective Phase Anatomy Of the Convection Region Proto- Neutron Star Upflow Accretion Shock Downflow Fryer & Warren 2002

12. Collapse of Massive Stars • The evolution is drastically altered by higher entropy – getting accurate entropy measurements essential. • Likely evolution for these massive stars predicts massive proto-black holes (at relatively low densities) that then collapse.

13. Collapse Calculations: Entropy of the Core Critical • Most of our intuition is based on black hole formation of stellar-massed systems. But as we move up in stellar mass, the entropy in the core increases. The intuition we’ve built up in the last decade may prove useless. Initial Entropy 105 solar mass star After 1s After 4s, just prior to BH formation

14. Entropy differences alter the cooling, which in turn, alters every phase of collapse. For example, low entropy cores need to be quite condensed to emit neutrinos. Only the inner core cools, leading to a collapse of the inner ~1Msun. For higher entropy, massive stars, a 40 solar mass proto-black hole forms. 290 ms 280 ms

15. Electron/positron pair annihilation (gamma-rays produce positrons, positrons annihilate with electrons producing neutrinos and anti-neutrinos) dominates the cooling at higher entropies. The lepton fraction remains higher and electron degeneracy pressure plays a stronger role at late times.

16. Neutrino signals from massive stars The neutrino signal from these stars peaks higher than normal supernovae (typically peaking at 1052 erg/s for 10ms) and the luminosity remains high for several seconds! Both a 105 solar mass star (top) and 300 solar mass star (left) produce strong neutrino signals with more than 104 times as much energy in the first few seconds.

17. Neutrino Detection • High fluxes, durations and energies may make these sources directly detectable beyond the Virgo cluster (by IceCube or Dusel). • But the most likely detection is through the diffuse neutrino background. If they occur at low redshift, they will fit directly in the GADZOOKS energy band. Preliminary results of a 105 solar mass black hole : neutrino fluxes (3 flavors) and gray energies. GADZOOKS sensitivity

18. Summary • Massive star evolution plagued by a few key uncertainties: mass loss from winds and stellar mixing. It is possible that BHs above 100Msun can form with metallicities above 0.1 solar. • Although we seem to be converging on an engine for “normal” supernovae, most of our intuition gained from these low-entropy models is not so useful for BH formation. • The neutrino signal from these collapses may be detectable. We need detailed spectra and details of formation. • We’re working on new models, requests taken.