Neutron Star Formation and the Supernova Engine

Bounce MassesMass at Explosion

Fallback

Neutron Star Masses• Observations of neutron star binaries provide a growing list of neutron star mass estimates.• Current observations predict a range of NS masses from 1.0 to >2 solar masses.• Can we explain these masses?

Lattim

er 2013

Lattim

er 2013

More accurate, smaller spread

Neutrino-Driven Supernova Mechanism

Temperature and Density of the Core Becomes so High that: Iron dissociates into alpha particles Electrons capture onto protonsCore collapses nearly at freefall!

Core reaches nuclear densities Nuclear forces and neutron degeneracy increase pressure

Bounce!

Radius (km)

Vel

ocit

y (c

)

Vel

ocit

y (c

)

Radius (km)

NS mass after the

shock stalls

•Depends upon the entropy of the core. • For Stars below ~15-20 solar masses, the stall is around 1.1 solar masses (using the latest MESA models).

The bounce depends upon the structure… Unfortunately this structure depends more on the stellar evolution code

than it does on metallicity or progenitor mass.• Total mass from stellar models:• Heger Solar – 12.9• Heger Zero – 24.9• Limongi Zero – 24.7

Neutrino-Driven Supernova Mechanism: Convection

Fryer 1999

Upflow

Downflow

Proto-NeutronStar

AnatomyOf the ConvectionRegion

Fryer & Warren 2002

AccretionShock

We can derive the explosion energy from the duration of this phase! Fryer 2006

Neutrino-Driven Supernova Mechanism: Convective Phase

EvolutionOf a CollapseSimulation

15 vs. 25 Solar Mass Collapse

Time steps: 50ms, 90ms, 140ms, 240ms

15 solar mass star explodesAt ~90ms.

25 solar mass star explodes At ~240ms.

~90ms~240ms

Neutrino-Driven Supernova Mechanism: Convection

Fryer 1999

• The mass at explosion depends on the delay.• The explosion energy also depends on the delay

Supernovae/HypernovaeNomoto et al. (2003)

Failed SN?

13M~15M

EK

(Jets!)

M(56Ni)/M Nomoto et al. (2003)

[/Fe] 0≫

BindingEnergyOf theOuterLayersOf theStar(Mstar

-3 solarMasses)

Fryer 1999

Anatomy of FallbackFallback Mechanism Rarefaction wave: As the neutron star cools, it accretes, producing a rarefaction wave that catches the shock and decelerates it (Colgate 1971): Accretion happens quickly (first 100s) PdV work: The initial ejecta decelerates as it drives an explosion through the star. If the velocity decelerates below the escape velocity, it falls back (Fryer 1999): Accretion happens quickly (first 100s) Reverse shock: The shock decelerates in the flat density gradient of the envelope, driving a reverse shock. This decelerates the material behind the shock sufficiently to fall back (Nomoto 1988, Woosley 1988): Accretion takes 1000-10,000s.

Fallback ratesIt is difficult to avoid fallback.Most happens at early times, but at the level of 10-4 Msun, this can happen even a year after the explosion.

Building a NS AtmosphereFree-fall Conditions

Gamma-law EOS

Radiation dominated Gas

Explosive Convection

Fallback Diagnostics - Nucleosynthesis

• Nuclear yields pervade many of the diagnostics discussed here (initial models, conditions for remnants)

• Detailed yields can also be compared to grains, stellar abundances, …

• r-process yields can also be used to constrain the conditions on the proto-neutron star (fallback, …)

Fryer et al. 2006

Neutrinos from Fallback

Neutrinos from cooling neutron stars emit below 1 foe/s at 10s with energies around 10MeV - Burrows 1988

Neutrinos from fallback are generally above 1 foe/s 5-10s after explosion with energies around 20 MeV – Fryer 2009

Moriya et al. 2010

Fallback Supernovae: a possible explanation for low energy supernovae

BH systems may place

constraints on fallback.

In the best observed systems, there exists an apparent gap in black hole masses from 3-5 Msun. Ozel et al. 2010 argue this gap is real!The gap argues for prompt explosions or some method to prevent fallback. But is this just an observational bias?

Compact Remnants•The masses of compact remnants can be measured in binary systems (e.g. binary pulsar systems and X-ray binaries) and these observations are producing a growing list of masses.• Advanced LIGO could dramatically increase these mass estimates

Binary Neutron Star Mass Distribution

• Gravitational mass determined by bounce – 1.0-1.5 solar masses

• Gravitational mass determined by engine depends on the delay (the explosion energy is an indicator).

• Fallback typically adds another >0.1 solar masses of material.

• We can not match all the observations (the observations seem contradictory).

• BNS mergers provide a potential probe if we can distinguish NS from BH collapse systems.

Conclusions

There are also issues with low-mass NSs

• The e-process (explosive burning of neutron-rich material – stellar cores will be neutron rich) will produce a lot of intermediate-mass elements.

• To avoid this, scientists have argued that all this must remain in the remnant.

• Unfortunately, if this occurs, we can’t make 1.0 solar mass neutron stars.

NS Atmospheres: Structure of Atmosphere

Atmosphere ExtentThe fallback atmosphere keeps expanding until neutrino cooling halts the expansion.This derivation assumes that the unstable entropy profile drives quick (and smooth) convection that equalizes the entropy.

Energy Conservation

Pair annihilation neutrino emission