Download Neutron Star Formation

Neutron Star Formation and the
Supernova Engine
Bounce Masses
Mass at Explosion
Fallback
• 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?
Lattimer 2013
Neutron
Star Masses
Lattimer 2013
More accurate,
smaller spread
Velocity (c)
Temperature and Density of the Core
Becomes so High that:
Iron dissociates into alpha particles
Electrons capture onto protons
Core collapses nearly at freefall!
Velocity (c)
Neutrino-Driven Supernova Mechanism
Radius (km)
Core reaches nuclear densities
Nuclear forces and neutron
degeneracy increase pressure
Bounce!
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
Neutrino-Driven Supernova Mechanism: Convective Phase
Anatomy
Of the
Convection
Region
ProtoNeutron
Star
Upflow
Accretion
Shock
Downflow
We can derive
the explosion
energy from
the duration of
this phase!
Fryer 2006
Fryer & Warren 2002
Evolution
Of a
Collapse
Simulation
15 vs. 25
Solar Mass Collapse
Time steps: 50ms, 90ms,
140ms, 240ms
15 solar mass star explodes
At ~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/Hypernovae
Nomoto et al. (2003)
EK
(Jets!)
Failed SN?
13M~15M
M(56Ni)/M
Nomoto et al. (2003)
[/Fe]≫0
Fryer 1999
Binding
Energy
Of the
Outer
Layers
Of the
Star
(Mstar
-3 solar
Masses)
Anatomy of Fallback
Fallback 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 rates
It 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 Atmosphere
Free-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 protoneutron star (fallback, …)
Fryer et al. 2006
Neutrinos from Fallback
Neutrinos from fallback are generally
above 1 foe/s 5-10s after explosion
with energies around 20 MeV – Fryer
2009
Neutrinos from cooling neutron
stars emit below 1 foe/s at 10s with
energies around 10MeV - Burrows
1988
Fallback Supernovae: a possible explanation for low energy supernovae
Moriya et al. 2010
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
Conclusions
• Gravitational mass determined by bounce – 1.01.5 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.
There are also issues with low-mass
NSs
• The e-process (explosive burning of neutronrich 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 Extent
The 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