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

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

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(?)

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

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

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.

H He O Fe Si

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.

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 ) Primary B and F from neutrino process Above M ~ 50, primary N production Nucleosynthesis sensitive to mixing and fall back

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.

Data from Cayrel et al, A&A, 416, 1117, (2004) Mg erg 1.2 x10 51 erg Na Al Si K Ca Sc Ti Cr Mn Fe Co Ni Zn Heger & Woosley (2004) for Sc and Zn see also Pruet et al (astroph )

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

0.3 x erg Masses of 56 Ni (solar masses):

0.3 x erg Masses of 56 Ni (solar masses):

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

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

Gravitational Binding Energy of the Presupernova Star 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. solar low Z

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

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 ….

Problematic: Their existence Mixing between H envelope and He convective core makes primary nitrogen resulting in radical restructuring of the star Sensitive 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

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

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

Initial mass: 150M  After explosion

Initial mass: 150M  Fe-deficient by 10 3 O Si S Ar Ca Mg C

Initial mass: 250M 

O Si S Ar Ca Fe iron-rich Mg C

O Si N C

N?

N?N?

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

Calculations by Sergei Blinnikov

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

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.

radial velocity 6.5 s after black hole formation Pair-instability collapse for M ~ 300 solar masses (Fryer, Woosley, & Heger ApJ, 550, 372, 2001) Temperature in 10 9 K just prior to black hole formation. about 90 solar masses quickly accretes into the black hole. Possible observational challenge: long time scale, soft spectrum. Is the envelope on or off? Does the star have enough rotation?

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 of 56 Ni – an important diagnostic of the central engine The supernovae may be, on the average, hyperenergetic (~10 52 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.

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 for beaming or ~ 0.02 GRB/sec The GRB rate is a very small fraction of the total supernova rate

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.

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 cm 2 sec -1

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

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

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

H He C,Ne O O Si C,Ne He never a red giant v rot = 400 km/s

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

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 Woosley & Heger astroph

For typical GRB (equivalent isotropic) energies, E 53 = 1 the relativistic jet with G ~ 100 gives up its energy at around cm. 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 Effect of Mass Loss on Burst Properties

Burning Phase Duration Wind Radius Probed by t 10 8 x t Hydrogen 19 My Optical observation Helium 0.5 My Optical observation Carbon/ 3400 y < cm SN Ib, GRB afterglow Neon Oxygen 7 mo < 2 x cm GRB Si 2 weeks < 2 x cm GRB For helium burning and beyond the wind radius is taken to be 1000 km/s times the duration of the burning phase. No WR star has ever been observed in any of the burning phases most appropriate to GRBs. Mass Loss

Summary Credible, though uncertain models can give – approximately – the observed rotation rate of young pulsars for stars that become red supergiants and have their differential rotation partially braked by internal magnetic torques. Using the same torques, but reducing WR mass loss by a factor of a few can give credible GRB progenitors if the stars thoroughly mix during hydrogen and helium burning. This may occur if the stars rotate on the main sequence considerably faster than usual – about 35% Keplerian, 400 km/s. GRBs will be favored by low metallicity. The threshold metallicity for making a GRB depends critically upon the size and Z-dependence of the mass loss rate.