View
223
Download
5
Category
Tags:
Preview:
Citation preview
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
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
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
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.
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.
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
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
• 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.
Recommended